Java并发——Semaphore

悠悠 2021-08-31 02:41 472阅读 0赞

### Semaphore ###

Semaphore即信号、信号系统。此类的主要作用就是限制线程并发的数量，如果不限制线程并发的数量，则CPU的资源很快就被耗尽，每个线程执行的任务是相当缓慢，因为CPU要把时间片分配给不同的线程对象，而且上下文切换也要耗时，最终造成系统运行效率大幅降低，所以限制并发线程的数量还是非常有必要的。

#### Semaphore的同步性 ####

public class Service { 
        private Semaphore semaphore = new Semaphore(1);
    
        public void testMethod() { 
            try { 
                semaphore.acquire();
                System.out.println(Thread.currentThread().getName() + "begin：" + System.currentTimeMillis());
                Thread.sleep(2000);
                System.out.println(Thread.currentThread().getName() + "end:" + System.currentTimeMillis());
                semaphore.release();
            } catch (InterruptedException e) { 
                e.printStackTrace();
            }
        }
    
        public static void main(String[] args) { 
            Service service = new Service();
            ThreadA a = new ThreadA(service);
            a.setName("A");
            a.start();
            ThreadB b = new ThreadB(service);
            b.setName("B");
            b.start();
            ThreadC c = new ThreadC(service);
            c.setName("C");
            c.start();
        }
    }
    
    class ThreadA extends Thread { 
        private Service service;
        public ThreadA(Service service) { 
            super();
            this.service = service;
        }
        @Override
        public void run() { 
            service.testMethod();
        }
    }
    class ThreadB extends Thread { 
        private Service service;
        public ThreadB(Service service) { 
            super();
            this.service = service;
        }
        @Override
        public void run() { 
            service.testMethod();
        }
    }
    class ThreadC extends Thread { 
        private Service service;
        public ThreadC(Service service) { 
            super();
            this.service = service;
        }
        @Override
        public void run() { 
            service.testMethod();
        }
    }

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NvbGRfX19wbGF5_size_16_color_FFFFFF_t_70]  
类Semaphore的构造函数参数permits是许可的意思，代表同一时间内，最多允许多少个线程同时执行acquire()和release()之间的代码。无参方法acquire(的作用是使用1个许可，是减法操作。

其实还可以传人>1的许可，代表同-时间内，最多允许有x个线程可以执行acquire()和release()之间的代码。

acquire(int permits)：  
每调用一次方法，就使用permits个许可。

release(int permits)：  
每调用一次方法，就增加permits个许可，可以动态增加许可的个数：

Semaphore semaphore = new Semaphore(5);
            semaphore.acquire();
            semaphore.acquire();
            semaphore.acquire();
            semaphore.acquire();
            semaphore.acquire();
            System.out.println(semaphore.availablePermits());
            semaphore.release();
            semaphore.release();
            semaphore.release();
            semaphore.release();
            semaphore.release();
            semaphore.release();
            System.out.println(semaphore.availablePermits());
            semaphore.release(4);
            System.out.println(semaphore.availablePermits());

![在这里插入图片描述][20200621235057360.png]

acquireUninterruptibly()：  
使等待进入acquire()方法的线程，不允许被中断。

public class Service2 { 
        private Semaphore semaphore = new Semaphore(1);
    
        public void test() { 
            try { 
                semaphore.acquire();
                System.out.println(Thread.currentThread().getName() + "begin:" + System.currentTimeMillis());
                for (int i = 0; i < Integer.MAX_VALUE / 100; i++) { 
                    String str = new String();
                    Math.random();
                }
                System.out.println(Thread.currentThread().getName() + "end:" + System.currentTimeMillis());
                semaphore.release();
            } catch (InterruptedException e) { 
                System.out.println("线程" + Thread.currentThread().getName() + "进入了Catch");
                e.printStackTrace();
            }
        }
    
        public static void main(String[] args) throws InterruptedException { 
            Service2 service2 = new Service2();
            ThreadA a = new ThreadA(service2);
            a.setName("A");
            a.start();
    
