通过在Java中使用反射来访问和修改私有类成员

Question

The following program is accessing and modifying a private field named privateKey declared within the class SimpleKeyPair . Let's have a look at it.

package mod;

import java.lang.reflect.Field;
import java.util.logging.Level;
import java.util.logging.Logger;

final class SimpleKeyPair
{
    private String privateKey = "Welcome SimpleKeyPair ";
}

final public class Main
{
    public static void main(String[] args)
    {
        SimpleKeyPair keyPair = new SimpleKeyPair();
        Class c = keyPair.getClass();

        try
        {
            Field field = c.getDeclaredField("privateKey");    // gets the reflected object
            field.setAccessible(true);

            System.out.println("Value of privateKey: " + field.get(keyPair));  // displays “Welcome SimpleKeyPair"

            field.set(keyPair, "Welcome PrivateMemberAccessTest");    // modifys the private member varaible

            System.out.println("Value of privateKey: " + field.get(keyPair));
        }
        catch (IllegalArgumentException ex)
        {
            Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);
        }
        catch (IllegalAccessException ex)
        {
            Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);
        }
        catch (NoSuchFieldException ex)
        {
            Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);
        }
        catch (SecurityException ex)
        {
            Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
}

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In the code above, the private field privateKey declared within the class SimpleKeyPair is being accessed and displayed on the console through the following statements.

Field field = c.getDeclaredField("privateKey"); 
field.setAccessible(true);
System.out.println("Value of privateKey: " + field.get(keyPair));

---

and that field is being modified and the new value of that field is being displayed through the following statements.

field.set(keyPair, "Welcome PrivateMemberAccessTest"); 
System.out.println("Value of privateKey: " + field.get(keyPair));

---

The actual output of the program would be as under.

Value of privateKey: Welcome SimpleKeyPair 
Value of privateKey: Welcome PrivateMemberAccessTest

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Means that the use of reflection in Java allows a direct access to private resources. If it is so then, declaring a member as private itself in Java is not safe though one of the aims of declaring class members as private is to hide them from outside the world. What is the actual use of reflection in Java?

Answer 1

You are correct that reflection can allow access to private (and package and protected) scoped members of a class. However, if you read the JavaDocs, you will find that all of the methods for fetching and invoking these accessors perform a SecurityManager check before performing the requested operation. So, in an environment with a SecurityManger, the operations will fail with SecurityExceptions being thrown.

Answer 2

What is the use of reflection? Introspection, the ability to make dynamic calls/resolutions, tooling, etc.

Do you mean what is the actual use of private ? To declare intent, and disallow casual abuse of private members. Heh.

Answer 3

In generally reflection is useful when you want work with metadata; an example are the orm framework like hibernate that use a lot of reflection capabilities and so on...