Java并发-类库新组件 - Semaphore 理解：计数信号量

Pool.java 类

package com.xyw.concurrent.blog;import java.util.*;import java.util.concurrent.*;/* * 对象池概念.管理和回收对象，可以签出和签入对象的概念 */public class Pool<T> {private int size;private List<T> items = new ArrayList<T>();private volatile boolean[] checkedOut;private Semaphore available; // 计数信号量 允许n个任务同时访问这个资源public Pool(Class<T> classObject, int size){this.size = size;checkedOut = new boolean[size]; // 用来表示某个任务是否被签出的情况available = new Semaphore(size, true);for(int i = 0; i < size; ++i){try{items.add(classObject.newInstance()); // 现在向其中添加对象}catch(Exception e){throw new RuntimeException(e);}}}public T checkOut() throws InterruptedException{available.acquire();return getItem(); // 对象签出对象池，这个是重新获得一个可用的对象从对象池中获得}public void checkIn(T x){if(releaseItem(x)){available.release(); // 对象签入对象池，用于回收对象池// 而 semaphore 的真正起到的作用 ，只是 利用并发，进行技术信号量}}private synchronized T getItem(){for(int i =0; i< size; ++i){if(!checkedOut[i]){checkedOut[i] = true;return items.get(i); }}return null;}private synchronized boolean releaseItem(T item){int index = items.indexOf(item);if (index == -1) return false;if (checkedOut[index]){checkedOut[index] = false;return true;}return false; // 这个表示，没有被签出的情况} }

Fat.java 类

package com.xyw.concurrent.blog;public class Fat {private volatile double  d;private static int counter = 0;private final int id = counter++;public Fat(){for(int i = 1; i < 10000; i++){d  += (Math.PI + Math.E) / (double) i;}}public void operation(){System.out.println(this);}public String toString(){return "Fat id:" + id;}} // 这是个创建代价高昂的对象类型，其构造器运行起来很是费事

SemaphoreDemo.java

package com.xyw.concurrent.blog;import java.util.*;import java.util.concurrent.*;class CheckoutTask<T> implements Runnable{private static int counter = 0;private final int id = counter++;private Pool<T> pool;public CheckoutTask(Pool<T> pool){this.pool = pool; // 创建的对象池，用于回收线程和产生线程}public void run(){try{T item = pool.checkOut(); // 签出一个线程System.out.println(this +"checked out" + item); // 产生出一个线程TimeUnit.SECONDS.sleep(1); // 产生一个对象，并运行一段时间System.out.println(this + "checking in " + item);pool.checkIn(item); // 回收对象，到对象池里面}catch(InterruptedException e){}}public String toString(){return "CheckedOutTask" + id + " ";}}public class SemaphoreDemo {final static int SIZE = 25;public static void main(String[] args) throws Exception{final Pool<Fat> pool = new Pool<Fat>(Fat.class,SIZE); // 创建对象池对象ExecutorService exec = Executors.newCachedThreadPool();for(int i = 0; i < SIZE; i++){exec.execute(new CheckoutTask<Fat>(pool));}System.out.println("All CheckoutTasks created");List<Fat> list = new ArrayList<Fat>();for(int i = 0; i< SIZE; i++){Fat f = pool.checkOut();System.out.println(i + " main() thread checked out");f.operation();list.add(f);}Future<?> blocked = exec.submit(new Runnable(){public void run(){try{pool.checkOut();}catch(InterruptedException e){System.out.println("checkOut() Interrupted"); }}}); // 一旦池中对象都被签出，Semaphore 将不再允许执行任何签出操作 block 的方法被Pool忽略。TimeUnit.SECONDS.sleep(2);blocked.cancel(true);System.out.println("checking in objects in" + list);for(Fat f : list)pool.checkIn(f);for(Fat f: list)pool.checkIn(f);exec.shutdown();}}

