Reflection in Java

Reflection is an API that is used to examine or modify the behavior of methods, classes, and interfaces at runtime. The required classes for reflection are provided under java.lang.reflect package which is essential in order to understand reflection. So we are illustrating the package with visual aids to have a better understanding as follows: 

• Reflection gives us information about the class to which an object belongs and also the methods of that class that can be executed by using the object.
• Through reflection, we can invoke methods at runtime irrespective of the access specifier used with them.

Reflection can be used to get information about class, constructors, and methods as depicted below in tabular format as shown:

Class	The getClass() method is used to get the name of the class to which an object belongs.
Constructors	The getConstructors() method is used to get the public constructors of the class to which an object belongs.
Methods	The getMethods() method is used to get the public methods of the class to which an object belongs.

We can invoke a method through reflection if we know its name and parameter types. We use two methods for this purpose as described below before moving ahead as follows:

1. getDeclaredMethod()
2. invoke()

Method 1: getDeclaredMethod(): It creates an object of the method to be invoked. 

Syntax: The syntax for this method

Class.getDeclaredMethod(name, parametertype)

Parameters:

• Name of a method whose object is to be created
• An array of Class objects

Method 2: invoke(): It invokes a method of the class at runtime we use the following method.

Syntax:

Method.invoke(Object, parameter)

Tip: If the method of the class doesn’t accept any parameter then null is passed as an argument.

Note: Through reflection, we can access the private variables and methods of a class with the help of its class object and invoke the method by using the object as discussed above. We use below two methods for this purpose.

Method 3: Class.getDeclaredField(FieldName): Used to get the private field. Returns an object of type Field for the specified field name. 

Method 4: Field.setAccessible(true): Allows to access the field irrespective of the access modifier used with the field.

Important observations Drawn From Reflection API

• Extensibility Features: An application may make use of external, user-defined classes by creating instances of extensibility objects using their fully-qualified names.
• Debugging and testing tools: Debuggers use the property of reflection to examine private members of classes.
• Performance Overhead: Reflective operations have slower performance than their non-reflective counterparts, and should be avoided in sections of code that are called frequently in performance-sensitive applications.
• Exposure of Internals: Reflective code breaks abstractions and therefore may change behavior with upgrades of the platform.

Example

Java

// Java Program to demonstrate the Use of Reflection

// Importing required classes
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;

// Class 1
// Of Whose object is to be created
class Test {
    // creating a private field
    private String s;

    // Constructor of this class

    // Constructor 1
    // Public constructor
    public Test() { s = "GeeksforGeeks"; }

    // Constructor 2
    // no arguments
    public void method1()
    {
        System.out.println("The string is " + s);
    }

    // Constructor 3
    // int as argument
    public void method2(int n)
    {
        System.out.println("The number is " + n);
    }

    // Constructor 4
    // Private method
    private void method3()
    {
        System.out.println("Private method invoked");
    }
}

// Class 2
// Main class
class GFG {

    // Main driver method
    public static void main(String args[]) throws Exception
    {
        // Creating object whose property is to be checked

        // Creating an object of class 1 inside main()
        // method
        Test obj = new Test();

        // Creating class object from the object using
        // getClass() method
        Class cls = obj.getClass();

        // Printing the name of class
        // using getName() method
        System.out.println("The name of class is "
                           + cls.getName());

        // Getting the constructor of the class through the
        // object of the class
        Constructor constructor = cls.getConstructor();

        // Printing the name of constructor
        // using getName() method
        System.out.println("The name of constructor is "
                           + constructor.getName());

        // Display message only
        System.out.println(
            "The public methods of class are : ");

        // Getting methods of the class through the object
        // of the class by using getMethods
        Method[] methods = cls.getMethods();

        // Printing method names
        for (Method method : methods)
            System.out.println(method.getName());

        // Creates object of desired method by
        // providing the method name and parameter class as
        //  arguments to the getDeclaredMethod() method
        Method methodcall1
            = cls.getDeclaredMethod("method2", int.class);

        // Invoking the method at runtime
        methodcall1.invoke(obj, 19);

        // Creates object of the desired field by
        // providing the name of field as argument to the
        // getDeclaredField() method
        Field field = cls.getDeclaredField("s");

        // Allows the object to access the field
        // irrespective of the access specifier used with
        // the field
        field.setAccessible(true);

        // Takes object and the new value to be assigned
        // to the field as arguments
        field.set(obj, "JAVA");

        // Creates object of desired method by providing the
        // method name as argument to the
        // getDeclaredMethod()
        Method methodcall2
            = cls.getDeclaredMethod("method1");

        // Invokes the method at runtime
        methodcall2.invoke(obj);

        // Creates object of the desired method by providing
        // the name of method as argument to the
        // getDeclaredMethod() method
        Method methodcall3
            = cls.getDeclaredMethod("method3");

        // Allows the object to access the method
        // irrespective of the access specifier used with
        // the method
        methodcall3.setAccessible(true);

        // Invoking the method at runtime
        methodcall3.invoke(obj);
    }
}

Output:

Related Courses

Java Programming Foundation – Self Paced Course

Start your programming journey the easy way! Learn Java basics and go from basics to advance with our Java Programming Foundation – Self Paced Course. This course covers topics like Variables and Data types, Operators, Loops, Functions in Java and a lot more, curated by leading industry experts having years of expertise. Get yourself enrolled and become an expert in JAVA!