本源码基于dubbo2.6.6
参考dubbo官网：dubbo官网SPI解读

dubbo的SPI使用

//加载SPI标签的类放到缓存中
    ExtensionLoader<MySPIDemo> extensionLoader = ExtensionLoader.getExtensionLoader(MySPIDemo.class);
    //获取SPI类
    MySPIDemo mySPIDemo = extensionLoader.getExtension("method");

加载SPI的入口getExtensionLoader
如果存在缓存中从缓存取，如果不存在那么就new ExtensionLoader(type)放入缓存并且返回

@SuppressWarnings("unchecked")
public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) { 
    //校验类
    if (type == null)
        throw new IllegalArgumentException("Extension type == null");
    //校验是否为接口，不是接口会报错
    if (!type.isInterface()) { 
        throw new IllegalArgumentException("Extension type(" + type + ") is not interface!");
    }
    //校验是否带有SPI注解
    if (!withExtensionAnnotation(type)) { 
        throw new IllegalArgumentException("Extension type(" + type +
                ") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");
    }
    //缓存中获取
    ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
    if (loader == null) { 
        //加入缓存
        EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
        loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
    }
    return loader;
}

这个构造方法如果type不是ExtensionFactory那么objectFactory就是ExtensionFactory创建的自适应拓展类（略有点绕）

private ExtensionLoader(Class<?> type) { 
    this.type = type;
    objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
}

从缓存(Holder)cachedAdaptiveInstance获取自适应拓展的实例
没有就通过createAdaptiveExtension()创建实例

这里T是ExtensionFactory

记住2点：

取缓存。

没缓存，创建

@SuppressWarnings(“unchecked”)
public T getAdaptiveExtension() {

//先从缓存Holder中取
Object instance = cachedAdaptiveInstance.get();
if (instance == null) { 
    if (createAdaptiveInstanceError == null) { 
        //加锁判断
        synchronized (cachedAdaptiveInstance) { 
            instance = cachedAdaptiveInstance.get();
            if (instance == null) { 
                try { 
                    //创建自适应拓展类，放入缓存
                    instance = createAdaptiveExtension();
                    cachedAdaptiveInstance.set(instance);
                } catch (Throwable t) { 
                    createAdaptiveInstanceError = t;
                    throw new IllegalStateException("fail to create adaptive instance: " + t.toString(), t);
                }
            }
        }
    } else { 
        throw new IllegalStateException("fail to create adaptive instance: " + createAdaptiveInstanceError.toString(), createAdaptiveInstanceError);
    }
}
return (T) instance;

}

createAdaptiveExtension通过反射创建实例
1.getAdaptiveExtensionClass找寻自适应拓展类的class对象
2.反射创建实例
3.injectExtension？？？

@SuppressWarnings("unchecked")
private T createAdaptiveExtension() { 
    try { 
        return injectExtension((T) getAdaptiveExtensionClass().newInstance());
    } catch (Exception e) { 
        throw new IllegalStateException("Can not create adaptive extension " + type + ", cause: " + e.getMessage(), e);
    }
}

获取自适应拓展类的字节码对象

private Class<?> getAdaptiveExtensionClass() { 
    getExtensionClasses();
    if (cachedAdaptiveClass != null) { 
        return cachedAdaptiveClass;
    }
    return cachedAdaptiveClass = createAdaptiveExtensionClass();
}

获取所有的自适应拓展类
先从缓存取，缓存没有就加载

private Map<String, Class<?>> getExtensionClasses() { 
    //从holder里面取缓存
    Map<String, Class<?>> classes = cachedClasses.get();
    if (classes == null) { 
        synchronized (cachedClasses) { 
            classes = cachedClasses.get();
            if (classes == null) { 
                //加载自适应拓展类字节码
                classes = loadExtensionClasses();
                //放入缓存
                cachedClasses.set(classes);
            }
        }
    }
    return classes;
}