运行结果：

CheckedOutTask0 checked outFat id:0

CheckedOutTask2 checked outFat id:2

CheckedOutTask1 checked outFat id:1

CheckedOutTask3 checked outFat id:3

CheckedOutTask4 checked outFat id:4

CheckedOutTask5 checked outFat id:5

CheckedOutTask6 checked outFat id:6

CheckedOutTask7 checked outFat id:7

CheckedOutTask8 checked outFat id:8

CheckedOutTask9 checked outFat id:9

CheckedOutTask10 checked outFat id:10

CheckedOutTask11 checked outFat id:11

CheckedOutTask12 checked outFat id:12

CheckedOutTask13 checked outFat id:13

CheckedOutTask14 checked outFat id:14

CheckedOutTask15 checked outFat id:15

CheckedOutTask16 checked outFat id:16

CheckedOutTask17 checked outFat id:17

CheckedOutTask18 checked outFat id:18

CheckedOutTask19 checked outFat id:19

CheckedOutTask20 checked outFat id:20

CheckedOutTask21 checked outFat id:21

CheckedOutTask22 checked outFat id:22

All CheckoutTasks created

CheckedOutTask23 checked outFat id:23

0 main() thread checked out

Fat id:24

CheckedOutTask2 checking in Fat id:2

CheckedOutTask5 checking in Fat id:5

CheckedOutTask24 checked outFat id:2

CheckedOutTask4 checking in Fat id:4

CheckedOutTask7 checking in Fat id:7

CheckedOutTask3 checking in Fat id:3

CheckedOutTask1 checking in Fat id:1

CheckedOutTask0 checking in Fat id:0

CheckedOutTask9 checking in Fat id:9

CheckedOutTask8 checking in Fat id:8

1 main() thread checked out

Fat id:5

CheckedOutTask11 checking in Fat id:11

CheckedOutTask6 checking in Fat id:6

2 main() thread checked out

Fat id:0

CheckedOutTask10 checking in Fat id:10

3 main() thread checked out

CheckedOutTask13 checking in Fat id:13

CheckedOutTask12 checking in Fat id:12

CheckedOutTask14 checking in Fat id:14

Fat id:1

4 main() thread checked out

Fat id:3

5 main() thread checked out

Fat id:4

6 main() thread checked out

Fat id:6

7 main() thread checked out

Fat id:7

8 main() thread checked out

Fat id:8

CheckedOutTask15 checking in Fat id:15

CheckedOutTask16 checking in Fat id:16

CheckedOutTask21 checking in Fat id:21

CheckedOutTask20 checking in Fat id:20

CheckedOutTask19 checking in Fat id:19

CheckedOutTask22 checking in Fat id:22

CheckedOutTask18 checking in Fat id:18

CheckedOutTask17 checking in Fat id:17

9 main() thread checked out

CheckedOutTask23 checking in Fat id:23

Fat id:9

10 main() thread checked out

Fat id:10

11 main() thread checked out

Fat id:11

12 main() thread checked out

Fat id:12

13 main() thread checked out

Fat id:13

14 main() thread checked out

Fat id:14

15 main() thread checked out

Fat id:15

16 main() thread checked out

Fat id:16

17 main() thread checked out

Fat id:17

18 main() thread checked out

Fat id:18

19 main() thread checked out

Fat id:19

20 main() thread checked out

Fat id:20

21 main() thread checked out

Fat id:21

22 main() thread checked out

Fat id:22

23 main() thread checked out

Fat id:23

CheckedOutTask24 checking in Fat id:2

24 main() thread checked out

Fat id:2

checkOut() Interrupted

checking in objects in[Fat id:24, Fat id:5, Fat id:0, Fat id:1, Fat id:3, Fat id:4, Fat id:6, Fat id:7, Fat id:8, Fat id:9, Fat id:10, Fat id:11, Fat id:12, Fat id:13, Fat id:14, Fat id:15, Fat id:16, Fat id:17, Fat id:18, Fat id:19, Fat id:20, Fat id:21, Fat id:22, Fat id:23, Fat id:2]