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

Javascript: Convert Strings to Binary (and representing in a nerdy way!)

I follow those GoogleDevelopers Videos . Sometime back, in one of the presentations on GoogleIO, there was this interesting string of dots at the bottom of each page of the presentation . They looked like random big and small dots. A similar bunch of dots were also on the T-shirt of a presenter was wearing in another presentation . While it seemed something in the pattern, I could not find what it was. Finally, another presenter cleared the matter that those dots are just binary representation of "GOOGLEIO" (So much for advertizing Google IO, Impressive!). So I wanna do it. Takes me back to days of those DSP classes at school. Nerdy me had to churn some old brain cells. I remember those first programming language classes in Pascal and C when you were asked to do fibonacci series and converting a binary string to ascii codes. That *experience* came handy here: Check it out! Text to Binarize: For those who came to copy the javascript code to convert string to binary,

MySql Copying Table Structures.

Some times you need to copy only table structures across databases. This article describes two ways of doing it. If the whole database schema need to be exported, mysqldump is very effective. A --nodata flag will dump all tables' schema. Like this. mysqldump --nodata -p -u username databaseName But if you want to copy a specific table, individually, you could use "create table like" feature. You could create it even from a different database. However it must be on the same mysqld instance. Like this. create table newtable like oldtable; --Or from a table in other database create table mytable like otherdatabase.tablename;

Add jquery in Chrome console

Many a time, a page you are debugging doesnot have jquery. This simple js will add(or prompt you to overwrite) jquery to any page from chrome console. javascript:if(!window.jQuery||confirm('Overwrite\x20current\x20version?\x20v'+jQuery.fn.jquery))(function(d,s){s=d.createElement('script');s.src='https://ajax.googleapis.com/ajax/libs/jquery/1.8/jquery.js';(d.head||d.documentElement).appendChild(s)})(document);