How can one tell if a local variable is 'final' from Java bytecode? (Related to BCEL)

Question

Where is information such as if a local variable is "final" stored in Java bytecode? I know that for fields (global variables) and methods these are found in the access flag bits, but cannot seem to find the equivalent in the local variable table.

I am interested in this question as I am using BCEL to check if a local variable is final, and have found the equivalent for fields, methods and classes in the class AccessFlags.

Thanks in advance.

Gordie

Answer 1

The finality of local variables is checked by the compiler and doesn't make it to the bytecode. This information isn't required at runtime, and hence isn't stored in the bytecode.

The JVM treats final and non-final local variables in the same way.

Enlil

I understand, but why is this not so for global variables too then please? – Gordie

Because global variables can be reused in other classes and/or programs. When the compiler is compiling a class using your final variable, it needs to know this – Enlil

Thank you that makes perfect sense now – Gordie

Answer 2

Short answer - you can't. The 'final' access flag for local variables only tells compiler that variable value can't be reassigned. See section 4.7.13 of the JVM specification.

Tertiary

Answer 3

I don't believe you can determine the finality of a local variable; this can be proven by writing a small method with and without the final keyword and comparing the bytecode.

Albaalbacete