Java IO流

前言
1 初识Java IO
- 1.1 IO流分类
- 1.2 案例实操
2 IO流对象
3 IO流方法
- 3.1 字节流方法
- 3.2 字符流方法
4 附加内容

前言

有人曾问fastjson的作者（阿里技术专家高铁）：“你开发fastjson，没得到什么好处，反而挨了骂背了锅，这种事情你为什么要做呢？”

高铁答道：“因为热爱本身，就是奖励啊！”

这个回答顿时触动了我。想想自己，又何尝不是如此。写作是个痛苦的过程，用心写作就更加煎熬，需字字斟酌，反复删改才有所成。然而，当一篇篇精良文章出自己手而呈现眼前时，那些痛苦煎熬就都那么值得。如果这些博文能有幸得大家阅读和认可，就更加是莫大的鼓舞了。技术人的快乐就是可以这么纯粹和简单。

如果各位能顺手点个赞，就更好了(●’◡’●)。

IO流是Java中的一个重要构成部分，也是我们经常打交道的。这篇关于Java IO的博文干货满满，堪称全网前三（请轻喷！）

下面几个问题（问题还会继续补充），如果你能对答如流，那么恭喜你，IO知识掌握得很好，可以立即关闭文章。反之，你可以在后面得文章中寻找答案。

Java IO流有什么特点？
Java IO流分为几种类型？
字节流和字符流的关系与区别？
字符流是否使用了缓冲？
缓冲流的效率一定高吗？为什么？
缓冲流体现了Java中的哪种设计模式思想？
为什么要实现序列化?如何实现序列化?
序列化数据后，再次修改类文件，读取数据会出问题，如何解决呢?

1 初识Java IO

IO，即in和out，也就是输入和输出，指应用程序和外部设备之间的数据传递，常见的外部设备包括文件、管道、网络连接。

Java 中是通过流处理IO 的，那么什么是流？

流（Stream），是一个抽象的概念，是指一连串的数据（字符或字节），是以先进先出的方式发送信息的通道。

当程序需要读取数据的时候，就会开启一个通向数据源的流，这个数据源可以是文件，内存，或是网络连接。类似的，当程序需要写入数据的时候，就会开启一个通向目的地的流。这时候你就可以想象数据好像在这其中“流”动一样。

一般来说关于流的特性有下面几点：

先进先出：最先写入输出流的数据最先被输入流读取到。
顺序存取：可以一个接一个地往流中写入一串字节，读出时也将按写入顺序读取一串字节，不能随机访问中间的数据。（RandomAccessFile除外）
只读或只写：每个流只能是输入流或输出流的一种，不能同时具备两个功能，输入流只能进行读操作，对输出流只能进行写操作。在一个数据传输通道中，如果既要写入数据，又要读取数据，则要分别提供两个流。

1.1 IO流分类

IO流主要的分类方式有以下3种：

按数据流的方向：输入流、输出流
按处理数据单位：字节流、字符流
按功能：节点流、处理流

1、输入流与输出流

输入与输出是相对于应用程序而言的，比如文件读写，读取文件是输入流，写文件是输出流，这点很容易搞反。

2、字节流与字符流

字节流和字符流的用法几乎完成全一样，区别在于字节流和字符流所操作的数据单元不同，字节流操作的单元是数据单元是8位的字节，字符流操作的是数据单元为16位的字符。

为什么要有字符流？

Java中字符是采用Unicode标准，Unicode 编码中，一个英文为一个字节，一个中文为两个字节。

而在UTF-8编码中，一个中文字符是3个字节。例如下面图中，“云深不知处”5个中文对应的是15个字节：-28-70-111-26-73-79-28-72-115-25-97-91-27-92-124

那么问题来了，如果使用字节流处理中文，如果一次读写一个字符对应的字节数就不会有问题，一旦将一个字符对应的字节分裂开来，就会出现乱码了。为了更方便地处理中文这些字符，Java就推出了字符流。

字节流和字符流的其他区别：

字节流一般用来处理图像、视频、音频、PPT、Word等类型的文件。字符流一般用于处理纯文本类型的文件，如TXT文件等，但不能处理图像视频等非文本文件。用一句话说就是：字节流可以处理一切文件，而字符流只能处理纯文本文件。
字节流本身没有缓冲区，缓冲字节流相对于字节流，效率提升非常高。而字符流本身就带有缓冲区，缓冲字符流相对于字符流效率提升就不是那么大了。详见文末效率对比。

