实验十七 线程同步控制
实验时间 2018-12-10
1、实验理论知识
多线程
多线程是进程执行过程中产生的多条执行线索。
进程
线程是比进程执行更小的单位。线程不能独立存在，必须存在于进程中，同一进程的各线程间共享进程空间的数据。每个线程有它自身的产生、存在和消亡的过程， 是一个动态的概念。线程创建、销毁和切换的负荷远小于进程，又称 为轻量级进程（lightweight process）。
Java实现多线程
-创建Thread类的子类
-在程序中定义实现Runnable接口的类
用Thread类的子类创建线程
首先需从Thread类派生出一个子类，在该子类中 重写run()方法。
class hand extends Thread { public void run() {……} }
然后用创建该子类的对象
Lefthand left=new Lefthand();
Righthand right=new Righthand();
最后用start()方法启动线程
left.start();
right.start();
用Runnable()接口实现线程
首先设计一个实现Runnable接口的类；
然后在类中根据需要重写run方法；
再创建该类对象，以此对象为参数建立Thread 类的对象；
调用Thread类对象的start方法启动线程，将 CPU执行权转交到run方法。
线程的终止：调用interrupt()方法。
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： - Java SE 5.0中引入ReentrantLock类。 - 在共享内存的类方法前加synchronized修饰符。
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。
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//Bank类
{
private final double[] accounts;//银行运转的基础数据
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);//调用initialBalance生成锁对象属性
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(); //临界区加锁
try
{
while (accounts[from] < amount)
sufficientFunds.await();//用锁对象生成条件对象sufficientFunds
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();
}
finally
{
bankLock.unlock();
}
} /**
* Gets the sum of all account balances.
* @return the total balance
*/
public double getTotalBalance()
{
bankLock.lock();
try
{
double sum = ; 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;
}
}
/**
* 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 = ;
public static final double INITIAL_BALANCE = ;
public static final double MAX_AMOUNT = ;
public static final int DELAY = ; public static void main(String[] args)
{
Bank bank = new Bank(NACCOUNTS, INITIAL_BALANCE);
for (int i = ; 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();
}
}
}
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。
/**
* 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 = ;
public static final double INITIAL_BALANCE = ;
public static final double MAX_AMOUNT = ;
public static final int DELAY = ; public static void main(String[] args)
{
Bank bank = new Bank(NACCOUNTS, INITIAL_BALANCE);//创建一个银行对象
for (int i = ; i < NACCOUNTS; i++)
{
int fromAccount = i;
Runnable r = () -> {
try
{
while (true)
{
int toAccount = (int) (bank.size() * Math.random());//拿出一个随机账户
double amount = MAX_AMOUNT * Math.random();//设定随机一笔钱
bank.transfer(fromAccount, toAccount, amount);//转账操作
Thread.sleep((int) (DELAY * Math.random()));//随机休眠时间
}
}
catch (InterruptedException e)
{
}
};
Thread t = new Thread(r);//创建一个线程
t.start();//线程处于可运行状态
}
}
}
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();//使线程处于等待集中
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());
notifyAll();//唤醒所有等待的线程
} /**
* Gets the sum of all account balances.
* @return the total balance
*/
public synchronized double getTotalBalance()
{
double sum = ; 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;
}
}
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(); } } |
class Cbank
{
private static int s=;//当类加载时s赋值为2000
public static void sub(int m)
{
int temp=s;
temp=temp-m;
try {
Thread.sleep((int)(*Math.random()));
}
catch (InterruptedException e) { }//捕获中断异常
s=temp;
System.out.println("s="+s);
}
} class Customer extends Thread//继承
{
public void run()//中值返回
{
for( int i=; i<=; i++)
Cbank.sub();
}
}
public class Thread3
{
public static void main(String args[])
{
Customer customer1 = new Customer();//把变量customer1的值设置为分配给新的Customer对象的内部地址
Customer customer2 = new Customer();
customer1.start();
customer2.start();
}
}
修改后的代码：
class Cbank
{
private static int s=;
public synchronized static void sub(int m)
{
int temp=s;
temp=temp-m;
try {
Thread.sleep((int)(*Math.random()));
}
catch (InterruptedException e) { }
s=temp;
System.out.println("s="+s);
}
}
class Customer extends Thread
{
public void run()
{
for( int i=; i<=; i++)
Cbank.sub();
}
}
public class Thread3
{
public static void main(String args[])
{
Customer customer1 = new Customer();
Customer customer2 = new Customer();
customer1.start();
customer2.start();
}
}
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
package tracket;
public class tracket {
public static void main(String[] args) {
Mythread mythread = new Mythread();
Thread t1 = new Thread(mythread);//实例化了一个Thread对象thread1
Thread t2 = new Thread(mythread);
Thread t3 = new Thread(mythread);
t1.start();//线程启动
t2.start();
t3.start();
}
}
class Mythread implements Runnable {
int t = ;
boolean flag = true;
@Override
public void run() {
// TODO Auto-generated method stub
while (flag) {
try {
Thread.sleep();//休眠时间500毫秒
} catch (Exception e) {
// TODO: handle exception
e.printStackTrace();
}
synchronized (this) {
if (t <= ) {
System.out.println(Thread.currentThread().getName() + "窗口售：第" + t + "张票");
t++;
}
if (t > ) {
flag = false;
}
}
}
}
}
实验总结：本周实验学习了线程的同步，对线程有了更能进一步的学习，最后的编程的练习题通过学长的讲解以及演示，代码语句的解释，深入了解线程同步问题；本周也是最后一周的实验，这学期通过老师和学长的教学学习到了不少java的知识，非常感谢。