JavaEE过滤器处理原理

     大家都知道JavaEE中的Filter是用了责任链模式来实现的，请求值先到达Filter，根据Filter在web.xml的配置顺序一个个按顺序地处理，处理完以后，再按反顺序来处理响应信息，有点像是一个栈（先处理请求的Filter，后处理响应）。下面就山寨一下它的实现，来说明它的实现原理。

package myself.filter;

/**
 * 请求
 * @author zyb
 * @since 2012-12-1
 *
 */
public class Request {

	private String value;

	public String getValue() {
		return value;
	}

	public void setValue(String value) {
		this.value = value;
	}

}

/**
 * 响应
 * 
 * @author zyb
 * @since 2012-12-1
 * 
 */
public class Response {

	private String value;

	public String getValue() {
		return value;
	}

	public void setValue(String value) {
		this.value = value;
	}

}

 package myself.filter; 

/**
 * 过滤器接口
 * @author zyb
 * @since 2012-12-1
 *
 */
public interface Filter {

	/**
	 * 处理请求和响应
	 * @param request		请求
	 * @param response		响应
	 * @param chain			过滤器链
	 */
	void doFilter(Request request, Response response, FilterChain chain);
	
}

 package myself.filter; 

import java.util.ArrayList;
import java.util.List;

/**
 * 过滤器链
 * @author zyb
 * @since 2012-12-1
 *
 */
public class FilterChain implements Filter { // 在JavaEE的FilterChain源码中并没有实现Filter接口
	
	// 用于保存所有要处理的过滤器
	private List<Filter> filters = new ArrayList<Filter>();
	
	// 索引，用于记数
	private int index = 0;
	
	@Override
	public void doFilter(Request request, Response response, FilterChain chain) {
		// 如果所有的过滤器都已经处理完
		if (index == filters.size()) {
			return;
		}
		// 取出相应的过滤器
		Filter filter = filters.get(index);
		// 索引加1
		index++;
		// 调用具体的过滤器处理请求和响应
		filter.doFilter(request, response, chain);
	}

	// 往过滤器链中添加过滤器
	public void addFilter(Filter filter) {
		this.filters.add(filter);
	}
}

 package myself.filter;

/**
 * 客户端程序
 * @author zyb
 * @since 2012-12-1
 *
 */
public class Client {

	public static void main(String[] args) {
		Request request = new Request();
		request.setValue("abcd");
		Response response = new Response();
		FilterChain filter = new FilterChain();
		
		// 将请求值变成大写字母
		filter.addFilter(new Filter() {
			
			@Override
			public void doFilter(Request request, Response response, FilterChain chain) {
				System.out.println("第一个Filter处理...请求值：" + request.getValue());
				StringBuilder sb = new StringBuilder();
				for (int i = 0; i < request.getValue().length(); i++) {
					sb.append((char) (request.getValue().charAt(i) - 32));
				}
				Response result = new Response();
				result.setValue(sb.toString());
				chain.doFilter(request, response, chain);
				System.out.println("第一个Filter响应...响应值：" + result.getValue());
				
			}
		});
		
		// 将请求值加1
		filter.addFilter(new Filter() {
			
			@Override
			public void doFilter(Request request, Response response, FilterChain chain) {
				System.out.println("第二个Filter处理...请求值：" + request.getValue());
				StringBuilder sb = new StringBuilder();
				for (int i = 0; i < request.getValue().length(); i++) {
					sb.append((char) (request.getValue().charAt(i) + 1));
				}
				Response result = new Response();
				result.setValue(sb.toString());
				chain.doFilter(request, response, chain);
				System.out.println("第二个Filter响应...响应值：" + result.getValue());
			}
		});
		
		filter.doFilter(request, response, filter);
	}
}

打印信息：

第一个Filter处理...请求值：abcd

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