以写文件为例，我们查看字符流的源码，发现确实有利用到缓冲区：

3、节点流和处理流

节点流：直接操作数据读写的流类，比如FileInputStream

处理流：对一个已存在的流的链接和封装，通过对数据进行处理为程序提供功能强大、灵活的读写功能，例如BufferedInputStream（缓冲字节流）

处理流和节点流应用了Java的装饰者设计模式。

下图就很形象地描绘了节点流和处理流，处理流是对节点流的封装，最终的数据处理还是由节点流完成的。

在诸多处理流中，有一个非常重要，那就是缓冲流。

我们知道，程序与磁盘的交互相对于内存运算是很慢的，容易成为程序的性能瓶颈。减少程序与磁盘的交互，是提升程序效率一种有效手段。缓冲流，就应用这种思路：普通流每次读写一个字节，而缓冲流在内存中设置一个缓存区，缓冲区先存储足够的待操作数据后，再与内存或磁盘进行交互。这样，在总数据量不变的情况下，通过提高每次交互的数据量，减少了交互次数。

联想一下生活中的例子，我们搬砖的时候，一块一块地往车上装肯定是很低效的。我们可以使用一个小推车，先把砖装到小推车上，再把这小推车推到车前，把砖装到车上。这个例子中，小推车可以视为缓冲区，小推车的存在，减少了我们装车次数，从而提高了效率。

需要注意的是，缓冲流效率一定高吗？不一定，某些情形下，缓冲流效率反而更低，具体请见IO流效率对比。

完整的IO分类图如下：

1.2 案例实操

接下来，我们看看如何使用Java IO。

文本读写的例子，也就是文章开头所说的，将“松下问童子，言师采药去。只在此山中，云深不知处。”写入本地文本，然后再从文件读取内容并输出到控制台。

1、FileInputStream、FileOutputStream（字节流）

字节流的方式效率较低，不建议使用

public class IOTest {
	public static void main(String[] args) throws IOException {
		File file = new File("D:/test.txt");
	<span class="token function">write</span><span class="token punctuation">(</span>file<span class="token punctuation">)</span><span class="token punctuation">;</span>
	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token function">read</span><span class="token punctuation">(</span>file<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">write</span><span class="token punctuation">(</span>File file<span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span>
	OutputStream os <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">FileOutputStream</span><span class="token punctuation">(</span>file<span class="token punctuation">,</span> <span class="token boolean">true</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token comment">// 要写入的字符串</span>
	String string <span class="token operator">=</span> <span class="token string">"松下问童子，言师采药去。只在此山中，云深不知处。"</span><span class="token punctuation">;</span>
	<span class="token comment">// 写入文件</span>
	os<span class="token punctuation">.</span><span class="token function">write</span><span class="token punctuation">(</span>string<span class="token punctuation">.</span><span class="token function">getBytes</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token comment">// 关闭流</span>
	os<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">public</span> <span class="token keyword">static</span> String <span class="token function">read</span><span class="token punctuation">(</span>File file<span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span>
	InputStream in <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">FileInputStream</span><span class="token punctuation">(</span>file<span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token comment">// 一次性取多少个字节</span>
	<span class="token keyword">byte</span><span class="token punctuation">[</span><span class="token punctuation">]</span> bytes <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">byte</span><span class="token punctuation">[</span><span class="token number">1024</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
	<span class="token comment">// 用来接收读取的字节数组</span>
	StringBuilder sb <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">StringBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token comment">// 读取到的字节数组长度，为-1时表示没有数据</span>
	<span class="token keyword">int</span> length <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
	<span class="token comment">// 循环取数据</span>
	<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>length <span class="token operator">=</span> in<span class="token punctuation">.</span><span class="token function">read</span><span class="token punctuation">(</span>bytes<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
		<span class="token comment">// 将读取的内容转换成字符串</span>
		sb<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span>bytes<span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> length<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token punctuation">}</span>
	<span class="token comment">// 关闭流</span>
	in<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token keyword">return</span> sb<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}

2、BufferedInputStream、BufferedOutputStream（缓冲字节流）

缓冲字节流是为高效率而设计的，真正的读写操作还是靠FileOutputStream和FileInputStream，所以其构造方法入参是这两个类的对象也就不奇怪了。

public class IOTest {
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">write</span><span class="token punctuation">(</span>File file<span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span>
	<span class="token comment">// 缓冲字节流，提高了效率</span>
	BufferedOutputStream bis <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">BufferedOutputStream</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">FileOutputStream</span><span class="token punctuation">(</span>file<span class="token punctuation">,</span> <span class="token boolean">true</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token comment">// 要写入的字符串</span>
	String string <span class="token operator">=</span> <span class="token string">"松下问童子，言师采药去。只在此山中，云深不知处。"</span><span class="token punctuation">;</span>
	<span class="token comment">// 写入文件</span>
	bis<span class="token punctuation">.</span><span class="token function">write</span><span class="token punctuation">(</span>string<span class="token punctuation">.</span><span class="token function">getBytes</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token comment">// 关闭流</span>
	bis<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">public</span> <span class="token keyword">static</span> String <span class="token function">read</span><span class="token punctuation">(</span>File file<span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span>
	BufferedInputStream fis <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">BufferedInputStream</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">FileInputStream</span><span class="token punctuation">(</span>file<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token comment">// 一次性取多少个字节</span>
	<span class="token keyword">byte</span><span class="token punctuation">[</span><span class="token punctuation">]</span> bytes <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">byte</span><span class="token punctuation">[</span><span class="token number">1024</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
	<span class="token comment">// 用来接收读取的字节数组</span>
	StringBuilder sb <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">StringBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token comment">// 读取到的字节数组长度，为-1时表示没有数据</span>
	<span class="token keyword">int</span> length <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
	<span class="token comment">// 循环取数据</span>
	<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>length <span class="token operator">=</span> fis<span class="token punctuation">.</span><span class="token function">read</span><span class="token punctuation">(</span>bytes<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
		<span class="token comment">// 将读取的内容转换成字符串</span>
		sb<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span>bytes<span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> length<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token punctuation">}</span>
	<span class="token comment">// 关闭流</span>
	fis<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token keyword">return</span> sb<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}


