synchronized is used in Java to control access to shared resources in a multi-threaded environment. Its main function is to ensure that only one thread can execute synchronous methods or code blocks at the same time, prevent race conditions and maintain data consistency. Specifically, 1. Synchronized realizes synchronization through a mutex mechanism, that is, threads must acquire object monitors (locks) before executing synchronous code. If the lock is held by other threads, it needs to wait until it is released; 2. It can be applied to methods or code blocks, where the synchronization method locks the entire method (instance method locks the current instance, and static method locks the class object), while the synchronization code block provides finer granular control and only locks the key parts; 3. Use recommendations include prioritizing the use of synchronous code blocks to improve performance, avoid deadlocks and excessive locking, and pay attention to the difference between static and instance method locks; 4. It is suitable for situations where multiple threads modify shared states, data inconsistency or need to avoid race conditions, such as concurrent modification of counters, collections, or implementation of thread-safe singletons, etc.; 5. Although synchronized is simple and easy to use, its performance overhead is high and its flexibility is not as good as modern concurrent tool classes such as ReentrantLock.
The synchronized keyword in Java is used to control access to shared resources in a multi-threaded environment. Its main purpose is to ensure that only one thread can execute a synchronized method or block at a time, preventing race conditions and keeping data consistency.
What does synchronized do exactly?
When you use synchronized , Java ensures mutual exclusion — meaning only one thread can be executing the synchronized code at any given moment. This applies either to an entire method or a specific block of code.
For example:
- If a method is declared as
public synchronized void update() { ... }, then any thread calling this method will first need to acquire the object's monitor (also known as its lock). - If another thread already holds the monitor, the new thread will wait until it's released.
This helps prevent multiple threads from modifying shared state simultaneously, which could lead to inconsistent or corrupted data.
How to use synchronized in Java
There are two common ways to apply synchronization:
-
Synchronized method
public synchronized void addData() { // some logic that modifies shared data }The entire method becomes locked per instance (for non-static methods) or per class (for static methods).
Synchronized block
public void processData() { synchronized(this) { // critical section } }This gives more fine-grained control. You can choose what object to lock on, and only part of the method gets locked.
A good practice is to prefer synchronized blocks over methods when possible — locking less code means better performance in concurrent applications.
When should you use synchronized?
Use synchronized when:
- Multiple threads are accessing and modifying the same object.
- You're seeing inconsistent behavior or data corruption in a multi-threaded context.
- You want to avoid race conditions without using external libraries.
Some typical cases include:
- Updating counters (
int count) shared between threads - Modifying lists or maps concurrently
- Implementing thread-safe singletons
Keep in mind: synchronization has a performance cost. Only synchronize what's necessary.
A few things to watch out for
- Deadlocks – if two threads each hold a lock and wait for the other to release theirs, neither can proceed.
- Lock granularity – don't lock the whole object if you can lock a smaller piece of data instead.
- Static vs instance methods – static synchronized methods lock on the class object, not on instances, so they're independent from instance-level locks.
Also, newer concurrency utilities like ReentrantLock or classes from java.util.concurrent offer more flexibility than synchronized , but they come with more complexity.
That's basically how synchronized works in Java. It's simple enough to use, but knowing when and where to apply it makes all the difference.
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