The snippet synchronized(X.class) uses the class instance as a monitor. As there is only one class instance (the object representing the class metadata at runtime) one thread can be in this block. With synchronized(this) the block is guarded by the instance. For every instance only one thread may enter the block. synchronized(X.class) is used to … Read more
Just to add a little detail to Oscar’s (pleasingly succinct!) answer, the relevant section on the Java Language Specification is 8.4.3.6, ‘synchronized Methods’: A synchronized method acquires a monitor (§17.1) before it executes. For a class (static) method, the monitor associated with the Class object for the method’s class is used. For an instance method, … Read more
Both methods lock the same monitor. Therefore, you can’t simultaneously execute them on the same object from different threads (one of the two methods will block until the other is finished).
While at first it might seem obvious that one would want to support the synchronized modifier on default methods, it turns out that doing so would be dangerous, and so was prohibited. Synchronized methods are a shorthand for a method which behaves as if the entire body is enclosed in a synchronized block whose lock … Read more
If you declare the method as synchronized (as you’re doing by typing public synchronized void addA()) you synchronize on the whole object, so two thread accessing a different variable from this same object would block each other anyway. If you want to synchronize only on one variable at a time, so two threads won’t block … Read more
It’s important to understand that there are two aspects to thread safety. execution control, and memory visibility The first has to do with controlling when code executes (including the order in which instructions are executed) and whether it can execute concurrently, and the second to do with when the effects in memory of what has … Read more
I’ll cover each point separately. Some evil code may steal your lock (very popular this one, also has an “accidentally” variant) I’m more worried about accidentally. What it amounts to is that this use of this is part of your class’ exposed interface, and should be documented. Sometimes the ability of other code to use … Read more
Can anyone tell me the advantage of the synchronized method over the synchronized block with an example? Thanks. There is not a clear advantage of using synchronized method over the block. Perhaps the only one ( but I wouldn’t call it an advantage ) is you don’t need to include the object reference this. Method: … Read more
The synchronized keyword is all about different threads reading and writing to the same variables, objects and resources. This is not a trivial topic in Java, but here is a quote from Sun: synchronized methods enable a simple strategy for preventing thread interference and memory consistency errors: if an object is visible to more than … Read more