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Semaphore 是一個計數信號量，它的本質是一個共享鎖。信號量維護了一個信號量許可集。線程可以通過調用acquire()來獲取信號量的許可；當信號量中有可用的許可時，線程能獲取該許可；否則線程必須等待，直到有可用的許可為止。 線程可以通過release()來釋放它所持有的信號量許可（用完信號量之后必須釋放，不然其他線程可能會無法獲取信號量）。
簡單示例：
package me.socketthread;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
public class SemaphoreLearn {
//信號量總數
private static final int SEM_MAX = 12;
public static void main(String[] args) {
Semaphore sem = new Semaphore(SEM_MAX);
//創建線程池
ExecutorService threadPool = Executors.newFixedThreadPool(3);
//在線程池中執行任務
threadPool.execute(new MyThread(sem, 7));
threadPool.execute(new MyThread(sem, 4));
threadPool.execute(new MyThread(sem, 2));
//關閉池
threadPool.shutdown();
}
}
class MyThread extends Thread {
private volatile Semaphore sem; // 信號量
private int count; // 申請信號量的大小
MyThread(Semaphore sem, int count) {
this.sem = sem;
this.count = count;
}
public void run() {
try {
// 從信號量中獲取count個許可
sem.acquire(count);
Thread.sleep(2000);
System.out.println(Thread.currentThread().getName() + " acquire count="+count);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
// 釋放給定數目的許可，將其返回到信號量。
sem.release(count);
System.out.println(Thread.currentThread().getName() + " release " + count + "");
}
}
}執行結果：
pool-1-thread-2 acquire count=4 pool-1-thread-1 acquire count=7 pool-1-thread-1 release 7 pool-1-thread-2 release 4 pool-1-thread-3 acquire count=2 pool-1-thread-3 release 2
線程1和線程2會并發執行，因為兩者的信號量和沒有超過總信號量，當前兩個線程釋放掉信號量之后線程3才能繼續執行。
源碼分析：
1、構造函數
在構造函數中會初始化信號量值，這值最終是作為鎖標志位state的值
Semaphore sem = new Semaphore(12);//簡單來說就是給鎖標識位state賦值為12
2、Semaphore.acquire(n);簡單理解為獲取鎖資源，如果獲取不到線程阻塞
Semaphore.acquire(n);//從鎖標識位state中獲取n個信號量，簡單來說是state = state-n 此時state大于0表示可以獲取信號量，如果小于0則將線程阻塞
public void acquire(int permits) throws InterruptedException {
if (permits < 0) throw new IllegalArgumentException();
//獲取鎖
sync.acquireSharedInterruptibly(permits);
}acquireSharedInterruptibly中的操作是獲取鎖資源，如果可以獲取則將state= state-permits，否則將線程阻塞
public final void acquireSharedInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (tryAcquireShared(arg) < 0)//tryAcquireShared中嘗試獲取鎖資源
doAcquireSharedInterruptibly(arg); //將線程阻塞
}tryAcquireShared中的操作是嘗試獲取信號量值，簡單來說就是state=state-acquires ，如果此時小于0則返回負值，否則返回大于新值，再判斷是否將當線程線程阻塞
protected int tryAcquireShared(int acquires) {
for (;;) {
if (hasQueuedPredecessors())
return -1;
//獲取state值
int available = getState();
//從state中獲取信號量
int remaining = available - acquires;
if (remaining < 0 ||
compareAndSetState(available, remaining))
//如果信號量小于0則直接返回，表示無法獲取信號量，否則將state值修改為新值
return remaining;
}
}doAcquireSharedInterruptibly中的操作簡單來說是將當前線程添加到FIFO隊列中并將當前線程阻塞。
/會將線程添加到FIFO隊列中，并阻塞
private void doAcquireSharedInterruptibly(int arg)
throws InterruptedException {
//將線程添加到FIFO隊列中
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head) {
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return;
}
}
//parkAndCheckInterrupt完成線程的阻塞操作
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}3、Semaphore.release(int permits)，這個函數的實現操作是將state = state+permits并喚起處于FIFO隊列中的阻塞線程。
public void release(int permits) {
if (permits < 0) throw new IllegalArgumentException();
//state = state+permits，并將FIFO隊列中的阻塞線程喚起
sync.releaseShared(permits);
}releaseShared中的操作是將state = state+permits，并將FIFO隊列中的阻塞線程喚起。
public final boolean releaseShared(int arg) {
//tryReleaseShared將state設置為state = state+arg
if (tryReleaseShared(arg)) {
//喚起FIFO隊列中的阻塞線程
doReleaseShared();
return true;
}
return false;
}tryReleaseShared將state設置為state = state+arg
protected final boolean tryReleaseShared(int releases) {
for (;;) {
int current = getState();
int next = current + releases;
if (next < current) // overflow
throw new Error("Maximum permit count exceeded");
//將state值設置為state=state+releases
if (compareAndSetState(current, next))
return true;
}
}doReleaseShared()喚起FIFO隊列中的阻塞線程
private void doReleaseShared() {
