Synchronized Methods and Blocks

Synchronized methods and blocks in Java prevent multiple threads from accessing the same resource simultaneously. This helps avoid issues like data races, where multiple threads attempt to modify a shared resource at the same time.

Real-life example: Imagine a warehouse with a single entrance and exit. If multiple people try to use the same door at once, it can cause chaos. Synchronization in programming acts like a controller at the door, allowing only one person to pass through at a time to prevent collisions.

Synchronized Methods

When a method is declared with the synchronized keyword, it automatically locks the object to which the method belongs for the duration of the method's execution.

Code example

In this example, the increment() method is synchronized, meaning only one thread can execute this method at any given time.

Synchronized Blocks

Synchronized blocks let you synchronize only a specific section of a method, which is useful if you only want to synchronize a particular part of the code.

Code example

In this example, only the code inside synchronized(this) is synchronized, which improves performance if the entire method does not need synchronization.

Note

In the synchronized(this) block, the keyword this means that the code block is synchronized on the current instance of the object, specifically on the object for which this method is called.

Object Monitors

When a thread acquires an object monitor, it gains exclusive access to synchronized methods or blocks of that object. Other threads attempting to access synchronized methods or blocks of the same object will be blocked until the monitor is released.

Methods wait(), notify(), notifyAll()

The wait() method is used to suspend the execution of the current thread and release the resources it occupies until another thread calls the notify() or notifyAll() method for the same monitor object.

The notify() and notifyAll() methods are used to resume the execution of one or all threads blocked on a monitor object.

Note

The wait(), notify(), and notifyAll() methods must be called inside a synchronized block associated with the same monitor object as the pending thread to ensure proper synchronization.

In the next chapter, we'll explore what can occur if you don't use synchronization.