详解JAVA 七种创建单例的方法

public class Singleton1 {
  //不能延迟加载 占用内存 耗费资源
  private static Singleton1 singleton1 = new Singleton1();

  public static Singleton1 getSingleton1() {
    return singleton1;
  }
}

public class Singleton2 {

  //延迟加载
  // 多线程下 不安全
  private static Singleton2 singleton1 = null;

  public Singleton2 getSingleton1() {


    if (singleton1==null){
      singleton1 = new Singleton2();
    }
    return singleton1;
  }
}

public class Singleton3 {

  //延迟加载
  // 多线程下 不安全
  private static Singleton3 singleton1 = null;

  //解决延迟加载 多线程安全问题，但存在读操作，加锁问题，线程排队，写操作只有一次 获取时需要排队等候问题
  public synchronized Singleton3 getSingleton1() {

    if (singleton1==null){
      singleton1 = new Singleton3();
    }
    return singleton1;
  }

/*
  等同方法前加锁
  public static Singleton3 getSingleton1() {
    synchronized(Singleton3.class){
      if (singleton1==null){
        singleton1 = new Singleton3();
      }
    }

    return singleton1;
  }
  */


}

public class Singleton4 {

  //延迟加载
  private static Singleton4 singleton1 = null;

  // 解决 读操作 多线程情况下 排队获取问题， 但是双重校验 也存在一个问题，jvm 重排序的问题下 会存在空指针问题
  public static Singleton4 getSingleton1() {

    if (singleton1==null){
      synchronized (Singleton4.class) {
        if (singleton1 == null) {
          singleton1 = new Singleton4();
        }
      }
    }
    return singleton1;
  }


}

public class Singleton5 {

  //延迟加载
  // volatile 关键字 禁止指令重排序
  // 解决 双重校验 也存在一个问题，jvm 重排序的问题下 会存在空指针问题
  private static volatile Singleton5 singleton1 = null;


  public static Singleton5 getSingleton1() {

    if (singleton1==null){
      synchronized (Singleton5.class) {
        if (singleton1 == null) {
          singleton1 = new Singleton5();
        }
      }
    }
    return singleton1;
  }


}

public class Singleton6 {

  //延迟加载
  //静态内部类 静态的始终在jvm中存在一份
  static class Singleton {
    private static Singleton6 singleton1 = new Singleton6();
  }


  public static Singleton6 get(){
    return Singleton.singleton1;
  }
}

public class Singleton7 {

  //枚举类型是 线程安全 构造方法只会被装载一次
  private enum Singleton {
    Singleton;

    private final Singleton7 singleton7;


    Singleton() {
      singleton7 = new Singleton7();
    }


    public Singleton7 getSingleton7() {
      return singleton7;
    }

  }

  //延迟加载
  public static Singleton7 get() {
    return Singleton.Singleton.getSingleton7();
  }


  //测试
  public static void main(String[] args) {

    IntStream.rangeClosed(1, 100).forEach(i -> {
      new Thread(String.valueOf(i)) {
        @Override
        public void run() {
          System.out.println(Singleton7.get());
        }
      }.start();
    });

  }
}