简单认识Semaphore
何为Semaphore?
- Semaphore顾名思义,叫信号量;
- Semaphore可用来控制同时访问特定资源的线程数量,以此来达到协调线程工作;
- Semaphore内部也有公平锁、非公平锁的静态内部类,就像ReentrantLock一样,Semaphore内部基本上是通过sync.xxx之类的这种调用方式的;
- Semaphore内部维护了一个虚拟的资源池,如果许可为0则线程阻塞等待,直到许可大于0时又可以有机会获取许可了;
Semaphore的state关键词
- 其实Semaphore的实现也恰恰很好利用了其父类AQS的state变量值;
- 初始化一个数量值作为许可池的资源,假设为N,那么当任何线程获取到资源时,许可减1,直到许可为0时后续来的线程就需要等待;
- Semaphore,简单大致意思为:A、B、C、D线程同时争抢资源,目前卡槽大小为2,若A、B正在执行且未执行完,那么C、D线程在门外等着,一旦A、B有1个执行完了,那么C、D就会竞争看谁先执行;state初始值假设为N,后续每tryAcquire()一次,state会CAS减1,当state为0时其它线程处于等待状态,直到state>0且<N后,进程又可以获取到锁进行各自操作了;
常用重要的方法
public Semaphore(int permits) // 创建一个给定许可数量的信号量对象,且默认以非公平锁方式获取资源 public Semaphore(int permits, boolean fair) // 创建一个给定许可数量的信号量对象,且是否公平方式由传入的fair布尔参数值决定 public void acquire() // 从此信号量获取一个许可,当许可数量小于零时,则阻塞等待 public void acquire(int permits) // 从此信号量获取permits个许可,当许可数量小于零时,则阻塞等待,但是当阻塞等待的线程被唤醒后发现被中断过的话则会抛InterruptedException异常 public void acquireUninterruptibly(int permits) // 从此信号量获取permits个许可,当许可数量小于零时,则阻塞等待,但是当阻塞等待的线程被唤醒后发现被中断过的话不会抛InterruptedException异常 public void release() // 释放一个许可 public void acquire(int permits) // 释放permits个许可 ### 设计与实现伪代码 #### 获取共享锁: ```java public final void acquireSharedInterruptibly(int arg) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); if (tryAcquireShared(arg) < 0) doAcquireSharedInterruptibly(arg); } 复制代码
acquire{ 如果检测中断状态发现被中断过的话,那么则抛出InterruptedException异常 如果尝试获取共享锁失败的话( 尝试获取共享锁的各种方式由AQS的子类实现 ), 那么就新增共享锁结点通过自旋操作加入到队列中,并且根据结点中的waitStatus来决定是否调用LockSupport.park进行休息 } 复制代码
释放共享锁:
public final boolean releaseShared(int arg) { if (tryReleaseShared(arg)) { doReleaseShared(); return true; } return false; } release{ 如果尝试释放共享锁失败的话( 尝试释放共享锁的各种方式由AQS的子类实现 ), 那么通过自旋操作唤完成阻塞线程的唤起操作 } 复制代码
Semaphore生活细节化理解
比如我们天天在外面吃快餐,我就以吃快餐为例生活化阐述该Semaphore原理:
- 1、场景:餐厅只有一个排队的走廊,只有十个打饭窗口;
- 2、开饭时间点,刚开始的时候,人数不多,屈指可数,窗口很多,打饭菜自然很快,但随着时间的推移人数会越来越多,会呈现阻塞拥挤状况,排起了慢慢长队;
- 3、人数越来越多,但窗口只有十个,后来的就只好按先来后到排队等待打饭菜咯,前面窗口空缺一个,排队最前的一个则上去打饭菜,秩序有条不紊;
- 4、总之大家都挨个挨个排队打饭,先来后到,相安无事的顺序打饭菜;
- 5、到此打止,1、2、3、4可以认为是一种公平方式的信号量共享锁;
- 6、但是呢,还有那么些紧急赶时间的人,来餐厅时刚好看到师傅刚刚打完一个人的饭菜,于是插入去打饭菜敢时间;
- 7、如果敢时间人的来的时候发现师傅还在打饭菜,那么就只得乖乖的排队等候打饭菜咯;
- 8、到此打止,1、2、6、7可以认为是一种非公平方式的信号量共享锁;
源码分析Semaphore
Semaphore构造器
构造器源码:
/** * Creates a {@code Semaphore} with the given number of * permits and nonfair fairness setting. * * @param permits the initial number of permits available. * This value may be negative, in which case releases * must occur before any acquires will be granted. */ public Semaphore(int permits) { sync = new NonfairSync(permits); } /** * Creates a {@code Semaphore} with the given number of * permits and the given fairness setting. * * @param permits the initial number of permits available. * This value may be negative, in which case releases * must occur before any acquires will be granted. * @param fair {@code true} if this semaphore will guarantee * first-in first-out granting of permits under contention, * else {@code false} */ public Semaphore(int permits, boolean fair) { sync = fair ? new FairSync(permits) : new NonfairSync(permits); } 复制代码
