Avoid synchronized(this) in Java?

Question

Whenever a question pops up on SO about Java synchronization, some people are very eager to point out that synchronized(this) should be avoided. Instead, they c

Answer 1

As already said here synchronized block can use user-defined variable as lock object, when synchronized function uses only "this". And of course you can manipulate with areas of your function which should be synchronized and so on.

But everyone says that no difference between synchronized function and block which covers whole function using "this" as lock object. That is not true, difference is in byte code which will be generated in both situations. In case of synchronized block usage should be allocated local variable which holds reference to "this". And as result we will have a little bit larger size of function (not relevant if you have only few number of functions).

More detailed explanation of the difference you can find here: http://www.artima.com/insidejvm/ed2/threadsynchP.html

Also usage of synchronized block is not good due to following point of view:

The synchronized keyword is very limited in one area: when exiting a synchronized block, all threads that are waiting for that lock must be unblocked, but only one of those threads gets to take the lock; all the others see that the lock is taken and go back to the blocked state. That's not just a lot of wasted processing cycles: often the context switch to unblock a thread also involves paging memory off the disk, and that's very, very, expensive.

For more details in this area I would recommend you read this article: http://java.dzone.com/articles/synchronized-considered