Friday, May 25, 2012

How to access private field and method using Reflection in Java

Reflection in Java is very powerful feature and allows you to access private method and fields which is not possible by any other means in Java and because of this feature of reflection many code coverage tool, static analysis tool and Java IDE like Eclipse and Netbeans has been so helpful. In last article we have seen details about private keyword in Java and learned  why we should always make fields and method private in Java. There we have mentioned that private fields and methods are only accessible in the class they are declared but with reflection you can call private method and access private fields outside the class. In this article we will see simple example of accessing private field using reflection and invoking private method using reflection in Java.

Accessing private fields in Java using reflection

invoke private method and field in java with reflection code exampleIn order to access private field using reflection, you need to know the name of field than by calling getDeclaredFields(String name) you will get a java.lang.reflect.Field instance representing that field. remember using getDclaredFields() method and not getFields() method which returns all non private fields both from sub class and super class. while getDeclaredFields() returns both private and non private fields declared in the class. Once you get the field reference you need to make it accessible by calling Field.setAccessible(true) because you are going to access private field. Now you can get value or private field by calling Field.get(String field_name).if you don't call setAccessible(true) and try to access private field using reflection you will get Exception as shown in below example

java.lang.IllegalAccessException: Class test.ReflectionTest can not access a member of class test.Person with modifiers "private"

        at sun.reflect.Reflection.ensureMemberAccess(Reflection.java:65)
        at java.lang.reflect.Field.doSecurityCheck(Field.java:960)
        at java.lang.reflect.Field.getFieldAccessor(Field.java:896)
        at java.lang.reflect.Field.get(Field.java:358)
        at test.ReflectionTest.main(ReflectionTest.java:31)

Calling private methods in Java using reflection

In our last Java tutorial on Reflection we have seen how to call a method by its String name and we will use that information here for invoking private method. Calling private method using reflection is similar to accessing private fields reflectively. Use getDeclaredMethods(String name, Class.. parameter) to get declared private method. pass all the argument type needed by method or nothing if method doesn't accept any argument. This will give you instance of java.lang.reflect.Method which can than be used to call private method using reflection, as shown in code example.

Code example of accessing private field and method using reflection

package test;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.Arrays;
import java.util.logging.Level;
import java.util.logging.Logger;

public class ReflectionTest {

    public static void main(String args[]) throws ClassNotFoundException {
     
        Class<Person> person = (Class<Person>) Class.forName("test.Person");
     
        //getFields() does not return private field
        System.out.println("Fields : " + Arrays.toString(person.getFields()));
     
        //getDeclaredFields() return both private and non private fields using reflection
        System.out.println("Declared Fields : " + Arrays.toString(person.getDeclaredFields()));      
           
        //getDeclaredMethods() return both private and non private methods using reflection
        System.out.println("Declared methods : " + Arrays.toString(person.getDeclaredMethods()));
     
        try {
         
            //accessing value of private field using reflection in Java
            Person privateRyan = new Person("John" , "8989736353");
            Field privateField = person.getDeclaredField("phone");
         
            //this call allows private fields to be accessed via reflection
            privateField.setAccessible(true);
         
            //getting value of private field using reflection
            String value = (String) privateField.get(privateRyan);          
         
            //print value of private field using reflection
            System.out.println("private field: " + privateField + " value: " + value);
         
         
            //accessing private method using reflection
            Method privateMethod = person.getDeclaredMethod("call");
         
            //making private method accessible using reflection
            privateMethod.setAccessible(true);
         
            //calling private method using reflection in java
            privateMethod.invoke(privateRyan);
         
         
        } catch (InvocationTargetException ex) {
            Logger.getLogger(ReflectionTest.class.getName()).log(Level.SEVERE, null, ex);
        } catch (NoSuchMethodException ex) {
            Logger.getLogger(ReflectionTest.class.getName()).log(Level.SEVERE, null, ex);
        } catch (IllegalArgumentException ex) {
            Logger.getLogger(ReflectionTest.class.getName()).log(Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            Logger.getLogger(ReflectionTest.class.getName()).log(Level.SEVERE, null, ex);
        } catch (NoSuchFieldException ex) {
            Logger.getLogger(ReflectionTest.class.getName()).log(Level.SEVERE, null, ex);
        } catch (SecurityException ex) {
            Logger.getLogger(ReflectionTest.class.getName()).log(Level.SEVERE, null, ex);
        }
     
     
    }
}

class Person{
    public String name;
    private String phone;
 
    public Person(String name, String phone){
        this.name = name;
        this.phone = phone;
    }
 
    private void call(){
        System.out.println("Calling " + this.name +" at " + this.phone);
    }
 
    public String getName(){
        return name;
    }
}

Output:
Fields : [public java.lang.String test.Person.name]
Declared Fields : [public java.lang.String test.Person.name, private java.lang.String test.Person.phone]
Declared methods : [public java.lang.String test.Person.getName(), private void test.Person.call()]
private field: private java.lang.String test.Person.phone value: 8989736353
Calling John at 8989736353


Few old Java article which you may like to read

1 comment :

Hemanth said...

I think Reflection breaks the purpose of using private variable. So, is Reflection a bad thing ? Is there any way we can avoid this ?

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