通过Filter和HttpServletResponseWrapper，实现Gzip压缩

实现定制输出的关键是对HttpServletResponse进行包装，截获所有输出，等过滤器链处理完后，
Filter.doFilter，在截获输出进行处理，在写入到真正的HttpServletResponse中。J2EE中已有
HttpServletResponseWrapper，使得包装HttpServletResponse更加容易

public class GZipFilter implements Filter {

  public void destroy() {

  }



  public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException,

      ServletException {

    HttpServletResponse resp = (HttpServletResponse) response;

    Wrapper wrapper = new Wrapper(resp);

    chain.doFilter(request, wrapper);

    byte[] gzipData = gzip(wrapper.getResponseData());

    resp.addHeader("Content-Encoding", "gzip");

    resp.setContentLength(gzipData.length);

    ServletOutputStream output = response.getOutputStream();

    output.write(gzipData);

    output.flush();

  }



  private byte[] gzip(byte[] data) {

    ByteArrayOutputStream byteOutput = new ByteArrayOutputStream(10240);

    GZIPOutputStream output = null;

    try {

      output = new GZIPOutputStream(byteOutput);

      output.write(data);

    } catch (IOException e) {

      e.printStackTrace();

    } finally {

      if (output != null) {

        try {

          output.close();

        } catch (IOException e) {

        }

      }

    }

    return byteOutput.toByteArray();

  }



  public void init(FilterConfig fConfig) throws ServletException {

  }

}





类Wrapper:

public class Wrapper extends HttpServletResponseWrapper {

  public static final int OT_NONE = 0, OT_WRITER = 1, OT_STREAM = 2;

  private int outputType = OT_NONE;

  private ServletOutputStream output = null;

  private PrintWriter writer = null;

  private ByteArrayOutputStream buffer = null;



  public Wrapper(HttpServletResponse resp) throws IOException {

    super(resp);

    buffer = new ByteArrayOutputStream();

  }



  public PrintWriter getWriter() throws IOException {

    if (outputType == OT_STREAM)

      throw new IllegalStateException();

    else if (outputType == OT_WRITER)

      return writer;

    else {

      outputType = OT_WRITER;

      writer = new PrintWriter(new OutputStreamWriter(buffer, getCharacterEncoding()));

      return writer;

    }

  }



  public ServletOutputStream getOutputStream() throws IOException {

    if (outputType == OT_WRITER)

      throw new IllegalStateException();

    else if (outputType == OT_STREAM)

      return output;

    else {

      outputType = OT_STREAM;

      output = new WrappedOutputStream(buffer);

      return output;

    }

  }



  public void flushBuffer() throws IOException {

    if (outputType == OT_WRITER)

      writer.flush();

    if (outputType == OT_STREAM)

      output.flush();

  }



  public void reset() {

    outputType = OT_NONE;

    buffer.reset();

  }



  public byte[] getResponseData() throws IOException {

    flushBuffer();

    return buffer.toByteArray();

  }



  //内部类，扩展ServletOutputStream


  class WrappedOutputStream extends ServletOutputStream {

    private ByteArrayOutputStream buffer;



    public WrappedOutputStream(ByteArrayOutputStream buffer) {

      this.buffer = buffer;

    }



    public void write(int b) throws IOException {

      buffer.write(b);

    }



    public byte[] toByteArray() {

      return buffer.toByteArray();

    }

  }

}