need somehelp ,the code of java at weblogic strange questious

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Java

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J2EE

good evening,everyone,i am come from china,my engilish is bad,Please forgive me.
i get into trouble,this is the code:

package com.wind.frame.base.wrapper;

import java.io.PrintWriter;
import java.io.StringWriter;

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


/**
 * 
 *
 */
public class ResponseWrapper extends HttpServletResponseWrapper {
	/**
	 */
	private StringWriter output;

	/**
	 * @param response
	 */
	public ResponseWrapper(HttpServletResponse response) {
		super(response);
		output = new StringWriter();
	}
	
	@Override
	public PrintWriter getWriter() {
        return new PrintWriter(output);
    }
	
	/**
	 *
	 * 
	 * @return
	 */
	public String getResult() {
	    return output.toString();
	}
	
}

in a filter,the method:

ResponseWrapper responseWrapper = new ResponseWrapper((HttpServletResponse) response);
filterChain.doFilter(request, responseWrapper);
String result = responseWrapper.getWriter();

the code in tomcat ,can get the correct results,in weblogic10,the resultis : String result="";

can understand what I said ?