java面试中单例问题

public class SingltonTest {
private SingltonTest() {
};

private static SingltonTest st;

public static SingltonTest getInstance() {
// 1.3 避过同步锁方法，性能提高
if (null != st) {
return st;
}
synchronized (SingltonTest.class) {
if (null == st) {
st = new SingltonTest();
}
}
return st;
/*
最开始的版本1.1	疏忽：如果多个线程进入if，还是会出现线程安全问题
if (null == st) {
synchronized (SingltonTest.class) {
st = new SingltonTest();
}
}

return st;
--------------------------------
第二个版本1.2		缺点：每次都要使用synchronized块，性能问题
synchronized (SingltonTest.class) {
if (null == st) {
st = new SingltonTest();
}
}

return st;
-------------------------------
1.3换装
if (null == st) {
synchronized (SingltonTest.class) {
if (null == st) {
st = new SingltonTest();
}
}
}
return st;
*/
}
}