使用反射，我可以确定Java静态最终字段是否内联？

Question

Is there any way, using reflection, to determine whether a static final field has an '=' value (so that the value will be in-lined when it is accessed) or whether its value is assigned by the static initializer? If the field does have a value, is there any way to retrieve that value without using the Field.get method (and therefore causing the static initializer to be executed)?

Answer 1

The field will be "inlined" only if it's a compile time constant - see the lengthy definition in the JLS .

So it's not enough for it to be assigned at the time of declaration. This is not a compile-time constant, although a valid declaration:

static final int INT_CONST = compute();

static int compute() {
    return 5;
}

Just by reflection you cannot determine how the field was initialized.

To access any value of a class field, that class needs to be loaded first, so you cannot prevent the static initializer from running.

Answer 2

You cannot do this by reflection, but you can do this using byte-code engineering library like ASM :

public class AsmTest {
    static final int a = 2; // constant
    static final String b = "string"; // constant
    static final String c = "foo "+"bar"; // constant: concatenation is allowed
    static final String d = " foobar ".trim(); // not constant: method called

    public static Object getFieldConstantValue(Class<?> clazz, final String field) {
        try(InputStream is = clazz.getResourceAsStream(clazz.getSimpleName()+".class")) {
            final Object[] value = {null};
            new ClassReader(is).accept(new ClassVisitor(Opcodes.ASM5) {
                @Override
                public FieldVisitor visitField(int access, String name, String desc,
                        String sig, Object val) {
                    if(name.equals(field))
                        value[0] = val;
                    return null;
                }
            }, ClassReader.SKIP_CODE | ClassReader.SKIP_DEBUG | ClassReader.SKIP_FRAMES);
            return value[0];
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    public static void main(String[] args) {
        for(String name : new String[] {"a", "b", "c", "d"}) {
            System.out.println(name+"="+getFieldConstantValue(AsmTest.class, name));
        }
    }
}

Output:

a=2
b=string
c=foo bar
d=null