前言:
此刻小伙伴们对“java面试pdai”大约比较讲究,同学们都需要学习一些“java面试pdai”的相关资讯。那么小编同时在网摘上汇集了一些有关“java面试pdai””的相关资讯,希望各位老铁们能喜欢,咱们一起来学习一下吧!AbstractQueuedSynchronizer抽象类是核心,需要重点掌握。它提供了一个基于FIFO队列,可以用于构建锁或者其他相关同步装置的基础框架。@pdai
带着BAT大厂的面试问题去理解
请带着这些问题继续后文,会很大程度上帮助你更好的理解相关知识点。@pdai
什么是AQS? 为什么它是核心?AQS的核心思想是什么? 它是怎么实现的? 底层数据结构等AQS有哪些核心的方法?AQS定义什么样的资源获取方式? AQS定义了两种资源获取方式:独占(只有一个线程能访问执行,又根据是否按队列的顺序分为公平锁和非公平锁,如ReentrantLock) 和共享(多个线程可同时访问执行,如Semaphore、CountDownLatch、 CyclicBarrier )。ReentrantReadWriteLock可以看成是组合式,允许多个线程同时对某一资源进行读。AQS底层使用了什么样的设计模式? 模板AQS的应用示例?AbstractQueuedSynchronizer简介
AQS是一个用来构建锁和同步器的框架,使用AQS能简单且高效地构造出应用广泛的大量的同步器,比如我们提到的ReentrantLock,Semaphore,其他的诸如ReentrantReadWriteLock,SynchronousQueue,FutureTask等等皆是基于AQS的。当然,我们自己也能利用AQS非常轻松容易地构造出符合我们自己需求的同步器。
AQS 核心思想
AQS核心思想是,如果被请求的共享资源空闲,则将当前请求资源的线程设置为有效的工作线程,并且将共享资源设置为锁定状态。如果被请求的共享资源被占用,那么就需要一套线程阻塞等待以及被唤醒时锁分配的机制,这个机制AQS是用CLH队列锁实现的,即将暂时获取不到锁的线程加入到队列中。
CLH(Craig,Landin,and Hagersten)队列是一个虚拟的双向队列(虚拟的双向队列即不存在队列实例,仅存在结点之间的关联关系)。AQS是将每条请求共享资源的线程封装成一个CLH锁队列的一个结点(Node)来实现锁的分配。
AQS使用一个int成员变量来表示同步状态,通过内置的FIFO队列来完成获取资源线程的排队工作。AQS使用CAS对该同步状态进行原子操作实现对其值的修改。
private volatile int state;//共享变量,使用volatile修饰保证线程可见性
状态信息通过procted类型的getState,setState,compareAndSetState进行操作
//返回同步状态的当前值protected final int getState() { return state;} // 设置同步状态的值protected final void setState(int newState) { state = newState;}//原子地(CAS操作)将同步状态值设置为给定值update如果当前同步状态的值等于expect(期望值)protected final boolean compareAndSetState(int expect, int update) { return unsafe.compareAndSwapInt(this, stateOffset, expect, update);}AQS 对资源的共享方式
AQS定义两种资源共享方式
Exclusive(独占):只有一个线程能执行,如ReentrantLock。又可分为公平锁和非公平锁: 公平锁:按照线程在队列中的排队顺序,先到者先拿到锁非公平锁:当线程要获取锁时,无视队列顺序直接去抢锁,谁抢到就是谁的Share(共享):多个线程可同时执行,如Semaphore/CountDownLatch。Semaphore、CountDownLatCh、 CyclicBarrier、ReadWriteLock 我们都会在后面讲到。
ReentrantReadWriteLock 可以看成是组合式,因为ReentrantReadWriteLock也就是读写锁允许多个线程同时对某一资源进行读。
不同的自定义同步器争用共享资源的方式也不同。自定义同步器在实现时只需要实现共享资源 state 的获取与释放方式即可,至于具体线程等待队列的维护(如获取资源失败入队/唤醒出队等),AQS已经在上层已经帮我们实现好了。
AQS底层使用了模板方法模式
同步器的设计是基于模板方法模式的,如果需要自定义同步器一般的方式是这样(模板方法模式很经典的一个应用):
使用者继承AbstractQueuedSynchronizer并重写指定的方法。(这些重写方法很简单,无非是对于共享资源state的获取和释放) 将AQS组合在自定义同步组件的实现中,并调用其模板方法,而这些模板方法会调用使用者重写的方法。
这和我们以往通过实现接口的方式有很大区别,AQS使用了模板方法模式,自定义同步器时需要重写下面几个AQS提供的模板方法:
isHeldExclusively()//该线程是否正在独占资源。只有用到condition才需要去实现它。tryAcquire(int)//独占方式。尝试获取资源,成功则返回true,失败则返回false。tryRelease(int)//独占方式。尝试释放资源,成功则返回true,失败则返回false。tryAcquireShared(int)//共享方式。尝试获取资源。负数表示失败;0表示成功,但没有剩余可用资源;正数表示成功,且有剩余资源。tryReleaseShared(int)//共享方式。尝试释放资源,成功则返回true,失败则返回false。
默认情况下,每个方法都抛出 UnsupportedOperationException。 这些方法的实现必须是内部线程安全的,并且通常应该简短而不是阻塞。AQS类中的其他方法都是final ,所以无法被其他类使用,只有这几个方法可以被其他类使用。
以ReentrantLock为例,state初始化为0,表示未锁定状态。A线程lock()时,会调用tryAcquire()独占该锁并将state+1。此后,其他线程再tryAcquire()时就会失败,直到A线程unlock()到state=0(即释放锁)为止,其它线程才有机会获取该锁。当然,释放锁之前,A线程自己是可以重复获取此锁的(state会累加),这就是可重入的概念。但要注意,获取多少次就要释放多么次,这样才能保证state是能回到零态的。
AbstractQueuedSynchronizer数据结构
AbstractQueuedSynchronizer类底层的数据结构是使用CLH(Craig,Landin,and Hagersten)队列是一个虚拟的双向队列(虚拟的双向队列即不存在队列实例,仅存在结点之间的关联关系)。AQS是将每条请求共享资源的线程封装成一个CLH锁队列的一个结点(Node)来实现锁的分配。其中Sync queue,即同步队列,是双向链表,包括head结点和tail结点,head结点主要用作后续的调度。而Condition queue不是必须的,其是一个单向链表,只有当使用Condition时,才会存在此单向链表。并且可能会有多个Condition queue。
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AbstractQueuedSynchronizer源码分析类的继承关系
AbstractQueuedSynchronizer继承自AbstractOwnableSynchronizer抽象类,并且实现了Serializable接口,可以进行序列化。
