冯志霞201771010107《面向对象程序设计（java）》第十七周学习总结

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实验十七  线程同步控制

实验时间 2018-12-10

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

View Code

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();
      try
      {
         while (accounts[from] < amount)
            sufficientFunds.await();//当循环条件有不满足的情况时需要用到条件对象
         System.out.print(Thread.currentThread());//获得当前线程名
         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();
      }
   }

   /**
    * 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;
   }
}

View Code

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
    */
   public synchronized void transfer(int from, int to, double amount) throws InterruptedException
   {
      while (accounts[from] < amount)
         wait();
      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();
   }
//与加锁过程基本相同，此时只需对方法体用 synchronized关键字修饰即可，无需在创建条件对象，其中notifyAll();与signalAll();作用一致
   /**
    * 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;
   }
}

　　运行结果与测试一相同

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 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();

  }

}

　　

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

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class Demo1{
    public static void main(String[] args) {
        demo d = new demo();
        Thread t1 = new Thread(d);
        Thread t2 = new Thread(d);
        Thread t3 = new Thread(d);
        t1.start();
        t2.start();
        t3.start();
        
    }
}
class demo implements Runnable{
  int t=1;
  Lock lock = new ReentrantLock();
    @Override
    public void run() {
        // TODO Auto-generated method stub
         
        while(true) {
            
            try {
                Thread.sleep(500);
                
            }catch (Exception e) {
                // TODO: handle exception
                e.printStackTrace();
            }
            lock.lock();
            
        
            if(t<=10)
            {
            
                System.out.println(Thread.currentThread().getName()+"窗口售：第"+t+"张票");
                t++;
            }
            
            
            
            
            
            lock.unlock();
        }
         
            
        }
        
    }
    


 

View Code

 

为何要使用同步？ 
    java允许多线程并发控制，当多个线程同时操作一个可共享的资源变量时（如数据的增删改查）， 
    将会导致数据不准确，相互之间产生冲突，因此加入同步锁以避免在该线程没有完成操作之前，被其他线程的调用， 
    从而保证了该变量的唯一性和准确性。

 

1.同步方法 
    即有synchronized关键字修饰的方法。 
    由于java的每个对象都有一个内置锁，当用此关键字修饰方法时， 
    内置锁会保护整个方法。在调用该方法前，需要获得内置锁，否则就处于阻塞状态。

    代码如： 
    public synchronized void save(){}

   注： synchronized关键字也可以修饰静态方法，此时如果调用该静态方法，将会锁住整个类

 

2.同步代码块 
    即有synchronized关键字修饰的语句块。 
    被该关键字修饰的语句块会自动被加上内置锁，从而实现同步

    代码如： 
    synchronized(object){ 
    }

3.使用重入锁实现线程同步

    
    ReentrantLock类是可重入、互斥、实现了Lock接口的锁， 
    它与使用synchronized方法和快具有相同的基本行为和语义，并且扩展了其能力

    ReenreantLock类的常用方法有：

        ReentrantLock() : 创建一个ReentrantLock实例 
        lock() : 获得锁 
        unlock() : 释放锁