Breaching Java Rules with Reflection

Continued from page: 1

Thursday, August 09, 2007

Putting Reflection in action
It's time to hit the ground running! We'll write a simple standalone Java client which will use the reflection API to call the private pop(int elementCount) method on the stack class. The following code fragment presents the Stackmain.java class:

package com.pcquest.medtracker.client;

import com.pcquest.medtracker.model.Patient;

import com.pcquest.medtracker.model.Stack;

import java.lang.reflect.*;

public class Stackmain {

public static void main(String[] args)throws Exception {

Stack s=new Stack(10);

s.push(new Patient("Samuel","Hoffman"));

s.push(new Patient("Richard","Ryan"));

s.push(new Patient("Steve","Beytel"));

System.out.println("The original list is: " + s);

Class klass = s.getClass();

Class[] paramTypes = { Integer.TYPE };

Method m = klass.getDeclaredMethod("pop", paramTypes);

Object[] arguments = { new Integer(2) };

m.setAccessible(true);

m.invoke(s, arguments);

System.out.println("The new list is: "

+ s);

}

}

In the Stackmain class we begin by instantiating the stack initially with 10 elements. Next, we get a Class object. The arguments to any method that is invoked through reflection is passed as an array of Class object. The getDeclaredMethod returns all methods declared by one class. In the above code fragment, we get the private pop(int elementCount) method handler. Then we call the setAccessible() method, passing 'true' to make it available to the caller program. Finally, the invoke() method invokes the underlying method represented by the Method object, on the specified object with specified parameters. The following screenshot depicts the output.

Conclusion
Reflection is a powerful tool in the hands of a developer with which she can modify the runtime behavior of applications running in the Java virtual machine. That said; reflection should only be used by experienced developers. According to the Sun Java Tutorial, “Reflection is powerful, but should not be used indiscriminately”. Reflection comes at a cost--non-reflective programs are more responsive. A security manager can prevent reflexive behavior at runtime. As a thumb rule, Reflection should only be used by experienced developers as indiscriminate usage could change semantics of a running application and introduce side effects.

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