A race condition in Java (and in computing in general) occurs when two or more threads (or processes) access shared resources (such as variables, objects, or memory) concurrently, and the final outcome or behavior depends on the order in which the threads execute. If the threads execute in an unpredictable or incorrect order, the outcome can be wrong, inconsistent, or unexpected.
Key Aspects of a Race Condition:
- Concurrent Access: Multiple threads try to access and modify the same shared resource at the same time.
- Uncontrolled Execution Order: The threads’ execution order is not controlled, meaning they can "race" to access and modify the shared resource.
- Inconsistent State: Due to the unpredictable timing of thread execution, the shared resource can end up in an inconsistent or incorrect state.
Example of Race Condition
Let’s look at a basic example of a race condition in Java:
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public class Counter { private int count = 0; public void increment() { count++; } public int getCount() { return count; } public static void main(String[] args) { Counter counter = new Counter(); // Creating 1000 threads, all trying to increment the counter at the same time for (int i = 0; i < 1000; i++) { new Thread(() -> counter.increment()).start(); } // Let threads finish (not the best way, just for illustration purposes) try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("Final count: " + counter.getCount()); } } |
Expected Output:
You might expect the count to be 1000 because you are incrementing it 1000 times. However, due to the race condition, the actual result will likely be less than 1000.
Why the Race Condition Occurs:
- Multiple threads are trying to execute the
increment()method at the same time. - The
count++operation is not atomic; it involves reading the current value ofcount, incrementing it, and writing the new value back. If two threads execute the operation at the same time, they might both read the same value before either has a chance to write the new value, leading to one of the increments being "lost."
For example:
- Thread A reads
count = 5. - Thread B reads
count = 5. - Thread A writes
count = 6. - Thread B writes
count = 6(overwriting the value set by Thread A).
This results in the counter only being incremented once, even though two threads tried to increment it.
How to Prevent Race Conditions
To avoid race conditions, synchronization mechanisms can be used to control the access to shared resources, ensuring that only one thread at a time can modify a shared resource.
1. Using synchronized Keyword
You can make the increment() method synchronized so that only one thread can execute it at a time:
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public class Counter { private int count = 0; public synchronized void increment() { count++; } public int getCount() { return count; } } |
2. Synchronized Block
A more granular way to control the synchronization is by using a synchronized block. This allows you to lock only a part of the method:
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public class Counter { private int count = 0; public void increment() { synchronized (this) { count++; } } public int getCount() { return count; } } |
3. Using Lock from java.util.concurrent.locks
The Lock interface provides more flexibility and control compared to synchronized blocks:
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import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock; public class Counter { private int count = 0; private final Lock lock = new ReentrantLock(); public void increment() { lock.lock(); try { count++; } finally { lock.unlock(); } } public int getCount() { return count; } } |
Other Mechanisms to Prevent Race Conditions:
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Atomic Variables: Use classes like
AtomicIntegerfrom thejava.util.concurrent.atomicpackage, which provide atomic operations on variables (increment, decrement, etc.).12345678910111213import 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();}} -
Concurrent Collections: Java provides thread-safe collections in the
java.util.concurrentpackage, such asConcurrentHashMap,CopyOnWriteArrayList, etc., which internally handle synchronization.
Conclusion:
A race condition in Java is a problem that arises when multiple threads access shared resources concurrently without proper synchronization, leading to incorrect or inconsistent results. To prevent race conditions, you can use synchronization mechanisms such as the synchronized keyword, Lock, or atomic variables. Proper synchronization ensures that shared resources are accessed by only one thread at a time, preventing race conditions.