《Java线程--10-5--北大计算机系java培训讲义.ppt

* 7.4 多线程问题--线程间的通信 . class Writer extends Thread { private PipedOutputStream outStream;//将数据输出 private String messages[ ]= { Monday, Tuesday , Wednsday, Thursday,Friday :, Saturday:,Sunday :}; public Writer(PipedOutputStream o) { outStream = o; } public void run() { PrintStream p = new PrintStream( outStream ); for (int i = 0; i messages.length; i++) { p.println(messages[ i ]); p.flush(); System.out.println(WrIte: + messages[i] ); } p.close(); p = null; }} Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. * 7.3 多线程问题---资源协调 1. 数据的完整性 线程1 线程2 线程10 资源 取过来 加1后送回去 withdrwal() withdrwal() 透支 余额 变量 Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. * 7.3 多线程问题---资源协调 对共享对象的访问必须同步,叫做条件变量. Java语言允许通过监视器(有的参考书称其为管程)使用条件变量实现线程同步. 监视器阻止两个线程同时访问同一个条件变量.它的如同锁一样作用在数据上. 线程1进入withdrawal方法时,获得监视器(加锁);当线程1的方法执行完毕返回时,释放监视器(开锁),线程2的withdrawal方能进入. withdrawal() 线程1 监视器 线程2 Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. * 7.3 多线程问题---资源协调 用synchronized来标识的区域或方法即为监视器监视的部分。 一个类或一个对象由一个监视器,如果一个程序内有两个方法使用synchronized标志,则他们在一个监视器管理之下. 一般情况下，只在方法的层次上使用关键区 read write 监 视 器 线程1 线程2 Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. * 7.3 多线程问题---资源协调 此处给出的例子演示两个线程在同步限制下工作的情况. class Account { statics int balance=1000; //为什么用static? statics int expense=0; public synchronized void withdrawl(int amount) { if (amount=balance) { balance-=amount; expense+=amount;} else { System.out.println(“bounced: “+amount);} } } Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. * 7.3 多线程问题---资源协调 2.