How do you create a synchronized method in Java?
Table of Contents
- Introduction
- Purpose of a Synchronized Method
- How to Create a Synchronized Method in Java
- Practical Use Cases for Synchronized Methods
- Conclusion
Introduction
In Java, when multiple threads access shared resources concurrently, it can lead to problems such as data inconsistency and race conditions. To avoid such issues, the synchronized keyword is used to control the access of multiple threads to a shared resource. A synchronized method ensures that only one thread can execute the method at a time, providing thread safety and protecting shared data from being corrupted. This guide will explain how to create a synchronized method in Java and the scenarios in which it is useful.
Purpose of a Synchronized Method
A synchronized method is used to ensure that a particular method can be executed by only one thread at a time. The purpose of using a synchronized method is to:
- Prevent Data Corruption: Ensure that shared data is accessed and modified by only one thread at a time, preventing race conditions.
- Thread Safety: Ensure that a method is thread-safe, so concurrent execution by multiple threads does not result in inconsistent or incorrect results.
- Mutual Exclusion: Ensure mutual exclusion, meaning no two threads can access the synchronized method simultaneously.
When a thread enters a synchronized method, it acquires a lock (or monitor) on the object or class the method belongs to. Once the thread finishes executing the method, it releases the lock, allowing other threads to access it.
How to Create a Synchronized Method in Java
In Java, you can make a method synchronized by adding the synchronized keyword before the method definition. Depending on whether the method is an instance method or a static method, the lock is applied differently:
1. Synchronized Instance Method
When a method is synchronized, it locks the instance of the class (the current object). This is the most common use case for synchronized methods.
Example: Synchronized Instance Method
Explanation:
- The method
increment()is marked assynchronized, which means only one thread can execute it at a time on the same object (Counter). - In this example, two threads are incrementing the
countvariable concurrently. However, because the method is synchronized, the threads will not interfere with each other, and the final count will be correct (2000).
2. Synchronized Static Method
A static synchronized method works similarly, but it locks the class itself rather than an instance of the class. This means that the method is synchronized for all instances of the class, ensuring that only one thread can execute it across all instances.
Example: Synchronized Static Method
Explanation:
- The
increment()method is synchronized with thestatic synchronizedkeyword. This means that only one thread can execute theincrement()method at a time for the entire class, not just individual instances. - Even though there might be multiple instances of
SharedCounter, the static method ensures that only one thread can access the method at a time across all instances.
Practical Use Cases for Synchronized Methods
- Incrementing Counters: When multiple threads are incrementing a shared counter (e.g., in multithreaded applications like games, simulations, or counters in web servers), synchronized methods ensure that the counter’s value is updated correctly.
- Bank Transactions: If multiple threads perform transactions on the same account, a synchronized method can ensure that the balance is updated correctly without data inconsistency or corruption.
- Producer-Consumer Problem: In scenarios where threads are producing and consuming data from a shared buffer, synchronized methods can be used to control access to the shared resource.
Conclusion
The synchronized keyword in Java is a powerful tool for ensuring thread safety in multi-threaded environments. By marking a method as synchronized, you prevent multiple threads from accessing the method concurrently, which can lead to data corruption and race conditions. You can create synchronized methods in Java for both instance and static methods, depending on whether the lock should be applied to individual objects or the entire class.
Understanding when and how to use synchronized methods is essential for writing robust, thread-safe applications in Java, especially when dealing with shared resources in a concurrent programming environment.