//对象池
public class Pool<T> {    private int size;    private List<T> items = new ArrayList<T>();    private volatile boolean[] checkedOut;    private Semaphore available;    public Pool(Class<T> classObject,int size) {        this.size = size;        this.checkedOut = new boolean[size];        this.available = new Semaphore(size, true);        for (int i = 0; i < size; i++) {            try {                items.add(classObject.newInstance());            } catch (Exception e) {                throw new RuntimeException(e);            }        }    }    public T checkOut() throws InterruptedException{        available.acquire();        return getItem();    }    public void checkIn(T x){        if(releaseItem(x)){            available.release();        }    }            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;    }}

//签出任务
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;    }        @Override    public void run() {        try {            T item = pool.checkOut();            System.out.println(this + "checked out " + item);            TimeUnit.SECONDS.sleep(1);            System.out.println(this + "checked in " + item);            pool.checkIn(item);        } catch (InterruptedException e) {            e.printStackTrace();        }    }    @Override    public String toString() {        return "CheckoutTask " + id + " ";    }}public class SemaphoreDemo {    final static int SIZE = 25;    public static void main(String[] args) throws InterruptedException {        final Pool<Fat> pool = new Pool<Fat>(Fat.class, SIZE);        ExecutorService exec = Executors.newCachedThreadPool();        for (int i = 0; i < SIZE; i++) {            exec.execute(new CheckoutTask<Fat>(pool));        }        System.out.println("All CheckoutTask created");        List<Fat> list = new ArrayList<Fat>();        System.out.println("kifjdskjfjdk");        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() {            @Override            public void run() {                try {                    pool.checkOut();                } catch (InterruptedException e) {                    System.out.println("checkedOut() Interrupted");                }            }        });        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();    }    }

public class Fat {    private volatile double d;    private static int counter = 0;    private final int id = counter++;    public Fat() {        for (int i = 0; i <10000; i++) {            d +=(Math.PI + Math.E) / i;        }    }    public void operation(){        System.out.println(this);    }    @Override    public String toString() {        return "Fat id: " + id;    }}