十七

理论部分

◆多个线程相对执行的顺序是不确定的。

◆线程执行顺序的不确定性会产生执行结果的不确定性。

◆在多线程对共享数据操作时常常会产生这种不确定性。

 

-多线程并发运行不确定性问题解决方案：引入线程同步机制，使得另一线程要使用该方法，就只能等待。

- 在Java中解决多线程同步问题的方法有两种：

- Java SE 5.0中引入ReentrantLock类

- 在共享内存的类方法前加synchronized修饰符

public synchronized static void sub(int m)

锁对象与条件对象

用ReentrantLock保护代码块的基本结构如下：

myLock.lock();

myLock.unlock(); 

有关锁对象和条件对象的关键要点:

锁用来保护代码片段，保证任何时刻只能有一个线程执行被保护的代码。

锁管理试图进入被保护代码段的线程。

锁可拥有一个或多个相关条件对象。
每个条件对象管理那些已经进入被保护的代码段但还不能运行的线程。

synchronized关键字

synchronized关键字作用：

某个类内方法用synchronized 修饰后，该方法被称为同步方法；
只要某个线程正在访问同步方法，其他线程欲要访问同步方法就被阻塞，直至线程从同步方法返回前唤醒被阻塞线程，其他线程方可能进入同步方法。

在同步方法中使用wait()、notify 和notifyAll()方法

 一个线程在使用的同步方法中时，可能根据问题的需要，必须使用wait()方法使本线程等待，暂时让出CPU的使用权，并允许其它线程使用这个同步方法。

➢线程如果用完同步方法，应当执行notifyAll()方法通知所有由于使用这个同步方法而处于等待的线程结束等待。

实验1：测试程序并进行代码注释。

测试程序1：

l 在Elipse环境下调试教材651页程序14-7，结合程序运行结果理解程序；

l 掌握利用锁对象和条件对象实现的多线程同步技术。

package synch;

/**
 * This program shows how multiple threads can safely access a data structure.
 * 
 * @version 1.31 2015-06-21
 * @author Cay Horstmann
 */
public class SynchBankTest {
    public static final int NACCOUNTS = 100;
    public static final double INITIAL_BALANCE = 1000;
    public static final double MAX_AMOUNT = 1000;
    public static final int DELAY = 10;

    public static void main(String[] args) {
        Bank bank = new Bank(NACCOUNTS, INITIAL_BALANCE);
        for (int i = 0; i < NACCOUNTS; i++) {
            int fromAccount = i;
            Runnable r = () -> {
                try {
                    while (true) {
                        int toAccount = (int) (bank.size() * Math.random());
                        double amount = MAX_AMOUNT * Math.random();
                        bank.transfer(fromAccount, toAccount, amount);
                        Thread.sleep((int) (DELAY * Math.random()));// 在指定的毫秒数内让当前正在执行的线程休眠
                    }
                } catch (InterruptedException e) {
                }
            };
            Thread t = new Thread(r);// 分配新的 Thread 对象
            t.start();// 开始线程
        }
    }
}

package synch;

import java.util.*;
import java.util.concurrent.locks.*;

/**
 * A bank with a number of bank accounts that uses locks for serializing access.
 * 
 * @version 1.30 2004-08-01
 * @author Cay Horstmann
 */
public class Bank {
    private final double[] accounts;
    private Lock bankLock;
    private Condition sufficientFunds;

    /**
     * Constructs the bank.
     * 
     * @param n
     *            the number of accounts
     * @param initialBalance
     *            the initial balance for each account
     */
    public Bank(int n, double initialBalance) {
        accounts = new double[n];
        Arrays.fill(accounts, initialBalance);
        bankLock = new ReentrantLock();
        sufficientFunds = bankLock.newCondition();
    }

    /**
     * Transfers money from one account to another.
     * 
     * @param from
     *            the account to transfer from
     * @param to
     *            the account to transfer to
     * @param amount
     *            the amount to transfer
     */
    public void transfer(int from, int to, double amount) throws InterruptedException {
        bankLock.lock();// 加锁
        try {
            while (accounts[from] < amount)
                sufficientFunds.await();// 锁对象的条件对象
            System.out.print(Thread.currentThread());// 返回对当前正在执行的线程对象的引用
            accounts[from] -= amount;
            System.out.printf(" %10.2f from %d to %d", amount, from, to);
            accounts[to] += amount;
            System.out.printf(" Total Balance: %10.2f%n", getTotalBalance());
            sufficientFunds.signalAll();// 唤醒所有等待线程
        } finally {
            bankLock.unlock();// 解锁
        }
    }

    /**
     * Gets the sum of all account balances.
     * 
     * @return the total balance
     */
    public double getTotalBalance() {
        bankLock.lock();// 加锁
        try {
            double sum = 0;

            for (double a : accounts)
                sum += a;

            return sum;
        } finally {
            bankLock.unlock();// 解锁
        }
    }

    /**
     * Gets the number of accounts in the bank.
     * 
     * @return the number of accounts
     */
    public int size() {
        return accounts.length;
    }
}

测试程序2：

l 在Elipse环境下调试教材655页程序14-8，结合程序运行结果理解程序；

l 掌握synchronized在多线程同步中的应用。

package synch2;

