饿汉式:静态成员变量

public class Singleton{
    private Singleton(){};

    private static Singleton instanse = mew Singleton();

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

饿汉式，静态代码块

public class Singleton{
    private Singleton(){};

    private static Singleton instance;

    static{
        instance = new Singleton();
    }

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

饿汉式，枚举

public enum Singleton{
    INSTANCE;
}

懒汉式：线程不安全

public class Singleton{
    private Singleton(){};

    private static Singleton instance;

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

懒汉式：线程安全

public class Singleton{
    private Singleton(){};

    private static Singleton instance;

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

懒汉式：双重检查锁

public class Singleton{
    private Singleton(){};

    private static volatile Singleton instance;

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

懒汉式：静态内部类

public class Singleton{
    private Singleton(){};

    public static class SingletonHolder{
        private static final Singleton INSTANCE = new Singleton();
    }

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