李曉菁201771010114《面向物件程式設計（java）》第十七週學習總結
1.Java通過多執行緒的併發執行提高系統資源利用率，改善系統性能。
2.假設有兩個或兩個以上的執行緒共享 某個物件，每個執行緒都呼叫了改變該物件類狀態的方法，就會引起的不確定性。
3.多執行緒併發執行中的問題
◆多個執行緒相對執行的順序是不確定的。
◆執行緒執行順序的不確定性會產生執行結果的不確定性。
◆在多執行緒對共享資料操作時常常會產生這種不確定性。
4.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。
5.（1）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();
}
（2）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。
實驗十七 執行緒同步控制
實驗時間 2018-12-10
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
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. * @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();//在等待條件前，鎖必須由當前執行緒保持。 } /** * 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();//獲取鎖 try { while (accounts[from] < amount) sufficientFunds.await();//造成當前執行緒在接到訊號或被中斷之前一直處於等待狀態。 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();//如果所有的執行緒都在等待此條件，則喚醒所有執行緒 } 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; } }bank
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(); } } }synch
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();//在其他執行緒呼叫此物件的 notify() 方法或 notifyAll() 方法前，導致當前執行緒等待 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();//喚醒在此物件監視器上等待的所有執行緒 } /** * 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(); } } }synch
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 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(); } } |
執行結果顯示兩個執行緒各自執行各自的：
修改後的程式碼：
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
import javax.swing.plaf.SliderUI; public class Demo1 { 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(); } } /* new Thread() { @Override public void run() { System.out.println(); }; }.start(); } }*/ class Mythread implements Runnable{ int t=1; boolean flag=true; @Override public void run() { while(flag) { try { Thread.sleep(500); } catch (InterruptedException 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<0) { flag=false; } } } } }
實驗總結;通過本次實驗，我學習到了執行緒同步的概念，以及如何處理。通過本學期的學習，由剛開始的新手小白，對Java一無所知，
到通過大量的練習慢慢對Java程式設計有所熟悉，雖然現在還不是很熟練，但在課程結束後仍需繼續關注學習Java的知識及程式設計。