Example¶

In the following example, the method spin repeatedly tests the field done in a loop. The method repeats the while-loop until the value of the field done is set by another thread. However, the compiler could optimize the code as shown in the second code snippet, because the field done is not marked as volatile and there are no statements in the body of the loop that could change the value of done. The optimized version of spin loops forever, even when another thread would set done to true.

class Spin {
    public boolean done = false;

    public void spin() {
        while(!done){
        }
    }
}

class Spin { // optimized
    public boolean done = false;

    public void spin() {
        boolean cond = done;
        while(!cond){
        }
    }
}

Recommendation¶

Ensure that access to this field is properly synchronized. Alternatively, avoid spinning on the field and instead use the wait and notifyAll methods or the java.util.concurrent library to communicate between threads.

References¶

• Java Language Specification: Threads and Locks.