避免死锁例子(Avoid Deadlock example)

2019-07-03 18:03发布

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我想知道什么是替代方法，以避免在下面的例子僵局。下面的例子是一个典型的银行帐户转帐死锁问题。什么是一些更好的去解决它在实践中的方法？

class Account {
     double balance;
     int id;
     public Account(int id, double balance){
          this.balance = balance;
          this.id = id;
     }
     void withdraw(double amount){
          balance -= amount;
     } 
     void deposit(double amount){
          balance += amount;
     }
}
class Main{
     public static void main(String [] args){
           final Account a = new Account(1,1000);
           final Account b = new Account(2,300);
           Thread a = new Thread(){
                 public void run(){
                     transfer(a,b,200);
                 }
           };
           Thread b = new Thread(){
                 public void run(){
                     transfer(b,a,300);
                 }
           };
           a.start();
           b.start();
     }
     public static void transfer(Account from, Account to, double amount){
          synchronized(from){
               synchronized(to){
                    from.withdraw(amount);
                    to.deposit(amount);
               }
          }
     }
}

我想知道它会解决这个僵局的问题，如果我喜欢跟着我转出的方法分离嵌套锁

 synchronized(from){
      from.withdraw(amount);
 }
 synchronized(to){
      to.deposit(amount);
 }

Answer 1:

排序的账户。死锁是从账户的顺序（A，B对B，A）。

因此，尝试：

 public static void transfer(Account from, Account to, double amount){
      Account first = from;
      Account second = to;
      if (first.compareTo(second) < 0) {
          // Swap them
          first = to;
          second = from;
      }
      synchronized(first){
           synchronized(second){
                from.withdraw(amount);
                to.deposit(amount);
           }
      }
 }

Answer 2:

这是一个经典的问题。我看到了两个可能的解决方案：

排序账户，并在具有比另一个低的ID帐户同步。在第10章中的实践并发Java并发的圣经中提到在这本书的作者这种方法使用系统的散列码来区分帐户。见java.lang.System中的＃identityHashCode 。
第二种解决方案是由你所提到的 - 是的，你能避免嵌套同步块和你的代码不会导致死锁。但是，在这种情况下，处理可能有一些问题，因为如果从第一个帐户中提款第二帐户可能被锁定任何显著时候，也许你会需要把钱退给第一个帐户。这是不好的，因为嵌套同步和两个帐户锁定较好，比较常用的解决方案。

Answer 3:

除了锁定的解决方案命令你也可以通过执行任何转账前的私有静态最终锁定对象上同步避免死锁。

 class Account{
 double balance;
 int id;
 private static final Object lock = new Object();
  ....




 public static void transfer(Account from, Account to, double amount){
          synchronized(lock)
          {
                    from.withdraw(amount);
                    to.deposit(amount);
          }
     }

该解决方案有一个私人静态锁系统限制在执行转让“顺序”的问题。

另外一个可能是，如果每个账户有一个ReentrantLock的：

private final Lock lock = new ReentrantLock();




public static void transfer(Account from, Account to, double amount)
{
       while(true)
        {
          if(from.lock.tryLock()){
            try { 
                if (to.lock.tryLock()){
                   try{
                       from.withdraw(amount);
                       to.deposit(amount);
                       break;
                   } 
                   finally {
                       to.lock.unlock();
                   }
                }
           }
           finally {
                from.lock.unlock();
           }

           int n = number.nextInt(1000);
           int TIME = 1000 + n; // 1 second + random delay to prevent livelock
           Thread.sleep(TIME);
        }

 }

因为这些锁将永远不会被无限期关押的僵局不会在这种方法中发生。如果当前对象的锁被获取，但第二锁不可用，则第一锁被释放，线程尝试重新获取锁之前休眠一定的时间规定量。

Answer 4:

您也可以为每个账户单独锁（在Account类），然后做交易前获得两个锁。看一看：

private boolean acquireLocks(Account anotherAccount) {
        boolean fromAccountLock = false;
        boolean toAccountLock = false;
        try {
            fromAccountLock = getLock().tryLock();
            toAccountLock = anotherAccount.getLock().tryLock();
        } finally {
            if (!(fromAccountLock && toAccountLock)) {
                if (fromAccountLock) {
                    getLock().unlock();
                }
                if (toAccountLock) {
                    anotherAccount.getLock().unlock();
                }
            }
        }
        return fromAccountLock && toAccountLock;
    }

