201771010113 李婷華 《面向物件程式設計(Java)》第十七週總結
一.理論知識部分
Java 的執行緒排程採用優先順序策略:優先順序高的先執行,優先順序低的後執行;多執行緒系統會自動為每個執行緒分配一個優先順序,預設時,繼承其父類的優先順序; 任務緊急的執行緒,其優先順序較高; 同優先順序的執行緒按“先進先出”的佇列原則。
呼叫setPriority(int a)重置當前執行緒的優先順序,a取值可以是前述的三個靜態量。呼叫getPriority()獲得當前執行緒優先順序。
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:J ava SE 5.0中引入ReentrantLock類。 在共享記憶體的類方法前加synchronized修飾符。
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。
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。
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。
1 package synch;
2
3 /**
4 * This program shows how multiple threads can safely access a data structure.
5 * @version 1.31 2015-06-21
6 * @author Cay Horstmann
7 */
8 public class SynchBankTest
9 {
10 public static final int NACCOUNTS = 100;
11 public static final double INITIAL_BALANCE = 1000;
12 public static final double MAX_AMOUNT = 1000;
13 public static final int DELAY = 10;
14
15 public static void main(String[] args)
16 {
17 Bank bank = new Bank(NACCOUNTS, INITIAL_BALANCE);
18 for (int i = 0; i < NACCOUNTS; i++)
19 {
20 int fromAccount = i;
21 Runnable r = () -> {
22 try
23 {
24 while (true)
25 {
26 int toAccount = (int) (bank.size() * Math.random());
27 double amount = MAX_AMOUNT * Math.random();
28 bank.transfer(fromAccount, toAccount, amount);
29 Thread.sleep((int) (DELAY * Math.random()));//隨機生成時間,使正在執行的執行緒休眠
30 }
31 }
32 catch (InterruptedException e)
33 {
34 }
35 };
36 Thread t = new Thread(r);
37 t.start();//開始執行緒
38 }
39 }
40 }
SynchBankTest
1 package synch;
2
3 import java.util.*;
4 import java.util.concurrent.locks.*;
5
6 /**
7 * A bank with a number of bank accounts that uses locks for serializing access.
8 * @version 1.30 2004-08-01
9 * @author Cay Horstmann
10 */
11 public class Bank
12 {
13 private final double[] accounts;
14 private Lock bankLock;
15 private Condition sufficientFunds;
16
17 /**
18 * Constructs the bank.
19 * @param n the number of accounts
20 * @param initialBalance the initial balance for each account
21 */
22 public Bank(int n, double initialBalance)
23 {
24 accounts = new double[n];
25 Arrays.fill(accounts, initialBalance);
26 bankLock = new ReentrantLock();
27 sufficientFunds = bankLock.newCondition();
28 }
29
30 /**
31 * Transfers money from one account to another.
32 * @param from the account to transfer from
33 * @param to the account to transfer to
34 * @param amount the amount to transfer
35 */
36 public void transfer(int from, int to, double amount) throws InterruptedException
37 {
38 bankLock.lock();//使用鎖物件,獲取鎖
39 try
40 {
41 while (accounts[from] < amount)
42 sufficientFunds.await();
43 System.out.print(Thread.currentThread());
44 accounts[from] -= amount;
45 System.out.printf(" %10.2f from %d to %d", amount, from, to);
46 accounts[to] += amount;
47 System.out.printf(" Total Balance: %10.2f%n", getTotalBalance());
48 sufficientFunds.signalAll();//喚醒所有執行緒
49 }
50 finally
51 {
52 bankLock.unlock();//釋放鎖
53 }
54 }
55
56 /**
57 * Gets the sum of all account balances.
58 * @return the total balance
59 */
60 public double getTotalBalance()
61 {
62 bankLock.lock();
63 try
64 {
65 double sum = 0;
66
67 for (double a : accounts)
68 sum += a;
69
70 return sum;
71 }
72 finally
73 {
74 bankLock.unlock();
75 }
76 }
77
78 /**
79 * Gets the number of accounts in the bank.
80 * @return the number of accounts
81 */
82 public int size()
83 {
84 return accounts.length;
85 }
86 }
Bank
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。
1 package synch2;
2
3 import java.util.*;
4
5 /**
6 * A bank with a number of bank accounts that uses synchronization primitives.
7 * @version 1.30 2004-08-01
8 * @author Cay Horstmann
9 */
10 public class Bank
11 {
12 private final double[] accounts;
13
14 /**
15 * Constructs the bank.