/**
 * This program shows how multiple threads can safely access a data structure,
 * using synchronized methods.
 * 
 * @version 1.31 2015-06-21
 * @author Cay Horstmann
 */
public class SynchBankTest2 {
    public static final int NACCOUNTS = 100;
    public static final double INITIAL_BALANCE = 1000;
    public static final double MAX_AMOUNT = 1000;
    public static final int DELAY = 10;

    public static void main(String[] args) {
        Bank bank = new Bank(NACCOUNTS, INITIAL_BALANCE);
        for (int i = 0; i < NACCOUNTS; i++) {
            int fromAccount = i;
            Runnable r = () -> {
                try {
                    while (true) {
                        int toAccount = (int) (bank.size() * Math.random());
                        double amount = MAX_AMOUNT * Math.random();
                        bank.transfer(fromAccount, toAccount, amount);
                        Thread.sleep((int) (DELAY * Math.random()));// 在指定的毫秒数内让当前正在执行的线程休眠
                    }
                } catch (InterruptedException e) {
                }
            };
            Thread t = new Thread(r);// 分配新的 Thread 对象
            t.start();// 使线程开始执行
        }
    }
}

package synch2;

import java.util.*;

/**
 * A bank with a number of bank accounts that uses synchronization primitives.
 * 
 * @version 1.30 2004-08-01
 * @author Cay Horstmann
 */
public class Bank {
    private final double[] accounts;

    /**
     * Constructs the bank.
     * 
     * @param n
     *            the number of accounts
     * @param initialBalance
     *            the initial balance for each account
     */
    public Bank(int n, double initialBalance) {
        accounts = new double[n];
        Arrays.fill(accounts, initialBalance);
    }

    /**
     * Transfers money from one account to another.
     * 
     * @param from
     *            the account to transfer from
     * @param to
     *            the account to transfer to
     * @param amount
     *            the amount to transfer
     */
    public synchronized void transfer(int from, int to, double amount) throws InterruptedException {
        while (accounts[from] < amount)
            wait();// 添加一个线程到等待集中
        System.out.print(Thread.currentThread());// 返回对当前正在执行的线程对象的引用
        accounts[from] -= amount;
        System.out.printf(" %10.2f from %d to %d", amount, from, to);
        accounts[to] += amount;
        System.out.printf(" Total Balance: %10.2f%n", getTotalBalance());
        notifyAll();// 解除等待线程的阻塞状态
    }

    /**
     * Gets the sum of all account balances.
     * 
     * @return the total balance
     */
    public synchronized double getTotalBalance() {
        double sum = 0;

        for (double a : accounts)
            sum += a;

        return sum;
    }

    /**
     * Gets the number of accounts in the bank.
     * 
     * @return the number of accounts
     */
    public int size() {
        return accounts.length;
    }
}

 

测试程序3：

l 在Elipse环境下运行以下程序，结合程序运行结果分析程序存在问题；

l 尝试解决程序中存在问题。

class Cbank
{
     private static int s=2000;
     public   static void sub(int m)
     {
           int temp=s;
           temp=temp-m;
          try {
                    Thread.sleep((int)(100*Math.random()));
              }
           catch (InterruptedException e)  {              }
           s=temp;
           System.out.println("s="+s);
         
   }
}
 
 
class Customer extends Thread
{
  public void run()
  {
   for( int i=1; i<=4; i++)
     Cbank.sub(100);
    }
 }
public class asfa
{
 public static void main(String args[])
  {
   Customer customer1 = new Customer();
   Customer customer2 = new Customer();
   customer1.start();
   customer2.start();
  }
}

 

 

 

 

import java.util.*;
import java.util.concurrent.locks.*;
class Cbank
{
     private static int s=2000;
     private static ReentrantLock lock = new ReentrantLock();
     public   static void sub(int m)
     {
        
         lock.lock();
           int temp=s;
           temp=temp-m;
          try {
                    Thread.sleep((int)(100*Math.random()));
              }
           catch (InterruptedException e)  {              }
           s=temp;
           System.out.println("s="+s);
           lock.unlock();
   }
}
 
 
class Customer extends Thread
{
  public void run()
  {
   for( int i=1; i<=4; i++)
     Cbank.sub(100);
    }
 }
public class asfa
{
 public static void main(String args[])
  {
   Customer customer1 = new Customer();
   Customer customer2 = new Customer();
   customer1.start();
   customer2.start();
  }
}

 

实验2 编程练习

利用多线程及同步方法，编写一个程序模拟火车票售票系统，共3个窗口，卖10张票，程序输出结果类似（程序输出不唯一，可以是其他类似结果）。

Thread-0窗口售：第1张票

Thread-0窗口售：第2张票

Thread-1窗口售：第3张票

Thread-2窗口售：第4张票

Thread-2窗口售：第5张票

Thread-1窗口售：第6张票

Thread-0窗口售：第7张票

Thread-2窗口售：第8张票

Thread-1窗口售：第9张票

Thread-0窗口售：第10张票

public class asfa {
    public static void main(String[] args) {
        Mythread mythread = new Mythread();
        Thread t1 = new Thread(mythread);
        Thread t2 = new Thread(mythread);
        Thread t3 = new Thread(mythread);
        t1.start();
        t2.start();
        t3.start();
    }
}
class Mythread implements Runnable {
    int t = 1;
    boolean flag = true;

    public void run() {
        while (flag) {
            try {
                Thread.sleep(500);
            } catch (Exception e) {
                e.printStackTrace();
            }
            synchronized (this) {
                if (t <= 10) {
                    System.out.println(Thread.currentThread().getName() + "窗口售：第" + t + "张票");
                    t++;
                }
                if (t > 10) {
                    flag = false;
                }
            }

        }

    }
}