Rest 概要

REST = RESTful = Representational State Transfer。是在因特网上提供计算机系统之间的互操作性的一种方式。基于Http协议的资源传递，包括JSON、XML和文本等。

统一接口

资源识别（Identification of resources）
- URI（Uniform Resource Identifier ）
资源操作（Manipulation of resources through representations）
- HTTP verbs：GET、PUT、POST、DELETE
自描述消息（Self-descriptive messages）
- Content-Type
- MIME-Type
- Media Type： application/javascript、 text/html
超媒体（HATEOAS）
- Hypermedia As The Engine Of Application State

REST 服务端实践

1、Spring Boot REST：

定义相关
- @Controller
- @RestController
映射相关
- @RequestMapping
- @PathVariable：获取路径参数
请求相关
- @RequestParam
- @RequestHeader
- @CookieValue
- RequestEntity：包括请求头和请求体等信息配置
响应相关
- @ResponseBody
- ResponseEntity：包括返回头和返回体等信息配置

以 @RequestMapping 为例讲解，源码如下，其他的源码亦可如此分析：

package org.springframework.web.bind.annotation;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.core.annotation.AliasFor;

@Target({ElementType.METHOD, ElementType.TYPE})     //注解作用范围 类或方法
@Retention(RetentionPolicy.RUNTIME)                 //作用范围，一般自定义注解使用 RetentionPolicy.RUNTIME
@Documented                                         //生成文档
@Mapping                                            //继承@Mapping注解
public @interface RequestMapping {
    String name() default "";

    @AliasFor("path")    //和path等效
    String[] value() default {};  

    @AliasFor("value")   //和value等效
    String[] path() default {};

    RequestMethod[] method() default {}; //可取值 RequestMethod.GET等

    String[] params() default {};    //参数名称

    String[] headers() default {};   //头信息

    String[] consumes() default {};  //客户端请求的数据要求，一般使用MediaType.APPLICATION_JSON_VALUE等

    String[] produces() default {};  //服务器返回的数据规定，一般使用MediaType.APPLICATION_JSON_VALUE等
}

举例：
导入依赖：

        <dependency>
            <groupId>com.fasterxml.jackson.dataformat</groupId>
            <artifactId>jackson-dataformat-xml</artifactId>
        </dependency>

返回 XML 格式的数据：

@RestController
public class RestTestController {

  @RequestMapping(method = RequestMethod.GET,value = "/test",produces = MediaType.APPLICATION_XML_VALUE)
    public User getUserData() {
        User user = new User();
        user.setName("zhangsan");
        user.setAge(18);
        return user;
    }
}

返回 json 格式的数据：

@RestController
public class RestTestController {

    @RequestMapping(method = RequestMethod.GET,value = "/test",produces = MediaType.APPLICATION_JSON_VALUE)
    public User getUserData() {
        User user = new User();
        user.setName("zhangsan");
        user.setAge(18);
        return user;
    }
}

2、Swagger、Spring RestDocs等，这里就不拓展了。

REST 客户端实践

1、Apache HttpClient
导入依赖：

        <!--httpclient-->
        <dependency>
            <groupId>org.apache.httpcomponents</groupId>
            <artifactId>httpclient</artifactId>
        </dependency>

HttpClient工具类代码：


import java.io.IOException;
import java.net.URI;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

import org.apache.http.NameValuePair;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.client.utils.URIBuilder;
import org.apache.http.entity.ContentType;
import org.apache.http.entity.StringEntity;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.message.BasicNameValuePair;
import org.apache.http.util.EntityUtils;

/**
 *@描述： http工具
 */
public class HttpClientUtil {

    public static String doGet(String url, Map<String, String> param) {

        // 创建Httpclient对象
        CloseableHttpClient httpclient = HttpClients.createDefault();

        String resultString = "";
        CloseableHttpResponse response = null;
        try {
            // 创建uri
            URIBuilder builder = new URIBuilder(url);
            if (param != null) {
                for (String key : param.keySet()) {
                    builder.addParameter(key, param.get(key));
                }
            }
            URI uri = builder.build();

            // 创建http GET请求
            HttpGet httpGet = new HttpGet(uri);

            // 执行请求
            response = httpclient.execute(httpGet);
            // 判断返回状态是否为200
            if (response.getStatusLine().getStatusCode() == 200) {
                resultString = EntityUtils.toString(response.getEntity(), "UTF-8");
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                if (response != null) {
                    response.close();
                }
                httpclient.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return resultString;
    }

    public static String doGet(String url) {
        return doGet(url, null);
    }

    public static String doPost(String url, Map<String, String> param) {
        // 创建Httpclient对象
        CloseableHttpClient httpClient = HttpClients.createDefault();
        CloseableHttpResponse response = null;
        String resultString = "";
        try {
            // 创建Http Post请求
            HttpPost httpPost = new HttpPost(url);
            // 创建参数列表
            if (param != null) {
                List<NameValuePair> paramList = new ArrayList<>();
                for (String key : param.keySet()) {
                    paramList.add(new BasicNameValuePair(key, param.get(key)));
                }
                // 模拟表单
                UrlEncodedFormEntity entity = new UrlEncodedFormEntity(paramList);
                httpPost.setEntity(entity);
            }
            // 执行http请求
            response = httpClient.execute(httpPost);
            resultString = EntityUtils.toString(response.getEntity(), "utf-8");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                response.close();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }

        return resultString;
    }

    public static String doPost(String url) {
        return doPost(url, null);
    }

    public static String doPostJson(String url, String json) {
        // 创建Httpclient对象
        CloseableHttpClient httpClient = HttpClients.createDefault();
        CloseableHttpResponse response = null;
        String resultString = "";
        try {
            // 创建Http Post请求
            HttpPost httpPost = new HttpPost(url);
            // 创建请求内容
            StringEntity entity = new StringEntity(json, ContentType.APPLICATION_JSON);
            httpPost.setEntity(entity);
            // 执行http请求
            response = httpClient.execute(httpPost);
            resultString = EntityUtils.toString(response.getEntity(), "utf-8");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                response.close();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }

        return resultString;
    }
}