本帖最后由 朱晓杰 于 2013-5-9 18:47 编辑
今天在整理死锁的时候,自己写了一个小例子,死锁的现象倒是出现了 ,可是在开发过程中要如何才能避免程序死锁呢?据我所知要定义锁的顺序,并在整个程序中都按照这个顺序来获取锁,请问该如何制定锁的顺序呢?下面是我的死锁的例子。
银行转账死锁示例- /*
- * 需求:简单的银行转账,它将资金从一个账户转到另一个账户
- * 在开始转账之前,需要获得两个Account的锁,以确保以原子的方式更新账户中的余额,且不能破坏不可变的条件,如账户的余额不能为负数
- *
- */
- /*账户类*/
- class Account{
- private String accountName;//账号
- private int balance;//资金总额
- public Account(String accountName,int balance){
- this.accountName = accountName;
- this.balance = balance;
- }
- /**/
- public String getAccountName() {//获取账号
- return accountName;
- }
- public int getBalance() {//获取账号余额
- return balance;
- }
-
- public void debit(int amount){//更新转出方余额
- this.balance -= amount;
- }
-
- public void credbit(int amount){//更新转入方余额
- this.balance += amount;
- }
- }
- class TransferAccount implements Runnable{
- public Account fromAccount;//转出账户
- public Account toAccount;//转入账户
- public int amount;//转出金额
- public TransferAccount(Account fromAccount,Account toAccount,int amount){
- this.fromAccount = fromAccount;
- this.toAccount = toAccount;
- this.amount = amount;
- }
- public void run(){
- while(true){
- synchronized(fromAccount){
- synchronized(toAccount){
- if(fromAccount.getBalance() <= 0){//转账进行的条件:判断转出账户的余额是否大于0
- System.out.println(fromAccount.getAccountName() + "账户余额不足,无法进行转账");
- return;
- }else{
- fromAccount.debit(amount);//更新转出账户的余额:-
- toAccount.credbit(amount);//更新转入账户的余额:+
- }
- }
- }
- System.out.println(fromAccount.getAccountName() + "......" + fromAccount.getBalance());//打印转出账户的余额
- System.out.println(toAccount.getAccountName() + "---" + toAccount.getBalance());//打印转入账户的余额
- }
- }
- }
- public class BankTransfer {
- public static void main(String[] args) {
- Account fromAccount = new Account("张三",100000);
- Account toAccount = new Account("李四",200000);
-
- Thread a = new Thread(new TransferAccount(fromAccount,toAccount,1));
- Thread b = new Thread(new TransferAccount(toAccount,fromAccount,2));
-
- a.start();
- b.start();
- }
- }
复制代码 分析发生死锁的原因 :所有的线程并不是按照相同的顺序来获取锁,锁的顺序取决于传递的参数顺序,而这些参数顺序又取决于外部输入,如果线程a和线程b同时转账,线程a从张三向李四转账,线程b从李四向张三转账,那么就会发生 死锁:
a:new TransferAccount(fromAccount,toAccount,1)
b:new TransferAccount(toAccount,fromAccount,2)
如果执行顺序不恰当,那么a可能获得fromAccount的锁并且等待toAccount的锁,然而此时b拥有toAccount的锁,并正在等待fromAccount的锁。 |