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Understanding Synchronized in Java

In Java, the `synchronized` keyword is used to ensure that only one thread can access a shared resource at a time. This is crucial in multithreaded programming, where multiple threads are executing concurrently and accessing shared data.

Why Do We Need Synchronized?

In a multithreaded environment, threads may interfere with each other when accessing shared resources. This can lead to inconsistent results, data corruption, or even crashes. Synchronization helps prevent these issues by ensuring that only one thread can access a shared resource at a time.

Types of Synchronization

There are two types of synchronization in Java:

  • **Method-level synchronization**: This type of synchronization is applied to an entire method. When a thread enters a synchronized method, it acquires a lock on the object that the method belongs to. No other thread can enter the same method until the lock is released.

  • **Block-level synchronization**: This type of synchronization is applied to a specific block of code within a method. When a thread enters a synchronized block, it acquires a lock on the object specified in the block. No other thread can enter the same block until the lock is released.

Method-Level Synchronization

Method-level synchronization is achieved by adding the `synchronized` keyword to a method declaration. Here's an example:


public class Counter {
  private int count = 0;

  public synchronized void increment() {
    count++;
  }

  public synchronized int getCount() {
    return count;
  }
}

In this example, the `increment()` and `getCount()` methods are synchronized. When a thread calls either of these methods, it acquires a lock on the `Counter` object. No other thread can call these methods until the lock is released.

Block-Level Synchronization

Block-level synchronization is achieved by using the `synchronized` keyword within a method. Here's an example:


public class Counter {
  private int count = 0;

  public void increment() {
    synchronized (this) {
      count++;
    }
  }

  public int getCount() {
    synchronized (this) {
      return count;
    }
  }
}

In this example, the `increment()` and `getCount()` methods use block-level synchronization. When a thread enters either of these methods, it acquires a lock on the `Counter` object. No other thread can enter the same block until the lock is released.

Best Practices

Here are some best practices to keep in mind when using synchronization:

  • **Use synchronization sparingly**: Synchronization can impact performance, so use it only when necessary.

  • **Avoid nested synchronization**: Nested synchronization can lead to deadlocks, so avoid it whenever possible.

  • **Use `synchronized` blocks instead of methods**: `synchronized` blocks provide more flexibility and can help reduce contention.

Conclusion

Synchronization is a crucial concept in Java programming, especially in multithreaded environments. By understanding how to use synchronization effectively, you can write more robust and efficient code. Remember to use synchronization sparingly, avoid nested synchronization, and prefer `synchronized` blocks over methods.

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