Ngtjyty

To my understanding following code should print "true", but when I run it it prints "false".

public class Test {

    public static boolean testTrue() {

        return true;

    }



    public static void main(String args) throws Exception {

        Object trueResult = Test.class.getMethod("testTrue").invoke(null);

        System.out.println(trueResult == Boolean.TRUE);

    }

}

According to JLS §5.1.7. Boxing Conversion:

If the value p being boxed is true, false, a byte, or a char in the range u0000 to u007f, or an int or short number between -128 and 127 (inclusive), then let r1 and r2 be the results of any two boxing conversions of p. It is always the case that r1 == r2.

However in case of method called via reflection boxed value is always created via new PrimitiveWrapper().

Please help me understand this.

invoke will always return a new Object. Any returned primitives are boxed.

...if the [return] value has a primitive type, it is first appropriately wrapped in an object.

Your issue is demonstrating the ambiguity of the term appropriately. i.e. during wrapping, it does not use Boolean.valueOf(boolean).

1.

The specific

in case of method called via reflection

is not covered by that part of the JLS you're quoting. That part you're quoting is about type conversion when you have a value of a type that you pass as another type. Here you're thinking of converting boolean to Boolean.

But type conversion means doing something like that:

Boolean b = true;

or

boolean b = true;

Boolean b2 = b;

Reflection is not a mechanism that applies type conversion.

When, by necessity, a reflective method call wraps a boolean return value into a Boolean object, it is not involved in the part of the JLS you quoted.

This explains why the JLS is not being violated here.

2.

As to why the reflection isn't choosing to be consistent with this behavior anyway:

That is because in older versions of Java, reflection existed before generics. And generics are the reason why autoboxing suddenly became convenient, and autoboxing is the reason why it seemed smart to not duplicate the "common" values of wrapped primitives.

All of this was defined after reflection already existed for a while, and was already behaving in a specific way. That means that there was already existing Java code that was using reflection, and most likely some existing code that was incorrectly relying on the existing behavior. Changing the existing behavior would have broken existing code, which was therefore avoided.

As you can see in java.lang.reflect.Method class, the invoke method has a signature as following:

 public Object invoke(Object obj, Object... args) { ... }

which returns an object as result.

Furthermore, Boolean.TRUE is defined as:

public static final Boolean TRUE = new Boolean(true);

which is a boxed object of true value.

By evaluating trueResult == Boolean.TRUE in your code, you are checking that whether the reference of trueResult and Boolean.TRUE are equal or not. Because == evaluates equality of values and in case of references, it means that are two references pointed to one Object in memory?

It is obvious that these two objects are not the same (they are two separate objects and instantiated in different parts of memory), so the result of trueResult == Boolean.TRUE is false.