            ThreadB b = new ThreadB(service2);
            b.setName("B");
            b.start();
    
            b.interrupt();
            System.out.println("main");
    
        }
    }
    
    class ThreadA extends Thread { 
        private Service2 service;
        public ThreadA(Service2 service) { 
            super();
            this.service = service;
        }
    
        @Override
        public void run() { 
            service.test();
        }
    }
    class ThreadB extends Thread { 
        private Service2 service;
        public ThreadB(Service2 service) { 
            super();
            this.service = service;
        }
    
        @Override
        public void run() { 
            service.test();
        }
    }

线程B成功被中断。  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NvbGRfX19wbGF5_size_16_color_FFFFFF_t_70 1]  
修改acquire方法：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NvbGRfX19wbGF5_size_16_color_FFFFFF_t_70 2]  
再次运行效果如下：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NvbGRfX19wbGF5_size_16_color_FFFFFF_t_70 3]  
acquireUninterruptibly()方法还有重载的写法`acquireUninterruptibly(int permits)`，此方法的作用是在等待许可的情况下不允许中断，如果成功获得锁，则取得指定的permits许可个数。

availablePermits()和drainPermits()方法：

`availablePermits()`返回此Semaphore对象中当前可用的许可数，此方法通常用于调试，因为许可的数量有可能实时在改变，并不是固定的数量。

`drainPermits()`可获取并返回立即可用的所有许可个数，并且将可用许可置0。

Semaphore semaphore = new Semaphore(10);
            semaphore.acquire();
            System.out.println(semaphore.drainPermits() + " " + semaphore.availablePermits());
            System.out.println(semaphore.drainPermits() + " " + semaphore.availablePermits());
            System.out.println(semaphore.drainPermits() + " " + semaphore.availablePermits());
            semaphore.release();

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NvbGRfX19wbGF5_size_16_color_FFFFFF_t_70 4]

getQueueLength()和hasQueuedThreads()：

方法`getQueueLength()`的作用是取得等待许可的线程个数。

方法`hasQueuedThreads()`的作用是判断有没有线程在等待这个许可。

这两个方法通常都是在判断当前有没有等待许可的线程信息时使用。

public class MyService { 
        private Semaphore semaphore = new Semaphore(1);
    
        public void test() { 
            try { 
                semaphore.acquire();
                Thread.sleep(1000);
                System.out.println("还有大约" + semaphore.getQueueLength() + "个线程在等待");
                System.out.println("是否有线程正在等待信号量？" + semaphore.hasQueuedThreads());
            } catch (InterruptedException e) { 
                e.printStackTrace();
            } finally { 
                semaphore.release();
            }
        }
    
        public static void main(String[] args) { 
            MyService myService = new MyService();
    
            MyThread firstThread = new MyThread(myService);
            firstThread.start();
    
            MyThread[] arr = new MyThread[4];
            for (int  i = 0; i < 4; i++) { 
                arr[i] = new MyThread(myService);
                arr[i].start();
            }
        }
    }

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NvbGRfX19wbGF5_size_16_color_FFFFFF_t_70 5]

公平与非公平信号量：

有些时候，获得许可的顺序与线程启动的顺序有关，这时信号量就要分为公平与非公平的。所谓的公平信号量是获得锁的顺序与线程启动的顺序有关，但不代表100%地获得信号量，仅仅是在概率上能得到保证。而非公平信号量就是无关的了。

public Semaphore(int permits, boolean fair) { 
            sync = fair ? new FairSync(permits) : new NonfairSync(permits);
        }