创建一个给定许可数量的信号量对象,默认使用非公平锁,当然也可通过fair布尔参数值决定是公平锁还是非公平锁;
Sync同步器
1、AQS --> Sync ---> FairSync // 公平锁 | |> NonfairSync // 非公平锁
2、Semaphore内的同步器都是通过Sync抽象接口来操作调用关系的,细看会发现基本上都是通过sync.xxx之类的这种调用方式的;
acquire()
1、源码:
/** * Acquires a permit from this semaphore, blocking until one is * available, or the thread is {@linkplain Thread#interrupt interrupted}. * * <p>Acquires a permit, if one is available and returns immediately, * reducing the number of available permits by one. * * <p>If no permit is available then the current thread becomes * disabled for thread scheduling purposes and lies dormant until * one of two things happens: * <ul> * <li>Some other thread invokes the {@link #release} method for this * semaphore and the current thread is next to be assigned a permit; or * <li>Some other thread {@linkplain Thread#interrupt interrupts} * the current thread. * </ul> * * <p>If the current thread: * <ul> * <li>has its interrupted status set on entry to this method; or * <li>is {@linkplain Thread#interrupt interrupted} while waiting * for a permit, * </ul> * then {@link InterruptedException} is thrown and the current thread's * interrupted status is cleared. * * @throws InterruptedException if the current thread is interrupted */ public void acquire() throws InterruptedException { sync.acquireSharedInterruptibly(1); // 调用父类AQS中的获取共享锁资源的方法 } 复制代码
acquire是信号量获取共享资源的入口,尝试获取锁资源,获取到了则立马返回并跳出该方法,没有获取到则该方法阻塞等待; 其主要也是调用sync的父类AQS的acquireSharedInterruptibly方法;
acquireSharedInterruptibly(int)
1、源码:
/** * Acquires in shared mode, aborting if interrupted. Implemented * by first checking interrupt status, then invoking at least once * {@link #tryAcquireShared}, returning on success. Otherwise the * thread is queued, possibly repeatedly blocking and unblocking, * invoking {@link #tryAcquireShared} until success or the thread * is interrupted. * @param arg the acquire argument. * This value is conveyed to {@link #tryAcquireShared} but is * otherwise uninterpreted and can represent anything * you like. * @throws InterruptedException if the current thread is interrupted */ public final void acquireSharedInterruptibly(int arg) throws InterruptedException { if (Thread.interrupted()) // 调用之前先检测该线程中断标志位,检测该线程在之前是否被中断过 throw new InterruptedException(); // 若被中断过的话,则抛出中断异常 if (tryAcquireShared(arg) < 0) // 尝试获取共享资源锁,小于0则获取失败,此方法由AQS的具体子类实现 doAcquireSharedInterruptibly(arg); // 将尝试获取锁资源的线程进行入队操作 } 复制代码
2、acquireSharedInterruptibly是共享模式下线程获取锁资源的基类方法,每当线程获取到一次共享资源,则共享资源数值就会做减法操作,直到共享资源值小于0时,则线程阻塞进入队列等待;
3、而且该线程支持中断,也正如方法名称所意,当该方法检测到中断后则立马会抛出中断异常,让调用该方法的地方立马感知线程中断情况;
tryAcquireShared(int)
1、公平锁tryAcquireShared源码:
