Reflection

1. Overview

It stands for modifying or examing the behavior of methods, classes, and interfaces at runtime.

• java.lang.reflect packages required classes for reflection
• It gives us information about the class to which an object belongs and also the methods of that class which can be executed by using the object.
• Being able to invoke methods at runtime irrespective of the access specifier used with them.
  • This can violate some policy like singleton using private constructor, and so on.

 

1.1 Where it used

• IDE e.g. Eclipse, NetBeans, etc.
• Debugger
• Test Tools
• Spring
• Hibernate

2. Usages

Reflection can be used to get information about

2.1 Class

• The getClass() is used to get the name of the class to which an object belongs.

2.2 Constructor

• The getConstructors() is used to get the public constructors of the class which an object belongs and what parameters they take, and etc.

2.3 Methods

• The getMethods() is used to get the public methods of the class to which an object belongs.

2.4 Fields

• If you have a reference to the Class object representing some class, you can see what fields that class contains.
• You can read its name, access modifier, etc.

Class myObjectClass = MyObject.class;

Field[] fields   = myObjectClass.getFields();

// A simple Java program to demonstrate the use of reflection 
import java.lang.reflect.Method; 
import java.lang.reflect.Field; 
import java.lang.reflect.Constructor; 
  
// class whose object is to be created 
class Test 
{ 
    // creating a private field 
    private String s; 
  
    // creating a public constructor 
    public Test()  {  s = "GeeksforGeeks"; } 
  
    // Creating a public method with no arguments 
    public void method1()  { 
        System.out.println("The string is " + s); 
    } 
  
    // Creating a public method with int as argument 
    public void method2(int n)  { 
        System.out.println("The number is " + n); 
    } 
  
    // creating a private method 
    private void method3() { 
        System.out.println("Private method invoked"); 
    } 
} 
  
class Demo 
{ 
    public static void main(String args[]) throws Exception 
    { 
        // Creating object whose property is to be checked 
        Test obj = new Test(); 
  
        // Creating class object from the object using 
        // getclass method 
        Class cls = obj.getClass(); 
        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(); 
        System.out.println("The name of constructor is " + 
                            constructor.getName()); 
  
        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 methodcall1 = cls.getDeclaredMethod("method2", 
                                                 int.class); 
  
        // invokes 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); 
  
        // invokes the method at runtime 
        methodcall3.invoke(obj); 
    } 
} 

The name of class is Test
The name of constructor is Test
The public methods of class are : 
method2
method1
wait
wait
wait
equals
toString
hashCode
getClass
notify
notifyAll
The number is 19
The string is JAVA
Private method invoked

3. Features

3.1 The .class syntax

• If a type is available but no instance then it is possible to obtain a Class by appending ".class" to the name of the type. it can be used for primitive data type so.

class Test{  
  public static void main(String args[]){  
   Class c = boolean.class;   
   System.out.println(c.getName());  
  
   Class c2 = Test.class;   
   System.out.println(c2.getName());  
 }  
}  

boolean
Test

3.2 invoke()

• Invoking a method of the class at runtime 

Method.invoke(Object, parameter)
// If the method of the class doesn’t accepts any 
// parameter then null is passed as argument.

 

3.3 Invoking regardless of access modifier

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. below for this purpose

3.3.1 Class.getDeclaredField(FieldName)

• Used to get the private field. Returns an object type Field for a specified field name

3.3.2 Field.setAccessible(true)

• Allows to access the field irrespective of the access modifier used with the field

4. Methods

Method	Description
public String getName()	returns the class name
public static Class forName(String className) throws ClassNotFoundException	loads the class and returns the reference of Class class.
public Object newInstance() throws Instantiation, IllegalAccessException	creates a new instance.
public boolean isInterface()	checks if it is an interface
public boolean isArray()	checks if it is an array.
public boolean isPrimitive()	checks if it is primitive
public Class getSuperclass()	returns the superclass class reference
public Field[] getDeclaredFields() throws SecurityException	returns the total number of fields of the class
public Method[] getDeclaredMethods() throws SecurityException	returns the total number of methods of this class.
public Consructor[] getDeclaredConstructor() throws SecurityException	returns the total number of constructors of this class.
public Method getDeclaredMethod(String name, Class[] parameterTypes) throws NosSuchMethodException, SecurityException	returns the method class instance

5. Pros and Cons

3.1 Advantages

• Extensibility Features
  • An application may make use of external, user-defined classes by creating instances of extensibility objects using their fully-qualified names
• Debugging the and testing tools
  • Debuggers use the property of reflection to examine private members in classes.

3.2 Drawbacks

• Performance Overhead
  • Reflective operations have slower performance than their non-reflective counterparts and should be avoided in sections of code which are called frequently in performance-sensitive applications
• Exposure of Internals
  • Reflective code breaks abstractions and therefore may change behavior with upgrades of the platform.

6. References

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

https://www.geeksforgeeks.org/reflection-in-java/

https://www.javatpoint.com/java-reflection

https://www.baeldung.com/java-reflection

http://tutorials.jenkov.com/java-reflection/index.html

http://www.java2s.com/Tutorials/Java/Java_Reflection/0020__Java_java.lang.Class.htm