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Java provides synchronization mechanisms to ensure that only one thread can access critical sections of code at a time, preventing concurrent modification issues. Two commonly used techniques for synchronization are synchronized blocks and synchronized methods. Additionally, Java offers class-level synchronization using synchronized static methods or blocks.

🔹 1. Synchronized Block vs Synchronized Method

Synchronized Method:

• When you declare a method as synchronized, the entire method is locked for a particular object or class.
• If an instance method is synchronized, only one thread can execute that method for a particular object instance at a time.
• If a static method is synchronized, it applies to the class-level lock, ensuring that only one thread can execute the synchronized static method for the entire class.

Example of synchronized method (instance method):

java

public synchronized void incrementCounter() {
    // Code inside this method is synchronized at the object level
    counter++;
}

Example of synchronized method (static method):

java

public static synchronized void incrementCounter() {
    // Code inside this method is synchronized at the class level
    counter++;
}

Synchronized Block:

• A synchronized block allows you to synchronize only a specific section of the code within a method, giving you more control over what part of the method is synchronized.
• You can specify an object to be used as a lock in the synchronized block. This allows you to limit the scope of synchronization, reducing contention among threads.

Example of synchronized block:

java

public void incrementCounter() {
    synchronized (lockObject) {
        // Code inside this block is synchronized
        counter++;
    }
}

• Advantages of Synchronized Block:
• Finer control: You can choose exactly what part of the code should be synchronized.
• Reduced contention: Since synchronization only applies to a smaller section of the code, threads can access other parts of the method concurrently, improving performance in multi-threaded environments.
• Advantages of Synchronized Method:
• Simplicity: The entire method is synchronized, which can be more straightforward and easier to manage in simple use cases.
• Automatically handles locking: No need to explicitly define a lock object.

When to Use Which?

• Use synchronized blocks when you want to optimize performance by limiting the scope of synchronized code to the smallest necessary block.
• Use synchronized methods for simplicity when the entire method needs to be synchronized, and the performance trade-off is acceptable.

🔹 2. Class Synchronization in Java

Class-level synchronization is used when you want to ensure that only one thread can access a particular static method or critical section of code in the entire class.

Synchronized Static Method:

• When a static method is synchronized, the lock is applied to the class itself, rather than an instance of the class. This means that all threads will synchronize on the class-level lock for that static method.
• This prevents more than one thread from executing the synchronized static method simultaneously across different instances of the class.

Example of synchronized static method:

java

public class MyClass {
    private static int sharedCounter = 0;

    public static synchronized void incrementCounter() {
        // Code inside this method is synchronized at the class level
        sharedCounter++;
    }
}

Synchronized Block Inside Static Method:

• You can also apply synchronization within a static method using a synchronized block. Here, you can specify an object (e.g., a lock object) to synchronize on, providing more control over synchronization.

Example of synchronized block in a static method:

java

public class MyClass {
    private static int sharedCounter = 0;
    private static final Object lock = new Object();

    public static void incrementCounter() {
        synchronized (lock) {
            // Code inside this block is synchronized at the class level
            sharedCounter++;
        }
    }
}

Class-Level Synchronization Use Cases:

• Thread-Safe Shared Resources: If you have shared static resources in a class (like a static variable), synchronizing the access to these resources ensures thread safety across the entire class.
• Global Resource Management: When dealing with global static resources or methods that can be accessed by multiple threads concurrently, class-level synchronization ensures that the shared resource is accessed in a thread-safe manner.

🧠 Conclusion:

• Synchronized Methods are easier to use but synchronize the entire method, which could cause unnecessary contention.
• Synchronized Blocks provide finer control, allowing synchronization to be applied only to specific code sections, leading to better performance in highly concurrent environments.
• Class-level synchronization is useful when you need to control access to shared resources or static methods at the class level, ensuring that only one thread can access these resources at a time.

By understanding when to use synchronized methods or blocks and how to apply class-level synchronization, you can optimize thread safety and performance in your Java programs.