Question

I have a Servlet class which do some dynamic class initialization and method invocation using reflection. It has the following code to do that.

@Override
protected void doGet( HttpServletRequest request, HttpServletResponse response ) throws ServletException, IOException {
    try {
        String reqActionContext = PageContextHelper.getFunctionName( "login" );
         // Trace reqActionContext
        System.out .println("reqActionContext : " + reqActionContext );
        // Get the page context related class object
        Object contextClass = PageContextHelper.getContextBoundedClass( "authentication");
        Class < ? >[ ] paramTypes = new Class < ? >[2];

        paramTypes[0] = HttpServletRequest.class ;
        paramTypes[1] = HttpServletResponse.class ;

        // Is there a class associated with the current context
        if ( contextClass != null ) {
            // Trace contextClassSystem. out .println("Contextclass : " + contextClass);
            // get the Class
            Class < ? > thisClass = contextClass.getClass();
            // Trace thisClass
            System.out .println("thisClass : " + thisClass );
            // get the method
            Method contextMethod = thisClass.getDeclaredMethod( reqActionContext, paramTypes );

            if ( contextMethod != null ) {
                contextMethod.invoke ( contextClass, request, response );
            }
        }
    }  catch ( IllegalArgumentException e ) {
        e.printStackTrace();
    } catch ( IllegalAccessException e ) {
        e.printStackTrace();
    } catch ( InvocationTargetException e ) {
        e.printStackTrace();
    } catch ( Exception e ) {
        e.printStackTrace();
    }
}

The class which get the context bounded class has the following code :
PageContextHelper.java

public class PageContextHelper {

    private static final String CONTEXT_CLASS_SUFFIX = "ContextHelper" ;

    private static final String CONTEXT_CLASS_PACKAGE = "com.proj.context." ;

    /**
    * Get the base path related context class object.
    *
    *@param basePath
    *@return
    */
    public static Object getContextBoundedClass ( String basePath ) {
        Class < ? > classOBJ = null ;

        try{

            if(basePath != null && !basePath.isEmpty() && basePath.length() > 1 ) {
                basePath = basePath .substring( 0,1 ).toUpperCase() + basePath.substring( 1 ).toLowerCase();
            }
            // Get the class object
            classOBJ = Class.forName( CONTEXT_CLASS_PACKAGE + basePath + CONTEXT_CLASS_SUFFIX);
        } catch ( ClassNotFoundException e ) {
            // Do nothing and return null object
        }
        return classOBJ;
    }

    .....

}

The class which is looking for is com.proj.context.AuthenticationContextHelper and it has the following content :
com.proj.context.AuthenticationContextHelper

package com.proj.context;

    public class AuthenticationContextHelper{

    public void loginProcess (HttpServletRequest request, HttpServletResponse response) throws Exception {
        try {
            // Do some processing here
        } catch ( Exception e ) {
            // TODO : Remove this trace line and do something meaningful
            e.printStackTrace();
        }
    }
}

When I run the code get the following trace :
Trace

reqActionContext :loginProcess 
Contextclass : class com.proj.context.AuthenticationContextHelper
thisClass : class java.lang.Class
java.lang.NoSuchMethodException: java.lang.Class.loginProcess(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse)
        at java.lang.Class.getDeclaredMethod(Class.java:1937)
        at com.proj.servlet.ContentServlet.doGet(ContentServlet.java:81)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
...

It says NoSuchMethod but the method is there in the class com.proj.context.AuthenticationContextHelper.

Am I missing any basic here or something wrong in the code?

The environment I am using is Java 1.6.0_32 and Tomcat 7

Answer 1

You are returning a Class object from getContextBoundedClass() and getting that into an Object here:

Object contextClass = PageContextHelper.getContextBoundedClass( "authentication");

Instead say this:

 Class contextClass = PageContextHelper.getContextBoundedClass( "authentication");

You dont call contextClass.getClass() again. Instead to get an instance of a class you call contextClass.newInstance() to create an instance of that object and use that while invoking the method. Read this tutorial:

http://docs.oracle.com/javase/tutorial/reflect/index.html