Trying to develop a blocking method

Question

This seems like it should be simple, but I've been trying to figure it out all morning. Basically, I want to develop a way to lock object across all instances to make it writeable. Creating a static boolean class variable will get me most of the way there, but I'm going for extra credit :) I want to have a method called "lock" that will ultimately guarantee an instance has exclusive write permission. If no instances currently hold the lock, it should simply set the boolean (I assume) and return. If another instance already has the lock, I want the lock method to block until it can get access to the lock. Ideally, this is queued so calls to lock are honored in the order they are received.
For example, I have two threads A and B. I have to ask you to use your imagination a little (since it is hard to demonstrate) and assume these blocks are overlapping on the respective threads.
//executing on thread A
LockableInstance a = new LockableInstance();
a.lock();
a.doSomething();
a.unlock();
//executing on thread B
LockableInstance b = new LockableInstance();
b.lock();  //thread A already holds a lock. this call waits until lock can be acquired.
b.doSomething();
b.unlock();The concept of blocking has really got me stumped. The one way I know this will work is to make lock() an infinite loop calling Thread.sleep until the class variable is unlocked. This seems inefficient for two reasons. For one, the sleep interval needs to be short, so it's just banging on the CPU (especially with many threads running). Another reason is that there is no guarantee locks will be obtained in the order they are requested.
Any ideas on how I could approach this?
Edited by: kgdoom on Nov 11, 2007 1:20 PM
Edited by: kgdoom on Nov 11, 2007 2:11 PM

Answer 1

I think I found what I needed :) For anyone interested, I can use an instance of java.util.concurrent.Semaphore. Specifically, I'm using a "binary" semaphore, which just an instance with 1allocated permit. Does exactly what I was trying to do!
New question... are semaphore objects hefty? If I have many classes using semaphores (one static instance per class), is it a drain on resources somehow? Is there something more efficient in binary cases?