作者：因情语写

链接：https://www.proprogrammar.com/article/440

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　　单例的实现有多种，有懒汉模式，饿汉模式，枚举方式，静态内部类方式等等，下面说说上面的四种

　　懒汉模式：加载类时就初始化单例

package creational.pattern.singleton;

public class HungrySingleton {
    // 类初始化时获得实例
    private static HungrySingleton INSTANCE = new HungrySingleton();;
    
    // 私有化构造器，只能通过getInstance方法获得单例
    private HungrySingleton(){}
    
    // 获取单例实例
    public static HungrySingleton getInstance()
    {
        return INSTANCE;
    }
}

　　饿汉模式（双重校验锁）

package creational.pattern.singleton;

import java.io.Serializable;

/*
 * jdk 1.5 后可以保证多线程下的安全
 */
public class DoubleCheckedLockingSingleton implements Serializable {
    
    private static final long serialVersionUID = -1926737179243132977L;
    
    private static DoubleCheckedLockingSingleton INSTANCE;

    private DoubleCheckedLockingSingleton() {
    }

    public static DoubleCheckedLockingSingleton getInstance() throws InstantiationException, IllegalAccessException, ClassNotFoundException {
        if (null == INSTANCE) {
            synchronized (DoubleCheckedLockingSingleton.class) {
                if (null == INSTANCE)
                    INSTANCE = (DoubleCheckedLockingSingleton) getClass("creational.pattern.singleton.DoubleCheckedLockingSingleton").newInstance();
            }
        }

        return INSTANCE;
    }

    // 应对不同的类加载器
    private static Class getClass(String classname) throws ClassNotFoundException {
        ClassLoader classLoader = Thread.currentThread().getContextClassLoader();

        if (classLoader == null)
            classLoader = DoubleCheckedLockingSingleton.class.getClassLoader();

        return (classLoader.loadClass(classname));
    }
    
    // 应对序列化与反序列化
    private Object readResolve() 
    {
        return INSTANCE;
    }
}

　　枚举方式，枚举只有一个private的构造方法，可以用来做单例模式

package creational.pattern.singleton;

enum EnumSingleton {
    INSTANCE;
}

　　静态内部类方式

package creational.pattern.singleton;

/*
 * 可以实现懒加载
 */
public class StaticInnerSingleton {
    private static class SingletonHolder{
        private static StaticInnerSingleton INSTANCE = new StaticInnerSingleton();
    }
    
    private StaticInnerSingleton(){}
    
    public static StaticInnerSingleton getInstance()
    {
        return SingletonHolder.INSTANCE;
    }
}

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