// FairSync 公平锁的 tryAcquireShared 方法 protected int tryAcquireShared(int acquires) { for (;;) { // 自旋的死循环操作方式 if (hasQueuedPredecessors()) // 检查线程是否有阻塞队列 return -1; // 如果有阻塞队列,说明共享资源的许可数量已经用完,返回-1乖乖进行入队操作 int available = getState(); // 获取锁资源的最新内存值 int remaining = available - acquires; // 计算得到剩下的许可数量 if (remaining < 0 || // 若剩下的许可数量小于0,说明已经共享资源了,返回负数然后乖乖进入入队操作 compareAndSetState(available, remaining)) // 若共享资源大于或等于0,防止并发则通过CAS操作占据最后一个共享资源 return remaining; // 不管得到remaining后进入了何种逻辑,操作了之后再将remaining返回,上层会根据remaining的值进行判断是否需要入队操作 } } 复制代码
2、非公平锁tryAcquireShared源码:
// NonfairSync 非公平锁的 tryAcquireShared 方法 protected int tryAcquireShared(int acquires) { return nonfairTryAcquireShared(acquires); // } // NonfairSync 非公平锁父类 Sync 类的 nonfairTryAcquireShared 方法 final int nonfairTryAcquireShared(int acquires) { for (;;) { // 自旋的死循环操作方式 int available = getState(); // 获取锁资源的最新内存值 int remaining = available - acquires; // 计算得到剩下的许可数量 if (remaining < 0 || // 若剩下的许可数量小于0,说明已经共享资源了,返回负数然后乖乖进入入队操作 compareAndSetState(available, remaining)) // 若共享资源大于或等于0,防止并发则通过CAS操作占据最后一个共享资源 return remaining; // 不管得到remaining后进入了何种逻辑,操作了之后再将remaining返回,上层会根据remaining的值进行判断是否需要入队操作 } } 复制代码
3、tryAcquireShared法是AQS的子类实现的,也就是Semaphore的两个静态内部类实现的,目的就是通过CAS尝试获取共享锁资源, 获取共享锁资源成功大于或等于0的自然数,获取共享锁资源失败则返回负数;
doAcquireSharedInterruptibly(int)
1、源码:
/** * Acquires in shared interruptible mode. * @param arg the acquire argument */ private void doAcquireSharedInterruptibly(int arg) throws InterruptedException { // 按照给定的mode模式创建新的结点,模式有两种:Node.EXCLUSIVE独占模式、Node.SHARED共享模式; final Node node = addWaiter(Node.SHARED); // 创建共享模式的结点 boolean failed = true; try { for (;;) { // 自旋的死循环操作方式 final Node p = node.predecessor(); // 获取结点的前驱结点 if (p == head) { // 若前驱结点为head的话,那么说明当前结点自然不用说了,仅次于老大之后的便是老二了咯 int r = tryAcquireShared(arg); // 而且老二也希望尝试去获取一下锁,万一头结点恰巧刚刚释放呢?希望还是要有的,万一实现了呢。。。 if (r >= 0) { // 若r>=0,说明已经成功的获取到了共享锁资源 setHeadAndPropagate(node, r); // 把当前node结点设置为头结点,并且调用doReleaseShared释放一下无用的结点 p.next = null; // help GC failed = false; return; } } if (shouldParkAfterFailedAcquire(p, node) && // 根据前驱结点看看是否需要休息一会儿 parkAndCheckInterrupt()) // 阻塞操作,正常情况下,获取不到共享锁,代码就在该方法停止了,直到被唤醒 // 被唤醒后,发现parkAndCheckInterrupt()里面检测了被中断了的话,则补上中断异常,因此抛了个异常 throw new InterruptedException(); } } finally { if (failed) cancelAcquire(node); } } 复制代码
2、doAcquireSharedInterruptibly也是采用一个自旋的死循环操作方式,直到正常返回或者被唤醒抛出中断异常为止;
release()
1、源码:
/** * Releases a permit, returning it to the semaphore. * * <p>Releases a permit, increasing the number of available permits by * one. If any threads are trying to acquire a permit, then one is * selected and given the permit that was just released. That thread * is (re)enabled for thread scheduling purposes. * * <p>There is no requirement that a thread that releases a permit must * have acquired that permit by calling {@link #acquire}. * Correct usage of a semaphore is established by programming convention * in the application. */ public void release() { sync.releaseShared(1); // 释放一个许可资源 } 复制代码
2、该方法是调用其父类AQS的一个释放共享资源的基类方法;
releaseShared(int)
1、源码:
