A race condition occurs when two or more threads access shared data at the same time, and the final result depends on the timing of their execution.
If one thread’s operations overlap with another thread’s operations without proper synchronization, unexpected and inconsistent results can occur.
A race condition happens when the program’s behavior changes based on which thread finishes first.
Consider the following example where two threads are incrementing a shared counter
//RaceConditionDemo.java
class Counter {
private int count = 0;
public void increment() {
count++;
}
public int getCount() {
return count;
}
}
public class RaceConditionDemo {
public static void main(String[] args) throws InterruptedException {
Counter counter = new Counter();
Runnable task = () -> {
for (int i = 0; i < 100; i++) {
counter.increment();
}
};
Thread thread1 = new Thread(task);
Thread thread2 = new Thread(task);
thread1.start();
thread2.start();
thread1.join();
thread2.join();
System.out.println("Final count: " + counter.getCount());
}
}Code language: PHP (php)In this example, the final count might not be 200 as expected because the increment method is not synchronized. This causes a race condition.
Preventing Race Conditions
To prevent race conditions, you can use synchronization mechanisms such as:
1.Synchronized Methods:
Use the synchronized keyword to make sure only one thread can execute a method at a time.
public synchronized void increment() {
count++;
}Code language: JavaScript (javascript)2.Synchronized Blocks:
Synchronize only the critical section of the code to improve performance.
public void increment() {
synchronized(this) {
count++;
}
}
Code language: JavaScript (javascript)3.Reentrant Locks:
Use ReentrantLock for more sophisticated thread synchronization.
import java.util.concurrent.locks.ReentrantLock;
public class Counter {
private int count = 0;
private ReentrantLock lock = new ReentrantLock();
public void increment() {
lock.lock();
try {
count++;
} finally {
lock.unlock();
}
}
public int getCount() {
return count;
}
}Code language: PHP (php)4.Atomic Variables:
Use classes from the java.util.concurrent.atomic package like AtomicInteger which provide methods that are atomic (thread-safe) for incrementing, decrementing, etc.
import java.util.concurrent.atomic.AtomicInteger;
public class Counter {
private AtomicInteger count = new AtomicInteger(0);
public void increment() {
count.incrementAndGet();
}
public int getCount() {
return count.get();
}
}Code language: PHP (php)Best Practices to Avoid Race Conditions
Always identify shared resources that can be accessed by multiple threads.
- Use appropriate synchronization mechanisms to protect shared resources.
- Avoid holding locks for long periods to minimize contention.
- Use higher-level concurrency utilities like java.util.concurrent package classes such as ConcurrentHashMap, CopyOnWriteArrayList, etc., which are designed for concurrent access.
- By carefully managing access to shared resources, you can avoid race conditions and ensure the correctness and reliability of your multithreaded Java applications.
Race conditions occur when multiple threads access and modify shared data simultaneously without proper synchronization, leading to unpredictable and inconsistent results. They are a major risk in concurrent programming and can cause difficult-to-detect bugs. To avoid race conditions, developers must use synchronization techniques like synchronized blocks, locks, or atomic classes to ensure that only one thread accesses the shared resource at a time, maintaining program correctness and data consistency.