Synchronization of static and instance methods in Java

Synchronization of static methods/fields in Java

What's an Intrinsic lock OR a monitor lock in Java?

An intrinsic lock, a monitor lock, or a monitor - all three refer to the same internal entity associated with every object in Java which enables the implementation of the Synchronization mechanism. This lock is used to enforce an exclusive and consistent access in Java as a monitor can be acquired by only one thread at a time.

A thread needs to acquire the monitor of an appropriate object before entering a synchronized method/block which it releases when the thread returns from the method (or completes the block). The monitor is released even if an uncaught exception is thrown from the method/block.

Synchronized methods vs Synchronized blocks

These are the two ways of achieving synchronized access in Java. In Java, a method is also a block only, but we normally refer to a block as a part of the method definition. As we know that a thread needs to acquire an appropriate monitor (which it releases when the method returns) before entering any synchronized method/block, so we can easily figure out that in case of a synchronized method a thread may need to have the lock for a longer period of time as compared to that in case of synchronized block.

Another difference is that we don't specify the particular object whose monitor is required to be obtained by a thread for entering a synchronized method whereas we can specify the particular object in case of a synchronized block.

static synchronized method vs instance synchronized method

When a thread needs to enter a static synchronized method, it acquires the monitor of the Class object associated with the particular class to which the static method belongs to whereas in case of a synchronized instance method, the thread requires to obtain the monitor of the particular object on which the method call is being made. Easy to understand the reason... right?

Synchronization of static fields in an instance synchronized method

As we just saw that a thread needs to acquire the monitor of the Class object of the class in case synchronized static method, but how can you handle the synchronization of static fields in instance methods?

In Java, an instance method can access static fields as well (vice versa is not possible for the obvious reason), suppose a synchronized instance method having code accessing static fields, is being accessed on different objects by different threads (which is quite possible as each of the different threads would have acquired monitor of one of the different objects) then in such a case we can't guarantee a exclusive access to those static fields. The reason is very simple - a static field doesn't belong to an instance instead to the class and hence the access to it is not controlled by monitors associated with the instances of the class instead the access to them is controlled by the monitor of the Class object associated with the class. Hence, we need to be very careful in such scenarios and any synchronized instance method should have an explicit synchronized block for accessing static fields if the static fields require an exclusive access. This synchronized block can specify the Class object associated with the class and hence a thread which enters the synchronized instance method after acquiring the monitor of the particular instance will first need to acquire the monitor of the Class object as well before executing the code which accesses static fields. And this way we can guarantee exclusive access to the static fields accessed inside a synchronized instance method.

Read next: Why synchronization of constructors is not allowed? Do you know Re-entrant Synchronization? You may like to read the article - What's Reentrant Synchronization in Java?

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