构造中传入true，即为公平信号量；反之为非公平。公平信号量运行的效果是线程启动的顺序与调用`semaphore.acquire()`的顺序有关，也就是先启动的线程优先获得许可。

tryAcquire()：  
无参方法tryAcquire()的作用是尝试地获得1个许可，如果获取不到则返回false,此方法通常与if语句结合使用，其具有无阻塞的特点。无阻塞的特点可以使线程不至于在同步处一直持续等待的状态，如果if语句判断不成立则线程会继续走else语句，程序会继续向下运行。

tryAcquire(int permits):  
有参方法tryAcquire(int permits)的作用是尝试地获得permits个许可，如果获取不到则返回false。

tryAcquire(long timeout, TimeUnit unit)：  
有参方法tryAcquire(int long timeout,TimeUnit unit)的作用是在指定的时间内尝试地获得1个许可，如果获取不到则返回false。

tryAcquire(int permits, long timeout, TimeUnit unit)：  
有参方法tryAcquire(int permits,long timeout,TimeUnit unit)的作用是在指定的时间内尝试地获得permits个许可，如果获取不到则返回false。

#### 使用Semaphore实现多生产者/多消费者模式 ####

public class RepastService { 
    
        volatile private Semaphore setSemaphore = new Semaphore(10);//10个厨师
        volatile private Semaphore getSemaphore = new Semaphore(20);//食客
        volatile private ReentrantLock lock = new ReentrantLock();
        volatile private Condition setCondition = lock.newCondition();
        volatile private Condition getCondition = lock.newCondition();
    
        //只有3个盘子放菜品
        volatile private Object[] producePosition = new Object[4];
    
        private boolean isEmpty() { 
            boolean isEmty = true;
            for (int i = 0; i < producePosition.length; i++) { 
                if (producePosition[i] != null) { 
                    isEmty = false;
                    break;
                }
            }
            if (isEmty == true) { 
                return true;
            } else { 
                return false;
            }
        }
    
        private boolean isFull() { 
            boolean isFull = true;
            for (int i = 0; i < producePosition.length; i++) { 
                if (producePosition[i] == null) { 
                    isFull = false;
                    break;
                }
            }
            return isFull;
        }
    
        public void set() { 
            try { 
                setSemaphore.acquire();//允许同时最多有10个厨师炒菜
                lock.lock();
                while (isFull()) { 
                    setCondition.await();
                }
                for (int i = 0; i < producePosition.length; i++) { 
                    if (producePosition[i] == null) { 
                        producePosition[i] = "菜";
                        System.out.println(Thread.currentThread().getName() + "生产了" + producePosition[i]);
                        break;
                    }
                }
                getCondition.signalAll();
                lock.unlock();
            } catch (InterruptedException e) { 
                e.printStackTrace();
            } finally { 
                setSemaphore.release();
            }
        }
    
        public void get() { 
            try { 
                getSemaphore.acquire();//允许同时最多20个食客
                lock.lock();
                while (isEmpty()) { 
                    getCondition.await();
                }
                for (int i = 0; i < producePosition.length; i++) { 
                    if (producePosition[i] != null) { 
                        System.out.println(Thread.currentThread().getName() + "吃了" + producePosition[i]);
                        producePosition[i] = null;
                        break;
                    }
                }
                setCondition.signalAll();
                lock.unlock();
            } catch (InterruptedException e) { 
                e.printStackTrace();
            } finally { 
                getSemaphore.release();
            }
        }
    
        public static void main(String[] args) throws InterruptedException { 
            RepastService service = new RepastService();
            ThreadP[] arrP = new ThreadP[60];
            ThreadC[] arrC = new ThreadC[60];
            for (int i = 0; i < 60; i++) { 
                arrP[i] = new ThreadP(service);
                arrC[i] = new ThreadC(service);
            }
            Thread.sleep(2000);
            for (int i = 0; i < 60; i++) { 
                arrP[i].start();
                arrC[i].start();
            }
        }
    }
    
    //厨师
    class ThreadP extends Thread { 
        private RepastService service;
    
        public ThreadP(RepastService service) { 
            this.service = service;
        }
    
        @Override
        public void run() { 
            service.set();
        }
    }
    
    //食客
    class ThreadC extends Thread { 
        private RepastService service;
    
        public ThreadC(RepastService service) { 
            this.service = service;
        }
    
        @Override
        public void run() { 
            service.get();
        }
    }

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