java - reordering inside synchronized block

Question

Is it possible for the reordering of statements inside a synchronized block? For example

synchronized(lock) {
   statement1;
   statement2;
}

In which, statement1 and statement2 are not dependent on each other. Can the processor or compiler reorder these statements?

Thank you.

Answer 1

Yes these statements can be reordered within synchronized block if optimizer decides so. But they can't be taken out of synchronized .

Answer 2

The compiler (optimizer, actually) might reorder things, or even eliminate code (like assigning to a variable that is not going to be referenced before going out of scope) if it knows of a certainty that there would be no side effects and it would speed things up. That will only happen within the synchronized block itself.

Answer 3

According to JSR-133, statements inside the synchronized block can not be reordered: http://www.cs.umd.edu/~pugh/java/memoryModel/jsr-133-faq.html , "What does synchronization do" section

"Each action in a thread happens before every action in that thread that comes later in the program's order."