Change private static final field using Java reflection

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);

Answer 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

• Using reflection to change static final File.separatorChar for unit testing
• How to limit setAccessible to only “legitimate” uses?
  • Has examples of messing with Integer's cache, mutating a String, etc

---

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
  • It's unlikely that this technique works with a primitive private static final boolean, because it's inlineable as a compile-time constant and thus the "new" value may not be observable

---

Appendix: On the bitwise manipulation

Essentially,

field.getModifiers() & ~Modifier.FINAL

turns off the bit corresponding to Modifier.FINAL from field.getModifiers(). & is the bitwise-and, and ~ is the bitwise-complement.

See also

• Wikipedia/Bitwise operation

---

Remember Constant Expressions

Still not being able to solve this?, have fallen onto depression like I did for it? Does your code looks like this?

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

Reading the comments on this answer, specially the one by @Pshemo, it reminded me that Constant Expressions are handled different so it will be impossible to modify it. Hence you will need to change your code to look like this:

public class A {
    private final String myVar;

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

if you are not the owner of the class... I feel you!

For more details about why this behavior read this?

Answer 2

If the value assigned to a static final boolean field is known at compile-time, it is a constant. Fields of primitive or String type can be compile-time constants. A constant will be inlined in any code that references the field. Since the field is not actually read at runtime, changing it then will have no effect.

The Java language specification says this:

If a field is a constant variable (§4.12.4), then deleting the keyword final or changing its value will not break compatibility with pre-existing binaries by causing them not to run, but they will not see any new value for the usage of the field unless they are recompiled. This is true even if the usage itself is not a compile-time constant expression (§15.28)

Here's an example:

class Flag {
  static final boolean FLAG = true;
}

class Checker {
  public static void main(String... argv) {
    System.out.println(Flag.FLAG);
  }
}

If you decompile Checker, you'll see that instead of referencing Flag.FLAG, the code simply pushes a value of 1 (true) onto the stack (instruction #3).

0:   getstatic       #2; //Field java/lang/System.out:Ljava/io/PrintStream;
3:   iconst_1
4:   invokevirtual   #3; //Method java/io/PrintStream.println:(Z)V
7:   return

Answer 3

A little curiosity from the Java Language Specification, chapter 17, section 17.5.4 "Write-protected Fields":

Normally, a field that is final and static may not be modified. However, System.in, System.out, and System.err are static final fields that, for legacy reasons, must be allowed to be changed by the methods System.setIn, System.setOut, and System.setErr. We refer to these fields as being write-protected to distinguish them from ordinary final fields.

Source: http://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.5.4

Answer 4

I also integrated it with joor library

Just use

      Reflect.on(yourObject).set("finalFieldName", finalFieldValue);

Also I fixed an issue with override which the previous solutions seem to miss. However use this very carefully, only when there's no other good solution.

Answer 5

Along with top ranked answer you may use a bit simpliest approach. Apache commons FieldUtils class already has particular method that can do the stuff. Please, take a look at FieldUtils.removeFinalModifier method. You should specify target field instance and accessibility forcing flag (if you play with non-public fields). More info you can find here.

Answer 6

In case of presence of a Security Manager, one can make use of AccessController.doPrivileged

Taking the same example from accepted answer above:

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");

        // wrapping setAccessible 
        AccessController.doPrivileged(new PrivilegedAction() {
            @Override
            public Object run() {
                modifiersField.setAccessible(true);
                return null;
            }
        });

        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"
    }
}

In lambda expression, AccessController.doPrivileged, can be simplified to:

AccessController.doPrivileged((PrivilegedAction) () -> {
    modifiersField.setAccessible(true);
    return null;
});

Answer 7

Even in spite of being final a field can be modified outside of static initializer and (at least JVM HotSpot) will execute the bytecode perfectly fine.

The problem is that Java compiler does not allow this, but this can be easily bypassed using objectweb.asm. Here is p̶e̶r̶f̶e̶c̶t̶l̶y̶ ̶v̶a̶l̶i̶d̶ ̶c̶l̶a̶s̶s̶f̶i̶l̶e̶ an invalid classfile from the JVMS specification standpoint, but it passes bytecode verification and then is successfully loaded and initialized under JVM HotSpot OpenJDK12:

ClassWriter cw = new ClassWriter(0);
cw.visit(Opcodes.V1_8, Opcodes.ACC_PUBLIC, "Cl", null, "java/lang/Object", null);
{
    FieldVisitor fv = cw.visitField(Opcodes.ACC_PRIVATE | Opcodes.ACC_STATIC | Opcodes.ACC_FINAL, "fld", "I", null, null);
    fv.visitEnd();
}
{
    // public void setFinalField1() { //... }
    MethodVisitor mv = cw.visitMethod(Opcodes.ACC_PUBLIC | Opcodes.ACC_STATIC, "setFinalField1", "()V", null, null);
    mv.visitMaxs(2, 1);
    mv.visitInsn(Opcodes.ICONST_5);
    mv.visitFieldInsn(Opcodes.PUTSTATIC, "Cl", "fld", "I");
    mv.visitInsn(Opcodes.RETURN);
    mv.visitEnd();
}
{
    // public void setFinalField2() { //... }
    MethodVisitor mv = cw.visitMethod(Opcodes.ACC_PUBLIC | Opcodes.ACC_STATIC, "setFinalField2", "()V", null, null);
    mv.visitMaxs(2, 1);
    mv.visitInsn(Opcodes.ICONST_2);
    mv.visitFieldInsn(Opcodes.PUTSTATIC, "Cl", "fld", "I");
    mv.visitInsn(Opcodes.RETURN);
    mv.visitEnd();
}
cw.visitEnd();

In Java, the class looks roughly speaking as follows:

public class Cl{
    private static final int fld;

    public static void setFinalField1(){
        fld = 5;
    }

    public static void setFinalField2(){
        fld = 2;
    }
}

which cannot be compiled with javac, but can be loaded and executed by JVM.