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3、InputStreamReader、OutputStreamWriter（字符流）

字符流适用于文本文件的读写，OutputStreamWriter类其实也是借助FileOutputStream类实现的，故其构造方法是FileOutputStream的对象

public class IOTest {
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">write</span><span class="token punctuation">(</span>File file<span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span>
	<span class="token comment">// OutputStreamWriter可以显示指定字符集，否则使用默认字符集</span>
	OutputStreamWriter osw <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">OutputStreamWriter</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">FileOutputStream</span><span class="token punctuation">(</span>file<span class="token punctuation">,</span> <span class="token boolean">true</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token string">"UTF-8"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token comment">// 要写入的字符串</span>
	String string <span class="token operator">=</span> <span class="token string">"松下问童子，言师采药去。只在此山中，云深不知处。"</span><span class="token punctuation">;</span>
	osw<span class="token punctuation">.</span><span class="token function">write</span><span class="token punctuation">(</span>string<span class="token punctuation">)</span><span class="token punctuation">;</span>
	osw<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">public</span> <span class="token keyword">static</span> String <span class="token function">read</span><span class="token punctuation">(</span>File file<span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span>
	InputStreamReader isr <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">InputStreamReader</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">FileInputStream</span><span class="token punctuation">(</span>file<span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token string">"UTF-8"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token comment">// 字符数组：一次读取多少个字符</span>
	<span class="token keyword">char</span><span class="token punctuation">[</span><span class="token punctuation">]</span> chars <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">char</span><span class="token punctuation">[</span><span class="token number">1024</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
	<span class="token comment">// 每次读取的字符数组先append到StringBuilder中</span>
	StringBuilder sb <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">StringBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token comment">// 读取到的字符数组长度，为-1时表示没有数据</span>
	<span class="token keyword">int</span> length<span class="token punctuation">;</span>
	<span class="token comment">// 循环取数据</span>
	<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>length <span class="token operator">=</span> isr<span class="token punctuation">.</span><span class="token function">read</span><span class="token punctuation">(</span>chars<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
		<span class="token comment">// 将读取的内容转换成字符串</span>
		sb<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span>chars<span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> length<span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token punctuation">}</span>
	<span class="token comment">// 关闭流</span>
	isr<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token keyword">return</span> sb<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>

}


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4、字符流便捷类

Java提供了FileWriter和FileReader简化字符流的读写，new FileWriter等同于new OutputStreamWriter(new FileOutputStream(file, true))

public class IOTest {
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">write</span><span class="token punctuation">(</span>File file<span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span>
	FileWriter fw <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">FileWriter</span><span class="token punctuation">(</span>file<span class="token punctuation">,</span> <span class="token boolean">true</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token comment">// 要写入的字符串</span>
	String string <span class="token operator">=</span> <span class="token string">"松下问童子，言师采药去。只在此山中，云深不知处。"</span><span class="token punctuation">;</span>
	fw<span class="token punctuation">.</span><span class="token function">write</span><span class="token punctuation">(</span>string<span class="token punctuation">)</span><span class="token punctuation">;</span>
	fw<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">public</span> <span class="token keyword">static</span> String <span class="token function">read</span><span class="token punctuation">(</span>File file<span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span>
	FileReader fr <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">FileReader</span><span class="token punctuation">(</span>file<span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token comment">// 一次性取多少个字节</span>
	<span class="token keyword">char</span><span class="token punctuation">[</span><span class="token punctuation">]</span> chars <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">char</span><span class="token punctuation">[</span><span class="token number">1024</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
	<span class="token comment">// 用来接收读取的字节数组</span>
	StringBuilder sb <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">StringBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token comment">// 读取到的字节数组长度，为-1时表示没有数据</span>
	<span class="token keyword">int</span> length<span class="token punctuation">;</span>
	<span class="token comment">// 循环取数据</span>
	<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>length <span class="token operator">=</span> fr<span class="token punctuation">.</span><span class="token function">read</span><span class="token punctuation">(</span>chars<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
		<span class="token comment">// 将读取的内容转换成字符串</span>
		sb<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span>chars<span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> length<span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token punctuation">}</span>
	<span class="token comment">// 关闭流</span>
	fr<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token keyword">return</span> sb<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}


