一、定义
是JUC包下的一个工具类，我们可以通过其限制执行的线程数量，达到限流的效果。
当一个线程执行时先通过其方法进行获取许可操作，获取到许可的线程继续执行业务逻辑，当线程执行完成后进行释放许可操作，未获取达到许可的线程进行等待或者直接结束。
可以把它简单的理解成我们停车场入口立着的那个显示屏，每有一辆车进入停车场显示屏就会显示剩余车位减1，每有一辆车从停车场出去，显示屏上显示的剩余车辆就会加1，当显示屏上的剩余车位为0时，停车场入口的栏杆就不会再打开，车辆就无法进入停车场了，直到有一辆车从停车场出去为止。
二、方法
构造方法
/**
* 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);
}
表示许可线程的数量
/**
* 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);
}
fair表示公平性，如果设置为true，表示公平，那么等待最久的线程先执行
/**
* 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);
}
表示线程获取1个许可，那么线程许可数量相应减少一个，获取一个令牌，在获取到令牌、或者被其他线程调用中断之前线程一直处于阻塞状态。
/**
* 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);
}
表示释放一个许可，那么线程许可数量相应增加，释放一个令牌，唤醒一个获取令牌不成功的阻塞线程。
/**
* Releases the given number of permits, returning them to the semaphore.
*
* <p>Releases the given number of permits, increasing the number of
* available permits by that amount.
* If any threads are trying to acquire permits, then one
* is selected and given the permits that were just released.
* If the number of available permits satisfies that thread's request
* then that thread is (re)enabled for thread scheduling purposes;
* otherwise the thread will wait until sufficient permits are available.
* If there are still permits available
* after this thread's request has been satisfied, then those permits
* are assigned in turn to other threads trying to acquire permits.
*
* <p>There is no requirement that a thread that releases a permit must
* have acquired that permit by calling {@link Semaphore#acquire acquire}.
* Correct usage of a semaphore is established by programming convention
* in the application.
*
* @param permits the number of permits to release
* @throws IllegalArgumentException if {@code permits} is negative
*/
public void release(int permits) {
if (permits < 0) throw new IllegalArgumentException();
sync.releaseShared(permits);
}
表示一个线程释放n个许可，这个数量有参数permits决定，获取一个令牌，在获取到令牌、或者被其他线程调用中断、或超时之前线程一直处于阻塞状态。
/**
* Acquires the given number of permits from this semaphore,
* blocking until all are available,
* or the thread is {@linkplain Thread#interrupt interrupted}.
*
* <p>Acquires the given number of permits, if they are available,
* and returns immediately, reducing the number of available permits
* by the given amount.
*
* <p>If insufficient permits are 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 one of the {@link #release() release}
* methods for this semaphore, the current thread is next to be assigned
* permits and the number of available permits satisfies this request; 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.
* Any permits that were to be assigned to this thread are instead
* assigned to other threads trying to acquire permits, as if
* permits had been made available by a call to {@link #release()}.
*
* @param permits the number of permits to acquire
* @throws InterruptedException if the current thread is interrupted
* @throws IllegalArgumentException if {@code permits} is negative
*/
public void acquire(int permits) throws InterruptedException {
if (permits < 0) throw new IllegalArgumentException();
sync.acquireSharedInterruptibly(permits);
}
表示一个线程获取n个许可，这个数量有参数permits决定
/**
* Returns the current number of permits available in this semaphore.
*
* <p>This method is typically used for debugging and testing purposes.
*
* @return the number of permits available in this semaphore
*/
public int availablePermits() {
return sync.getPermits();
}
返回当前信号量线程许可数量
/**
* Returns an estimate of the number of threads waiting to acquire.
* The value is only an estimate because the number of threads may
* change dynamically while this method traverses internal data
* structures. This method is designed for use in monitoring of the
* system state, not for synchronization control.
*
* @return the estimated number of threads waiting for this lock
*/
public final int getQueueLength() {
return sync.getQueueLength();
}
返回等待获取许可的线程数的预估值
/**
* Acquires a permit from this semaphore, blocking until one is
* available.
