关于java单例模式实现

public class SingletonTest {

private SingletonTest(){}

private static final SingletonTest instance = new SingletonTest();

public static SingletonTest getInstance(){

return instance;

}

}

public class SingletonTest1 {

private SingletonTest1(){}

public static SingletonTest1 getInstance(){

return A.INSTANCE;

}

private static class A{

private static final SingletonTest1 INSTANCE = new SingletonTest1();

}

}