What is mutual exclusion?

Mutual exclusion is a mechanism or protocol used to prevent simultaneous access to a shared resource (such as a variable, file, or memory block) by multiple competing threads or processes. It is necessary to ensure data integrity and prevent race conditions, where the outcome of an operation depends on the order of uncontrollable events.

The main idea of mutual exclusion is that at any given moment, only one thread or process can access the critical section — a part of the code that works with a shared resource.

Typical mechanisms for implementing mutual exclusion in Java include:

1. synchronized keyword:

   • Can be applied to methods or code blocks.
   • Instance methods are synchronized on the object instance.
   • Static methods are synchronized on the Class object.
   • Code blocks are synchronized on a specified monitor object.

   public class Counter {
       private int count = 0;
   
       public synchronized void increment() { // synchronized method
           count++;
       }
   
       public void decrement() {
           synchronized (this) { // synchronized block
               count--;
           }
       }
   }
   

2. Classes from the java.util.concurrent.locks package:

   • Provide more flexible and powerful tools than synchronized.
   • Examples: ReentrantLock, StampedLock, ReadWriteLock.
   • Allow explicit control over locking (acquire/release).

   import java.util.concurrent.locks.Lock;
   import java.util.concurrent.locks.ReentrantLock;
   
   public class CounterWithLock {
       private int count = 0;
       private final Lock lock = new ReentrantLock();
   
       public void increment() {
           lock.lock(); // Acquire lock
           try {
               count++;
           } finally {
               lock.unlock(); // Release lock
           }
       }
   }
   

3. Atomic variables (java.util.concurrent.atomic):

   • Provide thread-safe operations (such as increment, decrement, compare-and-swap) on single values without explicit locking.
   • Use low-level hardware instructions like Compare-and-Swap (CAS).
   • Examples: AtomicInteger, AtomicLong, AtomicReference.

   import java.util.concurrent.atomic.AtomicInteger;
   
   public class AtomicCounter {
       private AtomicInteger count = new AtomicInteger(0);
   
       public void increment() {
           count.incrementAndGet(); // Atomic operation
       }
   }
   

The choice of mechanism depends on specific requirements: simplicity (synchronized), flexibility (Locks), or performance for simple operations (Atomic).

Incorrect use of mutual exclusion mechanisms can lead to problems such as:

• Deadlock: Two or more threads wait for resources held by each other.
• Starvation: A thread cannot access a resource because other threads constantly acquire it.
• Livelock: Threads constantly change their state in response to each other but cannot perform useful work.