双重锁

public final class Singleton {
    private static volatile Singleton instance = null; // volatile 防止jvm字节码重排后出现DCL失效问题

    private Singleton(){

    }

    public static Singleton getSingleton(){
        if(instance==null){ //避免每次都要进入同步方法，提高效率
            synchronized (Singleton.class){
                if(instance==null){ //防止其他线程已经实例化
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

静态内部类

public class Singleton{
  private Singleton(){}
 
  private static class Holer{// 外部类加载时并不需要立即加载内部类，内部类不被加载则不去初始化INSTANCE，故而不占内存
     private static SingleTon instance = new SingleTon();
 }
 
  public static getSingleton(){
    return Holer.instance;
  }
}