Question

I found interesting thing while working with reflection. I tried to retrieve constructors of simple class and their modifiers.

public class Test {
    public Test(Object... args) {}
}

Here is the code to retrieve constructor modifiers:

Class<?> clazz = Test.class;
Constructor<?>[] ctors = clazz.getDeclaredConstructors();
for (Constructor<?> ctor : ctors) {        
    int mod = ctor.getModifiers();
    /*if not package-private modifier*/
    if(mod!=0) {
        System.out.println( Modifier.toString(mod)));
    }
}

The result is:

    public transient  

If I pass to constructor not variable parameters, but just array, it’s ok.

public class Test {
    public Test(Object[] args) {}
}

The result is:

    public  

The same happens regardless of constructor modifier (public, protected, private) or parameters type (primitive or reference). How could it be, whereas “transient” is not valid modifier for constructor?

Answer 1

Access modifiers are encoded as bit masks inside the class file. The JVM spec assigns different meaning to some of the bits depending on whether they appear in a method modifier or a field modifier. Bit 7 (0x0080) is one such bit.

For methods:

ACC_VARARGS    0x0080  Declared with variable number of arguments.

For fields:

ACC_TRANSIENT  0x0080  Declared transient; not written or read by a persistent
                       object manager.

Since you’re looking at a method, the correct interpretation of this modifier is ACC_VARARGS and not ACC_TRANSIENT.

However, the Modifier class only appears capable of dealing with a subset of modifiers defined in the JVM spec. Because all it takes is an int, it’s unable to tell ACC_VARARGS and ACC_TRANSIENT apart.