import java.io.CharArrayWriter;
import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpServletResponseWrapper;

public class UpperCaseFilter implements Filter {
  protected FilterConfig config;

  public void init(FilterConfig config) throws ServletException {
    this.config = config;
  }

  public void destroy() {
  }

  public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
      throws ServletException, IOException {
    ServletResponse newResponse = response;

    if (request instanceof HttpServletRequest) {
      newResponse = new CharResponseWrapper((HttpServletResponse) response);
    }

    chain.doFilter(request, newResponse);

    if (newResponse instanceof CharResponseWrapper) {
      String text = newResponse.toString();
      if (text != null) {
        response.getWriter().write(text);
      }
    }
  }
}

class CharResponseWrapper extends HttpServletResponseWrapper {
  protected CharArrayWriter charWriter;

  protected PrintWriter writer;

  protected boolean getOutputStreamCalled;

  protected boolean getWriterCalled;

  public CharResponseWrapper(HttpServletResponse response) {
    super(response);

    charWriter = new CharArrayWriter();
  }

  public ServletOutputStream getOutputStream() throws IOException {
    if (getWriterCalled) {
      throw new IllegalStateException("getWriter already called");
    }

    getOutputStreamCalled = true;
    return super.getOutputStream();
  }

  public PrintWriter getWriter() throws IOException {
    if (writer != null) {
      return writer;
    }
    if (getOutputStreamCalled) {
      throw new IllegalStateException("getOutputStream already called");
    }
    getWriterCalled = true;
    writer = new PrintWriter(charWriter);
    return writer;
  }

  public String toString() {
    String s = null;

    if (writer != null) {
      s = charWriter.toString();
    }
    return s;
  }
}

Simple response wrapper that utilizes a local CharArrayWriter for output


<filter>
  <filter-name>Upper Case Filter</filter-name>
  <filter-class>com.omh.filters.UpperCaseFilter</filter-class>
</filter>

<filter-mapping>
  <filter-name>Upper Case Filter</filter-name>
  <url-pattern>/*</url-pattern>
</filter-mapping>

*/