得到两把锁后，你可以做传输，而无需担心安全。

    public static void transfer(Acc from, Acc to, double amount) {
        if (from.acquireLocks(to)) {
            try {
                from.withdraw(amount);
                to.deposit(amount);
            } finally {
                from.getLock().unlock();
                to.getLock().unlock();
            }
        } else {
            System.out.println(threadName + " cant get Lock, try again!");
            // sleep here for random amount of time and try do it again
            transfer(from, to, amount);
        }
    }

Answer 5:

这里是陈述的问题的解决方案。

import java.util.Random;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class FixDeadLock1 {

    private class Account {

        private final Lock lock = new ReentrantLock();

        @SuppressWarnings("unused")
        double balance;
        @SuppressWarnings("unused")
        int id;

        public Account(int id, double balance) {
            this.balance = balance;
            this.id = id;
        }

        void withdraw(double amount) {
            this.balance -= amount;
        }

        void deposit(double amount) {
            balance += amount;
        }
    }

    private class Transfer {

        void transfer(Account fromAccount, Account toAccount, double amount) {
            /*
             * synchronized (fromAccount) { synchronized (toAccount) {
             * fromAccount.withdraw(amount); toAccount.deposit(amount); } }
             */

            if (impendingTransaction(fromAccount, toAccount)) {
                try {
                    System.out.format("Transaction Begins from:%d to:%d\n",
                            fromAccount.id, toAccount.id);
                    fromAccount.withdraw(amount);
                    toAccount.deposit(amount);
                } finally {
                    fromAccount.lock.unlock();
                    toAccount.lock.unlock();
                }

            } else {
                 System.out.println("Unable to begin transaction");
            }

        }

        boolean impendingTransaction(Account fromAccount, Account toAccount) {

            Boolean fromAccountLock = false;
            Boolean toAccountLock = false;

            try {
                fromAccountLock = fromAccount.lock.tryLock();
                toAccountLock = toAccount.lock.tryLock();
            } finally {
                if (!(fromAccountLock && toAccountLock)) {
                    if (fromAccountLock) {
                        fromAccount.lock.unlock();
                    }
                    if (toAccountLock) {
                        toAccount.lock.unlock();
                    }
                }
            }

            return fromAccountLock && toAccountLock;
        }

    }

    private class WrapperTransfer implements Runnable {
        private Account fromAccount;
        private Account toAccount;
        private double amount;

        public WrapperTransfer(Account fromAccount,Account toAccount,double amount){
            this.fromAccount = fromAccount;
            this.toAccount = toAccount; 
            this.amount = amount;
        }

        public void run(){
            Random random = new Random();
            try {
                int n = random.nextInt(1000);
                int TIME = 1000 + n; // 1 second + random delay to prevent livelock
                Thread.sleep(TIME);
            } catch (InterruptedException e) {}
            new Transfer().transfer(fromAccount, toAccount, amount);
        }

    }

    public void initiateDeadLockTransfer() {
        Account from = new Account(1, 1000);
        Account to = new Account(2, 300);       
        new Thread(new WrapperTransfer(from,to,200)).start();
        new Thread(new WrapperTransfer(to,from,300)).start();
    }

    public static void main(String[] args) {
        new FixDeadLock1().initiateDeadLockTransfer();
    }

}

Answer 6:

还有，你必须满足三个要求：

始终如一地由指定量减少一个帐户中的内容。
始终如一地由指定的数量增加其他帐户的内容。
如果上面的一个是成功的，对方也必须是成功的。

您可以通过使用达到1和2 原子能公司，但你将不得不使用其他的东西double ，因为没有AtomicDouble 。 AtomicLong很可能是你最好的选择。

所以你离开了你的第三个要求-如果一个成功的另一个必须成功。有一个简单的技术与原子能作品华丽，并且使用getAndAdd方法。

class Account {
  AtomicLong balance = new AtomicLong ();
}

...
Long oldDebtor = null;
Long oldCreditor = null;
try {
  // Increase one.
  oldDebtor = debtor.balance.getAndAdd(value);
  // Decrease the other.
  oldCreditor = creditor.balance.gtAndAdd(-value);
} catch (Exception e) {
  // Most likely (but still incredibly unlikely) InterruptedException but theoretically anything.
  // Roll back
  if ( oldDebtor != null ) {
    debtor.getAndAdd(-value);
  }
  if ( oldCreditor != null ) {
    creditor.getAndAdd(value);
  }
  // Re-throw after cleanup.
  throw (e);
}

文章来源: Avoid Deadlock example