将响应数据进行压缩处理的过滤器（CompressionFilter）

功能描述

        如果浏览器支持 gzip 压缩格式的数据，则将响应的数据使用 gzip 压缩后再输出。

使用方法

        在 java web 项目的 web.xml 文件中添加如下代码。

 < filter >
 < filter-name >CompressionFilter filter-name >
 < filter-class >com.hmw.filter.CompressionFilter filter-class >
 filter >

 < filter-mapping >
 < filter-name >CompressionFilter filter-name >
   < servlet-name >/LongServlet servlet-name >
 filter-mapping >

过滤器源码

CompressionFilter.java

package   com.hmw.filter;

import   java.io.ByteArrayOutputStream;
import   java.io.IOException;
import   java.io.OutputStream;
import   java.io.OutputStreamWriter;
import   java.util.zip.GZIPOutputStream;

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.http.HttpServletRequest;
import   javax.servlet.http.HttpServletResponse;

/**
 * 压缩过滤器
 * 如果浏览器支持 gzip 压缩格式的数据，则将响应的数据使用 gzip 压缩后再输出。
 */
public   class   CompressionFilter implements   Filter {

    @Override
    public   void   init(FilterConfig config) throws   ServletException {
    }

    /**
     * 如果浏览器不支持 gzip 压缩，则不做直接放行（不做压缩处理）
     * 反之，将HTTP响应头的编码设置为 gzip，然后将响应数据使用 gzip 进行压缩处理。
     */
    @Override
    public   void   doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws   ServletException, IOException {
        HttpServletRequest req = (HttpServletRequest) request;
        HttpServletResponse res = (HttpServletResponse) response;

        if   (!isGzipSupported(req)) { // Invoke resource normally.
            chain.doFilter(req, res);
            return ;
        }

        // 将响应头信息中的内容编码设置为 gzip
        res.setHeader( "Content-Encoding" , "gzip" );

        // 调用资源，使用 CharArrayWrapper 包装输出
        CharArrayWrapper responseWrapper = new   CharArrayWrapper(res);
        chain.doFilter(req, responseWrapper);
        // 取得存放输出数据的 char 型数组
        char [] responseChars = responseWrapper.toCharArray();

        // 将响应数据压缩后存入一个 byte 型的数组，然后输出到
        ByteArrayOutputStream byteStream = new   ByteArrayOutputStream();
        GZIPOutputStream zipOut = new   GZIPOutputStream(byteStream);
        OutputStreamWriter tempOut = new   OutputStreamWriter(zipOut);
        // 将原来的响应数据压缩后写入二字节输出流
        tempOut.write(responseChars);
        // 关闭输出流
        tempOut.close();

        // 更新响应头信息中 Content-Length 的值。
        res.setContentLength(byteStream.size());
        // 将压缩后的数据发送至客户端
        OutputStream realOut = res.getOutputStream();
        byteStream.writeTo(realOut);
    }

    @Override
    public   void   destroy() {
    }

    /**
     * 检测浏览器是否支持 Gzip 压缩
     *
     * @param req HTTP 请求对象
     * @return 如果浏览器支持 Gzip 压缩，则返回 true，反之，则返回 false
     */
    private   boolean   isGzipSupported(HttpServletRequest req) {
        String browserEncodings = req.getHeader( "Accept-Encoding" );
        return   ((browserEncodings != null ) && (browserEncodings.indexOf( "gzip" ) != - 1 ));
    }
}

CharArrayWrapper.java

package   com.hmw.filter;

import   java.io.CharArrayWriter;
import   java.io.PrintWriter;

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

/**
 * A response wrapper that takes everything the client would normally output and
 * saves it in one big character array.
 */
public   class   CharArrayWrapper extends   HttpServletResponseWrapper {
    private   CharArrayWriter charWriter;

    /**
     * Initializes wrapper.
     *
     * First, this constructor calls the parent constructor. That call is
     * crucial so that the response is stored and thus setHeader, *setStatus,
     * addCookie, and so forth work normally.
     *
     * Second, this constructor creates a CharArrayWriter that will be used to
     * accumulate the response.
     */
    public   CharArrayWrapper(HttpServletResponse response) {
        super (response);
        charWriter = new   CharArrayWriter();
    }

    /**
     * When servlets or JSP pages ask for the Writer, don't give them the real
     * one. Instead, give them a version that writes into the character array.
     * The filter needs to send the contents of the array to the client (perhaps
     * after modifying it).
     */
    @Override
    public   PrintWriter getWriter() {
        return   new   PrintWriter(charWriter);
    }

    /**
     * Get a String representation of the entire buffer.
     *
     * Be sure not to call this method multiple times on the same
     * wrapper. The API for CharArrayWriter does not guarantee that it
     * "remembers" the previous value, so the call is likely to make a new
     * String every time.
     */
    @Override
    public   String toString() {
        return   charWriter.toString();
    }

    /** Get the underlying character array. */
    public   char [] toCharArray() {
        return   charWriter.toCharArray();
    }
}