*
* <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
* some other thread invokes the {@link #release} method for this
* semaphore and the current thread is next to be assigned a permit.
*
* <p>If the current thread is {@linkplain Thread#interrupt interrupted}
* while waiting for a permit then it will continue to wait, but the
* time at which the thread is assigned a permit may change compared to
* the time it would have received the permit had no interruption
* occurred. When the thread does return from this method its interrupt
* status will be set.
*/
public void acquireUninterruptibly() {
sync.acquireShared(1);
}
获取一个令牌，在获取到令牌之前线程一直处于阻塞状态（忽略中断）
/**
* Acquires a permit from this semaphore, only if one is available at the
* time of invocation.
*
* <p>Acquires a permit, if one is available and returns immediately,
* with the value {@code true},
* reducing the number of available permits by one.
*
* <p>If no permit is available then this method will return
* immediately with the value {@code false}.
*
* <p>Even when this semaphore has been set to use a
* fair ordering policy, a call to {@code tryAcquire()} <em>will</em>
* immediately acquire a permit if one is available, whether or not
* other threads are currently waiting.
* This "barging" behavior can be useful in certain
* circumstances, even though it breaks fairness. If you want to honor
* the fairness setting, then use
* {@link #tryAcquire(long, TimeUnit) tryAcquire(0, TimeUnit.SECONDS) }
* which is almost equivalent (it also detects interruption).
*
* @return {@code true} if a permit was acquired and {@code false}
* otherwise
*/
public boolean tryAcquire() {
return sync.nonfairTryAcquireShared(1) >= 0;
}
尝试获得令牌，返回获取令牌成功或失败，不阻塞线程。
/**
* Acquires a permit from this semaphore, if one becomes available
* within the given waiting time and the current thread has not
* been {@linkplain Thread#interrupt interrupted}.
*
* <p>Acquires a permit, if one is available and returns immediately,
* with the value {@code true},
* 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 three 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; or
* <li>The specified waiting time elapses.
* </ul>
*
* <p>If a permit is acquired then the value {@code true} is returned.
*
* <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
* to acquire a permit,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* <p>If the specified waiting time elapses then the value {@code false}
* is returned. If the time is less than or equal to zero, the method
* will not wait at all.
*
* @param timeout the maximum time to wait for a permit
* @param unit the time unit of the {@code timeout} argument
* @return {@code true} if a permit was acquired and {@code false}
* if the waiting time elapsed before a permit was acquired
* @throws InterruptedException if the current thread is interrupted
*/
public boolean tryAcquire(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
尝试获得令牌，在超时时间内循环尝试获取，直到尝试获取成功或超时返回，不阻塞线程。
/**
* Queries whether any threads are waiting to acquire. Note that
* because cancellations may occur at any time, a {@code true}
* return does not guarantee that any other thread will ever
* acquire. This method is designed primarily for use in
* monitoring of the system state.
*
* @return {@code true} if there may be other threads waiting to
* acquire the lock
*/
public final boolean hasQueuedThreads() {
return sync.hasQueuedThreads();
}
等待队列里是否还存在等待线程。
/**
* Acquires and returns all permits that are immediately available.
*
* @return the number of permits acquired
*/
public int drainPermits() {
return sync.drainPermits();
}
清空令牌把可用令牌数置为0，返回清空令牌的数量。
三、使用Semaphore实现停车场指示牌功能
每个停车场入口都有一个提示牌，上面显示着停车场的剩余车位还有多少，当剩余车位为0时，不允许车辆进入停车场，直到停车场里面有车离开停车场，这时提示牌上会显示新的剩余车位数。
业务场景 ：
1、停车场容纳总停车量20。
2、当一辆车进入停车场后，显示牌的剩余车位数响应的减1.
