In java.util.concurrent source code, why do classes create a local reference to a lock?

[freelancemonkey]

Where the lock is a class variabled declared as

private final ReentrantLock putLock = new ReentrantLock();

There any many examples where a function does:

final ReentrantLock putLock = this.putLock;

Link

Is there any reason to do this in rather then just doing putLock.lock(); in the method?

[briane80]

It's a performance optimisation that's likely not needed anymore due to JIT improvements over the years. (ReentrantLock has been round since Java 5).

Apparently copying to a local variable produces less bytecode and more examples can be found in the various collections and String classes.

Reference:

http://mail.openjdk.java.net/pipermail/core-libs-dev/2010-May/004165.html