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Change private static final field using Java reflection Ask Question

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Change private static final field using Java reflection Ask Question

I have a class with a private static final field that, unfortunately, I need to change it at run-time.

Using reflection I get this error: java.lang.IllegalAccessException: Can not set static final boolean field

Is there any way to change the value?

Field hack = WarpTransform2D.class.getDeclaredField("USE_HACK");
hack.setAccessible(true);
hack.set(null, true);

ベストアンサー1

Assuming no SecurityManager is preventing you from doing this, you can use setAccessible to get around private and resetting the modifier to get rid of final, and actually modify a private static final field.

Here's an example:

import java.lang.reflect.*;

public class EverythingIsTrue {
   static void setFinalStatic(Field field, Object newValue) throws Exception {
      field.setAccessible(true);

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

      field.set(null, newValue);
   }
   public static void main(String args[]) throws Exception {      
      setFinalStatic(Boolean.class.getField("FALSE"), true);

      System.out.format("Everything is %s", false); // "Everything is true"
   }
}

Assuming no SecurityException is thrown, the above code prints "Everything is true".

What's actually done here is as follows:

  • The primitive boolean values true and false in main are autoboxed to reference type Boolean "constants" Boolean.TRUE and Boolean.FALSE
  • Reflection is used to change the public static final Boolean.FALSE to refer to the Boolean referred to by Boolean.TRUE
  • As a result, subsequently whenever a false is autoboxed to Boolean.FALSE, it refers to the same Boolean as the one refered to by Boolean.TRUE
  • Everything that was "false" now is "true"

Related questions

Caveats

Extreme care should be taken whenever you do something like this. It may not work because a SecurityManager may be present, but even if it doesn't, depending on usage pattern, it may or may not work.

JLS 17.5.3 Subsequent Modification of Final Fields

In some cases, such as deserialization, the system will need to change the final fields of an object after construction. final fields can be changed via reflection and other implementation dependent means. The only pattern in which this has reasonable semantics is one in which an object is constructed and then the final fields of the object are updated. The object should not be made visible to other threads, nor should the final fields be read, until all updates to the final fields of the object are complete. Freezes of a final field occur both at the end of the constructor in which the final field is set, and immediately after each modification of a final field via reflection or other special mechanism.

Even then, there are a number of complications. If a final field is initialized to a compile-time constant in the field declaration, changes to the final field may not be observed, since uses of that final field are replaced at compile time with the compile-time constant.

Another problem is that the specification allows aggressive optimization of final fields. Within a thread, it is permissible to reorder reads of a final field with those modifications of a final field that do not take place in the constructor.

See also

  • JLS 15.28 Constant Expression
    • この手法はプリミティブでは機能しない可能性が高いprivate static final boolean。なぜなら、プリミティブはコンパイル時定数としてインライン化可能であり、「新しい」値が観測できない可能性があるからである。

付録: ビット操作について

本質的には、

field.getModifiers() & ~Modifier.FINAL

Modifier.FINALからに対応するビットをオフにしますfield.getModifiers()。&はビットごとの論理積で、 は~ビットごとの補数です。

参照

定数式を覚える

まだ解決できないのですか? 私と同じように落ち込んでいませんか? あなたのコードは次のようになっていますか?

public class A {
    private final String myVar = "Some Value";
}

この回答に対するコメント、特に@Pshemoのコメントを読んで、私は次のことを思い出しました。定数式処理が異なるため、変更することはできません。したがって、コードを次のように変更する必要があります。

public class A {
    private final String myVar;

    private A() {
        myVar = "Some Value";
    }
}

あなたがクラスのオーナーでないなら...私も同じ気持ちです!

この動作の理由の詳細についてはこれを読む?

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