How to synchronize or lock upon variables in Java

Question

Let me use this small and simple sample   class Sample       private String msg   null       public void newmsg String x           msg   x             public String getmsg            String temp   msg          msg   null          return temp            Let s assume the function newmsg   is called by other threads that I don t have access to    I want to use the synchonize method to guarantee that the string msg is only used by one function per time  In other words  function newmsg   cannot run at the same time as getmsg

User · Answer

Use the synchronized keyword   class sample       private String msg null       public synchronized void newmsg String x           msg x             public synchronized string getmsg            String temp msg          msg null          return msg            Using the synchronized keyword on the methods will require threads to obtain a lock on the instance of sample  Thus  if any one thread is in newmsg    no other thread will be able to get a lock on the instance of sample  even if it were trying to invoke getmsg     On the other hand  using synchronized methods can become a bottleneck if your methods perform long-running operations - all threads  even if they want to invoke other methods in that object that could be interleaved  will still have to wait   IMO  in your simple example  it s ok to use synchronized methods since you actually have two methods that should not be interleaved  However  under different circumstances  it might make more sense to have a lock object to synchronize on  as shown in Joh Skeet s answer

User · Answer

In this simple example you can just put synchronized as a modifier after public in both method signatures   More complex scenarios require other stuff

User · Answer

That s pretty easy   class Sample       private String message   null      private final Object lock   new Object         public void newMessage String x            synchronized  lock                message   x                       public String getMessage             synchronized  lock                String temp   message              message   null              return temp                      Note that I didn t either make the methods themselves synchronized or synchronize on this  I firmly believe that it s a good idea to only acquire locks on objects which only your code has access to  unless you re deliberately exposing the lock  It makes it a lot easier to reassure yourself that nothing else is going to acquire locks in a different order to your code  etc

User · Answer

If on another occasion you re synchronising a Collection rather than a String  perhaps you re be iterating over the collection and are worried about it mutating  Java 5 offers    CopyOnWriteArrayList CopyOnWriteArraySet

User · Answer

From Java 1 5 it s always a good Idea to consider java util concurrent package  They are the state of the art locking mechanism in java right now  The synchronize mechanism is more heavyweight that the java util concurrent classes   The example would look something like this   import java util concurrent locks Lock  import java util concurrent locks ReentrantLock   public class Sample        private final Lock lock   new ReentrantLock         private String message   null       public void newmsg String msg            lock lock            try               message   msg            finally               lock unlock                         public String getmsg             lock lock            try               String temp   message              message   null              return temp            finally               lock unlock

User · Answer

For this functionality you are better off not using a lock at all  Try an AtomicReference   public class Sample       private final AtomicReference lt String gt  msg   new AtomicReference lt String gt          public void setMsg String x            msg set x              public String getMsg             return msg getAndSet null             No locks required and the code is simpler IMHO  In any case  it uses a standard construct which does what you want

[java] How to synchronize or lock upon variables in Java?

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