16 * @param n the number of accounts
17 * @param initialBalance the initial balance for each account
18 */
19 public Bank(int n, double initialBalance)
20 {
21 accounts = new double[n];
22 Arrays.fill(accounts, initialBalance);
23 }
24
25 /**
26 * Transfers money from one account to another.
27 * @param from the account to transfer from
28 * @param to the account to transfer to
29 * @param amount the amount to transfer
30 */
31 //使用synchronized修飾符
32 public synchronized void transfer(int from, int to, double amount) throws InterruptedException
33 {
34 while (accounts[from] < amount)
35 wait();//來自Object類
36 System.out.print(Thread.currentThread());
37 accounts[from] -= amount;
38 System.out.printf(" %10.2f from %d to %d", amount, from, to);
39 accounts[to] += amount;
40 System.out.printf(" Total Balance: %10.2f%n", getTotalBalance());
41 notifyAll();//解除所有執行緒的阻塞狀態
42 }
43
44 /**
45 * Gets the sum of all account balances.
46 * @return the total balance
47 */
48 public synchronized double getTotalBalance()
49 {
50 double sum = 0;
51
52 for (double a : accounts)
53 sum += a;
54
55 return sum;
56 }
57
58 /**
59 * Gets the number of accounts in the bank.
60 * @return the number of accounts
61 */
62 public int size()
63 {
64 return accounts.length;
65 }
66 }
Bank
1 package synch2;
2
3 /**
4 * This program shows how multiple threads can safely access a data structure,
5 * using synchronized methods.
6 * @version 1.31 2015-06-21
7 * @author Cay Horstmann
8 */
9 public class SynchBankTest2
10 {
11 public static final int NACCOUNTS = 100;
12 public static final double INITIAL_BALANCE = 1000;
13 public static final double MAX_AMOUNT = 1000;
14 public static final int DELAY = 10;
15
16 public static void main(String[] args)
17 {
18 Bank bank = new Bank(NACCOUNTS, INITIAL_BALANCE);
19 for (int i = 0; i < NACCOUNTS; i++)
20 {
21 int fromAccount = i;
22 Runnable r = () -> {
23 try
24 {
25 while (true)
26 {
27 int toAccount = (int) (bank.size() * Math.random());
28 double amount = MAX_AMOUNT * Math.random();
29 bank.transfer(fromAccount, toAccount, amount);
30 Thread.sleep((int) (DELAY * Math.random()));
31 }
32 }
33 catch (InterruptedException e)
34 {
35 }
36 };
37 Thread t = new Thread(r);
38 t.start();
39 }
40 }
41 }
SynchBankTest2
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(); } } |
1 class Cbank
2 {
3 private static int s=2000;
4 public static synchronized void sub(int m)
5 {
6
7 int temp=s;
8 temp=temp-m;
9 try {
10 Thread.sleep((int)(1000*Math.random()));
11 }
12 catch (InterruptedException e) { }
13 s=temp;
14 System.out.println("s="+s);
15 }
16 }
17
18
19 class Customer extends Thread
20 {
21 public void run()
22 {
23 for( int i=1; i<=4; i++)
24 Cbank.sub(100);
25 }
26 }
27 public class Thread3
28 {
29 public static void main(String args[])
30 {
31 Customer customer1 = new Customer();
32 Customer customer2 = new Customer();
33 customer1.start();
34 customer2.start();
35 }
36 }
Cbank
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
1 public class Demo {
2 public static void main(String[] args) {
3 Myrhread myrhread = new Myrhread();
4 Thread t1 = new Thread(myrhread);
5 Thread t2 = new Thread(myrhread);
6 Thread t3 = new Thread(myrhread);
7 t1.start();
8 t2.start();
9 t3.start();
10 }
11 }
12
13 class Myrhread implements Runnable {
14 int t = 1;
15 boolean flag = true;
16
17 public void run() {
18 while (flag) {
19 try {
20 Thread.sleep(500);
21 } catch (InterruptedException e) {
22 e.printStackTrace();
23 }
24 synchronized (this) {
25 if (t <= 10) {
26 System.out.println(Thread.currentThread().getName() + "視窗售:第" + t + "張票");
27 t++;
28 }
29 if (t > 10) {
30 flag = false;
31 }
32 }
33 }
34
35 }
36 }
Demo
3.實驗總結:
本週的實驗容量很少,實驗也相對來說簡單,完成的還算順利。學長也教了我們常用的一些快捷鍵,本週的收穫還是很大的。