从目录中获取

private static final String DUBBO_INTERNAL_DIRECTORY = DUBBO_DIRECTORY + “internal/”;
private static final String DUBBO_DIRECTORY = “META-INF/dubbo/”;
private static final String SERVICES_DIRECTORY = “META-INF/services/”;

private Map<String, Class<?>> loadExtensionClasses() { 
    //判断类是不是有SPI标签（ExtensionFactory）
    //ExtensionLoader.getExtensionLoader(ExtensionFactory.class)
    //type=ExtensionFactory 这个类是有SPI标签的
    final SPI defaultAnnotation = type.getAnnotation(SPI.class);
    if (defaultAnnotation != null) { 
        //获取spi的属性值
        String value = defaultAnnotation.value();
        if ((value = value.trim()).length() > 0) { 
            //根据正则表达式切分("\\s*[,]+\\s*")
            String[] names = NAME_SEPARATOR.split(value);
            //只允许有一个
            if (names.length > 1) { 
                throw new IllegalStateException("more than 1 default extension name on extension " + type.getName()
                        + ": " + Arrays.toString(names));
            }
            //获取缓存的名称
            if (names.length == 1) cachedDefaultName = names[0];
        }
    }
    //加载放入Map中，只有普通类才会加入到map中，自适应类不会加载进去
    Map<String, Class<?>> extensionClasses = new HashMap<String, Class<?>>();
    loadDirectory(extensionClasses, DUBBO_INTERNAL_DIRECTORY);
    loadDirectory(extensionClasses, DUBBO_DIRECTORY);
    loadDirectory(extensionClasses, SERVICES_DIRECTORY);
    return extensionClasses;
}

加载指定路径下的文件名是type的

private void loadDirectory(Map<String, Class<?>> extensionClasses, String dir) { 
    //获取路径下的指定名称的文件名，见下方的图
    String fileName = dir + type.getName();
    try { 
        Enumeration<java.net.URL> urls;
        ClassLoader classLoader = findClassLoader();
        if (classLoader != null) { 
            urls = classLoader.getResources(fileName);
        } else { 
            urls = ClassLoader.getSystemResources(fileName);
        }
        if (urls != null) { 
            while (urls.hasMoreElements()) { 
                java.net.URL resourceURL = urls.nextElement();
                loadResource(extensionClasses, classLoader, resourceURL);
            }
        }
    } catch (Throwable t) { 
        logger.error("Exception when load extension class(interface: " +
                type + ", description file: " + fileName + ").", t);
    }
}