public abstract class AbstractQueuedSynchronizer extends AbstractOwnableSynchronizer implements java.io.Serializable
其中AbstractOwnableSynchronizer抽象类的源码如下:
public abstract class AbstractOwnableSynchronizer implements java.io.Serializable { // 版本序列号 private static final long serialVersionUID = 3737899427754241961L; // 构造方法 protected AbstractOwnableSynchronizer() { } // 独占模式下的线程 private transient Thread exclusiveOwnerThread; // 设置独占线程 protected final void setExclusiveOwnerThread(Thread thread) { exclusiveOwnerThread = thread; } // 获取独占线程 protected final Thread getExclusiveOwnerThread() { return exclusiveOwnerThread; }}
AbstractOwnableSynchronizer抽象类中,可以设置独占资源线程和获取独占资源线程。分别为setExclusiveOwnerThread与getExclusiveOwnerThread方法,这两个方法会被子类调用。
AbstractQueuedSynchronizer类有两个内部类,分别为Node类与ConditionObject类。下面分别做介绍。
类的内部类 - Node类
static final class Node { // 模式,分为共享与独占 // 共享模式 static final Node SHARED = new Node(); // 独占模式 static final Node EXCLUSIVE = null; // 结点状态 // CANCELLED,值为1,表示当前的线程被取消 // SIGNAL,值为-1,表示当前节点的后继节点包含的线程需要运行,也就是unpark // CONDITION,值为-2,表示当前节点在等待condition,也就是在condition队列中 // PROPAGATE,值为-3,表示当前场景下后续的acquireShared能够得以执行 // 值为0,表示当前节点在sync队列中,等待着获取锁 static final int CANCELLED = 1; static final int SIGNAL = -1; static final int CONDITION = -2; static final int PROPAGATE = -3; // 结点状态 volatile int waitStatus; // 前驱结点 volatile Node prev; // 后继结点 volatile Node next; // 结点所对应的线程 volatile Thread thread; // 下一个等待者 Node nextWaiter; // 结点是否在共享模式下等待 final boolean isShared() { return nextWaiter == SHARED; } // 获取前驱结点,若前驱结点为空,抛出异常 final Node predecessor() throws NullPointerException { // 保存前驱结点 Node p = prev; if (p == null) // 前驱结点为空,抛出异常 throw new NullPointerException(); else // 前驱结点不为空,返回 return p; } // 无参构造方法 Node() { // Used to establish initial head or SHARED marker } // 构造方法 Node(Thread thread, Node mode) { // Used by addWaiter this.nextWaiter = mode; this.thread = thread; } // 构造方法 Node(Thread thread, int waitStatus) { // Used by Condition this.waitStatus = waitStatus; this.thread = thread; }}
每个线程被阻塞的线程都会被封装成一个Node结点,放入队列。每个节点包含了一个Thread类型的引用,并且每个节点都存在一个状态,具体状态如下。
CANCELLED,值为1,表示当前的线程被取消。SIGNAL,值为-1,表示当前节点的后继节点包含的线程需要运行,需要进行unpark操作。CONDITION,值为-2,表示当前节点在等待condition,也就是在condition queue中。PROPAGATE,值为-3,表示当前场景下后续的acquireShared能够得以执行。值为0,表示当前节点在sync queue中,等待着获取锁。类的内部类 - ConditionObject类
这个类有点长,耐心看下:
// 内部类public class ConditionObject implements Condition, java.io.Serializable { // 版本号 private static final long serialVersionUID = 1173984872572414699L; /** First node of condition queue. */ // condition队列的头结点 private transient Node firstWaiter; /** Last node of condition queue. */ // condition队列的尾结点 private transient Node lastWaiter; /** * Creates a new {@code ConditionObject} instance. */ // 构造方法 public ConditionObject() { } // Internal methods /** * Adds a new waiter to wait queue. * @return its new wait node */ // 添加新的waiter到wait队列 private Node addConditionWaiter() { // 保存尾结点 Node t = lastWaiter; // If lastWaiter is cancelled, clean out. if (t != null && t.waitStatus != Node.CONDITION) { // 尾结点不为空,并且尾结点的状态不为CONDITION // 清除状态为CONDITION的结点 unlinkCancelledWaiters(); // 将最后一个结点重新赋值给t t = lastWaiter; } // 新建一个结点 Node node = new Node(Thread.currentThread(), Node.CONDITION); if (t == null) // 尾结点为空 // 设置condition队列的头结点 firstWaiter = node; else // 尾结点不为空 // 设置为节点的nextWaiter域为node结点 t.nextWaiter = node; // 更新condition队列的尾结点 lastWaiter = node; return