Question

void myMethod(Object arg)
{
    arg.getThing().method1();
    arg.getThing().method2();
}

Will a basic Java 6 installation optimize this into a single call to the accessor (probably by storing the reference in a local). I realize if there are multiple threads the optimizer might have to refrain from doing it. Is there a list of common optimizations that I can expect from almost any Java 6 JVM?

example:

    {
       Object local = arg.getThing();
       local.method1();
       local.method2();
    }

Answer 1

I wouldn’t expect any optimisation here by the javac compiler, because the value of getThing() can change over time and between calls (for example, a random value, or the pop() operation on a stack, or the current time. You get the idea).

Maybe the JIT compiler can optimize this if it sees that the getThing() method returns always the same thing, but I wouldn’t count on that. Even the most simple return thing; statement can return a different value, if the value of thing is changed by another thread.