我们看一下这个文件的内容
找到META-INF下对应的文件除去注释并且加载class

private void loadResource(Map<String, Class<?>> extensionClasses, ClassLoader classLoader, java.net.URL resourceURL) { 
    try { 
        BufferedReader reader = new BufferedReader(new InputStreamReader(resourceURL.openStream(), "utf-8"));
        try { 
            String line;
            while ((line = reader.readLine()) != null) { 
                final int ci = line.indexOf('#');
                if (ci >= 0) line = line.substring(0, ci);
                line = line.trim();
                if (line.length() > 0) { 
                    try { 
                        String name = null;
                        int i = line.indexOf('=');
                        if (i > 0) { 
                            name = line.substring(0, i).trim();
                            line = line.substring(i + 1).trim();
                        }
                        if (line.length() > 0) { 
                            loadClass(extensionClasses, resourceURL, Class.forName(line, true, classLoader), name);
                        }
                    } catch (Throwable t) { 
                        IllegalStateException e = new IllegalStateException("Failed to load extension class(interface: " + type + ", class line: " + line + ") in " + resourceURL + ", cause: " + t.getMessage(), t);
                        exceptions.put(line, e);
                    }
                }
            }
        } finally { 
            reader.close();
        }
    } catch (Throwable t) { 
        logger.error("Exception when load extension class(interface: " +
                type + ", class file: " + resourceURL + ") in " + resourceURL, t);
    }
}
private void loadClass(Map<String, Class<?>> extensionClasses, java.net.URL resourceURL, Class<?> clazz, String name) throws NoSuchMethodException { 
    if (!type.isAssignableFrom(clazz)) { 
        throw new IllegalStateException("Error when load extension class(interface: " +
                type + ", class line: " + clazz.getName() + "), class "
                + clazz.getName() + "is not subtype of interface.");
    }
    //是否有适应注解
    if (clazz.isAnnotationPresent(Adaptive.class)) { 
        if (cachedAdaptiveClass == null) { 
            //缓存自适应类
            cachedAdaptiveClass = clazz;
        } else if (!cachedAdaptiveClass.equals(clazz)) { 
            throw new IllegalStateException("More than 1 adaptive class found: "
                    + cachedAdaptiveClass.getClass().getName()
                    + ", " + clazz.getClass().getName());
        }
        //是否是wrapper类型(clazz是否有一个参数是type的构造方法)
        //clazz=com.alibaba.dubbo.config.spring.extension.SpringExtensionFactory
        //type=com.alibaba.dubbo.common.extension.ExtensionFactory
    } else if (isWrapperClass(clazz)) { 
        //将clazz放入缓存中
        Set<Class<?>> wrappers = cachedWrapperClasses;
        if (wrappers == null) { 
            cachedWrapperClasses = new ConcurrentHashSet<Class<?>>();
            wrappers = cachedWrapperClasses;
        }
        wrappers.add(clazz);
    } else { 
        //既不是warpper类也没有Adaptive注解
        clazz.getConstructor();
        //name=spring
        if (name == null || name.length() == 0) { 
            //获取extension注解的value,如果没有就用小写的类名
            name = findAnnotationName(clazz);
            if (name.length() == 0) { 
                throw new IllegalStateException("No such extension name for the class " + clazz.getName() + " in the config " + resourceURL);
            }
        }
        String[] names = NAME_SEPARATOR.split(name);
        if (names != null && names.length > 0) { 
            //如果不为空，那么获取activate注解
            Activate activate = clazz.getAnnotation(Activate.class);
            //如果有注解就放入缓存中
            if (activate != null) { 
                cachedActivates.put(names[0], activate);
            }
            for (String n : names) { 
                //如果缓存的名字不存在那么放入缓存中
                if (!cachedNames.containsKey(clazz)) { 
                    cachedNames.put(clazz, n);
                }
                //再去缓存extensionClasses中找寻对应的字节码对象
                Class<?> c = extensionClasses.get(n);
                //如果不存在就把clazz放进缓存
                if (c == null) { 
                    extensionClasses.put(n, clazz);
                } else if (c != clazz) { 
                    throw new IllegalStateException("Duplicate extension " + type.getName() + " name " + n + " on " + c.getName() + " and " + clazz.getName());
                }
            }
        }
    }
}

获取extension注解的value,如果没有就用小写的类名

@SuppressWarnings("deprecation")
private String findAnnotationName(Class<?> clazz) { 
    com.alibaba.dubbo.common.Extension extension = clazz.getAnnotation(com.alibaba.dubbo.common.Extension.class);
    //获取extension注解的value,如果没有就用小写的类名
    if (extension == null) { 
        String name = clazz.getSimpleName();
        if (name.endsWith(type.getSimpleName())) { 
            name = name.substring(0, name.length() - type.getSimpleName().length());
        }
        return name.toLowerCase();
    }
    return extension.value();
}

getExtensionClasses（加载文件中的类）
获取自适应拓展类
injectExtension((T) getAdaptiveExtensionClass().newInstance());

最后ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension()获取的自适应拓展类是AdaptiveExtensionFactory

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

ExtensionLoader
private final Class<?> type=MySPIDemo.class;
private final ExtensionFactory objectFactory=AdaptiveExtensionFactory;

