写两种常用的单例模式实现，对比一下两种方式的实现有什么差异：
No1：
[java]
public class Test {
private static Test instance = null;
public static Test getInstance()
{
synchronized (Test.class) {
if (instance == null) {
instance = new Test();
}
}
return instance;
}
}
public class Test {
private static Test instance = null;
public static Test getInstance()
{
synchronized (Test.class) {
if (instance == null) {
instance = new Test();
}
}
return instance;
}
}
No2：
[java]
public class Test {
private static Test instance = null;
public synchronized static Test getInstance()
{
if (instance == null) {
instance = new Test();
}
return instance;
}
}
public class Test {
private static Test instance = null;
public synchronized static Test getInstance()
{
if (instance == null) {
instance = new Test();
}
return instance;
}
}