What is the purpose of the synchronized keyword in Java?

Table of Contents

• Introduction
• Purpose of the synchronized Keyword
  • 1. Synchronized Methods
    • Example: Synchronized Method
  • Explanation:
  • 2. Synchronized Blocks
    • Example: Synchronized Block
  • Explanation:
• Practical Use Cases for the synchronized Keyword
• Conclusion

Introduction

In Java, the synchronized keyword is used to control access to a critical section of code by multiple threads. When multiple threads try to access shared resources concurrently, it can lead to data inconsistency and unexpected behavior. The synchronized keyword helps ensure that only one thread can access the critical section at a time, preventing race conditions and ensuring thread safety. This guide explains the purpose and usage of the synchronized keyword in Java, including practical examples.

Purpose of the synchronized Keyword

The main purpose of the synchronized keyword is to provide a mechanism for thread synchronization, allowing Java programs to prevent concurrency issues. It ensures that:

1. Thread Safety: Ensures that a method or block of code is accessed by only one thread at a time, preventing conflicts when accessing shared data.
2. Mutual Exclusion: Provides mutual exclusion (mutex) by locking a shared resource for a thread, so no other thread can access it simultaneously.
3. Data Consistency: Prevents data corruption that may occur when multiple threads modify shared variables concurrently.

Java provides two ways to apply synchronization:

• Synchronized Methods
• Synchronized Blocks

1. Synchronized Methods

A synchronized method ensures that only one thread can execute the method at any given time. The lock for a synchronized method is obtained on the object (for instance methods) or the class (for static methods) that the method belongs to.

Example: Synchronized Method

Explanation:

• The method increment() is synchronized, meaning only one thread can execute it at a time.
• In the above example, two threads try to increment the count variable concurrently, but the synchronized keyword ensures that the updates to count are thread-safe. Without synchronization, the final value of count might not be correct due to race conditions.

2. Synchronized Blocks

In cases where you need more fine-grained control over the synchronization, you can use synchronized blocks. A synchronized block only locks a specific part of the code, rather than the entire method. This can improve performance, especially when you have methods with parts that don’t need to be synchronized.

Example: Synchronized Block

Explanation:

• The synchronized block inside the increment() method locks the critical section where the count variable is modified.
• The block is synchronized on this, meaning that it locks the current object (the instance of Counter).
• This approach ensures that only one thread can modify count at a time, preventing data inconsistency.

Practical Use Cases for the synchronized Keyword

1. Shared Data Access: When multiple threads access or modify shared data (e.g., counters, lists), synchronization is needed to avoid data corruption.
2. Bank Account Transactions: In a banking application, when multiple threads are performing transactions (deposit, withdraw) on the same account, synchronization ensures that account balance updates are done atomically.
3. Producer-Consumer Problem: In a multi-threaded environment where threads are producing and consuming data from a shared buffer, synchronization can be used to control access to the buffer.

Conclusion

The synchronized keyword in Java is an essential tool for managing thread safety in multi-threaded applications. By ensuring that only one thread can access a critical section at a time, it prevents race conditions and data inconsistency. Depending on the use case, you can choose to synchronize entire methods or specific code blocks. Using synchronization appropriately can help ensure that your Java applications behave correctly in a concurrent environment.