Java并发编程实战系列10之避免活跃性危险
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 implementation
public 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 {
@ThreadSafe
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) {
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;
}
}
@ThreadSafe
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 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。
