The Java programming language provides two basic synchronization idioms: synchronized methods and synchronized statements. The more complex of the two, synchronized statements, are described in the next section. This section is about synchronized methods.
To make a method synchronized, simply add the synchronized
keyword to its declaration:
public class SynchronizedCounter { private int c = 0; public synchronized void increment() { c++; } public synchronized void decrement() { c--; } public synchronized int value() { return c; } }
If count
is an instance of SynchronizedCounter
, then making these methods synchronized has two effects:
Note that constructors cannot be synchronized using the synchronized
keyword with a constructor is a syntax error. Synchronizing constructors doesn't make sense, because only the thread that creates an object should have access to it while it is being constructed.
List
called instances
containing every instance of class. You might be tempted to add the following line to your constructor:
instances.add(this);
instances
to access the object before construction of the object is complete.
Synchronized methods enable a simple strategy for preventing thread interference and memory consistency errors: if an object is visible to more than one thread, all reads or writes to that object's variables are done through synchronized
methods. (An important exception: final
fields, which cannot be modified after the object is constructed, can be safely read through non-synchronized methods, once the object is constructed) This strategy is effective, but can present problems with liveness, as we'll see later in this lesson.