javaweb过滤器修改Response的返回值

以前就听说javaweb中的过滤器可以修改请求过来和响应回去的数据，请求过来的数据修改比较简单，在此主要讨论的是响应回去的数据，咱们如何修改？

tomcat源码中提供了一个类HttpServletResponseWrapper，该类实际上实现了HttpServletResponse接口，也就是过滤器中直接封装该类的子类就可以调用，以下是该类部分源码。

public class HttpServletResponseWrapper extends ServletResponseWrapper
    implements HttpServletResponse
{

    public HttpServletResponseWrapper(HttpServletResponse response)
    {
        super(response);
    }

    private HttpServletResponse _getHttpServletResponse()
    {
        return (HttpServletResponse)super.getResponse();
    }

还记得输出时是通过response.getOutputStream()或者resp.getWriter()获取的输出流，然后将需要返回的数据写入数据流中；我们可以拿到这两个输出流，但是这两个流都是包装流，数据没办法直接从这两个输出流中读取出来，这该如何解决？那我们能不能自己封装这样的一个接口呢，看下面的代码：

package com.asiainfo.wrapper;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.io.PrintWriter;

import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpServletResponseWrapper;

/**
 * 返回值输出代理类
 * @author Administrator
 *
 */
public class ResponseWrapper extends HttpServletResponseWrapper {

    // 真正缓存数据的流
    private ByteArrayOutputStream buffer = null;
    private ServletOutputStream out = null;
    private PrintWriter writer = null;
    
    public ResponseWrapper(HttpServletResponse response) {
        super(response);
        // TODO Auto-generated constructor stub
        buffer = new ByteArrayOutputStream();
        out = new WrapperOutputStream(buffer);
        writer = new PrintWriter(buffer);
    }
    
    /**
     *     当获取字节输出流时，实际获取的是我们自己包装的字节输出流
     */
    public ServletOutputStream getOutputStream() {
        return out;
    }
    
    /** 
     *     当获取字符输出流时，实际获取的是我们自己包装的字符输出流
     */
    public PrintWriter getWriter() {
        return writer;
    }
    
    public void flushBuffer() throws IOException {
        if (out != null) {
            out.flush();
        }
        if (writer != null) {
            writer.flush();
        }
    }
    
    public byte[] getResponseData() throws IOException {
        flushBuffer();
        return buffer.toByteArray();
    }
    
    public String getContent() throws IOException {
        flushBuffer();
        return buffer.toString();
    }
}

class WrapperOutputStream extends ServletOutputStream {

    private OutputStream out;
    
    public WrapperOutputStream(OutputStream out) {
        super();
        this.out = out;
    }

    @Override
    public void write(int b) throws IOException {
        // TODO Auto-generated method stub
        out.write(b);
    }
}

写一个servlet处理调用请求

package com.asiainfo.servlet;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet("/demo")
public class FrontServlet extends HttpServlet {
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // TODO Auto-generated method stub
        ServletOutputStream outputStream = resp.getOutputStream();
        resp.setContentType("application/json;charset=UTF-8");
        String content = "{\"a\":\"前置servlet\"}";
        outputStream.write("{\"a\":\"前置servlet\"}".getBytes("utf-8"));
        System.out.println("servlet中写入的内容:"+content);
        outputStream.flush();
        outputStream.close();
    }
}

下面是写两个过滤器，DemoFilter2先对servlet中相应数据做修改，然后可以在DemoFilter1中查看修改后的内容，代码如下：

package com.asiainfo;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServletResponse;

import com.asiainfo.wrapper.ResponseWrapper;

@WebFilter("/*")
public class DemoFilter2 implements Filter {

    @Override
    public void destroy() {
        // TODO Auto-generated method stub

    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain chain)
            throws IOException, ServletException {
        ResponseWrapper responseWrapper = new ResponseWrapper((HttpServletResponse) servletResponse);
        chain.doFilter(servletRequest, responseWrapper);
        byte[] content = responseWrapper.getResponseData();
        String str = "";
        if (content != null && !"".equals(content)) {
            // 这里可以对相应数据做修改，修改后的内容
            str = "{\"a\":\"前置servlet\",\"b\":\"新增内容\"}";
            System.out.println("第二个过滤器修改后的内容:" + str);
            HttpServletResponse response = (HttpServletResponse) servletResponse;
            // 注意：这里的response还要传原来的，不然内容无法输出页面
            writeResponse(response, 200, str);
        }
    }

    public static void writeResponse(HttpServletResponse response, int status, String json) {
        try {
            response.setContentType("application/json;charset=UTF-8");
            response.setStatus(status);
            response.getWriter().write(json);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void init(FilterConfig arg0) throws ServletException {
        // TODO Auto-generated method stub

    }

}









package com.asiainfo;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServletResponse;

import com.asiainfo.wrapper.ResponseWrapper;

@WebFilter("/*")
public class DemoFilter1 implements Filter {

    @Override
    public void init(FilterConfig filterconfig) throws ServletException {
        // TODO Auto-generated method stub
        
    }

    @Override
    public void doFilter(ServletRequest servletrequest, ServletResponse servletresponse, FilterChain filterchain)
            throws IOException, ServletException {
        // TODO Auto-generated method stub
        ResponseWrapper responseWrapper = new ResponseWrapper((HttpServletResponse) servletresponse);
        filterchain.doFilter(servletrequest, responseWrapper);
        String content = responseWrapper.getContent();
        if (content != null) {
            System.out.println("第一个过滤器读取到的内容："+content);
        }
        HttpServletResponse response = (HttpServletResponse) servletresponse;
        writeResponse(response, 200, content);
    }
    
    public static void writeResponse(HttpServletResponse response, int status, String json) {
        try {
            response.setContentType("application/json;charset=UTF-8");
            response.setStatus(status);
            response.getWriter().write(json);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void destroy() {
        // TODO Auto-generated method stub

    }

}

将上面的web工程部署到tomcat中启动，浏览器中调用地址：http://localhost:8080/DemoFilter/demo

结果显示如下：

控制台结果：