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5、BufferedReader、BufferedWriter（字符缓冲流）
public class IOTest {
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">write</span><span class="token punctuation">(</span>File file<span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span>
	<span class="token comment">// BufferedWriter fw = new BufferedWriter(new OutputStreamWriter(new</span>
	<span class="token comment">// FileOutputStream(file, true), "UTF-8"));</span>
	<span class="token comment">// FileWriter可以大幅度简化代码</span>
	BufferedWriter bw <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">BufferedWriter</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">FileWriter</span><span class="token punctuation">(</span>file<span class="token punctuation">,</span> <span class="token boolean">true</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token comment">// 要写入的字符串</span>
	String string <span class="token operator">=</span> <span class="token string">"松下问童子，言师采药去。只在此山中，云深不知处。"</span><span class="token punctuation">;</span>
	bw<span class="token punctuation">.</span><span class="token function">write</span><span class="token punctuation">(</span>string<span class="token punctuation">)</span><span class="token punctuation">;</span>
	bw<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">public</span> <span class="token keyword">static</span> String <span class="token function">read</span><span class="token punctuation">(</span>File file<span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span>
	BufferedReader br <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">BufferedReader</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">FileReader</span><span class="token punctuation">(</span>file<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token comment">// 用来接收读取的字节数组</span>
	StringBuilder sb <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">StringBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token comment">// 按行读数据</span>
	String line<span class="token punctuation">;</span>
	<span class="token comment">// 循环取数据</span>
	<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>line <span class="token operator">=</span> br<span class="token punctuation">.</span><span class="token function">readLine</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">!=</span> null<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
		<span class="token comment">// 将读取的内容转换成字符串</span>
		sb<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span>line<span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token punctuation">}</span>
	<span class="token comment">// 关闭流</span>
	br<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token keyword">return</span> sb<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}


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2 IO流对象
第一节中，我们大致了解了IO，并完成了几个案例，但对IO还缺乏更详细的认知，那么接下来我们就对Java IO细细分解，梳理出完整的知识体系来。
Java种提供了40多个类，我们只需要详细了解一下其中比较重要的就可以满足日常应用了。
2.1 File类
File类是用来操作文件的类，但它不能操作文件中的数据。
public class File extends Object implements Serializable, Comparable<File>

 
 

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File类实现了Serializable、 Comparable<File>，说明它是支持序列化和排序的。
File类的构造方法




方法名
说明




File(File parent, String child)
根据 parent 抽象路径名和 child 路径名字符串创建一个新 File 实例。


File(String pathname)
通过将给定路径名字符串转换为抽象路径名来创建一个新 File 实例。


File(String parent, String child)
根据 parent 路径名字符串和 child 路径名字符串创建一个新 File 实例。


File(URI uri)
通过将给定的 file: URI 转换为一个抽象路径名来创建一个新的 File 实例。




File类的常用方法




方法
说明




createNewFile()
当且仅当不存在具有此抽象路径名指定名称的文件时，不可分地创建一个新的空文件。


delete()
删除此抽象路径名表示的文件或目录。


exists()
测试此抽象路径名表示的文件或目录是否存在。


getAbsoluteFile()
返回此抽象路径名的绝对路径名形式。


getAbsolutePath()
返回此抽象路径名的绝对路径名字符串。


length()
返回由此抽象路径名表示的文件的长度。


mkdir()
创建此抽象路径名指定的目录。




File类使用实例
public class FileTest {
	public static void main(String[] args) throws IOException {
		File file = new File("C:/Mu/fileTest.txt");
	<span class="token comment">// 判断文件是否存在</span>
	<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>file<span class="token punctuation">.</span><span class="token function">exists</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
		<span class="token comment">// 不存在则创建</span>
		file<span class="token punctuation">.</span><span class="token function">createNewFile</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token punctuation">}</span>
	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"文件的绝对路径："</span> <span class="token operator">+</span> file<span class="token punctuation">.</span><span class="token function">getAbsolutePath</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"文件的大小："</span> <span class="token operator">+</span> file<span class="token punctuation">.</span><span class="token function">length</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token comment">// 刪除文件</span>
	file<span class="token punctuation">.</span><span class="token function">delete</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}


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2.2 字节流
InputStream与OutputStream是两个抽象类，是字节流的基类，所有具体的字节流实现类都是分别继承了这两个类。
以InputStream为例，它继承了Object，实现了Closeable
public abstract class InputStream
extends Object
implements Closeable

 
 

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InputStream类有很多的实现子类，下面列举了一些比较常用的：
 
