设计模式中单例模式的双重验证模式的问题

Question

双重验证代码：
/**

• 双重验证懒汉模式（线程安全）
  *
  */
  public class LazyAgainSingletonPattern {

  private static volatile LazyAgainSingletonPattern pattern;

  private LazyAgainSingletonPattern(){

  }

  public static LazyAgainSingletonPattern getInstance() throws InterruptedException {
  if(pattern == null){
  Thread.sleep(1000);
  synchronized (LazyAgainSingletonPattern.class){
  if(pattern == null){
  pattern = new LazyAgainSingletonPattern();
  }
  }
  }
  return pattern;
  }
  }
  可以改为单重验证会有什么问题呢？
  如：public class LazyAgainSingletonPattern {

  private static volatile LazyAgainSingletonPattern pattern;

  private LazyAgainSingletonPattern(){

  }

  public static LazyAgainSingletonPattern getInstance() throws InterruptedException {
  Thread.sleep(1000);
  synchronized (LazyAgainSingletonPattern.class){
  if(pattern == null){
  pattern = new LazyAgainSingletonPattern();
  }
  }
  return pattern;
  }
  }
  少了一重验证，多线程下测试时结果都一样，都是单例对象
  测试代码：
  public class Test {

  public static void main(String[] args) {
  for(int i = 0;i < 10; i++){
  new Thread(()->{
  LazyAgainSingletonPattern singletonPattern = null;
  try {
  singletonPattern = LazyAgainSingletonPattern.getInstance();
  } catch (InterruptedException e) {
  e.printStackTrace();
  }
  System.out.println(singletonPattern);
  }).start();
  }
  }
  }
  输出结果都一样，请问双重验证为什么不能改为单重验证呢

Answer 1

就单例本身来讲，synchronized (LazyAgainSingletonPattern.class)已经能够保证了，我想你的疑惑是为啥在synchronized (LazyAgainSingletonPattern.class)这一句外层还要加一个是否为空的判断，需要说明下：懒加载的双重校验，外面一层是否为空的判断，目的是为了减少并发访问时线程阻塞所带来的问题，如果不为空则不再竞争锁。
我的博客，了解一下https://blog.csdn.net/msx985211/article/details/102715312