How do you use the synchronized keyword in JavaSE multithreading?

1 What is synchronization?

Synchronization is the process of coordinating the execution of multiple threads that access the same data or resources. Synchronization can ensure that only one thread at a time can modify or read a shared variable, object, or method, thus avoiding data corruption, race conditions, and inconsistent results. Synchronization can also help to establish a happens-before relationship between two or more threads, which means that the actions of one thread are visible to the other threads in a predictable order.

2 How to use the synchronized keyword?

The synchronized keyword can be used in two ways: as a modifier for a method or a block of code. When you declare a method as synchronized, it means that only one thread can execute that method at a time on the same instance of the class. For example, if you have a class Counter with a synchronized method increment(), then only one thread can call increment() on a Counter object at a time. If another thread tries to call increment() on the same object, it will have to wait until the first thread releases the lock on the object.

When you use the synchronized keyword for a block of code, you need to specify an object that will act as a lock or a monitor. Only one thread can enter the synchronized block at a time by acquiring the lock on the object. Any other thread that wants to enter the block will have to wait until the lock is released by the current thread. For example, you can use the synchronized keyword to protect a critical section of code that accesses a shared resource, such as a file or a collection.

3 What are the benefits of using the synchronized keyword?

The main benefit of using the synchronized keyword is that it can guarantee the atomicity and visibility of your operations on shared data. Atomicity means that your operations are indivisible and complete, and visibility means that your operations are reflected in the memory and seen by other threads. By using the synchronized keyword, you can ensure that your data is consistent and safe across multiple threads, and that your program behaves as expected.

Another benefit of using the synchronized keyword is that it is easy to use and understand. You don't need to worry about the low-level details of how locks and monitors work, or how to handle exceptions and interrupts. You just need to mark the methods or blocks of code that need synchronization, and let the Java runtime handle the rest.

4 What are the drawbacks of using the synchronized keyword?

The main drawback of using the synchronized keyword is that it can reduce the performance and scalability of your program. Synchronization introduces overhead and contention, which means that your threads have to spend extra time and resources to acquire and release locks, and to wait for their turn to access the shared data. This can slow down your program and limit its ability to take advantage of multiple cores and processors.

Another drawback of using the synchronized keyword is that it can cause deadlocks and livelocks, which are situations where two or more threads are stuck in a circular wait, preventing each other from progressing. For example, if thread A holds the lock on object X and waits for the lock on object Y, while thread B holds the lock on object Y and waits for the lock on object X, then both threads are deadlocked. To avoid this, you need to be careful about the order and scope of your locks, and use timeouts and interrupts to break the cycles.

5 What are some alternatives and best practices for using the synchronized keyword?

The synchronized keyword is not the only way to achieve synchronization in JavaSE multithreading; there are other classes and interfaces in the java.util.concurrent package that can provide more control, efficiency, and scalability. However, they require more knowledge and skill to use correctly. To ensure the correctness and reliability of your program, it is best practice to use the synchronized keyword only when necessary, minimize the amount of code and data that needs synchronization, use it at the appropriate level of granularity, follow a clear and simple locking policy, and be careful to avoid creating deadlocks and livelocks. Doing so can reduce overhead and contention, improve performance and scalability, increase concurrency and throughput, prevent data inconsistency, race conditions, and memory errors, and ensure responsiveness and availability.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?