When multiple threads are executing concurrently, they might try to access shared data. If two or more threads modify this data at the same time, it can lead to unpredictable behavior and errors. Synchronization helps by allowing only one thread to access the shared data or resource at a time. Java provides two primary ways to synchronize code:
- Synchronized Methods
- Synchronized Blocks
Both techniques ensure that only one thread can execute a critical section of code at a time.
Synchronized Methods
A synchronized method ensures that only one thread can execute it at a time. You can achieve this by using the synchronized keyword in the method signature. Here’s an example of a synchronized method:
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public class Counter { private int count = 0; // Synchronized method public synchronized void increment() { count++; } public int getCount() { return count; } } |
In the above example, the increment() method is synchronized. This means that when one thread is executing this method, no other thread can enter it. This prevents multiple threads from simultaneously modifying the count variable, ensuring its value remains consistent.
Synchronized Blocks
Sometimes, we may only need to synchronize a specific portion of code rather than the entire method. In such cases, we can use a synchronized block. This allows more fine-grained control over synchronization. Here’s an example using synchronized blocks:
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public class Counter { private int count = 0; public void increment() { synchronized (this) { count++; } } public int getCount() { return count; } } |
In this example, only the block of code that modifies the count variable is synchronized. This approach can lead to better performance when synchronization is only required for certain parts of the method.
synchronized keyword
The synchronized keyword works by acquiring a lock on the object it’s applied to. When a thread enters a synchronized method or block, it acquires the lock for the object or class. Other threads must wait for the lock to be released before they can enter the synchronized code.
- Instance Lock: When synchronizing a non-static method or block, the thread acquires a lock on the instance of the class.
- Class Lock: When synchronizing a static method or block, the thread acquires a lock on the class itself.
Importance of Synchronization
Without synchronization, we risk creating race conditions, where the behavior of the program depends on the timing and order of thread execution. This could lead to corrupted data or unexpected results. Synchronization helps to avoid these issues, but it comes with trade-offs, such as potential performance overhead due to lock contention.
Cons of Synchronization
While synchronization is essential for thread safety, improper use of synchronization can lead to issues:
- Deadlock: This occurs when two or more threads are blocked forever, waiting for each other to release resources.
- Thread Starvation: If one thread continuously acquires locks and doesn’t release them, other threads may never get a chance to execute.
To avoid these issues, always ensure your synchronized methods or blocks are as efficient as possible and use strategies like timed locks or lock ordering to prevent deadlocks.
