Behavior of the instance variable private & Final while working with reflection

Using reflection, I am able to successfully access the private members of the class A suite help extract:
package com.test;

public class A {

     
     private int _count = 0;
     public A() {
          
     }
     
     public void setCount(int count) {
          if ( count < 0){
               System.err.println("Value must be positive.");
               return;
          }
          _count = count;
     }
     
     public int getCount() {
          return _count;
     }
     
}
package com.test;

import java.lang.reflect.Field;

public class MainClass {

     private static Field f = null;
     A a  = null;
     static {
          try {
               
               f = A.class.getDeclaredField("_count");
               if ( f != null)
                    f.setAccessible(true);
               
          } catch (NoSuchFieldException e) {
               // TODO Auto-generated catch block
               e.printStackTrace();
          } catch (SecurityException e) {
               // TODO Auto-generated catch block
               e.printStackTrace();
          }
          
     }
     
     
     public MainClass() {
          
          a = new A();
          
          if ( f != null ) {
               try {
                    f.set(a, -99);
               } catch (IllegalArgumentException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
               } catch (IllegalAccessException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
               }
          }
          
     }
     
     public void printCount() {
          
          System.out.println("Total Count : " + a.getCount());
          
     }
     
     public static void main(String[] args) {
          
     
          MainClass cls = new MainClass();
          cls.printCount();
          
          
     }
     
}
The above code works fine, but when I tried the same thing with the class provided as java.lang.String and tried access private member of this one, he threw an exception to access. If access can be restricted by setting SecurityManager I've tried in my case also and worked, but also in java.lang.String, I was unable to find any implementation for the SecurityManager who refused access to private members, then how is it that I'm not able to access by using reflection?

Also in the same example above, when I did a final variable _count, and tried changing the value by using reflection, that he did not throw any exception when running, but don't not change either value.

Appreciate any help on the improvement of the understanding on this.

Thanks in advance.

I gave it a try with Java 1.7.0_21 - b11.

import java.lang.reflect.*;

class Y {
     public static void main(String[] args) throws Exception {
          String s = "123";
          System.out.println(s);
          Field stringValue = String.class.getDeclaredField("value");
          stringValue.setAccessible(true);
                //stringValue.set(s, "321");
          stringValue.set(s, new char[]{'3', '2', '1'});
          System.out.println(s);
     }
}

Output:

123
321

Jtahlborn is right and your code may not agree. You may have seen:

Exception in thread "main" java.lang.IllegalArgumentException: Can not set final [C field java.lang.String.value to java.lang.String
        at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(Unknown Source)
        at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(Unknown Source)
        at sun.reflect.UnsafeQualifiedObjectFieldAccessorImpl.set(Unknown Source)
        at java.lang.reflect.Field.set(Unknown Source)
        at Y.main(Y.java:8)

What is happening with the commented line.

Published by: baftos on May 16, 2013 15:07

Tags: Oracle

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