10.1 死锁

哲学家问题

有环

A等B，B等A

数据库往往可以检测和解决死锁//TODO

JVM不行，一旦死锁只有停止重启。

下面分别介绍了几种典型的死锁情况：

10.1.1 Lock ordering Deadlocks

下面是一个经典的锁顺序死锁：两个线程用不同的顺序来获得相同的锁，如果按照锁的请求顺序来请求锁，就不会发生这种循环依赖的情况。

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) {doSomethingElse();}}}void doSomething() {}void doSomethingElse() {}
}

10.1.1 Dynamic Lock Order Deadlocks

下面的转账例子，如果一个线程X向Y转，而另外一个线程Y向X也转，那么就会发生死锁。

public class DynamicOrderDeadlock {// Warning: deadlock-prone!public static void transferMoney(Account fromAccount,Account toAccount,DollarAmount amount)throws InsufficientFundsException {synchronized (fromAccount) {synchronized (toAccount) {if (fromAccount.getBalance().compareTo(amount) < 0)throw new InsufficientFundsException();else {fromAccount.debit(amount);toAccount.credit(amount);}}}}static class DollarAmount implements Comparable<DollarAmount> {// Needs implementationpublic DollarAmount(int amount) {}public DollarAmount add(DollarAmount d) {return null;}public DollarAmount subtract(DollarAmount d) {return null;}public int compareTo(DollarAmount dollarAmount) {return 0;}}static class Account {private DollarAmount balance;private final int acctNo;private static final AtomicInteger sequence = new AtomicInteger();public Account() {acctNo = sequence.incrementAndGet();}void debit(DollarAmount d) {balance = balance.subtract(d);}void credit(DollarAmount d) {balance = balance.add(d);}DollarAmount getBalance() {return balance;}int getAcctNo() {return acctNo;}}static class InsufficientFundsException extends Exception {}
}

解决办法还是顺序话锁，考虑针对两种情况取hashcode然后判断if-else里面决定锁顺序。

class Helper {public void transfer() throws InsufficientFundsException {if (fromAcct.getBalance().compareTo(amount) < 0)throw new InsufficientFundsException();else {fromAcct.debit(amount);toAcct.credit(amount);}}}int fromHash = System.identityHashCode(fromAcct);int toHash = System.identityHashCode(toAcct);if (fromHash < toHash) {synchronized (fromAcct) {synchronized (toAcct) {new Helper().transfer();}}} else if (fromHash > toHash) {synchronized (toAcct) {synchronized (fromAcct) {new Helper().transfer();}}} else {synchronized (tieLock) {synchronized (fromAcct) {synchronized (toAcct) {new Helper().transfer();}}}}

10.1.3 在协作对象之间发生死锁Deadlocks Between Cooperating Objects

下面的例子setLocation和getImage都会获取两把锁，会存在两个线程按照不同的顺序获取锁的情况。

public class CooperatingDeadlock {// Warning: deadlock-prone!class Taxi {@GuardedBy("this") 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.notifyAvailable(this);}public synchronized Point getDestination() {return destination;}public synchronized void setDestination(Point destination) {this.destination = destination;}}class Dispatcher {@GuardedBy("this") private final Set<Taxi> taxis;@GuardedBy("this") private final Set<Taxi> availableTaxis;public Dispatcher() {taxis = new HashSet<Taxi>();availableTaxis = new HashSet<Taxi>();}public synchronized void notifyAvailable(Taxi taxi) {availableTaxis.add(taxi);}public synchronized Image getImage() {Image image = new Image();for (Taxi t : taxis)image.drawMarker(t.getLocation());return image;}}class Image {public void drawMarker(Point p) {}}
}

10.1.4 开放调用

减小锁的力度，锁不嵌套。

class CooperatingNoDeadlock {@ThreadSafeclass Taxi {@GuardedBy("this") 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) {boolean reachedDestination;synchronized (this) {this.location = location;reachedDestination = location.equals(destination);}if (reachedDestination)dispatcher.notifyAvailable(this);}public synchronized Point getDestination() {return destination;}public synchronized void setDestination(Point destination) {this.destination = destination;}}@ThreadSafeclass Dispatcher {@GuardedBy("this") private final Set<Taxi> taxis;@GuardedBy("this") private final Set<Taxi> availableTaxis;public Dispatcher() {taxis = new HashSet<Taxi>();availableTaxis = new HashSet<Taxi>();}public synchronized void notifyAvailable(Taxi taxi) {availableTaxis.add(taxi);}public Image getImage() {Set<Taxi> copy;synchronized (this) {copy = new HashSet<Taxi>(taxis);}Image image = new Image();for (Taxi t : copy)image.drawMarker(t.getLocation());return image;}}class Image {public void drawMarker(Point p) {}}}

1.0.15 资源死锁

• 数据库连接池，A持有数据库D1连接，等待与D2连接，B持有D2的连接，等待与D1连接。
• 线程饥饿死锁，如8.1.1小节的例子。

10.2 死锁的避免与诊断

10.2.1 支持定时的锁

tryLock

10.2.2 kill -3 发信号给JVM dump线程

10.3 其他活跃性危险

10.3.1 饥饿

10.3.3 活锁Livelock

他不会阻塞线程，但是也不能继续执行，因为线程在不断的重复执行相同的操作，而且总会失败。

例如处理事务消，回滚后再次重新把任务放在队头。

又例如发送数据包，都选择1s后重试，那么总会冲突，所以可以考虑一个随机数时间间隔。