85. The synchronized modifier

The synchronized keyword is used in concurrent environments in which multiple threads are operating and there’s potential that code running on different threads are interacting on the same section of code. The synchronized modifier is used to indicate that the associated element can only be accessed by one caller at a time. In the code below you’ll see the various applications of synchronized available:

class PrizePool {

    synchronized amount

    static synchronized pools

    synchronized calculateTotalPool() {
        //synchronized code
    }

    def calculateWinnings() {
        //non-synchronized code
        synchronized (this) {
            //synchronized code
        }
        //non-synchronized code
    }

    static synchronized removePool() {
        //synchronized code
    }

    static addPool() {
        //non-synchronized code
        synchronized (PrizePool) {
            //synchronized code
        }
        //non-synchronized code
    }
}

• First of all, member and static variables can be marked as synchronized - only one thread can read/write the variable at a time.
• Instance and class methods can also be marked as synchronized. Once called by one thread, any other threads are prevented from accessing the method until the first thread’s call has completed.
• One or more blocks within an instance or class method can be synchronized¹:
  • Within an instance method the block is prefixed with synchronized (this) - note the use of an object instance (this)
  • For a class method, the block is prefixed with synchronized (PrizePool) - note the use of a class name (PrizePool)

A good starting place for understanding this topic is the Concurrency section of The Java Tutorials. Additionally, the GPars Framework provides a library to assist you in writing concurrent/parallel Groovy and Java code.