Querying Class Information at Runtime using Java Reflection API

Java Reflection API provides support for querying information about a class at run-time. This information includes the list of public as well as private members (methods) available for the class, the various public operations it supports etc. This section will guide you in making use of the Reflection API.

Suppose we have the following test class containing two methods,

TestClass.java

package tips.reflection;

public class TestClass {

    public int a = 10;
    private int b = 32;
    public String name = "TestClass";

    public void method1(){
        System.out.println("Method1 is called. a = " + a);
    }

    public void method3(){
        System.out.println("Method3 is called. name = " + name);
        privateMethod();
    }	

    private void privateMethod(){
        System.out.println("A private method ");
    }
}

If we want to get the class information for TestClass, then we have the following program. The following program dynamically loads the TestClass by calling the Class.forName("className"). It should be noted that while specifying the class name within Class.forName(), the fully qualified name of the class (i.e package name + class name) should be specified. Then to retrieve all the defined methods in the class, a call is made to Class.getDeclaredMethods() which returns all the methods as an array. Same happens while retrieving the fields.

The next block of code performs dynamic invocation of the method method1. It first gets a reference to the method method1 by calling Class.getMethod("method1"). Then using the Method.invoke() method, an invocation is performed on the method1 method by passing the original object along with an array of arguments the method is going to accept. Since in our case, the method method1 doesn't accept any arguments an empty Object array is passed to the Method.invoke() method.

ReflectionTest.java

package tips.reflection;

import java.lang.reflect.Field;
import java.lang.reflect.Method;

public class ReflectionTest {

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

        Class<?> classObject = Class.forName("tips.reflection.TestClass");

        Method allMethods[] = classObject.getDeclaredMethods();
        for(Method aMethod : allMethods){
            System.out.println("method : " +aMethod.getName());
        }

        Field allFields[] = classObject.getDeclaredFields();
        for(Field aField : allFields){
            System.out.println("field : " +aField.getName());
        }

        Object testClass = classObject.newInstance();		
        Method aMethod1 = classObject.getMethod("method1", new Class[]{});
        aMethod1.invoke(testClass, new Object[]{});		
    }
}

The output of the above code is,

method : method1
method : method3
method : privateMethod
field : a
field : b
field : name
Method1 is called. a = 10

In this section, we saw a simple example of using Reflection API. This API can be further explored and be effectively utilized based on our needs.