node; } /** * Removes and transfers nodes until hit non-cancelled one or * null. Split out from signal in part to encourage compilers * to inline the case of no waiters. * @param first (non-null) the first node on condition queue */ private void doSignal(Node first) { // 循环 do { if ( (firstWaiter = first.nextWaiter) == null) // 该节点的nextWaiter为空 // 设置尾结点为空 lastWaiter = null; // 设置first结点的nextWaiter域 first.nextWaiter = null; } while (!transferForSignal(first) && (first = firstWaiter) != null); // 将结点从condition队列转移到sync队列失败并且condition队列中的头结点不为空,一直循环 } /** * Removes and transfers all nodes. * @param first (non-null) the first node on condition queue */ private void doSignalAll(Node first) { // condition队列的头结点尾结点都设置为空 lastWaiter = firstWaiter = null; // 循环 do { // 获取first结点的nextWaiter域结点 Node next = first.nextWaiter; // 设置first结点的nextWaiter域为空 first.nextWaiter = null; // 将first结点从condition队列转移到sync队列 transferForSignal(first); // 重新设置first first = next; } while (first != null); } /** * Unlinks cancelled waiter nodes from condition queue. * Called only while holding lock. This is called when * cancellation occurred during condition wait, and upon * insertion of a new waiter when lastWaiter is seen to have * been cancelled. This method is needed to avoid garbage * retention in the absence of signals. So even though it may * require a full traversal, it comes into play only when * timeouts or cancellations occur in the absence of * signals. It traverses all nodes rather than stopping at a * particular target to unlink all pointers to garbage nodes * without requiring many re-traversals during cancellation * storms. */ // 从condition队列中清除状态为CANCEL的结点 private void unlinkCancelledWaiters() { // 保存condition队列头结点 Node t = firstWaiter; Node trail = null; while (t != null) { // t不为空 // 下一个结点 Node next = t.nextWaiter; if (t.waitStatus != Node.CONDITION) { // t结点的状态不为CONDTION状态 // 设置t节点的额nextWaiter域为空 t.nextWaiter = null; if (trail == null) // trail为空 // 重新设置condition队列的头结点 firstWaiter = next; else // trail不为空 // 设置trail结点的nextWaiter域为next结点 trail.nextWaiter = next; if (next == null) // next结点为空 // 设置condition队列的尾结点 lastWaiter = trail; } else // t结点的状态为CONDTION状态 // 设置trail结点 trail = t; // 设置t结点 t = next; } } // public methods /** * Moves the longest-waiting thread, if one exists, from the * wait queue for this condition to the wait queue for the * owning lock. * * @throws IllegalMonitorStateException if {@link #isHeldExclusively} * returns {@code false} */ // 唤醒一个等待线程。如果所有的线程都在等待此条件,则选择其中的一个唤醒。在从 await 返回之前,该线程必须重新获取锁。 public final void signal() { if (!isHeldExclusively()) // 不被当前线程独占,抛出异常 throw new IllegalMonitorStateException(); // 保存condition队列头结点 Node first = firstWaiter; if (first != null) // 头结点不为空 // 唤醒一个等待线程 doSignal(first); } /** * Moves all threads from the wait queue for this condition to * the wait queue for the owning lock. * * @throws IllegalMonitorStateException if {@link #isHeldExclusively} * returns {@code false} */ // 唤醒所有等待线程。如果所有的线程都在等待此条件,则唤醒所有线程。在从 await 返回之前,每个线程都必须重新获取锁。 