Double-checked locking and the Singleton pattern--双重检查加锁失效原因剖析

以下内容摘取自http://stackoverflow.com/questions/11195389/out-of-order-writes-for-double-checked-locking

Thread1 could  publish the [code]instance

reference to the main memory, but fail to publish any other data inside the

Singleton

object that wascreated. Thread2 will observe the object in an inconsistent state.

大概意思是Thread2有可能在Thread1构造函数执行一部分的时候读取Instance，比如Vector赋值，但inUser为false时，这时候就会造成两个线程获取的instance状态不一致。

[/code]

import java.util.Vector;

class Singleton {

private static Singleton instance;
private Vector v;
private boolean inUse;

private Singleton() {
v = new Vector();
v.addElement(new Object());
inUse = true;
}

public static Singleton getInstance() {
if (instance == null) {
synchronized (Singleton.class) { // 1
if (instance == null) // 2
instance = new Singleton(); // 3
}
}
return instance;
}
}