你的wrapper實現有問題。應當在內存中開辟一個ByteOutputStream，然后將攔截的響應寫入byte[]，寫入完畢后，再將wrapper的byte[]寫入真正的response對象

下面的例子是《Spring 2.0核心技術與最佳實踐》中的例子：

package net.livebookstore.web.filter;

import java.io.*;

import javax.servlet.*;
import javax.servlet.http.*;

/**
* This class is used for wrapped response for getting cached data.
*
* @author Xuefeng
*/
class CachedResponseWrapper extends HttpServletResponseWrapper {

/**
* Indicate that getOutputStream() or getWriter() is not called yet.
*/
public static final int OUTPUT_NONE = 0;

/**
* Indicate that getWriter() is already called.
*/
public static final int OUTPUT_WRITER = 1;

/**
* Indicate that getOutputStream() is already called.
*/
public static final int OUTPUT_STREAM = 2;

private int outputType = OUTPUT_NONE;

private int status = SC_OK;
private ServletOutputStream output = null;
private PrintWriter writer = null;
private ByteArrayOutputStream buffer = null;

public CachedResponseWrapper(HttpServletResponse resp) throws IOException {
super(resp);
buffer = new ByteArrayOutputStream();
}

public int getStatus() { return status; }

public void setStatus(int status) {
super.setStatus(status);
this.status = status;
}

public void setStatus(int status, String string) {
super.setStatus(status, string);
this.status = status;
}

public void sendError(int status, String string) throws IOException {
super.sendError(status, string);
this.status = status;
}

public void sendError(int status) throws IOException {
super.sendError(status);
this.status = status;
}

public void sendRedirect(String location) throws IOException {
super.sendRedirect(location);
this.status = SC_MOVED_TEMPORARILY;
}

public PrintWriter getWriter() throws IOException {
if(outputType==OUTPUT_STREAM)
throw new IllegalStateException();
else if(outputType==OUTPUT_WRITER)
return writer;
else {
outputType = OUTPUT_WRITER;
writer = new PrintWriter(new OutputStreamWriter(buffer, getCharacterEncoding()));
return writer;
}
}

public ServletOutputStream getOutputStream() throws IOException {
if(outputType==OUTPUT_WRITER)
throw new IllegalStateException();
else if(outputType==OUTPUT_STREAM)
return output;
else {
outputType = OUTPUT_STREAM;
output = new WrappedOutputStream(buffer);
return output;
}
}

public void flushBuffer() throws IOException {
if(outputType==OUTPUT_WRITER)
writer.flush();
if(outputType==OUTPUT_STREAM)
output.flush();
}

public void reset() {
outputType = OUTPUT_NONE;
buffer.reset();
}

/**
* Call this method to get cached response data.
* @return byte array buffer.
* @throws IOException
*/
public byte[] getResponseData() throws IOException {
flushBuffer();
return buffer.toByteArray();
}

/**
* This class is used to wrap a ServletOutputStream and
* store output stream in byte[] buffer.
*/
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();
}
}

}

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在Filter中調用：

public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException
{
HttpServletRequest httpRequest = (HttpServletRequest) request;
HttpServletResponse httpResponse = (HttpServletResponse)response;
CachedResponseWrapper wrapper = new CachedResponseWrapper(httpResponse);
// 寫入wrapper:
chain.doFilter(request, wrapper);
// 首先判斷status, 只對200狀態處理：
if(wrapper.getStatus()==HttpServletResponse.SC_OK) {
// 對響應進行處理，這里是進行GZip壓縮:
byte[] data = GZipUtil.gzip(wrapper.getResponseData());
httpResponse.setContentType(getContentType());
httpResponse.setContentLength(data.length);
httpResponse.setHeader("Content-Encoding", "gzip");
ServletOutputStream output = response.getOutputStream();
output.write(data);
output.flush();
}
}

需要判斷狀態，正確設置Content-Length，具體可參考《Spring 2.0核心技術與最佳實踐》第十一章12節：利用Filter實現內存緩存和靜態文件緩存
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@xuefeng
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