锁顺序死锁
public class LeftRightDeadlock {
private final Object left = new Object();
private final Object right = new Object();
public void leftRight() {
// 得到left锁
synchronized (left) {
// 得到right锁
synchronized (right) {
doSomething();
}
}
}
public void rightLeft() {
// 得到right锁
synchronized (right) {
// 得到left锁
synchronized (left) {
doSomethingElse();
}
}
}
}
动态锁顺序死锁
// 转账
public static void transferMoney(Account fromAccount,
Account toAccount,
DollarAmount amount)
throws InsufficientFundsException {
// 锁定汇账账户
synchronized (fromAccount) {
// 锁定来账账户
synchronized (toAccount) {
// 判余额是否大于0
if (fromAccount.getBalance().compareTo(amount) < 0) {
throw new InsufficientFundsException();
} else {
// 汇账账户减钱
fromAccount.debit(amount);
// 来账账户增钱
toAccount.credit(amount);
}
}
}
}
如果两个线程同时调用transferMoney()
线程A从X账户向Y账户转账
线程B从账户Y向账户X转账
那么就会发生死锁。
固定锁顺序避免死锁
public class InduceLockOrder {
// 额外的锁、避免两个对象hash值相等的情况(即使很少)
private static final Object tieLock = new Object();
public void transferMoney(final Account fromAcct,
final Account toAcct,
final DollarAmount amount)
throws InsufficientFundsException {
class Helper {
public void transfer() throws InsufficientFundsException {
if (fromAcct.getBalance().compareTo(amount) < 0)
throw new InsufficientFundsException();
else {
fromAcct.debit(amount);
toAcct.credit(amount);
}
}
}
// 得到锁的hash值
int fromHash = System.identityHashCode(fromAcct);
int toHash = System.identityHashCode(toAcct);
// 根据hash值来上锁
if (fromHash < toHash) {
synchronized (fromAcct) {
synchronized (toAcct) {
new Helper().transfer();
}
}
} else if (fromHash > toHash) {// 根据hash值来上锁
synchronized (toAcct) {
synchronized (fromAcct) {
new Helper().transfer();
}
}
} else {// 额外的锁、避免两个对象hash值相等的情况(即使很少)
synchronized (tieLock) {
synchronized (fromAcct) {
synchronized (toAcct) {
new Helper().transfer();
}
}
}
}
}
}
得到对应的hash值来固定加锁的顺序
协作对象之间发生死锁
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;
}
// setLocation 需要Taxi内置锁
public synchronized void setLocation(Point location) {
this.location = location;
if (location.equals(destination))
// 调用notifyAvailable()需要Dispatcher内置锁
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);
}
// 调用getImage()需要Dispatcher内置锁
public synchronized Image getImage() {
Image image = new Image();
for (Taxi t : taxis)
// 调用getLocation()需要Taxi内置锁
image.drawMarker(t.getLocation());
return image;
}
}
class Image {
public void drawMarker(Point p) {
}
}
}
上面的getImage()和setLocation(Point location)都需要获取两个锁的
并且在操作途中是没有释放锁的
这就是隐式获取两个锁(对象之间协作)..
这种方式也很容易就造成死锁.....
开放调用避免死锁
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 void setLocation(Point location) {
boolean reachedDestination;
// 加Taxi内置锁
synchronized (this) {
this.location = location;
reachedDestination = location.equals(destination);
}
// 执行同步代码块后完毕，释放锁
if (reachedDestination)
// 加Dispatcher内置锁
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;
// Dispatcher内置锁
synchronized (this) {
copy = new HashSet<Taxi>(taxis);
}
// 执行同步代码块后完毕，释放锁
Image image = new Image();
for (Taxi t : copy)
// 加Taix内置锁
image.drawMarker(t.getLocation());
return image;
}
}
class Image {
public void drawMarker(Point p) {
}
}
}
使用定时锁
使用显式Lock锁，在获取锁时使用tryLock()方法。当等待超过时限的时候，tryLock()不会一直等待，而是返回错误信息。
使用tryLock()能够有效避免死锁问题~~
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