一.理论知识部分
Java 的线程调度采用优先级策略:优先级高的先执行，优先级低的后执行；多线程系统会自动为每个线程分配一个优先级，缺省时，继承其父类的优先级； 任务紧急的线程，其优先级较高； 同优先级的线程按“先进先出”的队列原则。
调用setPriority(int a)重置当前线程的优先级，a取值可以是前述的三个静态量。调用getPriority()获得当前线程优先级。
多线程并发运行不确定性问题解决方案：引入线程同步机制，使得另一线程要使用该方法，就只能等待。
在Java中解决多线程同步问题的方法有两种：J ava SE 5.0中引入ReentrantLock类。 在共享内存的类方法前加synchronized修饰符。
有关锁对象和条件对象的关键要点:锁用来保护代码片段，保证任何时刻只能有一个线程执行被保护的代码。锁管理试图进入被保护代码段的线程。锁可拥有一个或多个相关条件对象。每个条件对象管理那些已经进入被保护的代码 段但还不能运行的线程。
synchronized关键字作用： 某个类内方法用synchronized 修饰后，该方法被称为同步方法；只要某个线程正在访问同步方法，其他线程欲要访问同步方法就被阻塞，直至线程从同 步方法返回前唤醒被阻塞线程，其他线程方可能进入同步方法。
在同步方法中使用wait()、notify 和notifyAll()方法：一个线程在使用的同步方法中时，可能根据问题的需要，必须使用wait()方法使本线程等待，暂时让出CPU的使用权，并允许其它线程使用这个同步方法。线程如果用完同步方法，应当执行notifyAll()方 法通知所有由于使用这个同步方法而处于等待的线程结束等待。
二.实验部分
1、实验目的与要求
(1) 掌握线程同步的概念及实现技术；
(2) 线程综合编程练习
2、实验内容和步骤
实验1：测试程序并进行代码注释。
测试程序1：
l 在Elipse环境下调试教材651页程序14-7，结合程序运行结果理解程序；
l 掌握利用锁对象和条件对象实现的多线程同步技术。
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()));//随机生成时间，使正在执行的线程休眠
30 }
31 }
32 catch (InterruptedException e)
33 {
34 }
35 };
36 Thread t = new Thread(r);
37 t.start();//开始线程
38 }
39 }
40 }
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();//使用锁对象,获取锁
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();//唤醒所有线程
49 }
50 finally
51 {
52 bankLock.unlock();//释放锁
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 }
测试程序2：
l 在Elipse环境下调试教材655页程序14-8，结合程序运行结果理解程序；
l 掌握synchronized在多线程同步中的应用。
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修饰符
32 public synchronized void transfer(int from, int to, double amount) throws InterruptedException
33 {
34 while (accounts[from] < amount)
35 wait();//来自Object类
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();//解除所有线程的阻塞状态
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 }
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 }
测试程序3：
l 在Elipse环境下运行以下程序，结合程序运行结果分析程序存在问题；
l 尝试解决程序中存在问题。
|
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 }
实验2 编程练习
利用多线程及同步方法，编写一个程序模拟火车票售票系统，共3个窗口，卖10张票，程序输出结果类似（程序输出不唯一，可以是其他类似结果）。
Thread-0窗口售：第1张票
Thread-0窗口售：第2张票
Thread-1窗口售：第3张票
Thread-2窗口售：第4张票
Thread-2窗口售：第5张票
Thread-1窗口售：第6张票
Thread-0窗口售：第7张票
Thread-2窗口售：第8张票
Thread-1窗口售：第9张票
Thread-0窗口售：第10张票
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() + "窗口售：第" + t + "張票");
27 t++;
28 }
29 if (t > 10) {
30 flag = false;
31 }
32 }
33 }
34
35 }
36 }
3.实验总结：
本周的实验容量很少，实验也相对来说简单，完成的还算顺利。学长也教了我们常用的一些快捷键，本周的收获还是很大的。