Java Servlet GZip Servlet Filter 详解-白红宇

GZip Servlet过滤器可用于GZip压缩内容从Java Web应用程序发送到浏览器。

###为什么要压缩

Gzip压缩HTML、js、css等，使得发送给浏览器的数据大小变得更小。提升上传速度，尤其是移动端带宽受限制的情况下，不过它可能带来服务器和浏览器的CPU消耗问题，但是响应速度会得道很大的改善。

###GZip 请求头

浏览器在发送到HTTP服务器（例如Java Web服务器）的请求中包含Accept-Encoding HTTP标头。 Accept-Encoding标头的内容告诉浏览器可以接受哪些内容编码。如果该标题包含gzip值，则浏览器可以接受GZip压缩内容。然后服务器可以将GZip压缩发送回浏览器的内容。

如果从服务器发回的内容是GZip压缩的，则服务器会在HTTP响应中包含带有值gzip的Content-Encoding HTTP标头。这样浏览器就知道内容是GZip压缩的。

###为什么使用GZip Servlet过滤器？

如果对每一个Servlet请求都设置压缩，那肯定在性能上会有差距，所以Gzip Servlet过滤器可以让我们对需要压缩的东西进行压缩，没必要压缩的就不去压缩。使性能最大化的提升。

###GZip Servlet滤波器设计

如图所示：首先需要一个Servlet过滤器类。该类映射到web.xml文件中的一组URL。当一个HTTP请求到达映射到过滤器的Servlet容器时，过滤器会在该请求被Servlet，JSP等处理之前截取请求所针对的目标。 GZip servlet过滤器检查客户端（浏览器）是否可以接受GZip压缩内容。如果可以接受，就对目标做压缩处理，然后将HttpServletResponse对象封装在GZipServletResponseWrapper进行压缩，最后将压缩内容写入HttpServletResponse。

###实例 The code consists of 3 classes. A GZipServletFilter, a GZipServletResponseWrapper and a GZipServletOutputStream.

The GZipServletOutputStream is what compresses the content written to it. It does so by using a GZIPOutputStream internally, which is a standard Java class.

主要用到三个类：GzipServletFilter、GzipServletResponseWrapper、GzipServletOutputStream。

GZipServletFilter 用来拦截请求，检查浏览器是否接受压缩。它将在HttpServletResponse传递给过滤器链之前将信息包装在GZipServletResponseWrapper中。

GZipServletResponseWrapper 用来返回一个输出流给Servlet或者Jsp，可以是GZipServletOutputStream 或者PrintWriter 类型的数据。

GZipServletOutputStream 是用来压缩并写入的内容的，通过内部使用GZIPOutputStream来实现。

示例：

public class GZipServletFilter implements Filter {  @Override  public void init(FilterConfig filterConfig) throws ServletException {  }  @Override  public void destroy() {  }  public void doFilter(ServletRequest request,                        ServletResponse response,                       FilterChain chain)   throws IOException, ServletException {    HttpServletRequest  httpRequest  = (HttpServletRequest)  request;    HttpServletResponse httpResponse = (HttpServletResponse) response;    if ( acceptsGZipEncoding(httpRequest) ) {      httpResponse.addHeader("Content-Encoding", "gzip");      GZipServletResponseWrapper gzipResponse =        new GZipServletResponseWrapper(httpResponse);            chain.doFilter(request, gzipResponse);      gzipResponse.close();    } else {      chain.doFilter(request, response);    }  }//判断 请求对象 是否接受压缩  private boolean acceptsGZipEncoding(HttpServletRequest httpRequest) {      String acceptEncoding =         httpRequest.getHeader("Accept-Encoding");      return acceptEncoding != null &&              acceptEncoding.indexOf("gzip") != -1;  }}复制代码

class GZipServletResponseWrapper extends HttpServletResponseWrapper {  private GZipServletOutputStream gzipOutputStream = null;  private PrintWriter             printWriter      = null;  public GZipServletResponseWrapper(HttpServletResponse response)          throws IOException {      super(response);  }  public void close() throws IOException {      //PrintWriter.close does not throw exceptions.      //Hence no try-catch block.      if (this.printWriter != null) {          this.printWriter.close();      }      if (this.gzipOutputStream != null) {          this.gzipOutputStream.close();      }  }  /**   * Flush OutputStream or PrintWriter   *   * @throws IOException   */  @Override  public void flushBuffer() throws IOException {    //PrintWriter.flush() does not throw exception    if(this.printWriter != null) {      this.printWriter.flush();    }    IOException exception1 = null;    try{      if(this.gzipOutputStream != null) {        this.gzipOutputStream.flush();      }    } catch(IOException e) {        exception1 = e;    }    IOException exception2 = null;    try {      super.flushBuffer();    } catch(IOException e){      exception2 = e;    }    if(exception1 != null) throw exception1;    if(exception2 != null) throw exception2;  }  @Override  public ServletOutputStream getOutputStream() throws IOException {    if (this.printWriter != null) {      throw new IllegalStateException(        "PrintWriter obtained already - cannot get OutputStream");    }    if (this.gzipOutputStream == null) {      this.gzipOutputStream = new GZipServletOutputStream(        getResponse().getOutputStream());    }    return this.gzipOutputStream;  }  @Override  public PrintWriter getWriter() throws IOException {     if (this.printWriter == null && this.gzipOutputStream != null) {       throw new IllegalStateException(         "OutputStream obtained already - cannot get PrintWriter");     }     if (this.printWriter == null) {       this.gzipOutputStream = new GZipServletOutputStream(         getResponse().getOutputStream());       this.printWriter      = new PrintWriter(new OutputStreamWriter(       this.gzipOutputStream, getResponse().getCharacterEncoding()));     }     return this.printWriter;  }  @Override  public void setContentLength(int len) {    //ignore, since content length of zipped content    //does not match content length of unzipped content.  }}复制代码

class GZipServletOutputStream extends ServletOutputStream {  private GZIPOutputStream    gzipOutputStream = null;  public GZipServletOutputStream(OutputStream output)        throws IOException {    super();    this.gzipOutputStream = new GZIPOutputStream(output);  }  @Override  public void close() throws IOException {    this.gzipOutputStream.close();  }  @Override  public void flush() throws IOException {    this.gzipOutputStream.flush();  }  @Override  public void write(byte b[]) throws IOException {    this.gzipOutputStream.write(b);  }  @Override  public void write(byte b[], int off, int len) throws IOException {    this.gzipOutputStream.write(b, off, len);  }  @Override  public void write(int b) throws IOException {     this.gzipOutputStream.write(b);  }}复制代码

###web.xml 配置

为了激活Gzip Servlet Filter，我们需要配置一些东西。


      
     
      GzipFilter
       
     
      com.xxx.GZipServletFilter
       #这里填写自己的GzipServletFilter类路径。
    
      
     
      GzipFilter
       
     
      *.js
     
    
      
     
      GzipFilter
       
     
      *.css
     
    
      
     
      GzipFilter
       
     
      *.html
     
    
      
     
      GzipFilter
       
     
      *.jsp
     
    
      
     
      GzipFilter
       
     
      /
     
    复制代码