What is the purpose of the synchronized keyword in Java

The synchronized keyword in Java is used to provide mutual exclusion and thread synchronization in concurrent programming. It is used to ensure that only one thread at a time can access a particular block of code or an object's critical section. The purpose of the synchronized keyword is to prevent multiple threads from concurrently accessing shared resources, thereby avoiding data inconsistencies and race conditions.

The synchronized keyword can be applied in three different ways:

1. Synchronized Methods: When a method is declared as synchronized, it means that only one thread can execute that method at a time for a particular instance of the class. Other threads attempting to execute the same method on the same instance will be blocked until the method is released by the executing thread. Synchronized methods are useful when protecting instance variables or ensuring atomicity of operations.

Example:

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public synchronized void synchronizedMethod() { // Synchronized code block // Only one thread can execute this method at a time }

1. Synchronized Statements (Code Blocks): Instead of synchronizing an entire method, it is also possible to synchronize specific blocks of code within a method. This allows more fine-grained control over synchronization. To synchronize a block of code, you can use the synchronized keyword followed by an object reference within parentheses. The specified object acts as a lock or monitor, ensuring that only one thread can execute the synchronized block at a time.

Example:

public void someMethod() { // Non-synchronized code synchronized (lockObject) { // Synchronized code block // Only one thread can execute this block at a time } // Non-synchronized code }

1. Synchronized Static Methods: Similar to synchronized instance methods, static methods can also be declared as synchronized. When a static method is synchronized, it ensures that only one thread can execute that method at a time across all instances of the class. This prevents multiple threads from interfering with static variables or static resources.

Example:

public static synchronized void synchronizedStaticMethod() { // Synchronized code block // Only one thread can execute this static method at a time }

The synchronized keyword is essential for managing thread safety and avoiding race conditions in multi-threaded Java applications. It provides a simple and effective way to coordinate the access of shared resources between multiple threads, ensuring that critical sections of code are executed atomically and in a mutually exclusive manner.