信號量Semaphore的介紹

我們以一個(gè)停車場運(yùn)作為例來說明信號量的作用。假設(shè)停車場只有三個(gè)車位，一開始三個(gè)車位都是空的。這時(shí)如果同時(shí)來了三輛車，看門人允許其中它們進(jìn)入進(jìn)入，然后放下車攔。以后來的車必須在入口等待，直到停車場中有車輛離開。這時(shí)，如果有一輛車離開停車場，看門人得知后，打開車攔，放入一輛，如果又離開一輛，則又可以放入一輛，如此往復(fù)。

在這個(gè)停車場系統(tǒng)中，車位是公共資源，每輛車好比一個(gè)線程，看門人起的就是信號量的作用。信號量是一個(gè)非負(fù)整數(shù)，表示了當(dāng)前公共資源的可用數(shù)目（在上面的例子中可以用空閑的停車位類比信號量），當(dāng)一個(gè)線程要使用公共資源時(shí)（在上面的例子中可以用車輛類比線程），首先要查看信號量，如果信號量的值大于1，則將其減1，然后去占有公共資源。如果信號量的值為0，則線程會將自己阻塞，直到有其它線程釋放公共資源。

在信號量上我們定義兩種操作： acquire（獲取） 和 release（釋放）。當(dāng)一個(gè)線程調(diào)用acquire操作時(shí)，它要么通過成功獲取信號量（信號量減1），要么一直等下去，直到有線程釋放信號量，或超時(shí)。release（釋放）實(shí)際上會將信號量的值加1，然后喚醒等待的線程。
信號量主要用于兩個(gè)目的，一個(gè)是用于多個(gè)共享資源的互斥使用，另一個(gè)用于并發(fā)線程數(shù)的控制。

示例

Semaphore包含公平鎖與非公平鎖2種實(shí)現(xiàn)

非公平鎖

package main.java.study; import java.util.Calendar; import java.util.Date; import java.util.concurrent.*;public class SemaphoreDemo {final static Semaphore sema = new Semaphore(5);public static class Runner implements Runnable{int i;public Runner(int i){this.i = i;}@Overridepublic void run() {try{sema.acquire();Thread.sleep(1000);System.out.println( "Runner【" + i + "】:" + new Date().toString() + " done!");}catch ( InterruptedException ex){ex.printStackTrace();}sema.release();}}public static void main (String[] args){ExecutorService service = Executors.newFixedThreadPool(10);for(int i = 0 ;i < 20;i++){service.execute(new Runner(i));}} }

結(jié)果如下：

Runner【2】:Sat May 25 14:27:55 CST 2019 done!
Runner【0】:Sat May 25 14:27:55 CST 2019 done!
Runner【4】:Sat May 25 14:27:55 CST 2019 done!
Runner【3】:Sat May 25 14:27:55 CST 2019 done!
Runner【1】:Sat May 25 14:27:55 CST 2019 done!
Runner【10】:Sat May 25 14:27:56 CST 2019 done!
Runner【12】:Sat May 25 14:27:56 CST 2019 done!
Runner【11】:Sat May 25 14:27:56 CST 2019 done!
Runner【13】:Sat May 25 14:27:56 CST 2019 done!
Runner【14】:Sat May 25 14:27:56 CST 2019 done!
Runner【5】:Sat May 25 14:27:57 CST 2019 done!
Runner【15】:Sat May 25 14:27:57 CST 2019 done!
Runner【7】:Sat May 25 14:27:57 CST 2019 done!
Runner【18】:Sat May 25 14:27:57 CST 2019 done!
Runner【19】:Sat May 25 14:27:57 CST 2019 done!
Runner【9】:Sat May 25 14:27:58 CST 2019 done!
Runner【6】:Sat May 25 14:27:58 CST 2019 done!
Runner【16】:Sat May 25 14:27:58 CST 2019 done!
Runner【17】:Sat May 25 14:27:58 CST 2019 done!
Runner【8】:Sat May 25 14:27:58 CST 2019 done!

?公平鎖

package main.java.study; import java.util.Calendar; import java.util.Date; import java.util.concurrent.*;public class SemaphoreDemo {final static Semaphore sema = new Semaphore(5,true);public static class Runner implements Runnable{int i;public Runner(int i){this.i = i;}@Overridepublic void run() {try{sema.acquire();Thread.sleep(1000);System.out.println( "Runner【" + i + "】:" + new Date().toString() + " done!");}catch ( InterruptedException ex){ex.printStackTrace();}sema.release();}}public static void main (String[] args){ExecutorService service = Executors.newFixedThreadPool(10);for(int i = 0 ;i < 20;i++){service.execute(new Runner(i));}} }

