王颖奇 20171010129《面向对象程序设计（java）》第十七周学习总结

实验十七  线程同步控制

实验时间 2018-12-10

学习总结：

1.Java通过多线程的并发运行提高系统资源利用 率，改善系统性能。

2.假设有两个或两个以上的线程共享 某个对象，每个线程都调用了改变该对象类状态的方法，就会引起的不确定性。

3.多线程并发执行中的问题

◆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。

4.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。

5.（1）锁对象与条件对象

用ReentrantLock保护代码块的基本结构如下：

myLock.lock();

try { critical section }

finally{

myLock.unlock();

}

（2）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。

具体用法参考博客：https://blog.csdn.net/sinat_32588261/article/details/72880159

1、实验目的与要求

(1) 掌握线程同步的概念及实现技术；

(2) 线程综合编程练习

2、实验内容和步骤

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。

代码：

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;//银行运转的基本数据
   private Lock bankLock;//锁对象
   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();
      try
      {//锁对象的引用条件对象
         while (accounts[from] < amount)
           sufficientFunds.await();
         System.out.print(Thread.currentThread());//打印出线程号
         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.signal();
      }
      finally
      {
         bankLock.unlock();
      }
   }

   /**
    * 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;
   }
}

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()));
               }
            }
            catch (InterruptedException e)
            {
            }
         };
         Thread t = new Thread(r);
         t.start();
      }
   }
}

运行结果：

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。

代码：

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();//导致线程进入等待状态直到它被通知。该方法只能在一个同步方法中调用。
      System.out.print(Thread.currentThread());//打印出线程号
      accounts[from] -= amount;
      System.out.printf(" %10.2f from %d to %d", amount, from, to);//第一个打印结果保留两位小数（最大范围是十位），
      accounts[to] += amount;
      System.out.printf(" Total Balance: %10.2f%n", getTotalBalance());
      notifyAll();//解除那些在该对象上调用wait方法的线程阻塞状态。该方法只能在同步方法或同步块内部调用。
   }

   /**
    * 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;
   }
}

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()));
               }
            }
            catch (InterruptedException e)
            {
            }
         };
         Thread t = new Thread(r);
         t.start();
      }
   }
}

运行结果：

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)(1000*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 Thread3

{

public static void main(String args[])

{

Customer customer1 = new Customer();

Customer customer2 = new Customer();

customer1.start();

customer2.start();

}

}

存在的问题：

两个线程各自运行自己的

运行的预期结果应该为：

最终的s=1200

更正思路：

增加语句

synchronized

代码：

package demo;

class Cbank

{

     private static int s=2000;

     public synchronized static void sub(int m)

     {

           int temp=s;

           temp=temp-m;

          try {

      Thread.sleep((int)(1000*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 Thread3

{

 public static void main(String args[])

  {

   Customer customer1 = new Customer();

   Customer customer2 = new Customer();

   customer1.start();

   customer2.start();

  }

}

运行结果：

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

代码：

package ThreadTest;

public class Demo {
    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() {
        // TODO Auto-generated method stub
        while (flag) {
            try {
                Thread.sleep(500);
            } catch (Exception e) {
                // TODO: handle exception
                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;
                }
            }

        }
    }
}

运行结果：

代码：

package ThreadTest;

public class Demo1 extends Thread {
    static int t = 1;

    public void run() {
        synchronized (this) {
            while (t <= 10) {
                if (t <= 10) {
                    System.out.println(Thread.currentThread().getName() + "窗口售：第" + t + "张票");
                    t++;
                } else {
                    System.out.println("票卖完了");
                }
            }
            try {
                sleep(500);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        Demo1 window1 = new Demo1();
        Demo1 window2 = new Demo1();
        Demo1 window3 = new Demo1();
        window1.start();
        window2.start();
        window3.start();
    }

}

运行结果：