synchronized

public synchronized void method(){}

ReentrantLock

private final ReentrantLock lock = new ReentrantLock();

public void method(){
	lock.lock();
    try{
		...
	}finally{
		lock.unlock();
	}
}

CountDownLatch

private final CountDownLatch latch = new CountDownLatch(3);
public void method() throws InterruptedException{
	for(String str:list){
        new Thread(()->{
			//doSomething
  			//计数器-1
            latch.countDown();
		}).start();
	}
    //主线程阻塞
    latch.await();
}

Semaphore

private final Semaphore semaphore = new Semaphore(3);

public void method(){
	semaphore.acquire();
    try{
		//doSomething
	}finally{
		semaphore.release();
	}
}