Java Programming/Reflection/Accessing Private Features with Reflection

Navigate Reflection topic: Reflection Overview Dynamic Class Loading Dynamic Method Invocation Accessing Private Features with Reflection

All features of a class can be obtained via reflection, including access to private methods & variables. But not always see [1]

Let us look at the following example, Secret.java:

public class Secret {

  private String secretCode = "It's a secret";

  private String getSecretCode(){
    return secretCode;     
  }
}

Although the field and method are marked private, the following class shows that it is possible to access the private features of a class:

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

public class Hacker {
 
  private static final Object[] EMPTY = {};

  public void reflect() throws IllegalAccessException, IllegalArgumentException, InvocationTargetException {
    Secret instance = new Secret();
    Class secretClass = instance.getClass();

    // Print all the method names & execution result
    Method methods[] = secretClass.getDeclaredMethods(); 
    System.out.println("Access all the methods"); 
    for (int i = 0; i < methods.length; i++) { 
       System.out.println("Method Name: " + methods[i].getName());
       System.out.println("Return type: " + methods[i].getReturnType());
       methods[i].setAccessible(true);
       System.out.println(methods[i].invoke(instance, EMPTY) + "\n");
    }

    //  Print all the field names & values
    Field fields[] = secretClass.getDeclaredFields();
    System.out.println("Access all the fields");
    for (int i = 0; i < fields.length; i++){ 
       System.out.println("Field Name: " + fields[i].getName()); 
       fields[i].setAccessible(true); 
       System.out.println(fields[i].get(instance) + "\n"); 
    }
 }

 public static void main(String[] args){

   Hacker newHacker = new Hacker();
 
   try { 
     newHacker.reflect();
   }
   catch (Exception e) {
     e.printStackTrace();
   }
 }
}

Output:

Access all the methods
Method Name: getSecretCode
Return type: class java.lang.String
It's a secret
Access all the fields
Field Name: secretCode
It's a secret

(We need to add some explanation of what is going on here. --djb 00:24, 14 March 2006 (UTC))

JUnit - Test Private methods

JUnit's are unit test cases, used to test the Java programs. Now you know how to test a private method using Reflection in JUnit.

There's a long-standing debate on whether testing private members is a good habit;There are cases where you want to make sure a class exhibited the right behavior while not making the fields that need checking to assert that public (as it's generally considered bad practice to create accessors to a class just for the sake of a unit test). There's also cases when you can greatly simplify a test case by using reflection to test all smaller private methods (and their various branches), then test the main function.

With dp4j it is possible to test private members without directly using the Reflection API but simply accessing them as if they were accessible from the testing method; dp4j injects the needed Reflection code at compile-time.