《Java并发编程实战》第十章避免活跃性危急读书笔记

Posted 2020-09-28 lytwajue

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一、死锁

所谓死锁：是指两个或两个以上的进程在运行过程中，因争夺资源而造成的一种互相等待的现象，若无外力作用，它们都将无法推进下去。

百科百科

当两个以上的运算单元，两方都在等待对方停止执行，以取得系统资源，可是没有一方提前退出时。这样的状况。就称为死锁。维基百科

1. 顺序死锁

最少有两个锁。一个线程获取到A锁须要获取B锁才干进行操作，而另外一个线程获取到了B锁，须要获取A锁才干运行操作，这样的情况下easy出现顺序死锁。

public class LeftRightDeadlock {

	private final Object left = new Object();
	private final Object right = new Object();

	public void leftRight() {
		synchronized (left) {
			synchronized (right) {
				// doSomething();
			}
		}
	}

	public void rightLeft() {
		synchronized (right) {
			synchronized (left) {
				// doSomething();
			}
		}
	}
}

2. 动态的锁顺序死锁

	public void transferMoney(Account fromAccount, Account toAccount, DollarAmount anount)
			throws InsufficientResourcesException {
		synchronized (fromAccount) {
			synchronized (toAccount) {
				if (fromAccount.getBalance().compareTo(amount) < 0) {
					throw new InsufficientResourcesException();
				} else {
					fromAccount.debit(anount);
					toAccount.credit(anount);
				}
			}
		}
	}

A: transferMoney(myAccount, yourAccount, 10);
B: transferMoney(yourAccount, myAccount, 20);

由外部传入的变量全部锁的条件，可是由以上传入的变量能够看到，这样的情况下会出现一个线程先获取myAccount锁在申请yourAccount锁，而另外一个线程相反先获取yourAccount锁在申请myAccount锁。

	private static final Object tieLock = new Object();

	public void transferMoney(final Account fromAccount, final Account toAccount, final DollarAmount anount)
			throws InsufficientResourcesException {
		class Helper{
		    public void transfer() throws InsufficientResourcesException {
		        if (fromAccount.getBalance().compareTo(amount) < 0){
		        	throw new InsufficientResourcesException();
		        } else{
					fromAccount.debit(anount);
					toAccount.credit(anount);
		        }
		    }
		}

		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();
		            }
		        }
		    }
		}
	}

3. 在协作对象之间发生的死锁

class Taxi {

	private Point location, destination;
	private final Dispatcher dispatcher;

	public Taxi(Dispatcher dispatcher) {
	    this.dispatcher = dispatcher;
	}
	
	public synchronized Point getLocation(){
	    return location;
	}

	public synchronized void setLocation(Point location){
	    this.location = location;
	    if (location.equals(destination)){
	        dispatcher.notifyAvaliable(this);
	    }
	}

}

 

class Dispatcher {

	private final Set<Taxi> taxis;
	private final Set<Taxi> avaliableTaxis;

	public Dispatcher(){
	    taxis = new HashSet<Taxi>();
	    avaliableTaxis = new HashSet<Taxi>();
	}

	public synchronized void notifyAvaliable(Taxi taxi) {
	    avaliableTaxis.add(taxi);
	}
	
	public synchronized Image getImage(){
	    Image image = new Image();
	    for (Taxi t :taxis){
	        image.drawMarker(t.getLocation());
	    }
	    return image;
	}

}

4. 开放调用

-- 待填充

5. 资源死锁

外部锁常被忽视而导致死锁，比如数据库的锁