public final void signalAll() { if (!isHeldExclusively()) // 不被当前线程独占,抛出异常 throw new IllegalMonitorStateException(); // 保存condition队列头结点 Node first = firstWaiter; if (first != null) // 头结点不为空 // 唤醒所有等待线程 doSignalAll(first); } /** * Implements uninterruptible condition wait. * <ol> * <li> Save lock state returned by {@link #getState}. * <li> Invoke {@link #release} with saved state as argument, * throwing IllegalMonitorStateException if it fails. * <li> Block until signalled. * <li> Reacquire by invoking specialized version of * {@link #acquire} with saved state as argument. * </ol> */ // 等待,当前线程在接到信号之前一直处于等待状态,不响应中断 public final void awaitUninterruptibly() { // 添加一个结点到等待队列 Node node = addConditionWaiter(); // 获取释放的状态 int savedState = fullyRelease(node); boolean interrupted = false; while (!isOnSyncQueue(node)) { // // 阻塞当前线程 LockSupport.park(this); if (Thread.interrupted()) // 当前线程被中断 // 设置interrupted状态 interrupted = true; } if (acquireQueued(node, savedState) || interrupted) // selfInterrupt(); } /* * For interruptible waits, we need to track whether to throw * InterruptedException, if interrupted while blocked on * condition, versus reinterrupt current thread, if * interrupted while blocked waiting to re-acquire. */ /** Mode meaning to reinterrupt on exit from wait */ private static final int REINTERRUPT = 1; /** Mode meaning to throw InterruptedException on exit from wait */ private static final int THROW_IE = -1; /** * Checks for interrupt, returning THROW_IE if interrupted * before signalled, REINTERRUPT if after signalled, or * 0 if not interrupted. */ private int checkInterruptWhileWaiting(Node node) { return Thread.interrupted() ? (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) : 0; } /** * Throws InterruptedException, reinterrupts current thread, or * does nothing, depending on mode. */ private void reportInterruptAfterWait(int interruptMode) throws InterruptedException { if (interruptMode == THROW_IE) throw new InterruptedException(); else if (interruptMode == REINTERRUPT) selfInterrupt(); } /** * Implements interruptible condition wait. * <ol> * <li> If current thread is interrupted, throw InterruptedException. * <li> Save lock state returned by {@link #getState}. * <li> Invoke {@link #release} with saved state as argument, * throwing IllegalMonitorStateException if it fails. * <li> Block until signalled or interrupted. * <li> Reacquire by invoking specialized version of * {@link #acquire} with saved state as argument. * <li> If interrupted while blocked in step 4, throw InterruptedException. * </ol> */ // // 等待,当前线程在接到信号或被中断之前一直处于等待状态 public final void await() throws InterruptedException { if (Thread.interrupted()) // 当前线程被中断,抛出异常 throw new InterruptedException(); // 在wait队列上添加一个结点 Node node = addConditionWaiter(); // int savedState = fullyRelease(node); int interruptMode = 0; while (!isOnSyncQueue(node)) { // 阻塞当前线程 LockSupport.park(this); if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) // 检查结点等待时的中断类型 break; } if (acquireQueued(node, savedState) && interruptMode != THROW_IE) interruptMode = REINTERRUPT; if (node.nextWaiter != null) // clean up if cancelled unlinkCancelledWaiters(); if (interruptMode != 0) reportInterruptAfterWait(interruptMode); } /** * Implements timed condition wait. * <ol> * <li> If current thread is interrupted, throw InterruptedException. * <li> Save lock state returned by {@link #getState}. * <li> Invoke {@link #release} with saved state as argument, * throwing IllegalMonitorStateException if it fails. * <li> Block until signalled, interrupted, or timed out. * <li> Reacquire by invoking specialized version of * {@link #acquire} with saved state as argument. * <li> If interrupted while blocked in step 4, throw