3、每有一辆车驶出停车场后，显示牌的剩余车位数响应的加1。
4、停车场剩余车位不足时，车辆只能在外面等待。
实现代码
package com.util;
import lombok.extern.slf4j.Slf4j;
import java.util.Random;
import java.util.concurrent.Semaphore;
import static java.lang.Thread.sleep;
/**
* @author : lssffy
* @Description : 线程的信号量控制
* @date : 2023/12/17 21:32
*/
@Slf4j
public class SemaphoreTest {
public static void main(String[] args) {
//创建Semaphore对象数量20
Semaphore sp = new Semaphore(20);
//100个线程同时运行
for (int i = 0; i < 8; i++) {
new Thread(new Runnable() {
@Override
public void run() {
try{
System.out.println("===" + Thread.currentThread().getName()+"来到停车场");
if(sp.availablePermits() == 0){
System.out.println("车位不足，请耐心等待");
}
sp.acquire();//获取令牌尝试进入停车场
System.out.println(Thread.currentThread().getName() + "成功进入停车场");
Thread.sleep(new Random().nextInt(10000));//模拟车辆在停车场停留的时间
System.out.println(Thread.currentThread().getName() + "驶出停车场");
sp.release();//释放令牌，腾出停车场车位
}catch (InterruptedException e){
e.printStackTrace();
}
}
},i+"号车").start();
}
}
}
执行结果
Semaphore实现原理
Semaphore初始化
Semaphore sp = new Semaphore(20);
1、当调用new Semaphore(20)方式时，默认会创建一个非公平的锁的同步阻塞队列
2、把初始化令牌数量赋值给同步队列的state状态，state的值代表当前所剩余的令牌数量
获取令牌
sp.acquire();
1、当前线程会尝试去同步队列获取一个令牌，获取令牌的过程就是使用原子的操作去修改同步队列的state，获取一个令牌则修改为state=state-1
2、当计算出来的state<0，则代表令牌数量不足，此时会创建一个Node节点加入阻塞队列，挂起当前线程
3、当计算出来的state>=0，则表示获取令牌成功
/**
* 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())
throw new InterruptedException();
//尝试获取令牌，arg为获取令牌个数，当可用令牌数量减去当前令牌数量结果小于0，则创建一个节点加入阻塞队列，挂起当前线程
if (tryAcquireShared(arg) < 0)
doAcquireSharedInterruptibly(arg);
}
/**
* 1、创建节点，加入阻塞队列
* 2、重双向链表的head、tail节点关系，清空无效节点
* 3、挂起当前节点线程
* Acquires in shared interruptible mode.
* @param arg the acquire argument
*/
private void doAcquireSharedInterruptibly(int arg)
throws InterruptedException {
//创建节点加入阻塞队列
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
for (;;) {
//获得当前节点pre节点
final Node p = node.predecessor();
if (p == head) {
//返回锁的state
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return;
}
}
//重组双向链表，清空无效节点，挂起当前线程
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
释放令牌
sp.release();
当调用sp.release();方法时
1、线程会尝试释放一个令牌，释放令牌的过程就是把同步队列的state修改为state=state+1的过程
2、释放令牌成功之后，同时唤醒同步队列的一个线程
3、被唤醒的节点会重新尝试去修改state=state-1的操作，如果state>=0则获取令牌成功，否则重新进入阻塞队列，挂起线程
/**
* Releases in shared mode. Implemented by unblocking one or more
* threads if {@link #tryReleaseShared} returns true.
* 释放共享锁，同时会唤醒同步队列的一个线程
* @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) {
//释放共享锁
if (tryReleaseShared(arg)) {
//唤醒所有共享节点线程
doReleaseShared();
return true;
}
return false;
}
/**
* 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.)
* 唤醒同步队列中的一个线程
*/
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 (;;) {
Node h = head;
if (h != null && h != tail) {
int ws = h.waitStatus;
//是否需要唤醒后续节点
if (ws == Node.SIGNAL) {
//修改状态为初始0
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
continue; // loop to recheck cases
unparkSuccessor(h);//唤醒h.nex节点线程
}
else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
continue; // loop on failed CAS
}
if (h == head) // loop if head changed
break;
}
}
