Synchronization

At any given time, we can have lots of threads running in an application. Unless we explicitly coordinate them, these threads will be executing methods without any regard for what the other threads are doing. Problems can arise when these methods share the same data. If one method is changing the value of some variables at the same time another method is reading these variables, it’s possible that the reading thread might catch things in the middle and get some variables with old values and some with new. Depending on the application, this situation could cause a critical error. In our HelloJava examples, both our paintComponent() and mouseDragged() methods access the messageX and messageY variables. Without knowing more about the implementation of the Java environment, we have to assume that these methods could conceivably be called by different threads and run concurrently. paintComponent() could be called while mouseDragged() is in the midst of updating messageX and messageY. At that point, the data is in an inconsistent state and if paintComponent() gets lucky, it could get the new x value with the old y value. Fortunately, Swing does not allow this to happen in this case because all event activity is handled by a single thread, and we probably would not even notice if it were to happen in this application anyway. We did, however, see another case in our changeColor() and currentColor() methods that is representative of the potential for a more serious “out of bounds” error. The synchronized modifier tells Java to acquire a lock for the object that contains the method before executing that method. Only one method in the object can have the lock at any given time, which means that only one synchronized method in that object can be running at a time. This allows a method to alter data and leave it in a consistent state before a concurrently running method is allowed to access it. When the method is done, it releases the lock on the class. Unlike synchronization in other languages, the synchronized keyword in Java provides locking at the language level. This means there is no way that you can forget to unlock a class. Even if the method throws an exception or the thread is terminated, Java will release the lock. This feature makes programming with threads in Java much easier than in other languages. We will discuss in more detail on coordinating threads and shared data. Well, it’s time to say goodbye to HelloJava. We hope that you have developed a feel for the major features of the Java language and that this will help you as you explore the details of programming with Java. If you are a bit bewildered by some of the material presented here, take heart. We’ll be covering all the major topics presented here again in their own chapters throughout the book. This tutorial was meant to be something of a “trial by fire” to get the important concepts and terminology into your brain so that the next time you hear them you’ll have a head start.