InterruptedException. * </ol> */ // 等待,当前线程在接到信号、被中断或到达指定等待时间之前一直处于等待状态 public final long awaitNanos(long nanosTimeout) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); Node node = addConditionWaiter(); int savedState = fullyRelease(node); final long deadline = System.nanoTime() + nanosTimeout; int interruptMode = 0; while (!isOnSyncQueue(node)) { if (nanosTimeout <= 0L) { transferAfterCancelledWait(node); break; } if (nanosTimeout >= spinForTimeoutThreshold) LockSupport.parkNanos(this, nanosTimeout); if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) break; nanosTimeout = deadline - System.nanoTime(); } if (acquireQueued(node, savedState) && interruptMode != THROW_IE) interruptMode = REINTERRUPT; if (node.nextWaiter != null) unlinkCancelledWaiters(); if (interruptMode != 0) reportInterruptAfterWait(interruptMode); return deadline - System.nanoTime(); } /** * Implements absolute timed condition wait. * <ol> * <li> If current thread is interrupted, throw InterruptedException. * <li> Save lock state returned by {@link #getState}. * <li> Invoke {@link #release} with saved state as argument, * throwing IllegalMonitorStateException if it fails. * <li> Block until signalled, interrupted, or timed out. * <li> Reacquire by invoking specialized version of * {@link #acquire} with saved state as argument. * <li> If interrupted while blocked in step 4, throw InterruptedException. * <li> If timed out while blocked in step 4, return false, else true. * </ol> */ // 等待,当前线程在接到信号、被中断或到达指定最后期限之前一直处于等待状态 public final boolean awaitUntil(Date deadline) throws InterruptedException { long abstime = deadline.getTime(); if (Thread.interrupted()) throw new InterruptedException(); Node node = addConditionWaiter(); int savedState = fullyRelease(node); boolean timedout = false; int interruptMode = 0; while (!isOnSyncQueue(node)) { if (System.currentTimeMillis() > abstime) { timedout = transferAfterCancelledWait(node); break; } LockSupport.parkUntil(this, abstime); if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) break; } if (acquireQueued(node, savedState) && interruptMode != THROW_IE) interruptMode = REINTERRUPT; if (node.nextWaiter != null) unlinkCancelledWaiters(); if (interruptMode != 0) reportInterruptAfterWait(interruptMode); return !timedout; } /** * Implements timed condition wait. * <ol> * <li> If current thread is interrupted, throw InterruptedException. * <li> Save lock state returned by {@link #getState}. * <li> Invoke {@link #release} with saved state as argument, * throwing IllegalMonitorStateException if it fails. * <li> Block until signalled, interrupted, or timed out. * <li> Reacquire by invoking specialized version of * {@link #acquire} with saved state as argument. * <li> If interrupted while blocked in step 4, throw InterruptedException. * <li> If timed out while blocked in step 4, return false, else true. * </ol> */ // 等待,当前线程在接到信号、被中断或到达指定等待时间之前一直处于等待状态。