for (;;) {
Node h = head;
if (h != null && h != tail) {
int ws = h.waitStatus;
if (ws == Node.SIGNAL) {
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
continue; // loop to recheck cases
//完成阻塞線程的喚起操作
unparkSuccessor(h);
}
else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
continue; // loop on failed CAS
}
if (h == head) // loop if head changed
break;
}
}總結：Semaphore簡單來說設置了一個信號量池state，當線程執行時會從state中獲取值，如果可以獲取則線程執行，并且在執行后將獲取的資源返回到信號量池中，并喚起其他阻塞線程；如果信號量池中的資源無法滿足某個線程的需求則將此線程阻塞。
Semaphore源碼：
public class Semaphore implements java.io.Serializable {
private static final long serialVersionUID = -3222578661600680210L;
private final Sync sync;
abstract static class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 1192457210091910933L;
//設置鎖標識位state的初始值
Sync(int permits) {
setState(permits);
}
//獲取鎖標識位state的值，如果state值大于其需要的值則表示鎖可以獲取
final int getPermits() {
return getState();
}
//獲取state值減去acquires后的值，如果大于等于0則表示鎖可以獲取
final int nonfairTryAcquireShared(int acquires) {
for (;;) {
int available = getState();
int remaining = available - acquires;
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}
//釋放鎖
protected final boolean tryReleaseShared(int releases) {
for (;;) {
int current = getState();
//將state值加上release值
int next = current + releases;
if (next < current) // overflow
throw new Error("Maximum permit count exceeded");
if (compareAndSetState(current, next))
return true;
}
}
//將state的值減去reductions
final void reducePermits(int reductions) {
for (;;) {
int current = getState();
int next = current - reductions;
if (next > current) // underflow
throw 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;
}
}
}
//非公平鎖
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);
}
}
//公平鎖
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())
return -1;
int available = getState();
int remaining = available - acquires;
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}
}
//設置信號量
public Semaphore(int permits) {
sync = new NonfairSync(permits);
}
public Semaphore(int permits, boolean fair) {
sync = fair ? new FairSync(permits) : new NonfairSync(permits);
}
//獲取鎖
public void acquire() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
public void acquireUninterruptibly() {
sync.acquireShared(1);
}
public boolean tryAcquire() {
return sync.nonfairTryAcquireShared(1) >= 0;
}
public boolean tryAcquire(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
public void release() {
sync.releaseShared(1);
}
//獲取permits值鎖
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);
}
public boolean tryAcquire(int permits) {
if (permits < 0) throw new IllegalArgumentException();
return sync.nonfairTryAcquireShared(permits) >= 0;
}
public boolean tryAcquire(int permits, long timeout, TimeUnit unit)
throws InterruptedException {
if (permits < 0) throw new IllegalArgumentException();
return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));
}
//釋放
public void release(int permits) {
if (permits < 0) throw new IllegalArgumentException();
sync.releaseShared(permits);
}
public int availablePermits() {
return sync.getPermits();
}
public int drainPermits() {
return sync.drainPermits();
}
protected void reducePermits(int reduction) {
if (reduction < 0) throw new IllegalArgumentException();
sync.reducePermits(reduction);
}
public boolean isFair() {
return sync instanceof FairSync;
}
public final boolean hasQueuedThreads() {
return sync.hasQueuedThreads();
}
public final int getQueueLength() {
return sync.getQueueLength();
}
protected Collection<Thread> getQueuedThreads() {
return sync.getQueuedThreads();
}
public String toString() {
return super.toString() + "[Permits = " + sync.getPermits() + "]";
}
}以上是“Java并發編程中Semaphore計數信號量的示例分析”這篇文章的所有內容，感謝各位的閱讀！相信大家都有了一定的了解，希望分享的內容對大家有所幫助，如果還想學習更多知識，歡迎關注億速云行業資訊頻道！
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