 详细说明一下上图中的类：

InputStream：InputStream是所有字节输入流的抽象基类，前面说过抽象类不能被实例化，实际上是作为模板而存在的，为所有实现类定义了处理输入流的方法。
FileInputSream：文件输入流，一个非常重要的字节输入流，用于对文件进行读取操作。
PipedInputStream：管道字节输入流，能实现多线程间的管道通信。
ByteArrayInputStream：字节数组输入流，从字节数组(byte[])中进行以字节为单位的读取，也就是将资源文件都以字节的形式存入到该类中的字节数组中去。
FilterInputStream：装饰者类，具体的装饰者继承该类，这些类都是处理类，作用是对节点类进行封装，实现一些特殊功能。
DataInputStream：数据输入流，它是用来装饰其它输入流，作用是“允许应用程序以与机器无关方式从底层输入流中读取基本 Java 数据类型”。
BufferedInputStream：缓冲流，对节点流进行装饰，内部会有一个缓存区，用来存放字节，每次都是将缓存区存满然后发送，而不是一个字节或两个字节这样发送，效率更高。
ObjectInputStream：对象输入流，用来提供对基本数据或对象的持久存储。通俗点说，也就是能直接传输对象，通常应用在反序列化中。它也是一种处理流，构造器的入参是一个InputStream的实例对象。

OutputStream类继承关系图：
 
OutputStream类继承关系与InputStream类似，需要注意的是PrintStream.
2.3 字符流
与字节流类似，字符流也有两个抽象基类，分别是Reader和Writer。其他的字符流实现类都是继承了这两个类。
以Reader为例，它的主要实现子类如下图：
 
 各个类的详细说明：

InputStreamReader：从字节流到字符流的桥梁（InputStreamReader构造器入参是FileInputStream的实例对象），它读取字节并使用指定的字符集将其解码为字符。它使用的字符集可以通过名称指定，也可以显式给定，或者可以接受平台的默认字符集。
BufferedReader：从字符输入流中读取文本，设置一个缓冲区来提高效率。BufferedReader是对InputStreamReader的封装，前者构造器的入参就是后者的一个实例对象。
FileReader：用于读取字符文件的便利类，new FileReader(File file)等同于new InputStreamReader(new FileInputStream(file, true),"UTF-8")，但FileReader不能指定字符编码和默认字节缓冲区大小。
PipedReader ：管道字符输入流。实现多线程间的管道通信。
CharArrayReader：从Char数组中读取数据的介质流。
StringReader ：从String中读取数据的介质流。

Writer与Reader结构类似，方向相反，不再赘述。唯一有区别的是，Writer的子类PrintWriter。
2.4 序列化
待续…
3 IO流方法
3.1 字节流方法
字节输入流InputStream主要方法：

read() ：从此输入流中读取一个数据字节。
read(byte[] b) ：从此输入流中将最多 b.length 个字节的数据读入一个 byte 数组中。
read(byte[] b, int off, int len) ：从此输入流中将最多 len 个字节的数据读入一个 byte 数组中。
close()：关闭此输入流并释放与该流关联的所有系统资源。

字节输出流OutputStream主要方法：

write(byte[] b) ：将 b.length 个字节从指定 byte 数组写入此文件输出流中。
write(byte[] b, int off, int len) ：将指定 byte 数组中从偏移量 off 开始的 len 个字节写入此文件输出流。
write(int b) ：将指定字节写入此文件输出流。
close() ：关闭此输入流并释放与该流关联的所有系统资源。

3.2 字符流方法
字符输入流Reader主要方法：

read()：读取单个字符。
read(char[] cbuf) ：将字符读入数组。
read(char[] cbuf, int off, int len) ： 将字符读入数组的某一部分。
read(CharBuffer target) ：试图将字符读入指定的字符缓冲区。
flush() ：刷新该流的缓冲。
close() ：关闭此流，但要先刷新它。

字符输出流Writer主要方法：

write(char[] cbuf) ：写入字符数组。
write(char[] cbuf, int off, int len) ：写入字符数组的某一部分。
write(int c) ：写入单个字符。
write(String str) ：写入字符串。
write(String str, int off, int len) ：写入字符串的某一部分。
flush() ：刷新该流的缓冲。
close() ：关闭此流，但要先刷新它。

另外，字符缓冲流还有两个独特的方法：

BufferedWriter类newLine() ：写入一个行分隔符。这个方法会自动适配所在系统的行分隔符。
BufferedReader类readLine() ：读取一个文本行。

4 附加内容
4.1 位、字节、字符
字节(Byte)是计量单位，表示数据量多少，是计算机信息技术用于计量存储容量的一种计量单位，通常情况下一字节等于八位。
字符(Character)计算机中使用的字母、数字、字和符号，比如’A’、‘B’、’$’、’&’等。
一般在英文状态下一个字母或字符占用一个字节，一个汉字用两个字节表示。
字节与字符：

ASCII 码中，一个英文字母（不分大小写）为一个字节，一个中文汉字为两个字节。
UTF-8 编码中，一个英文字为一个字节，一个中文为三个字节。
Unicode 编码中，一个英文为一个字节，一个中文为两个字节。
符号：英文标点为一个字节，中文标点为两个字节。例如：英文句号 . 占1个字节的大小，中文句号 。占2个字节的大小。
UTF-16 编码中，一个英文字母字符或一个汉字字符存储都需要 2 个字节（Unicode 扩展区的一些汉字存储需要 4 个字节）。
UTF-32 编码中，世界上任何字符的存储都需要 4 个字节。

