线程入门-临界区

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import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * Created by Administrator on 2017/9/13.
 */

class Pair {
    //not thread-safe
    private int x, y;

    public Pair(int x, int y) {
        this.x = x;
        this.y = y;
    }

    public Pair() {
        this(0, 0);
    }

    public int getX() {
        return x;
    }

    public int getY() {
        return y;
    }

    public void incrementX() {
        x++;
    }

    public void incrementY() {
        y++;
    }

    public String toString() {
        return "x:" + x + ",y:" + y;
    }


    // RuntimeException 运行时异常。是所有Java虚拟机正常操作期间可以被抛出的异常的父类
    public class PairValueNotEqualException extends RuntimeException {
        public PairValueNotEqualException() {
            super("Pair values not equal:" + Pair.this);
        }
    }

    public void checkState() {
        if (x != y)
            throw new PairValueNotEqualException();
    }

}

//Protect a Pair  inside a thread-safe class
abstract class PairManager {
    AtomicInteger checkCounter = new AtomicInteger(0);
    protected Pair p = new Pair();
    private List<Pair> storage = Collections.synchronizedList(new ArrayList<Pair>());

    public synchronized Pair getPair() {
        //make a copy to keep the original safe
        return new Pair(p.getX(), p.getY());
    }

    //Assume  this is a time consuming operation
    protected void store(Pair p) {
        storage.add(p);
        try {
            TimeUnit.MICROSECONDS.sleep(50);
        } catch (InterruptedException ignore) {
        }
    }

    public abstract void increment();
}

//synchronize the entire method
class PairManager1 extends PairManager {
    public synchronized void increment() {
        p.incrementX();
        p.incrementY();
        store(getPair());
    }
}

//use a critical section:
class PairManager2 extends PairManager {
    public void increment() {
        Pair temp;
        synchronized (this) {
            p.incrementX();
            p.incrementY();
            temp = getPair();
        }
        store(temp);
    }
}

class PairManipulator implements Runnable {
    private PairManager pm;

    public PairManipulator(PairManager pm) {
        this.pm = pm;
    }

    public void run() {
        while (true) {
            pm.increment();
        }
    }

    public String toString() {
        return "Pair: " + pm.getPair() + " checkCounter= " + pm.checkCounter.get();
    }
}

class PairChecker implements Runnable {
    private PairManager pm;

    public PairChecker(PairManager pm) {
        this.pm = pm;
    }

    public void run() {
        while (true) {
            pm.checkCounter.incrementAndGet();
            pm.getPair().checkState();
        }
    }
}

public class CriticalSection {
    //Test the two different approaches:
    static void testApproaches(PairManager pman1, PairManager pman2) {
        ExecutorService exec = Executors.newCachedThreadPool();
        PairManipulator pm1 = new PairManipulator(pman1);
        PairManipulator pm2 = new PairManipulator(pman2);

        PairChecker pcheck1 = new PairChecker(pman1);
        PairChecker pcheck2 = new PairChecker(pman2);

        exec.execute(pm1);
        exec.execute(pm2);
        exec.execute(pcheck1);
        exec.execute(pcheck2);

        try {
            TimeUnit.MICROSECONDS.sleep(500);
        } catch (InterruptedException e) {
            System.out.println("Sleep interrupted");
        }
        System.out.println("pm1: " + pm1 + "\npm2: " + pm2);
        System.exit(0);
    }

    public static void main(String[] args) {
        PairManager pman1 = new PairManager1();
        PairManager pman2 = new PairManager2();
        testApproaches(pman1, pman2);
    }
}

运行结果：

Pair不是线程安全的，不能保证一个Pair对象在多线程程序中不会被破坏。

创建PairManager类，PairManager类持有一个Pair对象并控制对它的一切访问。它的一些功能在基类中实现，并且其一个或多个抽象方法在派生类中定义，这种结构在设计模式中被称为模板方法。

通过checkCounter发现，同步控制块不加锁的时间短，这是宁愿使用同步控制块而不是对整个方法进行同步控制的典型原因，使得其他线程能够更多的访问。

 

