static and volatile

http://stackoverflow.com/questions/2423622/volatile-vs-static-in-java

Declaring a static variable in Java, means that there will be only one copy, no matter how many objects of the class are created. The variable will be accessible even with no Objects created at all. However threads may have locally cached values of it.

Declaring a variable as volatile, there will be one variable for each Object. So on the surface it seems there is no difference from a normal variable, and totally different from static. However, even with Object fields, a thread may cache a variable value locally.

This means that if two threads update a variable of the same Object concurrently, and the variable is not declared volatile, there could be a case in which one of the thread has in cache an old value.

Even if you access a static values through multiple thread each thread can have it's local cached copy! To avoid this you can declare the variable as static volatile and this will force the thread to read each time the global value.

However, volatile is not a substitute for proper synchronisation! For instance:

    private static volatile int counter = 0;

    private void concurrentMethodWrong() {
         counter = counter + 5;
         //do something
         counter = counter - 5;
    }

Executing concurrentMethodWrong concurrently many times may lead to a final value of counter different from zero! To solve the problem, you've to implement a lock:

    private static final Object counterLock = new Object();

    private static volatile int counter = 0;

    private void concurrentMethodRight() {
         synchronized (counterLock) {
             counter = counter + 5;
         }
         //do something
         synchronized (counterLock) {
             counter = counter - 5;
         }
    }

Or use the AtomicInteger class.