Skip to main content

Reentrant Locks - Synchronization on multiple Conditions

After looking at a way you can do a waited lock on a synchronised monitor in java, it needs a much more object oriented way of handling multiple scenarios.

The sychronised-wait-notify paradigm works if you wait and notify on the monitor based on one condition (wait before borrow if not available). The Give back doesnot wait ever, It just gives back. The pool kills any over flow. What if you need to put a conditional wait on that too? something like (give back only if object pool is not full and WAIT until space is available)

Here is where you have Java 5's Lock - Conditions paradim comes handy. A lock is objectification of the monitor, Threads that need to be synchronised on one monitor is can go through Locks instead. However, Locks are more useful if you have multiple conditions on which you what these threads to wait and be notified.

Take the above scenario, which is the case of a Bounded buffer where there are two conditions on which threads need to wait, empty and full. If buffer is empty, a get() need to wait until something comes up; And if it is full, a put() needs to wait until there is space. However since buffer is the same object they lock on to using synchronised will block both producers and consumers in the same wait set; So to make them wait in seperate queues, Lock and Conditions shall be used.

Look at an example in Conditions javadoc:
 class BoundedBuffer {
   final Lock lock = new ReentrantLock();
   final Condition notFull  = lock.newCondition(); 
   final Condition notEmpty = lock.newCondition(); 

   public void put(Object x) throws InterruptedException {
     lock.lock();
     try {
       while (this.isNotFull()) 
         notFull.await();
 //Do some logic to put object in buffer
       notEmpty.signal();
     } finally {
       lock.unlock();
     }
   }

   public Object take() throws InterruptedException {
     lock.lock(); //Same lock
     try {
       while (count == 0) 
         notEmpty.await();//Different Condition
 //Do some logic to put object in buffer
       notFull.signal();
       return x;
     } finally {
       lock.unlock();
     }
   } 
 }

Please bear in mind that use of Locks is comtemplated by some experts. However, I dont see anyother way this could be implemented atleast if not cleanly

Popular posts from this blog

One page Stock

Alright.. That was a long absence. The whole last week I dint blog. I dint go away. I was "occupied". I was learning stock trading. Its very fascinating. I have a good weeeked blog for you all. Here is my experience. I can literally hyper-link every word from the following paragraphs, but I am writing it as simple as I can so you can look up the italicised words in wikipedia . I got a paper trading account from a brokerage firm . You need one brokerage account first. Then it can be an Equity account where all your money is yours or a Margin account , where some of the money is lent by the brokerage firm. Then I get Buying power , which is the dollor value of how much stocks you can buy. I can make profit by simple rules. Buy when Price is low. Sell when price is high. There is another more intersting way of earning money. Selling short . Thats when price is not high, per say, but when are confident that the price WILL go down. then buy back when its lowest. This is what

Powered By

As it goes, We ought to give thanks to people who power us. This page will be updated, like the version page , to show all the tools, and people this site is Powered By! Ubuntu GIMP Firebug Blogger Google [AppEngine, Ajax and other Apis] AddtoAny Project Fondue jQuery

classpath*: making your Modular Spring Resources

Spring gives multiple options to load XML resources for building contexts. the reference documentation does explain this feature quite well. However, I am taking my shot at explaining the different practical scenarios ( by order of growing modularisation) For Example, A simplest Spring based web Context Loader can be configured with resources like this <context-param> <param-name>contextConfigLocation</param-name> <param-value>applicationContext.xml</param-value> </context-param> <listener> <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class> </listener> You just need to put applicationContext.xml in WEB-INF/ folder of your webapp. However, Typically an application is n-tiered. You can also have multiple files setup and in relative paths. like <param-value> context-files/applicationContext.xml context-files/dao.xml context-files/service.xml </param-value>