JVM HotSpot has special treatment of such classes in the sense that it prevents such "constants" from participating in constant folding. This check is done on the bytecode rewriting phase of class initialization:

// Check if any final field of the class given as parameter is modified
// outside of initializer methods of the class. Fields that are modified
// are marked with a flag. For marked fields, the compilers do not perform
// constant folding (as the field can be changed after initialization).
//
// The check is performed after verification and only if verification has
// succeeded. Therefore, the class is guaranteed to be well-formed.
InstanceKlass* klass = method->method_holder();
u2 bc_index = Bytes::get_Java_u2(bcp + prefix_length + 1);
constantPoolHandle cp(method->constants());
Symbol* ref_class_name = cp->klass_name_at(cp->klass_ref_index_at(bc_index));
if (klass->name() == ref_class_name) {
   Symbol* field_name = cp->name_ref_at(bc_index);
   Symbol* field_sig = cp->signature_ref_at(bc_index);

   fieldDescriptor fd;
   if (klass->find_field(field_name, field_sig, &fd) != NULL) {
      if (fd.access_flags().is_final()) {
         if (fd.access_flags().is_static()) {
            if (!method->is_static_initializer()) {
               fd.set_has_initialized_final_update(true);
            }
          } else {
            if (!method->is_object_initializer()) {
              fd.set_has_initialized_final_update(true);
            }
          }
        }
      }
    }
}

The only restriction that JVM HotSpot checks is that the final field should not be modified outside of the class that the final field is declared at.

Answer 8

The accepted answer worked for me until deployed on JDK 1.8u91. Then I realized it failed at field.set(null, newValue); line when I had read the value via reflection before calling of setFinalStatic method.

Probably the read caused somehow different setup of Java reflection internals (namely sun.reflect.UnsafeQualifiedStaticObjectFieldAccessorImpl in failing case instead of sun.reflect.UnsafeStaticObjectFieldAccessorImpl in success case) but I didn't elaborate it further.

Since I needed to temporarily set new value based on old value and later set old value back, I changed signature little bit to provide computation function externally and also return old value:

public static <T> T assignFinalField(Object object, Class<?> clazz, String fieldName, UnaryOperator<T> newValueFunction) {
    Field f = null, ff = null;
    try {
        f = clazz.getDeclaredField(fieldName);
        final int oldM = f.getModifiers();
        final int newM = oldM & ~Modifier.FINAL;
        ff = Field.class.getDeclaredField("modifiers");
        ff.setAccessible(true);
        ff.setInt(f,newM);
        f.setAccessible(true);

        T result = (T)f.get(object);
        T newValue = newValueFunction.apply(result);

        f.set(object,newValue);
        ff.setInt(f,oldM);

        return result;
    } ...

However for general case this would not be sufficient.

Answer 9

Just saw that question on one of the interview question, if possible to change final variable with reflection or in runtime. Got really interested, so that what I became with:

 /**
 * @author Dmitrijs Lobanovskis
 * @since 03/03/2016.
 */
public class SomeClass {

    private final String str;

    SomeClass(){
        this.str = "This is the string that never changes!";
    }

    public String getStr() {
        return str;
    }

    @Override
    public String toString() {
        return "Class name: " + getClass() + " Value: " + getStr();
    }
}

Some simple class with final String variable. So in the main class import java.lang.reflect.Field;

/**
 * @author Dmitrijs Lobanovskis
 * @since 03/03/2016.
 */
public class Main {


    public static void main(String[] args) throws Exception{

        SomeClass someClass = new SomeClass();
        System.out.println(someClass);

        Field field = someClass.getClass().getDeclaredField("str");
        field.setAccessible(true);

        field.set(someClass, "There you are");

        System.out.println(someClass);
    }
}

The output will be as follows:

Class name: class SomeClass Value: This is the string that never changes!
Class name: class SomeClass Value: There you are

Process finished with exit code 0

According to documentation https://docs.oracle.com/javase/tutorial/reflect/member/fieldValues.html

Answer 10

Many of the answers here are useful, but I've found none of them to work on Android, in particular. I'm even a pretty hefty user of Reflect by joor, and neither it nor apache's FieldUtils - both mentioned here in some of the answers, do the trick.

Problem with Android

The fundamental reason why this is so is because on Android there's no modifiers field in the Field class, which renders any suggestion involving this code (as in the marked answer), useless:

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

In fact, to quote from FieldUtils.removeFinalModifier():

// Do all JREs implement Field with a private ivar called "modifiers"?
final Field modifiersField = Field.class.getDeclaredField("modifiers");

So, the answer is no...

Solution

Pretty easy - instead of modifiers, the field name is accessFlags. This does the trick:

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

Side-note #1: this can work regardless of whether the field is static in the class, or not.

Side-note #2: Seeing that the field itself could be private, it's recommended to also enable access over the field itself, using field.setAccessible(true) (in addition to accessFlagsField.setAccessible(true).

Answer 11

If your field is simply private you can do this:

MyClass myClass= new MyClass();
Field aField= myClass.getClass().getDeclaredField("someField");
aField.setAccessible(true);
aField.set(myClass, "newValueForAString");

and throw/handle NoSuchFieldException

Answer 12

The whole point of a final field is that it cannot be reassigned once set. The JVM uses this guarentee to maintain consistency in various places (eg inner classes referencing outer variables). So no. Being able to do so would break the JVM!

The solution is not to declare it final in the first place.