此方法在行为上等效于: awaitNanos(unit.toNanos(time)) > 0 public final boolean await(long time, TimeUnit unit) throws InterruptedException { long nanosTimeout = unit.toNanos(time); if (Thread.interrupted()) throw new InterruptedException(); Node node = addConditionWaiter(); int savedState = fullyRelease(node); final long deadline = System.nanoTime() + nanosTimeout; boolean timedout = false; int interruptMode = 0; while (!isOnSyncQueue(node)) { if (nanosTimeout <= 0L) { timedout = transferAfterCancelledWait(node); break; } if (nanosTimeout >= spinForTimeoutThreshold) LockSupport.parkNanos(this, nanosTimeout); if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) break; nanosTimeout = deadline - System.nanoTime(); } if (acquireQueued(node, savedState) && interruptMode != THROW_IE) interruptMode = REINTERRUPT; if (node.nextWaiter != null) unlinkCancelledWaiters(); if (interruptMode != 0) reportInterruptAfterWait(interruptMode); return !timedout; } // support for instrumentation /** * Returns true if this condition was created by the given * synchronization object. * * @return {@code true} if owned */ final boolean isOwnedBy(AbstractQueuedSynchronizer sync) { return sync == AbstractQueuedSynchronizer.this; } /** * Queries whether any threads are waiting on this condition. * Implements {@link AbstractQueuedSynchronizer#hasWaiters(ConditionObject)}. * * @return {@code true} if there are any waiting threads * @throws IllegalMonitorStateException if {@link #isHeldExclusively} * returns {@code false} */ // 查询是否有正在等待此条件的任何线程 protected final boolean hasWaiters() { if (!isHeldExclusively()) throw new IllegalMonitorStateException(); for (Node w = firstWaiter; w != null; w = w.nextWaiter) { if (w.waitStatus == Node.CONDITION) return true; } return false; } /** * Returns an estimate of the number of threads waiting on * this condition. * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength(ConditionObject)}. * * @return the estimated number of waiting threads * @throws IllegalMonitorStateException if {@link #isHeldExclusively} * returns {@code false} */ // 返回正在等待此条件的线程数估计值 protected final int getWaitQueueLength() { if (!isHeldExclusively()) throw new IllegalMonitorStateException(); int n = 0; for (Node w = firstWaiter; w != null; w = w.nextWaiter) { if (w.waitStatus == Node.CONDITION) ++n; } return n; } /** * Returns a collection containing those threads that may be * waiting on this Condition. * Implements {@link AbstractQueuedSynchronizer#getWaitingThreads(ConditionObject)}. * * @return the collection of threads * @throws IllegalMonitorStateException if {@link #isHeldExclusively} * returns {@code false} */ // 返回包含那些可能正在等待此条件的线程集合 protected final Collection<Thread> getWaitingThreads() { if (!isHeldExclusively()) throw new IllegalMonitorStateException(); ArrayList<Thread> list = new ArrayList<Thread>(); for (Node w = firstWaiter; w != null; w = w.nextWaiter) { if (w.waitStatus == Node.CONDITION) { Thread t = w.thread; if (t != null) list.add(t); } } return list; }}
此类实现了Condition接口,Condition接口定义了条件操作规范,具体如下
public interface Condition { // 等待,当前线程在接到信号或被中断之前一直处于等待状态 void await() throws InterruptedException; // 等待,当前线程在接到信号之前一直处于等待状态,不响应中断 void awaitUninterruptibly(); //等待,当前线程在接到信号、被中断或到达指定等待时间之前一直处于等待状态 long awaitNanos(long nanosTimeout) throws InterruptedException; // 等待,当前线程在接到信号、被中断或到达指定等待时间之前一直处于等待状态。此方法在行为上等效于: awaitNanos(unit.toNanos(time)) > 0 boolean await(long time, TimeUnit unit) throws InterruptedException; // 等待,当前线程在接到信号、被中断或到达指定最后期限之前一直处于等待状态 boolean awaitUntil(Date deadline) throws InterruptedException; // 唤醒一个等待线程。如果所有的线程都在等待此条件,则选择其中的一个唤醒。在从 await 返回之前,该线程必须重新获取锁。 void signal(); // 唤醒所有等待线程。如果所有的线程都在等待此条件,则唤醒所有线程。在从 await 返回之前,每个线程都必须重新获取锁。 void signalAll();}
Condition接口中定义了await、signal方法,用来等待条件、释放条件。之后会详细分析CondtionObject的源码。
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