1.SPI介绍

参考此篇文章
https://www.cnblogs.com/huzi007/p/6679215.html

2.dubbo扩展点

参考此篇文章
http://blog.csdn.net/xiaoxufox/article/details/75117992

可以理解ExtensionLoader相比JDK SPI是扩展版本

ServiceLoader只能遍历加载所有实现，且非单实例
ExtensionLoader可以加载特定类,且是单实例

我们重点关注Adaptive的使用

3.Adaptive注解

3.1原理

可以先参考此篇文章
http://gaofeihang.cn/archives/278

其适配器有2种形式定义

静态代码适配
动态代码生成适配

3.2CacheFactory示例

我们来看内置的CacheFactory

代码示例1

@SPI("lru")
public interface CacheFactory {

    @Adaptive("cache")
    Cache getCache(URL url);

}

其动态生成的适配器代码如下:

代码示例2

package com.alibaba.dubbo.cache;
import com.alibaba.dubbo.common.extension.ExtensionLoader;

public class CacheFactory$Adpative implements com.alibaba.dubbo.cache.CacheFactory {
    public com.alibaba.dubbo.cache.Cache getCache(com.alibaba.dubbo.common.URL arg0) {
        if (arg0 == null) throw new IllegalArgumentException("url == null");
        com.alibaba.dubbo.common.URL url = arg0;
        String extName = url.getParameter("cache", "lru");
        if (extName == null)
            throw new IllegalStateException("Fail to get extension(com.alibaba.dubbo.cache.CacheFactory) name from url(" 
+ url.toString() + ") use keys([cache])");
        com.alibaba.dubbo.cache.CacheFactory extension = (com.alibaba.dubbo.cache.CacheFactory) ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.cache.CacheFactory.class).getExtension(extName);
        return extension.getCache(arg0);
    }
}

上面代码第8行是重点,URL的作用有2点

通过参数来选择适配对象
通过URL来传递参数,该功能作为可选项,如果只是当做适配器使用,URL参数就是显得比较多余

3.3自定义示例

通过自定义的例子再了来看一下

代码示例3

@SPI("dirFirst")
public interface DirExtension {

    @Adaptive({SpiConstants.DIR_DEFAULT})
    String sayHello(URL url, String name);
}

public class DirExtensionFirstImpl implements DirExtension {

    @Override
    public String sayHello(URL url, String name) {
        return name + "-first";
    }
}

public class DirExtensionDefaultImpl implements DirExtension {

    @Override
    public String sayHello(URL url, String name) {
        return name + "-defualt";
    }
}

public class DirExtensionTest {

    public static void main(String[] args){
        ExtensionLoader extensionLoader = ExtensionLoader.getExtensionLoader(DirExtension.class);
        DirExtension dirExtension = (DirExtension)extensionLoader.getAdaptiveExtension();

        Map<String, String> parameters = new HashMap<String, String>();
        //parameters.put("dirDefault", "dirFirst");
        parameters.put("dirDefault", "dirDefault");
        
        URL url = new URL("dubbo", "127.0.0.1", 8089, parameters);

        System.out.println(dirExtension.sayHello(url, "qbb"));
    }
}

上面的dirDefault参数作为参数key进行传值进行选择实例，以下是代码适配器自动生成的代码

代码示例4

package demo.dubbo;
import com.alibaba.dubbo.common.extension.ExtensionLoader;
public class DirExtension$Adpative implements demo.dubbo.DirExtension {
    public java.lang.String sayHello(com.alibaba.dubbo.common.URL arg0, java.lang.String arg1) {
        if (arg0 == null) throw new IllegalArgumentException("url == null");
        com.alibaba.dubbo.common.URL url = arg0;
        String extName = url.getParameter("dirDefault", "dirFirst");
        if(extName == null) throw new IllegalStateException("Fail to get extension(demo.dubbo.DirExtension) name from url(" 
+ url.toString() + ") use keys([dirDefault])");
        demo.dubbo.DirExtension extension = (demo.dubbo.DirExtension)ExtensionLoader.getExtensionLoader(demo.dubbo.DirExtension.class).getExtension(extName);
        return extension.sayHello(arg0, arg1);
    }
}

3.4自定义适配器

也可以不通过URL的方式进行传参数,可以来自定义参数来实现适配器

代码示例5

@SPI("dir2Default")
public interface Dir2Extension {

    String sayHello(String name, String type);
}

@Adaptive
public class Dir2ExtensionAdaptive implements Dir2Extension{


    @Override
    public String sayHello(String name, String type) {
        ExtensionLoader extensionLoader = ExtensionLoader.getExtensionLoader(Dir2Extension.class);
        Dir2Extension dir2Extension = (Dir2Extension)extensionLoader.getDefaultExtension();

        if (type != null && SpiConstants.DIR2_FIRST.equalsIgnoreCase(type.trim())) {
            dir2Extension = (Dir2Extension)extensionLoader.getExtension(SpiConstants.DIR2_FIRST);
        }

        return dir2Extension.sayHello(name, type);
    }

}

public class Dir2ExtensionTest {

    public static void main(String[] args){
        ExtensionLoader extensionLoader = ExtensionLoader.getExtensionLoader(Dir2Extension.class);
        Dir2Extension dirExtension = (Dir2Extension)extensionLoader.getAdaptiveExtension();

        String type = SpiConstants.DIR2_FIRST;

        //type = SpiConstants.DIR2_DEFAULT;
        //type = SpiConstants.DIR2_FIRST;

        System.out.println(dirExtension.sayHello("qbb2", type));
    }
}

实质还是没有变化，只不过把URL参数改成其他类型了而已

3.4对Adaptive的理解

如果没有引入该概念的时候,如何使用对象实例呢?如下代码

代码示例6

ExtensionLoader extensionLoader = ExtensionLoader.getExtensionLoader(DirExtension.class);

DirExtension dirExtension = (DirExtension)extensionLoader.getExtension(SpiConstants.DIR_DEFAULT);
System.out.println(dirExtension.sayHello(null, "qbb"));

dirExtension = (DirExtension)extensionLoader.getExtension(SpiConstants.DIR_FIRST);
System.out.println(dirExtension.sayHello(null, "qbb"));

上面先获取对象，然后再调用方法，此处没有Adaptive,参数URL是多余的

假设一个接口有很多方法，又需要对不同实现的实例进行调用,上面的方式就显得有些并不方便了,如果能够拿到一个适配对象，然后传入产生进行选择实例调用，将会大大减少代码量的编写，而且适配器的代码基本是比较类似的，为了抽象公共的适配器代码，所以使用了URL进行参数传值,并且采用动态代码生成适配器的方式来实现。
我们可以通过下面的代码再跟上面的比较一下,实质效果是相同的,主要是思维方式发生了变化

代码示例7

dirExtension = (DirExtension)extensionLoader.getAdaptiveExtension();
Map<String, String> parameters = new HashMap<String, String>();
parameters.put("dirDefault", SpiConstants.DIR_FIRST);
URL url = new URL("dubbo", "127.0.0.1", 8089, parameters);
System.out.println(dirExtension.sayHello(url, "qbb"));

代码示例
https://github.com/sevenXin/javaDemo/tree/master/demo-dubbo

dubbo扩展点-适配器