JAVA单例模式的几种方法

public class Elvis
{
public static final Elvis INSTANCE = new Elvis();
private Elvis(){...}
public static Elvis getInstance()
{
return INSTANCE;
}
}

public class Elvis
{
public static final Elvis INSTANCE = null;
private static ReentrantLock lock = new ReentrantLock();
private Elvis()
{

}
public static Elvis getInstance()
{
lock.lock();
if(INSTANCE == NULL)
{
INSTANCE = new Elvis();
}
lock.unlock();
return INSTANCE;
}
}