將響應數據進行壓縮處理的過濾器（CompressionFilter）

    本系列文章導讀：

        通用的用戶登錄過濾器（SessionFilter）

        設置不使用緩存的過濾器（ClearCacheFilter）

        設置站點黑名單的過濾器（BannedAccessFilter）

        將響應數據進行壓縮處理的過濾器（CompressionFilter）

        替換禁用語（指定關鍵字）的過濾器（StopWordsFilter）

功能描述

        如果瀏覽器支持 gzip 壓縮格式的數據，則將響應的數據使用 gzip 壓縮后再輸出。

使用方法

        在 java web 項目的 web.xml 文件中添加如下代碼。

 <!--壓縮過濾器的配置  開始 -->
  <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>
 <!--壓縮過濾器的配置  結束 -->

過濾器源碼

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;

/**
 * 壓縮過濾器 

 * 如果瀏覽器支持 gzip 壓縮格式的數據，則將響應的數據使用 gzip 壓縮后再輸出。
 * 
 * @author 何明旺
 */
public class CompressionFilter implements Filter {

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

    /**
     * 如果瀏覽器不支持 gzip 壓縮，則不做直接放行（不做壓縮處理）

     * 反之，將HTTP響應頭的編碼設置為 gzip，然后將響應數據使用 gzip 進行壓縮處理。
     */
    @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;
        }

        // 將響應頭信息中的內容編碼設置為 gzip
        res.setHeader("Content-Encoding", "gzip");
        
        // 調用資源，使用 CharArrayWrapper 包裝輸出
        CharArrayWrapper responseWrapper = new CharArrayWrapper(res);
        chain.doFilter(req, responseWrapper);
        // 取得存放輸出數據的 char 型數組
        char[] responseChars = responseWrapper.toCharArray();
        
        // 將響應數據壓縮后存入一個 byte 型的數組，然后輸出到
        ByteArrayOutputStream byteStream = new ByteArrayOutputStream();
        GZIPOutputStream zipOut = new GZIPOutputStream(byteStream);
        OutputStreamWriter tempOut = new OutputStreamWriter(zipOut);
        // 將原來的響應數據壓縮后寫入二字節輸出流
        tempOut.write(responseChars);
        // 關閉輸出流
        tempOut.close();

        // 更新響應頭信息中 Content-Length 的值。
        res.setContentLength(byteStream.size());
        // 將壓縮后的數據發送至客戶端
        OutputStream realOut = res.getOutputStream();
        byteStream.writeTo(realOut);
    }

    @Override
    public void destroy() {
    }

    /**
     * 檢測瀏覽器是否支持 Gzip 壓縮
     * 
     * @param req HTTP 請求對象
     * @return 如果瀏覽器支持 Gzip 壓縮，則返回 true，反之，則返回 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.
	 * <P>
	 * 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.
	 * <P>
	 * 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.
	 * <P>
	 * Be sure <B>not</B> 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();
	}
}