Synchronized blocks are generally better when you need fine-grained control, flexibility in choosing lock objects, and improved performance by locking only critical sections. 1. Synchronized methods lock the entire method and use the object or class as the lock, leading to broader locking than necessary. 2. Synchronized blocks allow specifying exactly which object to lock and limit the locked code section, enabling more precise synchronization. 3. Blocks offer greater flexibility by using custom lock objects, preventing unrelated operations from interfering. 4. Blocks improve performance by reducing lock contention and avoiding unnecessary blocking of external code.
When you're working with multithreading in Java, you'll often run into the question of whether to use a synchronized method or a synchronized block. Both are used to control access to shared resources, but they differ in scope, flexibility, and performance.
1. Scope of Locking
One of the main differences is how much code gets locked when a thread enters.
Synchronized Method: When a thread calls a synchronized method on an object, it acquires the lock on the entire object (
thisfor instance methods, or the class object for static methods). This means even if only a small part of the method needs synchronization, the whole method is blocked for other threads.Synchronized Block: You can specify exactly which object's lock you want to acquire, and only the code inside the block is locked. That gives you more fine-grained control over what’s being synchronized.
For example:
// Synchronized method
public synchronized void doSomething() {
// Entire method is locked
}
// Synchronized block
public void doSomethingElse() {
synchronized(this) {
// Only this part is locked
}
}So if you don't need to lock the whole method, using a block can reduce contention and improve performance.
2. Flexibility in Choosing the Lock Object
This is a big one that often tips the scales toward using blocks.
Synchronized Method: The lock is always tied to the object itself (or the class for static methods), which can be limiting. If multiple methods synchronize on the same object, they’ll block each other even if they’re not related.
Synchronized Block: You can choose any object as the lock monitor. That lets you separate locks for different parts of your code. For example, you could have two unrelated operations each synchronized on their own private lock objects — so they won’t interfere with each other.
Example:
private final Object lock1 = new Object();
private final Object lock2 = new Object();
public void operationA() {
synchronized(lock1) {
// Do something
}
}
public void operationB() {
synchronized(lock2) {
// Do something else
}
}This kind of separation isn’t possible with synchronized methods.
3. Performance Considerations
While the difference might seem minor, it can matter in high-concurrency environments.
- Synchronized Methods tend to be slower when only part of the method needs synchronization because they lock the whole method.
- Synchronized Blocks allow you to keep the synchronized section as small as possible, reducing lock contention and improving throughput.
Also, since blocks let you use private lock objects, they prevent external code from accidentally acquiring the same lock, which can help avoid deadlocks or unnecessary blocking.
4. Static vs Instance Context
Another subtle point is how each approach behaves with static methods.
- Static Synchronized Method: Acquires a lock on the class object (
ClassName.class). - Instance Synchronized Method: Acquires a lock on the instance (
this).
With synchronized blocks, you can explicitly choose the same behavior:
// Static block equivalent
synchronized(MyClass.class) { ... }
// Instance block equivalent
synchronized(this) { ... }This makes it easier to understand and manage locking at a glance.
So, basically, the choice comes down to control. If you want minimal locking and precise control over what’s locked and by whom, go for synchronized blocks. If simplicity and clarity are more important — and the whole method really does need synchronization — then synchronized methods are perfectly fine.
It's not complicated, but easy to misuse if you don't understand the underlying locking behavior.
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