/** * Releases in shared mode. Implemented by unblocking one or more * threads if {@link #tryReleaseShared} returns true. * * @param arg the release argument. This value is conveyed to * {@link #tryReleaseShared} but is otherwise uninterpreted * and can represent anything you like. * @return the value returned from {@link #tryReleaseShared} */ public final boolean releaseShared(int arg) { if (tryReleaseShared(arg)) { // 尝试释放共享锁资源,此方法由AQS的具体子类实现 doReleaseShared(); // 自旋操作,唤醒后继结点 return true; } return false; } 复制代码
2、releaseShared主要是进行共享锁资源释放,如果释放成功则唤醒队列等待的结点,如果失败则返回false,由上层调用方决定如何处理;
tryReleaseShared(int)
1、源码:
// NonfairSync 和 FairSync 的父类 Sync 类的 tryReleaseShared 方法 protected final boolean tryReleaseShared(int releases) { for (;;) { // 自旋的死循环操作方式 int current = getState(); // 获取最新的共享锁资源值 int next = current + releases; // 对许可数量进行加法操作 // int类型值小于0,是因为该int类型的state状态值溢出了,溢出了的话那得说明这个锁有多难释放啊,可能出问题了 if (next < current) // overflow throw new Error("Maximum permit count exceeded"); if (compareAndSetState(current, next)) // return true; // 返回成功标志,告诉上层该线程已经释放了共享锁资源 } } 复制代码
2、tryReleaseShared主要通过CAS操作对state锁资源进行加法操作,腾出多余的共享锁资源供其它线程竞争;
doReleaseShared()
1、源码:
/** * Release action for shared mode -- signals successor and ensures * propagation. (Note: For exclusive mode, release just amounts * to calling unparkSuccessor of head if it needs signal.) */ private void doReleaseShared() { /* * Ensure that a release propagates, even if there are other * in-progress acquires/releases. This proceeds in the usual * way of trying to unparkSuccessor of head if it needs * signal. But if it does not, status is set to PROPAGATE to * ensure that upon release, propagation continues. * Additionally, we must loop in case a new node is added * while we are doing this. Also, unlike other uses of * unparkSuccessor, we need to know if CAS to reset status * fails, if so rechecking. */ for (;;) { // 自旋的死循环操作方式 Node h = head; // 每次都是取出队列的头结点 if (h != null && h != tail) { // 若头结点不为空且也不是队尾结点 int ws = h.waitStatus; // 那么则获取头结点的waitStatus状态值 if (ws == Node.SIGNAL) { // 若头结点是SIGNAL状态则意味着头结点的后继结点需要被唤醒了 // 通过CAS尝试设置头结点的状态为空状态,失败的话,则继续循环,因为并发有可能其它地方也在进行释放操作 if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0)) continue; // loop to recheck cases unparkSuccessor(h); // 唤醒头结点的后继结点 } // 如头结点为空状态,则把其改为PROPAGATE状态,失败的则可能是因为并发而被改动过,则再次循环处理 else if (ws == 0 && !compareAndSetWaitStatus(h, 0, Node.PROPAGATE)) continue; // loop on failed CAS } // 若头结点没有发生什么变化,则说明上述设置已经完成,大功告成,功成身退 // 若发生了变化,可能是操作过程中头结点有了新增或者啥的,那么则必须进行重试,以保证唤醒动作可以延续传递 if (h == head) // loop if head changed break; } } 复制代码
2、doReleaseShared主要是释放共享许可,但是最重要的目的还是保证唤醒后继结点的传递,来让这些线程释放他们所持有的信号量;
总结
1、在分析了AQS之后,再来分析Semaphore是不是变得比较简单了;
2、在这里我简要总结一下Semaphore的流程的一些特性:
- • 管理一系列许可证,即state共享资源值;
- • 每acquire一次则state就减1一次,直到许可证数量小于0则阻塞等待;
- • 释放许可的时候要保证唤醒后继结点,以此来保证线程释放他们所持有的信号量;
- • 是Synchronized的升级版,因为Synchronized是只有一个许可,而Semaphore就像开了挂一样,可以有多个许可;