Java中的門閂和柵欄是一種同步工具，用于管理多線程程序中的并發問題。

門閂是一種阻塞對象，它允許線程一直等待，直到其他線程完成某些操作后才能繼續執行。在Java中，門閂可以使用CountDownLatch類來實現。

public class CountDownLatchExample {
public static void main(String[] args) throws InterruptedException {
final CountDownLatch latch = new CountDownLatch(3);
new Thread(new Runnable() {
public void run() {
System.out.println("Thread1 is executing");
latch.countDown();
}
}).start();
new Thread(new Runnable() {
public void run() {
System.out.println("Thread2 is executing");
latch.countDown();
}
}).start();
new Thread(new Runnable() {
public void run() {
System.out.println("Thread3 is executing");
latch.countDown();
}
}).start();
latch.await();
System.out.println("All threads have finished executing");
}
}

在上面的代碼中，我們創建了一個CountDownLatch實例，并將其初始化為3。然后我們啟動了三個線程，并在每個線程的run()方法中調用了countDown()方法。最后，我們調用await()方法來等待所有線程執行完成，這樣才能輸出"All threads have finished executing"。

柵欄是一種同步點，它允許一組線程在一個確定的點上等待彼此，并且只有當所有線程都到達這個點時才能繼續執行。在Java中，柵欄可以使用CyclicBarrier類來實現。

public class CyclicBarrierExample {
public static void main(String[] args) throws InterruptedException, BrokenBarrierException {
final CyclicBarrier barrier = new CyclicBarrier(3, new Runnable() {
public void run() {
System.out.println("All threads have reached the barrier, starting...");
}
});
new Thread(new Runnable() {
public void run() {
try {
System.out.println("Thread1 is waiting at the barrier");
barrier.await();
System.out.println("Thread1 has started running");
} catch(Exception e) {}
}
}).start();
new Thread(new Runnable() {
public void run() {
try {
System.out.println("Thread2 is waiting at the barrier");
barrier.await();
System.out.println("Thread2 has started running");
} catch(Exception e) {}
}
}).start();
new Thread(new Runnable() {
public void run() {
try {
System.out.println("Thread3 is waiting at the barrier");
barrier.await();
System.out.println("Thread3 has started running");
} catch(Exception e) {}
}
}).start();
Thread.sleep(5000);
new Thread(new Runnable() {
public void run() {
try {
System.out.println("Thread4 is waiting at the barrier");
barrier.await();
System.out.println("Thread4 has started running");
} catch(Exception e) {}
}
}).start();
}
}

在上面的代碼中，我們創建了一個CyclicBarrier實例，并將其初始化為3。我們啟動了三個線程，并在每個線程的run()方法中調用了await()方法。由于柵欄的容量為3，所以線程1到線程3必須都到達柵欄之后才能繼續執行。最后，我們啟動了第四個線程，并在等待5秒后再次調用await()方法。