Accessing Java static final variable value through reflection

Question

Can the value of a Java static final class variable be retrieved through reflection?

Answer 1

Yes. (Only there is no such thing as static, instance. It's static, non-instance.)

> If the underlying field is a static field, the obj argument is ignored; it may be null.

(include standard warning that most uses of reflection are a bad idea)

Answer 2

I would guess that it depends on the type and the compiler (on second thought, it had sure better not!). Sun's compiler inlines primitive constants, but I don't know if they remove the entry from the class entirely. I'll find out.

Edit: Yes, you can still access them even if they are inlined. Test class:

public class ReflectionConstantTest {
    private static final int CONST_INT = 100;
    private static final String CONST_STRING = "String";
    private static final Object CONST_OBJECT = new StringBuilder("xyz");
    public static void main(String[] args) throws Exception {
        int testInt = CONST_INT;
        String testString = CONST_STRING;
        Object testObj = CONST_OBJECT;
        for (Field f : ReflectionConstantTest.class.getDeclaredFields()) {
            f.setAccessible(true);
            System.out.println(f.getName() + ": " + f.get(null));
        }
    }
}

Output:

CONST_INT: 100
CONST_STRING: String
CONST_OBJECT: xyz

javap -c output:

Compiled from "ReflectionConstantTest.java"
public class scratch.ReflectionConstantTest extends java.lang.Object{
public scratch.ReflectionConstantTest();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."":()V
   4:   return

public static void main(java.lang.String[])   throws java.lang.Exception;
  Code:
   0:   bipush  100
   2:   istore_1
   3:   ldc     #2; //String String
   5:   astore_2
   6:   getstatic       #3; //Field CONST_OBJECT:Ljava/lang/Object;
   9:   astore_3
   10:  ldc_w   #4; //class scratch/ReflectionConstantTest
   13:  invokevirtual   #5; //Method java/lang/Class.getDeclaredFields:()[Ljava/lang/reflect/Field;
   16:  astore  4
   18:  aload   4
   20:  arraylength
   21:  istore  5
   23:  iconst_0
   24:  istore  6
   26:  iload   6
   28:  iload   5
   30:  if_icmpge       90
   33:  aload   4
   35:  iload   6
   37:  aaload
   38:  astore  7
   40:  aload   7
   42:  iconst_1
   43:  invokevirtual   #6; //Method java/lang/reflect/Field.setAccessible:(Z)V
   46:  getstatic       #7; //Field java/lang/System.out:Ljava/io/PrintStream;
   49:  new     #8; //class java/lang/StringBuilder
   52:  dup
   53:  invokespecial   #9; //Method java/lang/StringBuilder."":()V
   56:  aload   7
   58:  invokevirtual   #10; //Method java/lang/reflect/Field.getName:()Ljava/lang/String;
   61:  invokevirtual   #11; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   64:  ldc     #12; //String :
   66:  invokevirtual   #11; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   69:  aload   7
   71:  aconst_null
   72:  invokevirtual   #13; //Method java/lang/reflect/Field.get:(Ljava/lang/Object;)Ljava/lang/Object;
   75:  invokevirtual   #14; //Method java/lang/StringBuilder.append:(Ljava/lang/Object;)Ljava/lang/StringBuilder;
   78:  invokevirtual   #15; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   81:  invokevirtual   #16; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   84:  iinc    6, 1
   87:  goto    26
   90:  return

static {};
  Code:
   0:   new     #8; //class java/lang/StringBuilder
   3:   dup
   4:   ldc     #17; //String xyz
   6:   invokespecial   #18; //Method java/lang/StringBuilder."":(Ljava/lang/String;)V
   9:   putstatic       #3; //Field CONST_OBJECT:Ljava/lang/Object;
   12:  return

}

You can see that CONST_INT is inlined, but CONST_STRING and CONST_OBJECT (of course) are not. Yet CONST_INT is still available reflectively.

Answer 3

Just getting the name and value does not require setAccessible(true). Here's a useful example when you have to deal with constants declared in an interface, and want the symbolic names:

interface Code {
   public static final int FOO = 0;
   public static final int BAR = 1;
}

...

try {
   for (Field field : Code.class.getDeclaredFields()) {
      String name = field.getName();
      int value = field.getInt(null);
      System.out.println(name + "=" + value);
   }
}
catch (IllegalAccessException e) {
   System.out.println(e);
}

Answer 4

If open-source libraries are allowed on your project you can use

FieldUtils.readDeclaredStaticField

public class Test {
    public final static String CONSTANT="myConstantValue";
}

In another class you can use:

Object value = FieldUtils.readDeclaredStaticField(Test.class, "CONSTANT");
System.out.println(value);

You will see "myConstantValue" in the console.