【JUC源码解析】Semaphore

简介

Semaphore（信号量），概念上讲，一个信号量持有一组许可(permits)。

概述

线程可调用它的acquire()方法获取一个许可，不成功则阻塞；调用release()方法来归还一个许可，前提是已经拿到过一个许可。然而，并没有什么实际的许可对象，Semaphore只是记录了一个数字，并根据这个数字管控线程。

应用

描述

有一组数据items，其实是26个大写的英文字母，有52个线程从中取数据（getItem），每个线程取一个数据，有一个线程不停地往里面放数据（putItem）直至取数据的52个线程都取到了数据。有一个信号量（Semaphore）,要求同时只能有10个线程从中取数据（取数据之前会拿一个许可，共有10许可），除非，某个线程取得的数据，又被放数据的线程放进去了，模拟取数据线程操作结束，把许可证归还，其他取数据线程才能从中拿到许可，然后取数据。

代码

  1 public class Pool {
  2     private static final int MAX_AVAILABLE = 10;
  3     private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);
  4 
  5     public String getItem() throws InterruptedException {
  6         available.acquire();
  7         return getNextAvailableItem();
  8     }
  9 
 10     public void putItem(String x) {
 11         if (markAsUnused(x))
 12             available.release();
 13     }
 14 
 15     private static void sleep(int sleep) {
 16         try {
 17             Thread.sleep(sleep);
 18         } catch (InterruptedException e) {
 19             Thread.currentThread().interrupt();
 20         }
 21     }
 22 
 23     protected boolean[] used = new boolean[MAX_AVAILABLE];
 24 
 25     protected synchronized String getNextAvailableItem() {
 26         for (int i = 0; i < MAX_AVAILABLE; ++i) {
 27             if (!used[i]) {
 28                 used[i] = true;
 29                 return items[i];
 30             }
 31         }
 32         return null;
 33     }
 34 
 35     int len = 26;
 36     protected String[] items = new String[len];
 37     {
 38         initial(0);
 39     }
 40 
 41     void initial(int sleep) {
 42         for (int i = 0; i < len; i++) {
 43             sleep(sleep);
 44             items[i] = String.valueOf((char) (i + 65));
 45             putItem(items[i]);
 46         }
 47     }
 48 
 49     protected synchronized boolean markAsUnused(String item) {
 50         for (int i = 0; i < MAX_AVAILABLE; ++i) {
 51             if (item == items[i]) {
 52                 if (used[i]) {
 53                     used[i] = false;
 54                     return true;
 55                 } else
 56                     return false;
 57             }
 58         }
 59         return false;
 60     }
 61 
 62     public static void main(String[] args) {
 63         final Pool pool = new Pool();
 64 
 65         final AtomicBoolean hasDone = new AtomicBoolean(false);
 66 
 67         ExecutorService es = Executors.newFixedThreadPool(52);
 68 
 69         new Thread(new Runnable() { // 计时器，每10毫秒记录一次
 70             @Override
 71             public void run() {
 72                 while (!hasDone.get()) {
 73                     System.out.print(".");
 74                     sleep(10);
 75                 }
 76 
 77             }
 78         }).start();
 79         new Thread(new Runnable() { // 1秒后，每20毫秒添加一个元素
 80             @Override
 81             public void run() {
 82                 sleep(1000);
 83                 while (!hasDone.get()) {
 84                     pool.initial(20);
 85                 }
 86             }
 87         }).start();
 88 
 89         final CyclicBarrier barrier = new CyclicBarrier(52, new Runnable() {
 90             @Override
 91             public void run() {
 92                 hasDone.set(true);
 93             }
 94         });
 95         for (int i = 0; i < 52; i++) {
 96             es.execute(new Runnable() {
 97                 @Override
 98                 public void run() {
 99                     try {
100                         System.out.println(Thread.currentThread() + " << " + pool.getItem());
101                         barrier.await();
102                     } catch (InterruptedException e) {
103                         Thread.currentThread().interrupt();
104                     } catch (BrokenBarrierException e) {
105                         e.printStackTrace();
106                     }
107                 }
108             });
109         }
110         es.shutdown();
111     }
112 }