結(jié)果如下：

Runner【2】:Sat May 25 14:33:49 CST 2019 done!
Runner【1】:Sat May 25 14:33:49 CST 2019 done!
Runner【3】:Sat May 25 14:33:49 CST 2019 done!
Runner【0】:Sat May 25 14:33:49 CST 2019 done!
Runner【4】:Sat May 25 14:33:49 CST 2019 done!
Runner【5】:Sat May 25 14:33:51 CST 2019 done!
Runner【6】:Sat May 25 14:33:51 CST 2019 done!
Runner【8】:Sat May 25 14:33:51 CST 2019 done!
Runner【9】:Sat May 25 14:33:51 CST 2019 done!
Runner【7】:Sat May 25 14:33:51 CST 2019 done!
Runner【10】:Sat May 25 14:33:53 CST 2019 done!
Runner【12】:Sat May 25 14:33:53 CST 2019 done!
Runner【11】:Sat May 25 14:33:53 CST 2019 done!
Runner【13】:Sat May 25 14:33:53 CST 2019 done!
Runner【14】:Sat May 25 14:33:53 CST 2019 done!
Runner【15】:Sat May 25 14:33:55 CST 2019 done!
Runner【17】:Sat May 25 14:33:55 CST 2019 done!
Runner【19】:Sat May 25 14:33:55 CST 2019 done!
Runner【16】:Sat May 25 14:33:55 CST 2019 done!
Runner【18】:Sat May 25 14:33:55 CST 2019 done!

源碼

Semaphore：

public Semaphore(int permits) {sync = new NonfairSync(permits);}public Semaphore(int permits, boolean fair) {sync = fair ? new FairSync(permits) : new NonfairSync(permits);}public boolean tryAcquire() {return sync.nonfairTryAcquireShared(1) >= 0;}public boolean tryAcquire(int permits) {if (permits < 0) throw new IllegalArgumentException();return sync.nonfairTryAcquireShared(permits) >= 0;}public void acquire() throws InterruptedException {sync.acquireSharedInterruptibly(1);}public void acquireUninterruptibly() {sync.acquireShared(1);}public void acquire(int permits) throws InterruptedException {if (permits < 0) throw new IllegalArgumentException();sync.acquireSharedInterruptibly(permits);}public void acquireUninterruptibly(int permits) {if (permits < 0) throw new IllegalArgumentException();sync.acquireShared(permits);}//釋放1個(gè)許可public void release() {sync.releaseShared(1);}//釋放指定數(shù)量許可public void release(int permits) {if (permits < 0) throw new IllegalArgumentException();sync.releaseShared(permits);}//減少許可protected void reducePermits(int reduction) {if (reduction < 0) throw new IllegalArgumentException();sync.reducePermits(reduction);}//清空許可public int drainPermits() {return sync.drainPermits();}

Semaphore可以：

• acqure 1個(gè)或者多個(gè)許可
• release 1個(gè)或者多個(gè)許可
• 默認(rèn)是可中斷acquire
• 默認(rèn)是非公平鎖

Sync：

abstract static class Sync extends AbstractQueuedSynchronizer {private static final long serialVersionUID = 1192457210091910933L;Sync(int permits) {setState(permits);}final int getPermits() {return getState();}final int nonfairTryAcquireShared(int acquires) {for (;;) {int available = getState();int remaining = available - acquires;if (remaining < 0 ||compareAndSetState(available, remaining)) 【1】return remaining;}}protected final boolean tryReleaseShared(int releases) {for (;;) {int current = getState();int next = current + releases;if (next < current) // overflowthrow new Error("Maximum permit count exceeded");if (compareAndSetState(current, next))return true;}}final void reducePermits(int reductions) {for (;;) {int current = getState();int next = current - reductions;if (next > current) // underflowthrow new Error("Permit count underflow");if (compareAndSetState(current, next))return;}}final int drainPermits() {for (;;) {int current = getState();if (current == 0 || compareAndSetState(current, 0))return current;}}}

Fair和NoFair

static final class NonfairSync extends Sync {private static final long serialVersionUID = -2694183684443567898L;NonfairSync(int permits) {super(permits);}protected int tryAcquireShared(int acquires) {return nonfairTryAcquireShared(acquires);}}/*** Fair version*/static final class FairSync extends Sync {private static final long serialVersionUID = 2014338818796000944L;FairSync(int permits) {super(permits);}protected int tryAcquireShared(int acquires) {for (;;) {if (hasQueuedPredecessors()) //【2】return -1;int available = getState();int remaining = available - acquires;if (remaining < 0 ||compareAndSetState(available, remaining))return remaining;}}} public final boolean hasQueuedPredecessors() {// The correctness of this depends on head being initialized// before tail and on head.next being accurate if the current// thread is first in queue.Node t = tail; // Read fields in reverse initialization orderNode h = head;Node s;//隊(duì)列為空，或者h(yuǎn)ead節(jié)點(diǎn)后置為null，表示本線程未在等待隊(duì)列return h != t &&((s = h.next) == null //head節(jié)點(diǎn)的后置節(jié)點(diǎn)為null|| s.thread != Thread.currentThread() //或者后置節(jié)點(diǎn)關(guān)聯(lián)線程不為本線程);}

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比較：

• 非公平鎖，只要許可滿足就acquire成功（見【1】）
• 公平鎖，僅當(dāng)本線程在隊(duì)列中沒有前置節(jié)點(diǎn)時(shí)，才acquire成功（見【2】）

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AbstractQueuedSynchronizer源碼參考：https://blog.csdn.net/demon7552003/article/details/90105335

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