单例模式几种实现

1.单线程模式下，推荐懒加载

public class Singleton1 {
  private static Singleton1 singleton = null;
  private Singleton1() {
  }
  public static Singleton1 getInstance() {
    if (singleton == null) {
      singleton = new Singleton1();
    }
    return singleton;
  }
}

2.上述1演示，线程不安全，最容易想到的变为线程安全的方式就是加关键字synchronized

public class Singleton1_1 {
  private static Singleton1_1 singleton = null;
  private Singleton1_1() {
  }
  public synchronized static Singleton1_1 getInstance() {
    if (singleton == null) {
      singleton = new Singleton1_1();
    }
    return singleton;
  }
}

3.“双重检查锁”（Double Check Lock，简称DCL）

public class Singleton1_2 {
  private static Singleton1_2 singleton = null;
  private Singleton1_2() {
  }
  public static Singleton1_2 getInstance() {
    if (singleton == null) {
      synchronized (Singleton1_2.class) {
        if (singleton == null) {
          singleton = new Singleton1_2();
        }
      }
    }
    return singleton;
  }
}

4.由于指令重排，上述3，仍然可能不安全，volatile关键字可以解决此问题

public class Singleton1_3 {
  private static volatile Singleton1_3 singleton = null;
  private Singleton1_3() {
  }
  public static Singleton1_3 getInstance() {
    if (singleton == null) {
      synchronized (Singleton1_3.class) {
        if (singleton == null) {
          singleton = new Singleton1_3();
        }
      }
    }
    return singleton;
  }
}

5.饿汉模式

public class Singleton2 {
  private static final Singleton2 singleton = new Singleton2();
  private Singleton2() {
  }
  public static Singleton2 getInstance() {
    return singleton;
  }
}