【Some Java Concurrency Tips。。。】

熊猫儿

Below is an example of the use of a semaphore to control access to pool of resources. Multiple threads can request the use of a resource and return it when they have finished with it.

In the creator we create a new semaphore with the same size as the pool of resources we're creating.

In the getResource() method the semaphore aquire method is called to try to aquire a permit to use a resource. If there are resources available this will return and a resource will be returned from the pool. If all the resources are in use the call to aquire will block until another thread calls release on the semaphore.  When a thread finishes with a resource the resource is returned to the pool and the release method is called. Both aquire and release can be considered atomic operations.

熊猫儿

Multithreaded Lazy Initialization is tricky
When threads share a lazily initialized field, access to the field must be synchronized, or non-determinism type bugs can result.

熊猫儿

Prefer Normal initialization
Don't use lazy initialization unless an object or field is costly to initialize and not used often. Normally normal initialization is best Below is a thread safe example of eager initialization for a singleton, the private final instance field and the private constructor make it immutable.

熊猫儿

If you need to use lazy initialization for performance on a static field, use the initialize-on-demand holder pattern. This pattern takes advantage of the guarantee that a class will not be initialized until it is used :

References and More Information:
Effective Java, Second Edition by Joshua Bloch
Java Concurrency in Practice by Brian Goetz
Robust and Scalable Concurrent Programming: Lessons from the Trenches
Concurrency: Past and Present
http://weblogs.java.net/blog/car ... va-concurrency-tips

justforregister

nice job