Thread Synchronization in Java

Thread Synchronization in Java | Generated by AI

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Let’s dive into thread synchronization in Java! When multiple threads access shared resources (like variables, collections, or files), you need to prevent race conditions—where threads interfere with each other, leading to unpredictable results. Java provides several tools for this: synchronized, Lock, and Semaphore. I’ll explain each, show examples, and compare them so you can pick the right one for your needs.

1. `synchronized` Keyword

The synchronized keyword ensures only one thread can execute a block of code or method at a time. It’s the simplest synchronization tool, built into Java’s core.

How It Works

Locks an object’s monitor (every Java object has one).
Other threads wait until the lock is released.

Example: Counter with `synchronized`

class Counter {
    private int count = 0;

    public synchronized void increment() {
        count++;
    }

    public int getCount() {
        return count;
    }
}

public class Main {
    public static void main(String[] args) throws InterruptedException {
        Counter counter = new Counter();
        Runnable task = () -> {
            for (int i = 0; i < 1000; i++) {
                counter.increment();
            }
        };

        Thread t1 = new Thread(task);
        Thread t2 = new Thread(task);
        t1.start();
        t2.start();
        t1.join();
        t2.join();

        System.out.println("Final count: " + counter.getCount()); // Always 2000
    }
}

Without synchronized, count++ (read-modify-write) could overlap, giving a result less than 2000.
synchronized locks the Counter instance, ensuring atomic updates.

Synchronized Block

You can also lock specific code instead of a whole method:

public void increment() {
    synchronized (this) {
        count++;
    }
}

Use a different object as the lock if you want finer control:

private final Object lock = new Object();
public void increment() {
  synchronized (lock) {
      count++;
  }
}

Pros and Cons

Pros: Simple, built-in, good for basic use.
Cons: No flexibility (e.g., can’t interrupt a waiting thread), blocks until released.

2. `Lock` Interface (java.util.concurrent.locks)

The Lock interface (e.g., ReentrantLock) offers more control than synchronized. It’s part of the java.util.concurrent package and was introduced to address synchronized limitations.

Key Features

Explicit lock() and unlock() calls.
Supports try-locks, timeouts, and interruptible locking.
Fairness option (threads wait in order).

Example: Counter with `ReentrantLock`

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

class Counter {
    private int count = 0;
    private final Lock lock = new ReentrantLock();

    public void increment() {
        lock.lock();
        try {
            count++;
        } finally {
            lock.unlock(); // Always unlock in finally
        }
    }

    public int getCount() {
        return count;
    }
}

public class Main {
    public static void main(String[] args) throws InterruptedException {
        Counter counter = new Counter();
        Runnable task = () -> {
            for (int i = 0; i < 1000; i++) {
                counter.increment();
            }
        };

        Thread t1 = new Thread(task);
        Thread t2 = new Thread(task);
        t1.start();
        t2.start();
        t1.join();
        t2.join();

        System.out.println("Final count: " + counter.getCount()); // Always 2000
    }
}

try-finally ensures the lock is released even if an exception occurs.

Advanced Features

Try Lock: Non-blocking attempt to acquire the lock:

if (lock.tryLock()) {
  try {
      count++;
  } finally {
      lock.unlock();
  }
} else {
  System.out.println("Couldn’t acquire lock");
}

Timeout: Wait for a limited time:

if (lock.tryLock(1, TimeUnit.SECONDS)) { ... }

Interruptible: Allow waiting threads to be interrupted:
```
lock.lockInterruptibly();
```

Pros and Cons

Pros: Flexible, supports advanced features, explicit control.
Cons: More verbose, manual unlock required (risk of forgetting).

3. `Semaphore`

A Semaphore controls access to a resource by maintaining a set of permits. It’s great for limiting concurrency (e.g., max 5 threads can access a resource).

How It Works

Threads acquire permits with acquire().
Release permits with release().
If no permits are available, threads wait.

Example: Limiting Database Connections

import java.util.concurrent.Semaphore;

class ConnectionPool {
    private final Semaphore semaphore = new Semaphore(3); // Max 3 connections

    public void connect() {
        try {
            semaphore.acquire();
            System.out.println(Thread.currentThread().getName() + " connected");
            Thread.sleep(1000); // Simulate work
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        } finally {
            semaphore.release();
            System.out.println(Thread.currentThread().getName() + " disconnected");
        }
    }
}

public class Main {
    public static void main(String[] args) {
        ConnectionPool pool = new ConnectionPool();
        Runnable task = () -> pool.connect();

        Thread[] threads = new Thread[10];
        for (int i = 0; i < 10; i++) {
            threads[i] = new Thread(task, "Thread-" + i);
            threads[i].start();
        }
    }
}

Only 3 threads can “connect” at once; others wait until a permit is released.

Advanced Features

Fairness: new Semaphore(3, true) ensures FIFO order.
Try Acquire: semaphore.tryAcquire() checks without blocking.

Pros and Cons

Pros: Ideal for resource pools, flexible permit system.
Cons: More complex than synchronized or Lock, not for simple mutual exclusion.

Combining with `ExecutorService`

Here’s how these tools work with a thread pool:

import java.util.concurrent.*;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

class SharedResource {
    private int value = 0;
    private final Lock lock = new ReentrantLock();

    public void increment() {
        lock.lock();
        try {
            value++;
        } finally {
            lock.unlock();
        }
    }

    public int getValue() {
        return value;
    }
}

public class Main {
    public static void main(String[] args) throws InterruptedException {
        SharedResource resource = new SharedResource();
        ExecutorService executor = Executors.newFixedThreadPool(4);

        for (int i = 0; i < 10; i++) {
            executor.submit(() -> {
                for (int j = 0; j < 100; j++) {
                    resource.increment();
                }
            });
        }

        executor.shutdown();
        executor.awaitTermination(5, TimeUnit.SECONDS);
        System.out.println("Final value: " + resource.getValue()); // Always 1000
    }
}

Comparison

When to Use What?

synchronized: Quick, simple mutual exclusion (e.g., protecting a single variable).
Lock: Need try-locks, timeouts, or interruptibility (e.g., complex workflows).
Semaphore: Limiting concurrent access to a resource (e.g., connection pools, rate limiting).

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