實驗十七 線程同步控制
實驗十七 線程同步控制
實驗時間 2018-12-10
一、理論知識部分：
? 線程同步
（1）多線程並發運行不確定性問題解決方案：引入線 程同步機制，使得另一線程要使用該方法，就只 能等待
（2）在Java中解決多線程同步問題的方法有兩種：
解決方案一：鎖對象與條件對象
用ReentrantLock保護代碼塊的基本結構如下： myLock.lock();
try { critical section }
finally{ myLock.unlock(); }
（3）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；
? 只要某個線程正在訪問同步方法，其他線程欲要訪問同步方法就被阻塞，直至線程從同步方法返回前喚醒被阻塞線程，其他線程方可能進入同步方法。
（4）在同步方法中使用wait()、notify 和notifyAll()方法
一個線程在使用的同步方法中時，可能根據問題 的需要，必須使用wait()方法使本線程等待，暫 時讓出CPU的使用權，並允許其它線程使用這個 同步方法。
線程如果用完同步方法，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 線程結束等待。
二 實驗部分
1、實驗目的與要求
(1) 掌握線程同步的概念及實現技術；
(2) 線程綜合編程練習
2、實驗內容和步驟
實驗1：測試程序並進行代碼註釋。
測試程序1：
l 在Elipse環境下調試教材651頁程序14-7，結合程序運行結果理解程序；
l 掌握利用鎖對象和條件對象實現的多線程同步技術。
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 Horstmannbank*/ 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(); 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.signal(); } 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; } }
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(); } } }SynchBankTest
測試程序2：
l 在Elipse環境下調試教材655頁程序14-8，結合程序運行結果理解程序；
l 掌握synchronized在多線程同步中的應用。
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())); } } catch (InterruptedException e) { } }; Thread t = new Thread(r); t.start(); } } }SynchBankTest2
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(); 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.signal(); } 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; } }bank
測試程序3：
l 在Elipse環境下運行以下程序，結合程序運行結果分析程序存在問題；
l 嘗試解決程序中存在問題。
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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(); } }
實驗2 編程練習
利用多線程及同步方法，編寫一個程序模擬火車票售票系統，共3個窗口，賣10張票，程序輸出結果類似（程序輸出不唯一，可以是其他類似結果）。
Thread-0窗口售：第1張票
Thread-0窗口售：第2張票
Thread-1窗口售：第3張票
Thread-2窗口售：第4張票
Thread-2窗口售：第5張票
Thread-1窗口售：第6張票
Thread-0窗口售：第7張票
Thread-2窗口售：第8張票
Thread-1窗口售：第9張票
Thread-0窗口售：第10張票
public class shi { 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; boolean flag = true; @Override public void run() { // TODO Auto-generated method stub while (flag) { try { Thread.sleep(500); } catch (Exception e) { // TODO: handle exception } synchronized (this) { if (t <= 10) { System.out.println(Thread.currentThread().getName() + "窗口售：第" + t + "張票"); t++; } else if(t>10) { flag = false; } } } } }
實驗總結 掌握了線程同步的概念及實現技術；
實驗十七 線程同步控制