實驗十七  執行緒同步控制

實驗時間 2018-12-10

1. 知識總結：

多執行緒併發執行不確定性問題解決方案：引入執行緒同步機制，使得另一執行緒要使用該方法，就只能等待。

 在Java中解決多執行緒同步問題的方法有兩種：

- Java SE 5.0中引入ReentrantLock類。

- 在共享記憶體的類方法前加synchronized修飾符。

解決方案一：鎖物件與條件物件：

用ReentrantLock保護程式碼塊的基本結構。

有關鎖物件和條件物件的關鍵要點:

➢ 鎖用來保護程式碼片段，保證任何時刻只能有一

個執行緒執行被保護的程式碼。

➢ 鎖管理試圖進入被保護程式碼段的執行緒。

➢ 鎖可擁有一個或多個相關條件物件。

➢ 每個條件物件管理那些已經進入被保護的程式碼段但還不能執行的執行緒。

解決方案二： synchronized關鍵字。

synchronized關鍵字作用：

➢ 某個類內方法用synchronized 修飾後，該方法被稱為同步方法；

➢ 只要某個執行緒正在訪問同步方法，其他執行緒欲要訪問同步方法就被阻塞，直至執行緒從同

步方法返回前喚醒被阻塞執行緒，其他執行緒方可能進入同步方法。

在同步方法中使用wait()、notify 和notifyAll()方法。

➢ 一個執行緒在使用的同步方法中時，可能根據問題的需要，必須使用wait()方法使本執行緒等待，暫時讓出

➢ 執行緒如果用完同步方法，應當執行notifyAll()方法通知所有由於使用這個同步方法而處於等待的執行緒結束等待。

2、實驗目的與要求

(1) 掌握執行緒同步的概念及實現技術；

(2) 執行緒綜合程式設計練習

3、實驗內容和步驟

實驗1：測試程式並進行程式碼註釋。

測試程式1：

l 在Elipse環境下除錯教材651頁程式14-7，結合程式執行結果理解程式；

l 掌握利用鎖物件和條件物件實現的多執行緒同步技術。

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();//鎖物件初始化 
      sufficientFunds = bankLock.newCondition();//newCondition方法生成了鎖物件的條件物件
   }

   /**
    * 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
   {//加鎖
      bankLock.lock();
      try
      {
         while (accounts[from] < amount)
            sufficientFunds.await();//將執行緒放到條件的等待集中
         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();//解除該條件的等待集中隨機的所有執行緒的阻塞狀態
      }
      finally
      {
         bankLock.unlock();//釋放鎖
      }
   }

   /**
    * 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;
   }
}

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
{//定義四個公共屬性
   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//丟擲異常
            {
               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();
      }
   }
}

 測試程式2：

l 在Elipse環境下除錯教材655頁程式14-8，結合程式執行結果理解程式；

l 掌握synchronized在多執行緒同步中的應用。

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();
   }

   /**
    * Gets the sum of all account balances.
    * @return the total balance
    */
   public synchronized double getTotalBalance()
   {//計算過程
      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();
      }
   }
}

測試程式3：

l 在Elipse環境下執行以下程式，結合程式執行結果分析程式存在問題；

l 嘗試解決程式中存在問題。

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();
    }
}

實驗2 程式設計練習

利用多執行緒及同步方法，編寫一個程式模擬火車票售票系統，共3個視窗，賣10張票，程式輸出結果類似（程式輸出不唯一，可以是其他類似結果）。

Thread-0視窗售：第1張票

Thread-0視窗售：第2張票

Thread-1視窗售：第3張票

Thread-2視窗售：第4張票

Thread-2視窗售：第5張票

Thread-1視窗售：第6張票

Thread-0視窗售：第7張票

Thread-2視窗售：第8張票

Thread-1視窗售：第9張票

Thread-0視窗售：第10張票

package DFDF;

public class DFDF {
    public class Demo {
        public 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;

        @Override
        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() + "視窗售：第" + t + "張票");
                        t++;
                    }
                    if (t > 10) {
                        flag = false;
                    }
                }

            }

        }
    }
}

    

4、實驗總結：這次實驗中學會了在Java中解決多執行緒同步問題。

掌握利用鎖物件和條件物件實現的多執行緒同步技術。

掌握synchronized在多執行緒同步中的應用。