1.懒汉式,线程不安全

 public class Singleton{
    private Singleton() {}

    private static Singleton instance;

    public static Singleton getInstance(){
        if (null == instance){
            instance = new Singleton();
        }
        return instance;
    }
}

2.懒汉式,线程安全

 public class Singleton{
    private Singleton() {}

    private static Singleton instance;

    public static synchronized Singleton getInstance(){
        if (null == instance){
            instance = new Singleton();
        }
        return instance;
    }
}

3.饿汉式,线程安全

public class Singleton{
    private Singleton() {}

    private static Singleton instance = new Singleton();

    public static Singleton getInstance(){
        return instance;
    }
}

4.枚举,线程安全

public enum Singleton {  
    INSTANCE;  
    public void whateverMethod() {  
    }  
}

5.双检锁/双重校验锁

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