JAVA并发编程学习笔记------锁顺序死锁

一、需求描述：

　　将资金从一个账户转移到另一个账户。

二、程序实现：

（1）账户类：

public class Account { private long account; public Account(String user, long account) { this.account = account; } public Account() { super(); } public long getAccount() { return account; } public void setAccount(long account) { this.account = account; } public void debit(long money){ this.account -= money; } public void credit(long money){ this.account += money; } }

　　（2）资产转移类：

public class TransMoney { private static final Object tieLock = new Object(); public static void transferMoney(Account fromAccount,Account toAccount,long amount){ synchronized (fromAccount){ synchronized (toAccount){ fromAccount.debit(amount); toAccount.credit(amount); } } } }

　　（3）测试类：

public class DemonstrateDeadLock { private static final int NUM_THREADS = 20; private static final int NUM_ACCOUNTS = 5; private static final int NUM_ITERATIONS = 1000000; public static void main(String[] args) { final Random rdn = new Random(); final Account[] accounts = new Account[NUM_ACCOUNTS]; for(int i=0;i<accounts.length;i++){ accounts[i] = new Account(); } class TransferThread extends Thread{ public void run(){ for(int i=0;i<NUM_ITERATIONS;i++){ int fromAccount = rdn.nextInt(NUM_ACCOUNTS); int toAccount = rdn.nextInt(NUM_ACCOUNTS); TransMoney.transferMoney(accounts[fromAccount],accounts[toAccount],rdn.nextInt(1000)); } } } for(int i=0;i<NUM_THREADS;i++){ new TransferThread().start(); } } }

　　（4）解析：

上述程序容易形成死锁，原因在于多账户调用TransMoney.transferMoney时，存在锁顺序冲突，

解决方案是使用System.identityHashCode来定义锁的顺序，消除死锁的可能性，代码实现如下：

public static void transferMoney(final Account fromAccount,final Account toAccount,final long amount){ class Helper{ public void transfer(){ fromAccount.debit(amount); toAccount.credit(amount); } } int fromHash = System.identityHashCode(fromAccount); int toHash = System.identityHashCode(toAccount); if(fromHash < toHash){ synchronized (fromAccount){ synchronized (toAccount){ new Helper().transfer(); } } }else if(fromHash > toHash){ synchronized (toAccount){ synchronized (fromAccount){ new Helper().transfer(); } } }else{ synchronized (tieLock){ synchronized (fromAccount){ synchronized (toAccount){ new Helper().transfer(); } } } } }

 

转载于:https://www.cnblogs.com/hunterCecil/p/6189171.html