窮吉201771010119
一,理論知識
1.ava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
二、實驗部分
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 匯入; 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;//銀行運轉的基本資料 private Lock bankLock;//鎖物件 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 {//鎖物件的引用條件物件 while (accounts[from] < amount) sufficientFunds.await(); System.out.print(Thread.currentThread());//打印出執行緒號 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();//加鎖 try { double sum = 0; for (double a : accounts) sum += a; return sum; } finally { bankLock.unlock();//解鎖 } } /** * Gets the number of accounts in the bank. * @return the number of accounts */ public int size() { return accounts.length; } }
package 匯入; /** * 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(); } } }
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 匯入; import java.util.*; /** * 使用同步原語的具有多個銀行帳戶的銀行 * @version 1.30 2004-08-01 * @author Cay Horstmann */ public class Bank { private final double[] accounts; /** * 構建了銀行。 * @param 賬戶數量 * @param 每個賬戶的初始餘額 */ public Bank(int n, double initialBalance) { accounts = new double[n]; Arrays.fill(accounts, initialBalance); } /** * 把錢從一個賬戶轉到另一個賬戶。 * @param 從賬戶轉出 * @param 到賬轉到 * @param 轉帳金額 */ public synchronized void transfer(int from, int to, double amount) throws InterruptedException { while (accounts[from] < amount) wait();//Object類 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(); } /** *獲取所有帳戶餘額的總和。 * @return 總平衡 */ public synchronized double getTotalBalance() { double sum = 0; for (double a : accounts) sum += a; return sum; } /** * 獲取銀行中的帳戶編號。 * @return 賬戶數量 */ public int size() { return accounts.length; } }
package 匯入; /** * 這個程式展示了多執行緒如何安全地訪問一個數據結構,使用同步方法。 * @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 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(); } } |
源程式執行結果
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(); } }
結果圖:
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
public class Demo { public static void main(String[] args) { Mythread mythread = new Mythread(); Thread ticket1 = new Thread(mythread); Thread ticket2 = new Thread(mythread); Thread ticket3 = new Thread(mythread); ticket1.start(); ticket2.start(); ticket3.start(); } } class Mythread implements Runnable { int ticket = 1; boolean flag = true; @Override public void run() { while (flag) { try { Thread.sleep(500); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } synchronized (this) { if (ticket <= 10) { System.out.println(Thread.currentThread().getName() + "視窗售:第" + ticket + "張票"); ticket++; } if (ticket > 10) { flag = false; } } } } }實驗總結:這次的試驗程式設計題較比以前寫的簡單的多,對於我來說這門學課完全是一個新的知識以前所沒有接觸過得。雖然學起來有的程式碼寫不出,有的理解不了。但還是在盡力去理解。儘管不知道學到了多好但至少接觸了一門以前一直沒接觸的知識。
結果圖: