Filter to intercept server output - servlet-filters

Question

I have to write a filter that will change the server's output. Here is my code:
#WebServlet("/ServerClass")
public class ServerClass extends HttpServlet {
private static final long serialVersionUID = 1L;
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String text = request.getParameter("text");
PrintWriter pw = new PrintWriter(response.getOutputStream());
pw.write(text);
pw.close();
}
}
The filter implementation:
#WebFilter("/TextFilter")
public class TextFilter implements Filter {
public TextFilter() {
}
public void destroy() {
}
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
PrintWriter out = response.getWriter();
TextResponseWrapper wrapper = new TextResponseWrapper((HttpServletResponse)response);
chain.doFilter(request, wrapper);
String responseText = wrapper.toString();
responseText.replaceAll("text", "****");
response.setContentLength(responseText.getBytes().length);
out.println(responseText);
out.close();
}
public void init(FilterConfig fConfig) throws ServletException {
}
}
class TextResponseWrapper extends HttpServletResponseWrapper {
private CharArrayWriter output;
public String toString() {
return output.toString();
}
public TextResponseWrapper(HttpServletResponse response){
super(response);
output = new CharArrayWriter();
}
public PrintWriter getWriter(){
return new PrintWriter(output);
}
}
web.xml
<filter>
<filter-name>TextFilter</filter-name>
<filter-class>TextFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>TextFilter</filter-name>
<servlet-name>ServerClass</servlet-name>
</filter-mapping>
The problem is that I am getting the following error:
java.lang.IllegalStateException: getWriter() has already been called for this response
at org.apache.catalina.connector.Response.getOutputStream(Response.java:604)
at org.apache.catalina.connector.ResponseFacade.getOutputStream(ResponseFacade.java:197)
at javax.servlet.ServletResponseWrapper.getOutputStream(ServletResponseWrapper.java:96)
at ServerClass.doPost(ServerClass.java:19)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at TextFilter.doFilter(TextFilter.java:33)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:953)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1041)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:603)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:310)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
So from what I understood after some research, is that this error is caused by the fact that I open the OutputStream from both ServerClass and TextFilter. But how can I intercept the server output without doing so? Which is the correct approach in this situation?

Answer 1

The servlet is calling getOutputStream, not getWriter. Your class TextResponseWrapper does not override getOutputStream.
So you have two choices:
Override getOutputStream to return your wrapping stream.
Change the servlet's code from PrintWriter pw = new PrintWriter(response.getOutputStream()); to PrintWriter pw = new PrintWriter(response.getWriter());