单例模式的两种较好的实现方式

public class A {
private static A instance=null;

private A(){
}

public static A getInstance(){
if(instance==null){
synchronized(A.class){
if(instance==null){
instance=new A();
}
}
}
return instance;
}
}

二:

public class B {
private B() {
}

private static class BHolder {
private final static B instance = new B();
}

public static B getInstance() {
return BHolder.instance;
}
}