输出

.Thread[pool-1-thread-1,5,main] << A
Thread[pool-1-thread-2,5,main] << B
Thread[pool-1-thread-3,5,main] << C
Thread[pool-1-thread-4,5,main] << D
Thread[pool-1-thread-5,5,main] << E
Thread[pool-1-thread-6,5,main] << F
Thread[pool-1-thread-7,5,main] << G
Thread[pool-1-thread-8,5,main] << H
Thread[pool-1-thread-10,5,main] << I
Thread[pool-1-thread-9,5,main] << J
...............................................................................................Thread[pool-1-thread-12,5,main] << A
..Thread[pool-1-thread-11,5,main] << B
..Thread[pool-1-thread-13,5,main] << C
..Thread[pool-1-thread-14,5,main] << D
..Thread[pool-1-thread-15,5,main] << E
..Thread[pool-1-thread-16,5,main] << F
..Thread[pool-1-thread-17,5,main] << G
..Thread[pool-1-thread-18,5,main] << H
..Thread[pool-1-thread-19,5,main] << I
.Thread[pool-1-thread-20,5,main] << J
...............................Thread[pool-1-thread-21,5,main] << A
..Thread[pool-1-thread-22,5,main] << B
..Thread[pool-1-thread-24,5,main] << C
..Thread[pool-1-thread-23,5,main] << D
..Thread[pool-1-thread-25,5,main] << E
..Thread[pool-1-thread-26,5,main] << F
..Thread[pool-1-thread-27,5,main] << G
..Thread[pool-1-thread-28,5,main] << H
..Thread[pool-1-thread-29,5,main] << I
..Thread[pool-1-thread-30,5,main] << J
.................................Thread[pool-1-thread-31,5,main] << A
...Thread[pool-1-thread-32,5,main] << B
.Thread[pool-1-thread-33,5,main] << C
..Thread[pool-1-thread-34,5,main] << D
..Thread[pool-1-thread-35,5,main] << E
..Thread[pool-1-thread-36,5,main] << F
..Thread[pool-1-thread-37,5,main] << G
..Thread[pool-1-thread-38,5,main] << H
..Thread[pool-1-thread-39,5,main] << I
..Thread[pool-1-thread-42,5,main] << J
.................................Thread[pool-1-thread-41,5,main] << A
..Thread[pool-1-thread-40,5,main] << B
..Thread[pool-1-thread-43,5,main] << C
..Thread[pool-1-thread-44,5,main] << D
..Thread[pool-1-thread-52,5,main] << E
..Thread[pool-1-thread-45,5,main] << F
..Thread[pool-1-thread-51,5,main] << G
..Thread[pool-1-thread-50,5,main] << H
..Thread[pool-1-thread-49,5,main] << I
..Thread[pool-1-thread-48,5,main] << J
.................................Thread[pool-1-thread-47,5,main] << A
..Thread[pool-1-thread-46,5,main] << B

从上面输出可知，每10条记录，都要等一段时间，恰好体现了信号量管控的现象

源码分析

同步器对象

1     private final Sync sync; // 同步器对象

内部类

Sync

 1     abstract static class Sync extends AbstractQueuedSynchronizer { // 内部类，继承自AQS
 2         private static final long serialVersionUID = 1192457210091910933L;
 3 
 4         Sync(int permits) { // 构造方法
 5             setState(permits); // 初始化许可的个数，对应state
 6         }
 7 
 8         final int getPermits() { // 获取当前许可的个数
 9             return getState();
10         }
11 
12         final int nonfairTryAcquireShared(int acquires) { // 获取锁，默认非公平锁
13             for (;;) {
14                 int available = getState(); // 获取当前state，可用permits个数
15                 int remaining = available - acquires; // 可用 - 获取 = 剩余
16                 if (remaining < 0 || compareAndSetState(available, remaining)) // 如果remaining小于0，直接返回；否则CAS
17                                                                                 // state为remaining，成功则返回，失败，则重新获取
18                     return remaining;
19             }
20         }
21 
22         protected final boolean tryReleaseShared(int releases) { // 释放锁
23             for (;;) {
24                 int current = getState(); // 获取当前状态
25                 int next = current + releases; // 当前值 + 释放值 = 下次可用值
26                 if (next < current) // 溢出，说明releases的值小于0
27                     throw new Error("Maximum permit count exceeded");
28                 if (compareAndSetState(current, next)) // CAS state
29                                                         // 为next，成功则返回；否则，重新获取并释放
30                     return true;
31             }
32         }
33 
34         final void reducePermits(int reductions) { // 减少许可个数
35             for (;;) {
36                 int current = getState(); // 当前可用的个数
37                 int next = current - reductions; // 剩余
38                 if (next > current) // 说明reductions小于0
39                     throw new Error("Permit count underflow");
40                 if (compareAndSetState(current, next)) // CAS state为next，成功则返回；否则，重新获取并减少
42                     return;
43             }
44         }
45 
46         final int drainPermits() { // 排空许可，即情况所有的许可
47             for (;;) {
48                 int current = getState(); // 获取当前状态
49                 if (current == 0 || compareAndSetState(current, 0)) // CAS state
50                                                                     // 为0
51                     return current;
52             }
53         }
54     }

