设计模式-单例模式

反编译工具jad
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4、单例模式
饿汉式，比较消耗内存。当程序需要创建大量单例时，会影响程序启动速度。

package com.jdwa.singleton;

public class HungrySingleton {
    private static final HungrySingleton instance= new HungrySingleton();

    private HungrySingleton(){}

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

简单的懒汉式，解决了内存消耗问题，但是存在线程安全问题。

package com.jdwa.singleton;

public class LazySingleton {
    private static LazySingleton instance;

    private LazySingleton(){}

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

带synchronized关键字的懒汉式，解决了内存消耗以及线程安全问题，但是创建大量单例会因为锁而导致性能下降。

package com.jdwa.singleton;

public class LazySingletonWithSync {
    private static LazySingletonWithSync instance;

    private LazySingletonWithSync(){}

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

双检索单例，在一定程度上解决了性能问题。

package com.jdwa.singleton;

public class LazyDoudleCheckSingleton {
    private static LazyDoudleCheckSingleton instance = new LazyDoudleCheckSingleton();

    private LazyDoudleCheckSingleton(){}

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

静态内部类，同时解决了上述问题，到那时会被通过反射创建。默认不加载静态内部类，只有使用的时候才会加载。

package com.jdwa.singleton;

public class LazyInnerClassSingleton {
    private LazyInnerClassSingleton(){}
    
    public static LazyInnerClassSingleton getInstance(){
        return LazyHolder.INSTANCE;
    }
    
    private static class LazyHolder {
        private static final LazyInnerClassSingleton INSTANCE = new LazyInnerClassSingleton();
    }
}

反射会破坏单例，以静态内部类举例

package com.jdwa.singleton;

import java.lang.reflect.Constructor;

public class LazyInnerClassTest {
    public static void main(String[] args) throws Exception{
        Class<?> clazz = LazyInnerClassSingleton.class;
        Constructor constructor = clazz.getDeclaredConstructor();//返回无参的public和非public的

        constructor.setAccessible(true); //强制访问
        Object o1 = constructor.newInstance();
        Object o2 = constructor.newInstance();
        System.out.println(o1==o2); //false
    }
}

优化静态内部类：私有的构造器添加判断

package com.jdwa.singleton;

public class LazyInnerClassWithImProvedCon {
    private LazyInnerClassWithImProvedCon(){
        if (LazyHolder.INSTANCE != null) {
            throw new RuntimeException("不允许创建多个实例");
        }
    }

    public static LazyInnerClassWithImProvedCon getInstance(){
        return LazyInnerClassWithImProvedCon.LazyHolder.INSTANCE;
    }

    private static class LazyHolder {
        private static final LazyInnerClassWithImProvedCon INSTANCE = new LazyInnerClassWithImProvedCon();
    }
}

当单例模式支持序列化时，也可能被破坏，解决方案：加一个readResolve方法
代码与测试如下：

package com.jdwa.singleton;

import java.io.*;

public class SeriableSingleton implements Serializable {
    private final static SeriableSingleton INSTANCE = new SeriableSingleton();
    private SeriableSingleton(){}

    public static SeriableSingleton getInstance(){
        return INSTANCE;
    }

    private Object readResolve(){
        return INSTANCE;
    }

    public static void main(String[] args) {
        SeriableSingleton s1 = null;
        SeriableSingleton s2 = getInstance();

        FileOutputStream fos = null;
        try {
            fos = new FileOutputStream("SeriableSingleton.obj");
            ObjectOutputStream oos = new ObjectOutputStream(fos);
            oos.writeObject(s2);
            oos.flush();
            oos.close();

            FileInputStream fis = new FileInputStream("SeriableSingleton.obj");
            ObjectInputStream ois = new ObjectInputStream(fis);
            s1 = (SeriableSingleton) ois.readObject();
            ois.close();

            System.out.println(s1 == s2); //true
        }catch (Exception e){
            e.printStackTrace();
        }
    }
}

通过该方法可以避免单例对象被序列化破坏，但是同样的，通过源码可以看出，该实例是被实例化了两次，只不过只返回了一次。当有大量单例对象被实例化，就会造成内存开销的增加。

要解决这个问题，可以使用注册式单例。其思路是将每一个实例都登记到某一个地方，使用唯一标记符来获取单例。有两种方式：枚举式，容器式。
枚举式单例：

package com.jdwa.singleton;

public enum EnumSingleton {
    INSTANCE;
    private Object data;

    public Object getData() {
        return data;
    }

    public void setData(Object data) {
        this.data = data;
    }
    
    public static EnumSingleton getInstance(){
        return INSTANCE;
    }
    
}

容器式：

package com.jdwa.singleton;

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

public class ContainerSingleton {
    private ContainerSingleton(){}
    private static Map<String,Object> ioc = new ConcurrentHashMap<>();
    public static Object getBean(String className){
        synchronized (ioc) {
            if (!ioc.containsKey(className)){
                Object obj = null;
                try {
                    obj = Class.forName(className).newInstance();
                    ioc.put(className,obj);
                }catch (Exception e){
                    e.printStackTrace();
                }
                return obj;
            } else {
                return ioc.get(className);
            }
        }
    }
    
}

ThreadLocal 线程单例实现

package com.jdwa.singleton;

public class ThreadLocalSingleton {
    private static final ThreadLocal<ThreadLocalSingleton> threadlocalInstance = new ThreadLocal<ThreadLocalSingleton>(){
        @Override
        protected ThreadLocalSingleton initialValue(){
            return new ThreadLocalSingleton();
        }
    };
    
    private ThreadLocalSingleton(){}
    
    public static ThreadLocalSingleton getInstance(){
        return threadlocalInstance.get();
    }
}

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