MINIMIZING SYNCHRONIZATION OVERHEAD ACROSS CLASSES OR METHODS
Synchronization is key to writing well-behaved multithreaded 
applications in Java. Synchronization prevents multiple threads from 
entering code sections or accessing variables simultaneously. However, 
synchronization also introduces a fair amount of overhead, dragging 
down the performance of your application. By using lock objects created 
solely for the purpose of synchronization, you can provide arbitrarily 
fine-grained control over how multiple threads access your code, 
thereby reducing the overhead of thread-safe programming.
For example, suppose you want to synchronize code across multiple 
methods, or even multiple classes. For multiple methods in a single 
class, you can synchronize on the entire class or instance, but that 
can introduce a lot of unnecessary overhead. Synchronizing across 
multiple classes is impossible using the standard methods without 
creating some kind of proxy class to control access. The most efficient 
way to handle either case is to introduce an Object instance used 
solely for the purpose of synchronization. First, you need to create 
the object, usually as a static variable accessible from the affected code:
static final public Object lockObject = new Object();
Now you can synchronize on that object anywhere you need. Here's an 
example using two methods that would work equally well if the methods 
were in different classes as long as the lockObject instance was 
accessible to both:void method1() {    synchronized(lockObject) { 
       //protected code    } }  void method2() {    synchronized(lockObject) { 
       //protected code    } }
You can use both methods with the assurance that the protected code in 
each method will never execute at the same time as any other code 
protected by the lockObject instance. By taking advantage of the 
ability to synchronize on any arbitrary object, you can achieve the 
flexibility and efficiency needed to create high-performance 
multithreaded Java applications.------------------------------------------