NonfairSync

 1     static final class NonfairSync extends Sync { // 非公平
 2         private static final long serialVersionUID = -2694183684443567898L;
 3 
 4         NonfairSync(int permits) {
 5             super(permits); // 调用父类构造方法
 6         }
 7 
 8         protected int tryAcquireShared(int acquires) {
 9             return nonfairTryAcquireShared(acquires); // 直接调用父类方法
10         }
11     }

FairSync

 1     static final class FairSync extends Sync { // 公平
 2         private static final long serialVersionUID = 2014338818796000944L;
 3 
 4         FairSync(int permits) {
 5             super(permits); // 调用父类构造方法
 6         }
 7 
 8         protected int tryAcquireShared(int acquires) {
 9             for (;;) {
10                 if (hasQueuedPredecessors()) // 如果等待队列里有元素，直接返回失败，目的是让线程入队等待，保证公平性
11                     return -1;
12                 int available = getState(); // 后面同非公平逻辑
13                 int remaining = available - acquires;
14                 if (remaining < 0 || compareAndSetState(available, remaining))
15                     return remaining;
16             }
17         }
18     }

构造方法

1     public Semaphore(int permits) {
2         sync = new NonfairSync(permits); // 默认非公平
3     }
4 
5     public Semaphore(int permits, boolean fair) { // 根据fair参数选择公平或非公平
6         sync = fair ? new FairSync(permits) : new NonfairSync(permits);
7     }

其它方法

 1     public void acquire() throws InterruptedException {
 2         sync.acquireSharedInterruptibly(1); // 获取许可，响应中断
 3     }
 4 
 5     public void acquireUninterruptibly() {
 6         sync.acquireShared(1); // 不响应中断
 7     }
 8 
 9     public boolean tryAcquire() {
10         return sync.nonfairTryAcquireShared(1) >= 0; // false: < 0   true: > 0
11     }
12 
13     public boolean tryAcquire(long timeout, TimeUnit unit) throws InterruptedException {
14         return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); // 支持超时
15     }
16 
17     public void release() {
18         sync.releaseShared(1); // 释放许可
19     }
20 
21     public void acquire(int permits) throws InterruptedException { // 获取多个
22         if (permits < 0)
23             throw new IllegalArgumentException();
24         sync.acquireSharedInterruptibly(permits); // 响应中断
25     }
26 
27     public void acquireUninterruptibly(int permits) { // 不响应中断
28         if (permits < 0)
29             throw new IllegalArgumentException();
30         sync.acquireShared(permits);
31     }
32 
33     public boolean tryAcquire(int permits) { //  false: < 0   true: > 0
34         if (permits < 0)
35             throw new IllegalArgumentException();
36         return sync.nonfairTryAcquireShared(permits) >= 0;
37     }
38 
39     public boolean tryAcquire(int permits, long timeout, TimeUnit unit) throws InterruptedException {
40         if (permits < 0)
41             throw new IllegalArgumentException();
42         return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout)); // 支持超时
43     }
44 
45     public void release(int permits) { // 释放多个
46         if (permits < 0)
47             throw new IllegalArgumentException();
48         sync.releaseShared(permits);
49     }
50 
51     public int availablePermits() { // 可用的许可个数
52         return sync.getPermits();
53     }
54 
55     public int drainPermits() { // 排空
56         return sync.drainPermits();
57     }
58 
59     protected void reducePermits(int reduction) { // 减少
60         if (reduction < 0)
61             throw new IllegalArgumentException();
62         sync.reducePermits(reduction);
63     }
64 
65     public boolean isFair() { // 是否是公平的
66         return sync instanceof FairSync;
67     }
68 
69     public final boolean hasQueuedThreads() {
70         return sync.hasQueuedThreads(); // 队列里是否还有等待的线程
71     }
72 
73     public final int getQueueLength() {
74         return sync.getQueueLength(); // 队列长度，等待线程的个数
75     }
76 
77     protected Collection<Thread> getQueuedThreads() {
78         return sync.getQueuedThreads(); // 获取在对列里等待的线程
79     }
80 
81     public String toString() { // toString()方法
82         return super.toString() + "[Permits = " + sync.getPermits() + "]";
83     }

源码比较简单。

行文至此结束。

尊重他人的劳动，转载请注明出处：http://www.cnblogs.com/aniao/p/aniao_semaphore.html

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原文地址：https://www.cnblogs.com/aniao/p/aniao_semaphore.html