Final variables in class file format

bytecode, final, java, jit, performance

Solution

No, there is no such information encoded in class file.

You may easily verify this by compiling a source file with a `final` local variable and without `final` - the result classes will be identical.

However, Java 8 added `MethodParameters` attribute that records information about names and access flags of method arguments. This means, you can check whether a method argument is `final` or not.

Just-in-time compilers do not need to know about `final` locals - they can easily determine the actual scope of any expression. Even if a variable is not final, e.g.

    int x = 1;
    // ... code A ...

    x = 2;
    // ... code B ...

the compilers will optimize the code `A` as if `x` is always `1`, and the code `B` as if `x` is always `2`.

Problem

Does class file format provide support for final keyword in case of using it with variables? Or does it just deduce the effective finality of a variable from code and JIT compiler perform optimization based on it? Here, in class file format documentation, they mentioned about final keyword, but only in case of using it with final block and final class. There is nothing about final variables.

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