Synchronizers in Java

In multithreaded applications, threads often need to coordinate, wait for certain conditions, or exchange data safely. Java provides synchronizers in the java.util.concurrent package to handle these scenarios efficiently.

Synchronizers help control thread execution without low-level wait()/notify() methods, making concurrent programming safer, easier, and faster.

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What Are Synchronizers?

Synchronizers are concurrency utilities that help threads coordinate their actions. They build on locks but provide higher-level abstractions such as counting, barriers, and signaling.

Think of them like traffic signals for threads—they determine when a thread can proceed and when it must wait.

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Common Synchronizers in Java

Java provides high-level synchronizers in the java.util.concurrent package to coordinate threads safely and efficiently.

1. CountDownLatch

• Allows threads to wait until a set of operations completes.
• Initialized with a count; each thread decrements it with countDown().
• Threads calling await() wait until the count reaches zero.
• Use Case: Waiting for multiple services or tasks to finish before proceeding.

Example:

import java.util.concurrent.CountDownLatch;

CountDownLatch latch = new CountDownLatch(3);

Runnable task = () -> {
    System.out.println(Thread.currentThread().getName() + " finished work");
    latch.countDown();
};

new Thread(task).start();
new Thread(task).start();
new Thread(task).start();

latch.await(); // Main thread waits
System.out.println("All tasks completed!");

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2. CyclicBarrier

• Makes threads wait for each other at a common barrier point.
• Reusable after all threads reach the barrier (cyclic).
• Use Case: Coordinating parallel tasks in phases.

Example:

import java.util.concurrent.CyclicBarrier;

CyclicBarrierbarrier=newCyclicBarrier(3,
    () -> System.out.println("All threads reached the barrier"));

Runnabletask= () -> {
    System.out.println(Thread.currentThread().getName() +" is waiting at barrier");
    barrier.await();// Wait for other threads
    System.out.println(Thread.currentThread().getName() +" passed barrier");
};

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3. Semaphore

• Controls access to a fixed number of resources using permits.
• Threads acquire a permit with acquire() and release it with release().
• Use Case: Database connection pools, API rate limiting.

Example:

import java.util.concurrent.Semaphore;

Semaphoresemaphore=newSemaphore(2);// Only 2 threads allowed concurrently

Runnabletask= () -> {
    semaphore.acquire();
try {
        System.out.println(Thread.currentThread().getName() +" is working");
        Thread.sleep(2000);
    }finally {
        semaphore.release();
    }
};

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4. Exchanger

• Allows two threads to exchange data at a synchronization point.
• Both threads wait until the other arrives and swaps objects.
• Use Case: Producer-consumer scenarios with data swapping.

Example:

import java.util.concurrent.Exchanger;

Exchanger<String> exchanger =newExchanger<>();

Runnabletask1= () -> {
Stringdata="Data from Thread1";
Stringreceived= exchanger.exchange(data);
    System.out.println(Thread.currentThread().getName() +" received: " + received);
};

Runnabletask2= () -> {
Stringdata="Data from Thread2";
Stringreceived= exchanger.exchange(data);
    System.out.println(Thread.currentThread().getName() +" received: " + received);
};

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5. Phaser

• Flexible barrier for multi-phase thread execution.
• Threads register and wait for others at each phase.
• Use Case: Complex multi-step workflows with dynamic thread participation.

Example:

import java.util.concurrent.Phaser;

Phaserphaser=newPhaser(3);

phaser.arriveAndAwaitAdvance();// Wait for all parties at this phase

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Advantages of Synchronizers

• Simplify complex thread coordination
• Reduce manual use of wait() and notify()
• Thread-safe and optimized for performance
• Useful for high-concurrency applications

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Summary Table of Synchronizers

Synchronizer	Main Purpose	Use Case
CountDownLatch	Wait for multiple threads to finish	Initialization, batch completion
CyclicBarrier	Wait for threads at a barrier	Phase-wise task coordination
Semaphore	Limit concurrent access	Database connections, API calls
Exchanger	Exchange data between two threads	Data swapping, producer-consumer
Phaser	Multi-phase coordination	Complex multi-step workflows

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Conclusion

Java synchronizers are powerful tools for thread coordination and control.

• Use CountDownLatch for one-time waits
• Use CyclicBarrier or Phaser for repeated coordination
• Use Semaphore to limit concurrent resource access
• Use Exchanger to swap data safely between threads

Synchronizers make multithreaded programming simpler, safer, and more efficient, especially in high-concurrency applications.