Servlet之Filter实现原理(责任链设计模式)

小半 • 2023年1月19日下午6:09 • 技术随笔 • 阅读 244

环境：Java8

Servlet中Filter使用的设计模式是责任链设计模式。我们可以定义一组Filter然后对数据进行依次的处理。责任链设计模式属于行为型模式。UML图如下：

Filter执行流程如下：

接下来我们来实现一个Filter

Filter.java接口

public interface Filter {
	
	void doFilter(Request request, Response response, FilterChain chain) ;
	
}

FilterChain.java 关键的调用链

public class FilterChain implements Filter{

	private List<Filter> filters = new ArrayList<>() ;
	
	private int position = 0 ;
	
	public void doFilter(Request request, Response response, FilterChain chain) {
		if (position > filters.size() - 1) {
			return ;
		}
		Filter filter = filters.get(this.position++) ;
		filter.doFilter(request, response, this) ;
	}
	// 构建需要调用的Filter链
	public FilterChain addFilter(Filter filter) {
		this.filters.add(filter) ;
		return this ;
	}
	
}

Filter相关子类

public class LoginFilter implements Filter {

	@Override
	public void doFilter(Request request, Response response, FilterChain chain) {
		System.out.println("LoginFilter entry...") ;
		chain.doFilter(request, response, chain) ;
		System.out.println("LoginFilter over...") ;
	}

}

public class CharFilter implements Filter {

	@Override
	public void doFilter(Request request, Response response, FilterChain chain) {
		System.out.println("CharFilter entry...") ;
		chain.doFilter(request, response, chain) ;
		System.out.println("CharFilter over...") ;
	}

}

public class GrantFilter implements Filter {

	@Override
	public void doFilter(Request request, Response response, FilterChain chain) {
		System.out.println("GrantFilter entry...") ;
		chain.doFilter(request, response, chain) ;
		System.out.println("GrantFilter over...") ;
	}

}

客户端

public static void main(String[] args) {
		LoginFilter loginFilter = new LoginFilter() ;
		CharFilter charFilter = new CharFilter() ;
		GrantFilter grantFilter = new GrantFilter() ;
		FilterChain chain = new FilterChain() ;
		chain.addFilter(loginFilter).addFilter(charFilter).addFilter(grantFilter) ;
		Request request = new Request() ;
		Response response = new Response() ;
		chain.doFilter(request, response, null) ;
	}

运行结果