Condition介面提供了與Object阻塞(wait())與喚醒(notify()或notifyAll())相似的功能，只不過Condition介面提供了更為豐富的功能，如：限定等待時長等。Condition需要與Lock結合使用，需要通過鎖物件獲取Condition。
一、基本使用
基於Condition實現生產者、消費者模式。程式碼基本與Object#wait()和Object#notify()類似，只不過我們使用Lock替換了synchronized關鍵字。
生產者
public class Producer implements Runnable {
private Lock lock;
private Condition condition;
private Queue<String> queue;
private int maxSize;
public Producer(Lock lock, Condition condition, Queue<String> queue, int maxSize) {
this.lock = lock;
this.condition = condition;
this.queue = queue;
this.maxSize = maxSize;
}
@Override
public void run() {
int i = 0;
for (; ; ) {
lock.lock();
// 如果滿了，則阻塞
while (queue.size() == maxSize) {
System.out.println("生產者佇列滿了，等待...");
try {
condition.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
try {
TimeUnit.SECONDS.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
queue.add("一個訊息:" + ++i);
System.out.printf("生產者%s生產了一個訊息:%s\n", Thread.currentThread().getName(), i);
condition.signal();
lock.unlock();
}
}
}
消費者
public class Consumer implements Runnable {
private Lock lock;
private Condition condition;
private Queue<String> queue;
private int maxSize;
public Consumer(Lock lock, Condition condition, Queue<String> queue, int maxSize) {
this.lock = lock;
this.condition = condition;
this.queue = queue;
this.maxSize = maxSize;
}
@Override
public void run() {
for (; ; ) {
lock.lock();
while (queue.isEmpty()) {
System.out.println("消費者佇列為空，等待...");
try {
condition.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
String obj = queue.remove();
System.out.printf("消費者%s消費一個訊息：%s\n", Thread.currentThread().getName(), obj);
condition.signal();
lock.unlock();
}
}
}
測試類
public class ConditionProducerConsumer {
public static void main(String[] args) {
Lock lock = new ReentrantLock();
Condition condition = lock.newCondition();
Queue<String> queue = new LinkedBlockingQueue<>();
int maxSize = 10;
Producer producer = new Producer(lock, condition, queue, maxSize);
Consumer consumer = new Consumer(lock, condition, queue, maxSize);
new Thread(producer).start();
new Thread(consumer).start();
}
}
二、原始碼分析
上述示例中使用的Lock是ReentrantLock，關於它的lock方法與unlock方法的原理詳見ReentrantLock實現原理。上述示例中的Condition物件是呼叫了Lock#newCondition()方法，原始碼如下：
public class ReentrantLock implements Lock, java.io.Serializable {
...
public Condition newCondition() {
return sync.newCondition();
}
abstract static class Sync extends AbstractQueuedSynchronizer {
...
final ConditionObject newCondition() {
return new ConditionObject();
}
...
}
...
}
上述的ConditionObject定義在AQS中，如下:
public abstract class AbstractQueuedSynchronizer
extends AbstractOwnableSynchronizer
implements java.io.Serializable {
...
public class ConditionObject implements Condition, java.io.Serializable {
...
}
...
}
首先來分析下Condition#await()方法
public final void await() throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
int interruptMode = 0;
while (!isOnSyncQueue(node)) {
LockSupport.park(this);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null) // clean up if cancelled
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
}
private Node addConditionWaiter() {
Node t = lastWaiter;
// If lastWaiter is cancelled, clean out.
if (t != null && t.waitStatus != Node.CONDITION) {
unlinkCancelledWaiters();
t = lastWaiter;
}
Node node = new Node(Thread.currentThread(), Node.CONDITION);
if (t == null)
firstWaiter = node;
else
t.nextWaiter = node;
lastWaiter = node;
return node;
}
根據AQS佇列的特性，若有多個執行緒執行lock#lock()方法，會將處於阻塞狀態的執行緒維護到一個雙向連結串列中，如下：
假設當前是執行緒A獲取到鎖，其他執行緒執行lock#lock()方法時，將會構建成一個上述連結串列。
若獲取鎖的執行緒（執行緒A）執行Condition#await()方法，則會將當前執行緒新增至Condition佇列中，如下：
然後在呼叫fullyRelease()方法時會釋放當前執行緒的鎖，然後喚醒處於阻塞佇列中的下一個執行緒：
在呼叫isOnSyncQueue()方法時會檢查當前節點是否在同步佇列中，若不存在，則會呼叫LockSupport.park()進行阻塞。
假設當前執行緒A是生產者執行緒，呼叫await()方法後，會釋放鎖，並且將當前執行緒加入到Condition佇列中。此時，消費者能獲取到鎖資源，然後繼續執行。假設執行緒B是消費者執行緒，當新增一個元素後會呼叫condition#signal()方法，定義如下：
public final void signal() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
Node first = firstWaiter;
if (first != null)
doSignal(first);
}
private void doSignal(Node first) {
do {
if ( (firstWaiter = first.nextWaiter) == null)
lastWaiter = null;
first.nextWaiter = null;
} while (!transferForSignal(first) &&
(first = firstWaiter) != null);
}
final boolean transferForSignal(Node node) {
/*
* If cannot change waitStatus, the node has been cancelled.
*/
if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
return false;
/*
* Splice onto queue and try to set waitStatus of predecessor to
* indicate that thread is (probably) waiting. If cancelled or
* attempt to set waitStatus fails, wake up to resync (in which
* case the waitStatus can be transiently and harmlessly wrong).
*/
Node p = enq(node);
int ws = p.waitStatus;
if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
LockSupport.unpark(node.thread);
return true;
}
執行signal()方法，會將Condition佇列中的第一個節點移除，將其變為同步佇列中的尾結點，如下：
至此，完成了Condition佇列轉換為同步佇列的過程。後續流程基本就是重複以上操作。
本文詳細介紹了單個Condition佇列的執行流程，其實一個Lock中可以有多個Condition佇列，比如：JUC中提供的LinkedBlockingDeque、ArrayBlockingQueue等