Fail to modify private final static variable without Exception

Question

I tried to modify private final static variable like this:

    ...try {

        Field f =TargetA.class.getDeclaredField("RECV_TIMEOUT");
        f.setAccessible(true);

        Field modifiersField = Field.class.getDeclaredField("modifiers");
        modifiersField.setAccessible(true);
        modifiersField.setInt(f, f.getModifiers() & ~Modifier.FINAL);

        f.set(null, 12L);

    } catch (Exception e) {
        e.printStackTrace();//not reach here!
    } ...

    class TargetA{
        private static final long RECV_TIMEOUT = 180000L;
     }

However TargetA.RECV_TIMEOUT is still 180000L, without any Exception. I searched the problem in StackOverflow, but couldn’t find solution.

I guess the Java version1.6 has more restriction in reflection which breaks OO rules. Thanks for your advice!

Peter Lawrey · Accepted Answer · 2011-08-30 09:58:00Z

7

You can change the static final field this way and if you look at the value using reflection it will be changed. The problem you have is that the compiler does one and only one optimisation which is to inline constants know at compile time. This means the value can be change, however the places where the constant is used is not changed.

A way around this is to use a wrapper method to "confuse" the compiler, which avoid having to change how you use the constant.

public static final long RECV_TIMEOUT = runtime(180000L);

public static final <T> T runtime(T t) { return t; }

edited Aug 30, 2011 at 9:58

answered Aug 30, 2011 at 9:48

Peter Lawrey

535k83 gold badges770 silver badges1.2k bronze badges

Sign up to request clarification or add additional context in comments.

Comments

axtavt · Accepted Answer · 2011-08-30 10:21:10Z

5

Modification of final fields via reflection has many limitations. In particular, if final field is initialized by a compile-time constant, its new value may not be observed (JLS 17.5.3 Subsequent Modification of Final Fields).

You can use the following workaround:

class TargetA{
     private static final long RECV_TIMEOUT = defaultTimeout();     

     private static long defaultTimeout() { return 180000L; }
}

edited Aug 30, 2011 at 10:21

answered Aug 30, 2011 at 9:48

axtavt

243k42 gold badges518 silver badges486 bronze badges

Comments

user425367 · Accepted Answer · 2011-08-30 09:54:18Z

0

Try to add the following code after your change and you will see that it has changed.

Field e =TargetA.class.getDeclaredField("RECV_TIMEOUT");
e.setAccessible(true);
System.out.println(e.getLong(modifiersField));

However as quoted from Peter

This means the value can be change, however the places where the constant is used is not changed.

answered Aug 30, 2011 at 9:54

user425367

Collectives™ on Stack Overflow

Fail to modify private final static variable without Exception

3 Answers 3

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related