Synchronization is crucial in multithreading to ensure that shared resources are accessed by only one thread at a time, preventing inconsistent or corrupted data.
🔍 What is it?
- A mechanism to control access to shared resources by multiple threads to avoid data inconsistency and ensure thread safety.
- Ensures that only one thread can access a critical section of code or a shared resource at any given time by using locks or monitors.
```java
public class Counter {
private int count \= 0;
}
```
- Simple Analogy: Like having a single key to a locker shared by multiple people; only one person can open the locker at a time.
Why Synchronization is Needed
When multiple threads access shared resources (like variables, objects, or files) concurrently, they can interfere with each other, leading to inconsistent or erroneous results. Synchronization is needed to:
- Prevent Race Conditions: Occur when the outcome depends on the sequence or timing of uncontrollable events.
- Ensure Consistency: Guarantees that the shared resource maintains a consistent state.
- Avoid Deadlocks: A situation where two or more threads are blocked forever, waiting for each other to release resources.
❓ How Synchronization Works
Synchronization can be achieved using various mechanisms:
Synchronized Methods
- Explanation: Locks the instance or class to prevent other threads from accessing the method until the lock is released.
- Usage: Lock the entire method, preventing other threads from entering the method simultaneously.
Example:
java public synchronized void mySynchronizedMethod() { // critical section code }
Synchronized Blocks
- Explanation: Allows finer control by synchronizing only a part of the code block, using a specific object as the lock.
- Usage: Lock a specific section of code to improve performance and reduce lock contention.
Example:
java public void myMethod() { synchronized (this) { // critical section code } }
Locks and Monitors
- Explanation: A monitor is a synchronization construct that controls access to an object. Java uses intrinsic locks and monitors to achieve this.
- Example:
public void increment() { lock.lock(); try { count++; } finally { lock.unlock(); } }
Key Concepts
- Mutual Exclusion: Ensures that only one thread can access the critical section of code at a time.
- Atomicity: Guarantees that a series of operations on a shared resource appear to be instantaneous or indivisible.
- Visibility: Ensures that changes made by one thread are visible to other threads in a timely manner.
Common Issues
- Deadlock: Occurs when two or more threads are blocked forever, each waiting for the other to release a resource.
- Starvation: A situation where a thread is perpetually denied access to resources.
- Livelock: Similar to deadlock, but the states of threads keep changing in response to each other without making progress.
Summary
- Synchronization is a critical concept in concurrent programming to ensure that multi-threaded applications function correctly and efficiently.
- It helps manage access to shared resources, maintains consistency, and prevents conflicts between threads.
- It is achieved through synchronized methods, blocks, and the use of locks/monitors.
Follow-up Questions
- What is a potential drawback of using synchronization?
Overusing synchronization can lead to performance issues and thread contention, causing slower execution.
Can synchronization be used to avoid race conditions?
Yes, synchronization helps prevent race conditions by ensuring that only one thread can access critical sections of code at a time.
What is a deadlock in the context of synchronization?
- A deadlock occurs when two or more threads are blocked forever, each waiting for the other to release a lock.
// Synchronized method public synchronized void increment() { count++; }
public int getCount() {
return count;
}