201771010135 楊蓉慶《面對物件程式設計（java）》第十七週學習總結
阿新 • • 發佈：2018-12-20
1、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
一、理論知識
⚫ 執行緒同步
（1）多執行緒併發執行不確定性問題解決方案：引入線 程同步機制，使得另一執行緒要使用該方法，就只 能等待
（2）在Java中解決多執行緒同步問題的方法有兩種：
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。
（4）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CPU的使用權，並允許其它執行緒使用這個 同步方法。
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. * @versionbank1.30 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();//釋放這個鎖 } } /** * 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(); } } }SynchBank
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();//解除那些在該物件上呼叫wait方法的執行緒的阻塞狀態 } /** * 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; } }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
結果如下：
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；
嘗試解決程式中存在問題。
package B; class Cbank { private static int s=2000; public synchronized static void sub(int m)//加鎖物件 { 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(); } }Thread3
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
程式碼如下：
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() { while (flag) { try { Thread.sleep(500); } catch(InterruptedException e) { 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; } } } } }Demo
結果如下：
三、實驗總結
本次作業我們學習了執行緒同步的知識，在測試程式中更深一步的理解了理論知識，發現了執行緒的多變性，希望在接下來額學習中可以更好的運用。