Exception message in Servlet

Question

I have JSON-RPC Service (a class with methods invoked using Reflection) and I want to pass exception as data to error object. So I try to catch Exception in Servlet but e.getMessage() return null, showing backtrace don't show my code at all (only function that get Stack trace and a place where I call it):

public Object loadService() throws InstantiationException, ClassNotFoundException, IllegalAccessException {
    ClassLoader parentClassLoader = ServiceReloader.class.getClassLoader();
    ServiceReloader classLoader = new ServiceReloader(parentClassLoader);
    //ClassLoader classLoader = init.class.getClassLoader();
    Class aClass = classLoader.loadClass("pl.jcubic.Service");
    return aClass.newInstance();
}
public String backTrace() {
    StackTraceElement[] stack = (new Throwable()).getStackTrace();
    String trace = "";
    for (int i=0; i<stack.length; ++i) {
        trace += stack[i].toString() + "\n";
    }
    return trace;
}
public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    Object id = null;
    try {
        JSONRPC2Request reqIn = JSONRPC2Request.parse(this.getInputData(request));
        id = reqIn.getID();

        Object service = loadService();

        Method[] methods = service.getClass().getMethods();

        Object result = method.invoke(service, params);
        JSONRPC2Response respOut = new JSONRPC2Response(result, id);
        out.println(respOut);

    } catch (InvocationTargetException e) {
        out.println(internalError(id, e.getMessage(), backTrace()));
    }
}

when a method throw Exception like throw new Exception("Error occured"); (that message I want to send using JSON-RPC) I got this backtrace,

init.stackTrace(init.java:65)
init.doPost(init.java:119)
javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)
org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)
org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:99)
org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:936)
org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:407)
org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1004)
org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:589)
org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:312)
java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
java.lang.Thread.run(Thread.java:724)

Is there any way to display my Exception I throw?

Answer 1

I have two errors here:

1. I use new Trowable to create backtrace I should use existing exception:

   public String backTrace(Throwable e) {
       StackTraceElement[] stack = e.getStackTrace();
       String trace = "";
       for (int i=0; i<stack.length; ++i) {
           trace += stack[i].toString() + "\n";
       }
       return trace;
   }
   

2. And I need to use getCause() method of Throwable to get real Exception (don't know if I only need to use it because of reflection):

   } catch (InvocationTargetException e) {
       Throwable cause = e.getCause();
       out.println(internalError(id, cause.getMessage(), backTrace(cause)));
   }