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

/**
 * Created by Administrator on 2017/9/13.
 */

class Pair {
    //not thread-safe
    private int x, y;

    public Pair(int x, int y) {
        this.x = x;
        this.y = y;
    }

    public Pair() {
        this(0, 0);
    }

    public int getX() {
        return x;
    }

    public int getY() {
        return y;
    }

    public void incrementX() {
        x++;
    }

    public void incrementY() {
        y++;
    }

    public String toString() {
        return "x:" + x + ",y:" + y;
    }


    // RuntimeException 运行时异常。是所有Java虚拟机正常操作期间可以被抛出的异常的父类
    public class PairValueNotEqualException extends RuntimeException {
        public PairValueNotEqualException() {
            super("Pair values not equal:" + Pair.this);
        }
    }

    public void checkState() {
        if (x != y)
            throw new PairValueNotEqualException();
    }

}

//Protect a Pair  inside a thread-safe class
abstract class PairManager {
    AtomicInteger checkCounter = new AtomicInteger(0);
    protected Pair p = new Pair();
    private List<Pair> storage = Collections.synchronizedList(new ArrayList<Pair>());

    public synchronized Pair getPair() {
        //make a copy to keep the original safe
        return new Pair(p.getX(), p.getY());
    }

    //Assume  this is a time consuming operation
    protected void store(Pair p) {
        storage.add(p);
        try {
            TimeUnit.MICROSECONDS.sleep(50);
        } catch (InterruptedException ignore) {
        }
    }

    public abstract void increment();
}

//synchronize the entire method
class PairManager1 extends PairManager {
    public synchronized void increment() {
        p.incrementX();
        p.incrementY();
        store(getPair());
    }
}

//use a critical section:
class PairManager2 extends PairManager {
    public void increment() {
        Pair temp;
        synchronized (this) {
            p.incrementX();
            p.incrementY();
            temp = getPair();
        }
        store(temp);
    }
}

//synchronize the entire method
class ExplicitPairManager1 extends PairManager {
    private Lock lock = new ReentrantLock();

    public synchronized void increment() {
        lock.lock();
        try {
            p.incrementX();
            p.incrementY();
            store(getPair());
        } finally {
            lock.unlock();
        }
    }
}

//use a critical section
class ExplicitPairManager2 extends PairManager {
    private Lock lock = new ReentrantLock();

    public void increment() {
        Pair temp;
        lock.lock();
        try {
            p.incrementX();
            p.incrementY();
            temp = getPair();
        } finally {
            lock.unlock();
        }
        store(temp);
    }

}

class PairManipulator implements Runnable {
    private PairManager pm;

    public PairManipulator(PairManager pm) {
        this.pm = pm;
    }

    public void run() {
        while (true) {
            pm.increment();
        }
    }

    public String toString() {
        return "Pair: " + pm.getPair() + " checkCounter= " + pm.checkCounter.get();
    }
}

class PairChecker implements Runnable {
    private PairManager pm;

    public PairChecker(PairManager pm) {
        this.pm = pm;
    }

    public void run() {
        while (true) {
            pm.checkCounter.incrementAndGet();
            pm.getPair().checkState();
        }
    }
}

public class CriticalSection {
    //Test the two different approaches:
    static void testApproaches(PairManager pman1, PairManager pman2) {
        ExecutorService exec = Executors.newCachedThreadPool();
        PairManipulator pm1 = new PairManipulator(pman1);
        PairManipulator pm2 = new PairManipulator(pman2);

        PairChecker pcheck1 = new PairChecker(pman1);
        PairChecker pcheck2 = new PairChecker(pman2);

        exec.execute(pm1);
        exec.execute(pm2);
        exec.execute(pcheck1);
        exec.execute(pcheck2);

        try {
            TimeUnit.MICROSECONDS.sleep(500);
        } catch (InterruptedException e) {
            System.out.println("Sleep interrupted");
        }
        System.out.println("pm1: " + pm1 + "\npm2: " + pm2);
        System.exit(0);
    }

    public static void main(String[] args) throws Exception{
        /*PairManager pman1 = new PairManager1();
        PairManager pman2 = new PairManager2();*/
        PairManager pman1 = new ExplicitPairManager1();
        PairManager pman2 = new ExplicitPairManager2();
        testApproaches(pman1, pman2);
    }
}

转载于:https://my.oschina.net/u/560971/blog/1548944