4.2 IO流效率对比
首先，对比下普通字节流和缓冲字节流的效率：
public class MyTest {
	public static void main(String[] args) throws IOException {
		File file = new File("C:/Mu/test.txt");
		StringBuilder sb = new StringBuilder();
	<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> <span class="token number">3000000</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
		sb<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span><span class="token string">"abcdefghigklmnopqrstuvwsyz"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token punctuation">}</span>
	<span class="token keyword">byte</span><span class="token punctuation">[</span><span class="token punctuation">]</span> bytes <span class="token operator">=</span> sb<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getBytes</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token keyword">long</span> start <span class="token operator">=</span> System<span class="token punctuation">.</span><span class="token function">currentTimeMillis</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token function">write</span><span class="token punctuation">(</span>file<span class="token punctuation">,</span> bytes<span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token keyword">long</span> end <span class="token operator">=</span> System<span class="token punctuation">.</span><span class="token function">currentTimeMillis</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token keyword">long</span> start2 <span class="token operator">=</span> System<span class="token punctuation">.</span><span class="token function">currentTimeMillis</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token function">bufferedWrite</span><span class="token punctuation">(</span>file<span class="token punctuation">,</span> bytes<span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token keyword">long</span> end2 <span class="token operator">=</span> System<span class="token punctuation">.</span><span class="token function">currentTimeMillis</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"普通字节流耗时："</span> <span class="token operator">+</span> <span class="token punctuation">(</span>end <span class="token operator">-</span> start<span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">" ms"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"缓冲字节流耗时："</span> <span class="token operator">+</span> <span class="token punctuation">(</span>end2 <span class="token operator">-</span> start2<span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">" ms"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token punctuation">}</span>

<span class="token comment">// 普通字节流</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">write</span><span class="token punctuation">(</span>File file<span class="token punctuation">,</span> <span class="token keyword">byte</span><span class="token punctuation">[</span><span class="token punctuation">]</span> bytes<span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span>
	OutputStream os <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">FileOutputStream</span><span class="token punctuation">(</span>file<span class="token punctuation">)</span><span class="token punctuation">;</span>
	os<span class="token punctuation">.</span><span class="token function">write</span><span class="token punctuation">(</span>bytes<span class="token punctuation">)</span><span class="token punctuation">;</span>
	os<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// 缓冲字节流</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">bufferedWrite</span><span class="token punctuation">(</span>File file<span class="token punctuation">,</span> <span class="token keyword">byte</span><span class="token punctuation">[</span><span class="token punctuation">]</span> bytes<span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span>
	BufferedOutputStream bo <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">BufferedOutputStream</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">FileOutputStream</span><span class="token punctuation">(</span>file<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	bo<span class="token punctuation">.</span><span class="token function">write</span><span class="token punctuation">(</span>bytes<span class="token punctuation">)</span><span class="token punctuation">;</span>
	bo<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}


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运行结果：
普通字节流耗时：250 ms
缓冲字节流耗时：268 ms

 
 

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这个结果让我大跌眼镜，不是说好缓冲流效率很高么？要知道为什么，只能去源码里找答案了。翻看字节缓冲流的write方法：
public synchronized void write(byte b[], int off, int len) throws IOException {
    if (len >= buf.length) {
        /* If the request length exceeds the size of the output buffer,
           flush the output buffer and then write the data directly.
           In this way buffered streams will cascade harmlessly. */
        flushBuffer();
        out.write(b, off, len);
        return;
    }
    if (len > buf.length - count) {
        flushBuffer();
    }
    System.arraycopy(b, off, buf, count, len);
    count += len;
}

 
 

