What is synchronized statement used for?

Question

What is the usage of synchronized statements?

Answer 1

These are used for when you are building a program with many "threads". When main starts, it starts with one thread, which executes the steps in a sequence. You can start many more threads, which can then execute code at the same time. If you're executing the same code at the same time, things might behave in ways you don't want:

y = x+20;
// at this moment, before the next instruction starts, some other thread performs 
// the above step, which sets 'y' (an object property) to something different.
int b = y+10; // this would not be x+20, as you might expect.

What you want to do is put a 'lock' over this block of code, to make sure that no other thread can start executing any code that is "synchronized on" the variable y.

synchronized (y) {
    y = x+20;
    int b = y+10;
} // lock gets released here

Now, all other threads have to wait for whichever thread got there first to exit the block and release the lock, at which point another thread grabs the lock, enters the block of code, executes it, and releases the lock. Note that y has to be an object (Integer), not a primitive type.

You can also add 'synchronized' to methods, which synchronizes on 'this' (the instance object), or the class in the case of a static method.

Writing multi-threaded code is hard, because of problems like this. Synchronization is one tool, though it has one major problem - deadlocks. There is a lot of information online about deadlocks.