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单例模式实现

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单例模式的实现方式

单例模式实现

懒汉式

第一次调用时才实例化对象

线程不安全

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/**
 * 懒汉式非线程安全单例
 */
public class Singleton_01 {

    private static Singleton_01 instance;

    private Singleton_01() {
    }

    public static Singleton_01 getInstance(){
        if (null != instance) return instance;
        return new Singleton_01();
    }
}

线程安全

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public class Singleton_02 {

    private static Singleton_02 instance;

    private Singleton_02() {
    }

    public static synchronized Singleton_02 getInstance(){
        if (null != instance) return instance;
        return new Singleton_02();
    }

}

双检索

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public class Singleton_05 {

    private static volatile Singleton_05 instance;

    private Singleton_05() {
    }

    public static Singleton_05 getInstance(){
       if(null != instance) return instance;
       synchronized (Singleton_05.class){
           if (null == instance){
               instance = new Singleton_05();
           }
       }
       return instance;
    }

}

内部类

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public class Singleton_04 {

    private static class SingletonHolder {
        private static Singleton_04 instance = new Singleton_04();
    }

    private Singleton_04() {
    }

    public static Singleton_04 getInstance() {
        return SingletonHolder.instance;
    }

}

饿汉式

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public class Singleton_03 {

    private static Singleton_03 instance = new Singleton_03();

    private Singleton_03() {
    }

    public static Singleton_03 getInstance() {
        return instance;
    }

}

枚举

通过枚举实现单例,序列化和反序列化后持有对象哈希码相同，还是同一个对象。

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public enum Singleton_07 {

    INSTANCE;
    private User instance;

    Singleton_07() {
        this.instance = new User();
    }

    public User getInstance() {
        return instance;
    }
}

校验代码

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@Test
public void test() throws IOException, ClassNotFoundException {
    System.out.println(Singleton_07.INSTANCE.getInstance().hashCode());
    ObjectOutputStream oos=new ObjectOutputStream(new FileOutputStream("D:\\object.txt"));
    oos.writeObject(Singleton_07.INSTANCE);
    oos.flush();
    oos.close();

    ObjectInputStream ois=new ObjectInputStream(new FileInputStream("D:\\object.txt"));
    Object o = ois.readObject();
    Singleton_07 s= (Singleton_07) o;
    System.out.println(s.getInstance().hashCode());
    ois.close();
}

输出结果:

image-20220223172244271