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注释里说得很明白：如果请求长度超过输出缓冲区的大小，刷新输出缓冲区，然后直接写入数据。这样，缓冲流将无害地级联。
但是，至于为什么这么设计，我没有想明白，有哪位明白的大佬可以留言指点一下。
基于上面的情形，要想对比普通字节流和缓冲字节流的效率差距，就要避免直接读写较长的字符串，于是，设计了下面这个对比案例：用字节流和缓冲字节流分别复制文件。
public class MyTest {
	public static void main(String[] args) throws IOException {
		File data = new File("C:/Mu/data.zip");
		File a = new File("C:/Mu/a.zip");
		File b = new File("C:/Mu/b.zip");
	StringBuilder sb <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">StringBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token keyword">long</span> start <span class="token operator">=</span> System<span class="token punctuation">.</span><span class="token function">currentTimeMillis</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token function">copy</span><span class="token punctuation">(</span>data<span class="token punctuation">,</span> a<span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token keyword">long</span> end <span class="token operator">=</span> System<span class="token punctuation">.</span><span class="token function">currentTimeMillis</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token keyword">long</span> start2 <span class="token operator">=</span> System<span class="token punctuation">.</span><span class="token function">currentTimeMillis</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token function">bufferedCopy</span><span class="token punctuation">(</span>data<span class="token punctuation">,</span> b<span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token keyword">long</span> end2 <span class="token operator">=</span> System<span class="token punctuation">.</span><span class="token function">currentTimeMillis</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"普通字节流耗时："</span> <span class="token operator">+</span> <span class="token punctuation">(</span>end <span class="token operator">-</span> start<span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">" ms"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"缓冲字节流耗时："</span> <span class="token operator">+</span> <span class="token punctuation">(</span>end2 <span class="token operator">-</span> start2<span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">" ms"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// 普通字节流</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">copy</span><span class="token punctuation">(</span>File in<span class="token punctuation">,</span> File out<span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span>
	<span class="token comment">// 封装数据源</span>
	InputStream is <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">FileInputStream</span><span class="token punctuation">(</span>in<span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token comment">// 封装目的地</span>
	OutputStream os <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">FileOutputStream</span><span class="token punctuation">(</span>out<span class="token punctuation">)</span><span class="token punctuation">;</span>
	
	<span class="token keyword">int</span> by <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
	<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>by <span class="token operator">=</span> is<span class="token punctuation">.</span><span class="token function">read</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
		os<span class="token punctuation">.</span><span class="token function">write</span><span class="token punctuation">(</span>by<span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token punctuation">}</span>
	is<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	os<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// 缓冲字节流</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">bufferedCopy</span><span class="token punctuation">(</span>File in<span class="token punctuation">,</span> File out<span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span>
	<span class="token comment">// 封装数据源</span>
	BufferedInputStream bi <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">BufferedInputStream</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">FileInputStream</span><span class="token punctuation">(</span>in<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token comment">// 封装目的地</span>
	BufferedOutputStream bo <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">BufferedOutputStream</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">FileOutputStream</span><span class="token punctuation">(</span>out<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	
	<span class="token keyword">int</span> by <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
	<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>by <span class="token operator">=</span> bi<span class="token punctuation">.</span><span class="token function">read</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
		bo<span class="token punctuation">.</span><span class="token function">write</span><span class="token punctuation">(</span>by<span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token punctuation">}</span>
	bo<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	bi<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}


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运行结果：
普通字节流耗时：184867 ms
缓冲字节流耗时：752 ms

 
 

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这次，普通字节流和缓冲字节流的效率差异就很明显了，达到了245倍。
再看看字符流和缓冲字符流的效率对比：
public class IOTest {
	public static void main(String[] args) throws IOException {
		// 数据准备
		dataReady();
	File data <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">File</span><span class="token punctuation">(</span><span class="token string">"C:/Mu/data.txt"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	File a <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">File</span><span class="token punctuation">(</span><span class="token string">"C:/Mu/a.txt"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	File b <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">File</span><span class="token punctuation">(</span><span class="token string">"C:/Mu/b.txt"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	File c <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">File</span><span class="token punctuation">(</span><span class="token string">"C:/Mu/c.txt"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token keyword">long</span> start <span class="token operator">=</span> System<span class="token punctuation">.</span><span class="token function">currentTimeMillis</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token function">copy</span><span class="token punctuation">(</span>data<span class="token punctuation">,</span> a<span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token keyword">long</span> end <span class="token operator">=</span> System<span class="token punctuation">.</span><span class="token function">currentTimeMillis</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token keyword">long</span> start2 <span class="token operator">=</span> System<span class="token punctuation">.</span><span class="token function">currentTimeMillis</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token function">copyChars</span><span class="token punctuation">(</span>data<span class="token punctuation">,</span> b<span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token keyword">long</span> end2 <span class="token operator">=</span> System<span class="token punctuation">.</span><span class="token function">currentTimeMillis</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token keyword">long</span> start3 <span class="token operator">=</span> System<span class="token punctuation">.</span><span class="token function">currentTimeMillis</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token function">bufferedCopy</span><span class="token punctuation">(</span>data<span class="token punctuation">,</span> c<span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token keyword">long</span> end3 <span class="token operator">=</span> System<span class="token punctuation">.</span><span class="token function">currentTimeMillis</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"普通字节流1耗时："</span> <span class="token operator">+</span> <span class="token punctuation">(</span>end <span class="token operator">-</span> start<span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">" ms,文件大小："</span> <span class="token operator">+</span> a<span class="token punctuation">.</span><span class="token function">length</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">/</span> <span class="token number">1024</span> <span class="token operator">+</span> <span class="token string">" kb"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"普通字节流2耗时："</span> <span class="token operator">+</span> <span class="token punctuation">(</span>end2 <span class="token operator">-</span> start2<span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">" ms,文件大小："</span> <span class="token operator">+</span> b<span class="token punctuation">.</span><span class="token function">length</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">/</span> <span class="token number">1024</span> <span class="token operator">+</span> <span class="token string">" kb"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"缓冲字节流耗时："</span> <span class="token operator">+</span> <span class="token punctuation">(</span>end3 <span class="token operator">-</span> start3<span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">" ms,文件大小："</span> <span class="token operator">+</span> c<span class="token punctuation">.</span><span class="token function">length</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">/</span> <span class="token number">1024</span> <span class="token operator">+</span> <span class="token string">" kb"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// 普通字符流不使用数组</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">copy</span><span class="token punctuation">(</span>File in<span class="token punctuation">,</span> File out<span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span>
	Reader reader <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">FileReader</span><span class="token punctuation">(</span>in<span class="token punctuation">)</span><span class="token punctuation">;</span>
	Writer writer <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">FileWriter</span><span class="token punctuation">(</span>out<span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token keyword">int</span> ch <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
	<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>ch <span class="token operator">=</span> reader<span class="token punctuation">.</span><span class="token function">read</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
		writer<span class="token punctuation">.</span><span class="token function">write</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token keyword">char</span><span class="token punctuation">)</span> ch<span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token punctuation">}</span>
	reader<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	writer<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// 普通字符流使用字符流</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">copyChars</span><span class="token punctuation">(</span>File in<span class="token punctuation">,</span> File out<span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span>
	Reader reader <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">FileReader</span><span class="token punctuation">(</span>in<span class="token punctuation">)</span><span class="token punctuation">;</span>
	Writer writer <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">FileWriter</span><span class="token punctuation">(</span>out<span class="token punctuation">)</span><span class="token punctuation">;</span>

