Determining whether a field is a compile-time constant at run time?

Perhaps I need to be more specific.

I'm trying to do some simple source-code analysis, and I need to know whether a field is a constant variable because, in conjunction with its value, whether a field is a constant variable potentially alters the call graph. For example:

public class C {

staticint v = 0;

static final int c = 0;

static void foo(byte b) { System.out.println("byte"); }

static void foo(long i) { System.out.println("long"); }

public static void main(String[] args) {

foo(true ? v : (byte)(0));

foo(true ? c : (byte)(0));

}

}

Despite the fact that the values of both "v" and "c" are zero, this will print:

long

byte

In the first call, the result type of the ternary operator is "long" because the second and third operands undergo numeric promotion, since "v" is not a compile-time constant. In the second call, the result type of the ternary operator is "byte" because the second operand undergoes a narrowing conversion, since "c" is a compile-time constant and its value is losslessly convertible to "byte".

So, whether a field is a constant variable potentially affects my analysis, and I therefore need to determine whether a field (for which no source code may be available) is a constant variable.

scudobuioa at 2007-7-15 4:02:26 >

> Reflection will do that for you.

For a field to be a constant variable, it must be both final and initialized with a compile-time constant expression. Reflection can tell me the first part, but not the second.

For example, consider the following field declaration:

public static final String s = "Hello World!";

The field "s" is a constant variable. However, if I amend this a bit:

public static final String s = "Hello World!".intern();

Now the field "s" is no longer a constant variable, because its initializer is no longer a compile-time constant expression.

scudobuioa at 2007-7-15 4:02:26 >

> You'll have to look at the classfile itself, using a

> tool such as

> [url=http://jakarta.apache.org/bcel/]BCEL[/url].

Yeah, I was afraid of that. I was hoping there would be something easier that I just overlooked, like the [url=http://java.sun.com/javase/6/docs/api/javax/lang/model/element/VariableElement.html#getConstantValue()]mechanism[/url] in the 1.6 API.

I'll probably take another look at the BCEL, though, thanks.

scudobuioa at 2007-7-15 4:02:26 >