一、死锁
1. 顺序死锁
最少有两个锁，一个线程获取到A锁需要获取B锁才能进行操作，而另外一个线程获取到了B锁，需要获取A锁才能执行操作，这种情况下容易出现顺序死锁。
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);
B: transferMoney(yourAccount, myAccount, 20);
由外部传入的变量所有锁的条件，但是由以上传入的变量可以看到，这种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，例如数据库的锁
二、死锁的避免与诊断
1. 支持定时的死锁
存在一些预防死锁的手段，比如Lock的tryLock，JDK 7中引入的Phaser等。
2. 通过线程转储信息来分析死锁
通过Dump线程的StackTrace，例如linux下执行命令 kill -3 <pid>，或者jstack –l <pid>，或者使用Jconsole连接上去查看线程的StackTrace，由此来诊断死锁问题。
三、其他活跃性危险
1. 饥饿
2. 糟糕的响应性
3. 活锁
四、锁的使用
使用支持CAS的数据结构，避免使用锁，如：AtomicXXX、ConcurrentMap、CopyOnWriteList、ConcurrentLinkedQueue
死锁经常是无法完全避免的，鸵鸟策略被很多基础框架所采用。
存在检测死锁的办法
五、参考资料：
《温绍锦 - Java并发程序设计教程》