dcl = Double-checked locking如下：
public static Singleton getInstance()
{
if (instance == null)
{
synchronized(Singleton.class) { //1
Singleton inst = instance; //2
if (inst == null)
{
synchronized(Singleton.class) { //3
//inst = new Singleton(); //4
instance = new Singleton();
}
//instance = inst; //5
}
}
}
return instance;
}
我觉得没必要 dcl，两次 synchronized 难道不花费更多的时间么？本来为了提高性能，反而把问题复杂化了。
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 synchronized Singleton getInstance()
{
if (instance == null) //1
instance = new Singleton(); //2
return instance; //3
}
}
这样不会影响多少性能，不就是等待一个 return 的时间么。
class Singleton
{
private Vector v;
private boolean inUse;
private static Singleton instance = new Singleton();
private Singleton()
{
v = new Vector();
inUse = true;
//...
}
public static Singleton getInstance()
{
return instance;
}
}
也可以，不用等，只是没有了 lazy initialize 好处。
