单例模式

懒汉（线程不安全)

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

懒汉（线程安全）

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

饿汉

public class Singleton{
    private static Singleton instance =new Singleton();
    private Singleton(){}
    public static Singleton getInstance(){
        return instance;
    }
  }

饿汉（变种）

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

静态内部类

public class Singleton{
    private static class SingletonHolder{
        private static final Singleton instance=new Singleton();
    }
    private Singleton(){}
    public static final Singleton getInstance(){
        return SingletonHolder.instance;
    }
}

枚举

public enum Singleton{
    instance;
    Singleton(){
        instance=Singleton();
    }
    public void getInstance(){
        return instance;
    }
}

双重校验锁

public class Singleton{
    private volatile static Singleton instance;
    private Singleton(){}
    public static Singleton getInstance(){
        if(instance==null){
            synchronized(Singleton.class){
                if(instance==null){
                        instance=new Singleton();
                }
              }
        }
}