	<span class="token keyword">char</span><span class="token punctuation">[</span><span class="token punctuation">]</span> chs <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">char</span><span class="token punctuation">[</span><span class="token number">1024</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
	<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>reader<span class="token punctuation">.</span><span class="token function">read</span><span class="token punctuation">(</span>chs<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
		writer<span class="token punctuation">.</span><span class="token function">write</span><span class="token punctuation">(</span>chs<span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token punctuation">}</span>
	reader<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	writer<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// 缓冲字符流</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">bufferedCopy</span><span class="token punctuation">(</span>File in<span class="token punctuation">,</span> File out<span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span>
	BufferedReader br <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">BufferedReader</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">FileReader</span><span class="token punctuation">(</span>in<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	BufferedWriter bw <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">BufferedWriter</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">FileWriter</span><span class="token punctuation">(</span>out<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	String line <span class="token operator">=</span> null<span class="token punctuation">;</span>
	<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>line <span class="token operator">=</span> br<span class="token punctuation">.</span><span class="token function">readLine</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">!=</span> null<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
		bw<span class="token punctuation">.</span><span class="token function">write</span><span class="token punctuation">(</span>line<span class="token punctuation">)</span><span class="token punctuation">;</span>
		bw<span class="token punctuation">.</span><span class="token function">newLine</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
		bw<span class="token punctuation">.</span><span class="token function">flush</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token punctuation">}</span>

	<span class="token comment">// 释放资源</span>
	bw<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	br<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// 数据准备</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">dataReady</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span>
	StringBuilder sb <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">StringBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> <span class="token number">600000</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
		sb<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span><span class="token string">"abcdefghijklmnopqrstuvwxyz"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token punctuation">}</span>
	OutputStream os <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">FileOutputStream</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">File</span><span class="token punctuation">(</span><span class="token string">"C:/Mu/data.txt"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	os<span class="token punctuation">.</span><span class="token function">write</span><span class="token punctuation">(</span>sb<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getBytes</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

	os<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"完毕"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}
运行结果：
普通字符流1耗时：1337 ms,文件大小：15234 kb
普通字符流2耗时：82 ms,文件大小：15235 kb
缓冲字符流耗时：205 ms,文件大小：15234 kb

 
测试多次，结果差不多，可见字符缓冲流效率上并没有明显提高，我们更多的是要使用它的readLine()和newLine()方法。
4.3 NIO
待续…

方法名	说明
`File(File parent, String child)`	根据 parent 抽象路径名和 child 路径名字符串创建一个新 File 实例。
`File(String pathname)`	通过将给定路径名字符串转换为抽象路径名来创建一个新 File 实例。
`File(String parent, String child)`	根据 parent 路径名字符串和 child 路径名字符串创建一个新 File 实例。
`File(URI uri)`	通过将给定的 file: URI 转换为一个抽象路径名来创建一个新的 File 实例。

方法	说明
`createNewFile()`	当且仅当不存在具有此抽象路径名指定名称的文件时，不可分地创建一个新的空文件。
`delete()`	删除此抽象路径名表示的文件或目录。
`exists()`	测试此抽象路径名表示的文件或目录是否存在。
`getAbsoluteFile()`	返回此抽象路径名的绝对路径名形式。
`getAbsolutePath()`	返回此抽象路径名的绝对路径名字符串。
`length()`	返回由此抽象路径名表示的文件的长度。
`mkdir()`	创建此抽象路径名指定的目录。