|
Java™
by example!
|
|
|
When should I use the synchronized modifier?
If you have more than one of your threads running at the same time that
access shared objects, it may cause your object to be in an inconsitent state.
The following example shows what strange behaviour may result from not thinking
carefully about the implications of thread programming.
It creates one object
Person and two threads that infinitely sets its member variable name to
respectively alicia and to joris. Then it goes ahead and calls
the printName method that contains some illogical code: print out the name
only if it contains the string alicia and it contains the string joris.
That should never happen, right?
Well, consider this. We use two threads that operate on the same person object.
Every thread gets to use the processor for a certain time slice. Suppose the time
slice of the first thread ends right after the statement if (name.equals("alicia")),
which validates to true. Then the second thread gets its chance to run and
calls setName("joris") on the same object. What happens now if the first thread gets
again control over the CPU? Right! The member name has the value joris
and will output so.
Java provides locks (also called monitors) to solve this problem. Making a
method synchronized will lock the object. This means that no other thread can call
a synchronized method on that object. In our example, you not only have to make
the method printName synchronized, but also setName or the problem
will still arise. If you don't make setName synchronized, it may still be
called by the second thread, while the first thread is in the synchronized method
printName.
So remember: every method that accesses a critical shared resource should have
the synchronized modifier.
Solution:
Sometimes, you don't want to synchronize the entire method, only the statements that
may modify shared resources (the "critical section"). You can achieve this kind of
functionality by using:
Further Information
Author of answer: Joris Van den Bogaert
Comments to this answer are only viewable by members. Login or become a member!
|
|
|
|
|
In-house 
