三、程序示例 例一、 让我们看看下面的例子。取钱的流程是输入密码,然后确定要取得金额,如果所取的金额小于或等于可以取出的金额,WITHDRAW则返回TRUE,然后ATM机出钱,然后打印清单;否则返回FALSE,然后打印清单。如下图: public class AutomatedTellerMachine extends Teller { public void withdraw(float amount) { Account a = getAccount(); if (a.deduct(amount)) dispense(amount); printReceipt(); } }
public class Account { private float total; public boolean deduct(float t) { if (t <= total) { total -= t; return true; } return false; } } 就这个例子而言,假设有这种情况,对同一个账号可以在不同的地方取钱,在同一时间,不同地点,妻子和丈夫取钱,妻子输入了账号上的最大金额,丈夫也是一样,假如妻子输入后已经得到true的返回值,但是丈夫的线程所得到的值还没有更新,这样丈夫也能够得到true的返回值,这样就出现了问题!这个问题怎么解决呢?在java里面提供了控制机制以保证deduct操作时的原子性,那就是关键字synchronized。 在Account的deduct方法加入synchronized就可以解决这个问题。 例二、 在这里我们用多线程中最典型的例子,生产者与消费者问题。在这个例子里面我们定义了生产者Producer,消费者Consumer和仓库Warehouse三个类,在整个程序的生命周期里,生产者随机地制造出产品放到仓库中,消费者也是随即地从仓库中取出产品。 import exception.ProducerConsumerException;
/** * Consumer.java * Consumer * By: Jiabo * Date: Mar 21, 2004 * Time: 2:47:58 PM */ public class Consumer extends Thread {
private Warehouse warehouse; private String id;
public Consumer(Warehouse warehouse, String id) { this.warehouse = warehouse; this.id = id; }
public void run() {
int tmp = (int) Math.random() * 10;
try { warehouse.get(tmp); System.out.println("Consumer # " + this.id + " get " + tmp); } catch (ProducerConsumerException e) { e.printStackTrace(); }
try { sleep((int) (Math.random() * 100)); } catch (InterruptedException e) { e.printStackTrace(); } } } 在这个类中,值得注意的一点是run方法中必须使用try-catch,因为,消费者从仓库中取东西时有可能诸如仓库中的储量不够得异常,在消费者里面也是一样,只不过异常变为仓库已满。 import exception.*;
/** * Producer.java * Producer * By: Jiabo * Date: Mar 21, 2004 * Time: 2:47:45 PM */ public class Producer extends Thread {
private Warehouse warehouse; private String id;
public Producer(Warehouse warehouse, String id) { this.warehouse = warehouse; this.id = id; }
public void run() {
int tmp = (int) Math.random() * 10;
if (tmp != 0) { try { warehouse.put(tmp); System.out.println("Consumer # " + this.id + " put " + tmp); } catch (ProducerConsumerException e) { e.printStackTrace(); } }
try { sleep((int) (Math.random() * 100)); } catch (InterruptedException e) { e.printStackTrace(); } } } 最重要的一部分在Warehouse类,如上所说为了保证get何set的原子性,在这里使用了synchronized关键字,并且在操作时抛出了可能跑出的异常。 import exception.*;
/** * Warehouse * By: Jiabo * Date: Mar 21, 2004 * Time: 2:48:10 PM */ public class Warehouse {
// max capability of the warehouse private int MAX; private int contents;
// init with max capacity public Warehouse(int max) { this.MAX = max; this.contents = 0; }
public synchronized void get(int amount) throws ProducerConsumerException {
// the amount you want to get is bigger than the contends that the warehouse stores if (amount > this.contents) { throw new NotEnoughGoodsException(); }
amount -= contents; }
public synchronized void put(int amount) throws ProducerConsumerException {
// the amount you want to put is out of the capability of the warehouse if (amount > (this.MAX - this.contents)) { throw new WarehouseFullException(); } else if (this.contents == 0) { // warehouse is empty throw new WarehouseEmptyException(); }
amount += contents; } } 致谢: 非常感谢挚友eflyer在病中为本文的部分程序提出了宝贵建议,在此表示诚挚的谢意。 参考: § http://www-900.ibm.com/developerWorks/cn/java/j-thread/index.shtml § Java Threads, 2nd edition Scott Oaks & Henry Wong 2nd Edition January 1999 ISBN: 1-56592-418-5, 332 pages
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