201771010113 李婷华 《面向对象程序设计（Java）》第十七周总结

一.理论知识部分

Java 的线程调度采用优先级策略:优先级高的先执行，优先级低的后执行；多线程系统会自动为每个线程分配一个优先级，缺省时，继承其父类的优先级； 任务紧急的线程，其优先级较高； 同优先级的线程按“先进先出”的队列原则。

调用setPriority(int a)重置当前线程的优先级，a取值可以是前述的三个静态量。调用getPriority()获得当前线程优先级。

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：J ava SE 5.0中引入ReentrantLock类。 在共享内存的类方法前加synchronized修饰符。

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。

二.实验部分

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。

 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()));//随机生成时间，使正在执行的线程休眠
                }
             }
             catch (InterruptedException e)
             {
             }
          };
          Thread t = new Thread(r);
          t.start();//开始线程
       }
    }
 }

SynchBankTest

 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();//使用锁对象,获取锁
       try
       {
          while (accounts[from] < amount)
             sufficientFunds.await();
          System.out.print(Thread.currentThread());
          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();//唤醒所有线程
       }
       finally
       {
          bankLock.unlock();//释放锁
       }
    }

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

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

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

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

Bank

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;

 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
     */
    //使用synchronized修饰符
    public synchronized void transfer(int from, int to, double amount) throws InterruptedException
    {
       while (accounts[from] < amount)
          wait();//来自Object类
       System.out.print(Thread.currentThread());
       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();//解除所有线程的阻塞状态
    }

    /**
     * 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;
    }
 }

Bank

 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();
       }
    }
 }

SynchBankTest2

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();

}

}

 class Cbank
 {
      private static int s=2000;
      public   static synchronized 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();
   }
 }

Cbank

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 Demo {
     public static void main(String[] args) {
         Myrhread myrhread = new Myrhread();
         Thread t1 = new Thread(myrhread);
         Thread t2 = new Thread(myrhread);
         Thread t3 = new Thread(myrhread);
         t1.start();
         t2.start();
         t3.start();
     }
 }

 class Myrhread implements Runnable {
     int t = 1;
     boolean flag = true;

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

     }
 }

Demo

3.实验总结：

本周的实验容量很少，实验也相对来说简单，完成的还算顺利。学长也教了我们常用的一些快捷键，本周的收获还是很大的。