Dubbo扩展点机制在项目中的应用

改写自：

Dubbo 源码

实现效果为两种调用方式:

//1. 指定实现类 
Message msg = ExtensionLoader.getExtensionLoader(Serializer.class).getExtension(SerializationTypeEnum.getName(serializeType)).deserialize(clazz, bytes);

//2. 注解上指定接口实现类  
Message msg = ExtensionLoader.getExtensionLoader(Serializer.class).getExtension().deserialize(clazz, bytes);

ExtensionLoader:

package com.fastrpc.extension;

import com.sun.org.apache.bcel.internal.generic.IF_ACMPEQ;
import lombok.extern.slf4j.Slf4j;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.URL;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;


@Slf4j
public class ExtensionLoader {
    private static final String SERVICES_DIRECTORY = "meta-INF/services/";
    //为每一个接口保存一个加载类
    private static final ConcurrentMap, ExtensionLoader> EXTENSION_LOADERS = new ConcurrentHashMap, ExtensionLoader>();
    private static final ConcurrentMap, Object> EXTENSION_INSTANCES = new ConcurrentHashMap, Object>();

    private final Class type;



    //缓存接口的实现类的Class
    private final Holder>> cachedClasses = new Holder>>();
    // 保存Class实例化过的类
    private final ConcurrentMap> cachedInstances = new ConcurrentHashMap>();
    private volatile String cachedDefaultName;

    private ExtensionLoader(Class type) {
        this.type = type;
    }

    
    public static  ExtensionLoader getExtensionLoader(Class type)
    {
        if (type==null)
            throw new IllegalArgumentException("Extension type == null");
        if (!type.isInterface())
            throw new IllegalArgumentException("Extenison type ("+type+") is not interface!");
        if (!withExtensionAnnotation(type))
            throw new IllegalArgumentException("Extension type(" + type +
                    ") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");
        ExtensionLoader loader= (ExtensionLoader) EXTENSION_LOADERS.get(type);
        if (loader==null)
        {
            EXTENSION_LOADERS.putIfAbsent(type,new ExtensionLoader(type));
            loader= (ExtensionLoader) EXTENSION_LOADERS.get(type);
        }
        return loader;

    }

    
    public static  boolean withExtensionAnnotation(Class type)
    {
        return  (type.isAnnotationPresent(SPI.class));
    }

    
    public T getExtension()
    {
        return getExtension("default");
    }
    
    public T getExtension(String name)
    {
        if (name==null||name.length()==0)
            throw new IllegalArgumentException("Extension name == null");
        //没有指定具体实现类则从SPI中获取指定的实现类
        if("default".equalsIgnoreCase(name))
        {
            return getDefaultExtension();
        }
        Holder