分析完成

getExtensionLoader总结：

获取ExtensionLoader会先从EXTENSION_LOADERS缓存中取。
如果是ExtensionFactory类objectFactory对象创建工厂就是空，否则是ExtensionFactory的自适应拓展类
获取自适应拓展类，先查询缓存cachedAdaptiveInstance，获取自适应的实例，没有就创建拓展类
先获取字节码对象，再通过反射创建实例
先获取所有的拓展字节码对象，先从缓存cachedClasses取如果不存在loadExtensionClasses，并且加入缓存
获取type的SPI注解的值作为缓存cachedDefaultName，然后从不同的目录加载（DUBBO_INTERNAL_DIRECTORY、DUBBO_DIRECTORY、SERVICES_DIRECTORY）
private static final String DUBBO_INTERNAL_DIRECTORY = DUBBO_DIRECTORY + “internal/”;
private static final String DUBBO_DIRECTORY = “META-INF/dubbo/”;
private static final String SERVICES_DIRECTORY = “META-INF/services/”;
通过文件路径去加载资源loadResource并且放入缓存extensionClasses
loadClass，如果类上有Adaptive注解，放入缓存（Class类自适应拓展字节码类）cachedAdaptiveClass
如果类是wrapper类，构造函数有一个type类型的入参。将字节码对象放入cachedWrapperClasses缓存
如果类是一个普通类，判断有没有Activate注解，如果有的话，name不为空就把name作为key，如果name为空把extension注解的value,如果没有就用小写的类名作为key(name)，Activate作为值放入缓存cachedActivates中。
如果没有Activate注解将字节码对象作为key，name作为value放入缓存cachedNames，查询缓存extensionClasses中是否有name对应的字节码对象，如果不存在就放进去。

缓存类

EXTENSION_LOADERS（ConcurrentMap, ExtensionLoader<?>>）缓存，拓展加载类缓存
cachedAdaptiveInstance（Holder）自适应拓展类实例缓存
cachedClasses（Holder>>所有拓展类的字节码缓存，加载文件的拓展类存放的地方）
cachedAdaptiveClass（Class<?>）缓存拓展类的字节码对象
cachedWrapperClasses（ConcurrentHashSet>）缓存wrapper对象
cachedActivates（Map）缓存Activate
cachedNames（ConcurrentMap, String>）缓存字节码对象和名称

接下来分析
MySPIDemo mySPIDemo = extensionLoader.getExtension(“method”);

@SuppressWarnings("unchecked")
public T getExtension(String name) { 
    if (name == null || name.length() == 0)
        throw new IllegalArgumentException("Extension name == null");
    if ("true".equals(name)) { 
        return getDefaultExtension();
    }
    //从缓存cachedInstances（ConcurrentMap<String, Holder<Object>>）中获取该名称的Holder
    Holder<Object> holder = cachedInstances.get(name);
    if (holder == null) { 
        //将对象放入缓存，取出来放入holder
        cachedInstances.putIfAbsent(name, new Holder<Object>());
        holder = cachedInstances.get(name);
    }
    //缓存的实例
    Object instance = holder.get();
    if (instance == null) { 
        synchronized (holder) { 
            instance = holder.get();
            if (instance == null) { 
                //创建拓展类
                instance = createExtension(name);
                holder.set(instance);
            }
        }
    }
    return (T) instance;
}

Dubbo IOC

Dubbo IOC 是通过 setter 方法注入依赖。Dubbo 首先会通过反射获取到实例的所有方法，然后再遍历方法列表，检测方法名是否具有 setter 方法特征。若有，则通过 ObjectFactory 获取依赖对象，最后通过反射调用 setter 方法将依赖设置到目标对象中。整个过程对应的代码如下：

private T injectExtension(T instance) { 
    try { 
        if (objectFactory != null) { 
            for (Method method : instance.getClass().getMethods()) { 
                if (method.getName().startsWith("set")
                        && method.getParameterTypes().length == 1
                        && Modifier.isPublic(method.getModifiers())) { 
                    //这个方法可不可以进行反射
                    if (method.getAnnotation(DisableInject.class) != null) { 
                        continue;
                    }
                    //获取参数类型
                    Class<?> pt = method.getParameterTypes()[0];
                    try { 
                        //获取set后的参数名称：比如setAge,那么property=age;
                        String property = method.getName().length() > 3 ? method.getName().substring(3, 4).toLowerCase() + method.getName().substring(4) : "";
                        Object object = objectFactory.getExtension(pt, property);
                        if (object != null) { 
                            method.invoke(instance, object);
                        }
                    } catch (Exception e) { 
                        logger.error("fail to inject via method " + method.getName()
                                + " of interface " + type.getName() + ": " + e.getMessage(), e);
                    }
                }
            }
        }
    } catch (Exception e) { 
        logger.error(e.getMessage(), e);
    }
    return instance;
}

objectFactory.getExtension有3种实现方式
ExtensionFactory的实现类
通过SPI获取

public class SpiExtensionFactory implements ExtensionFactory { 
    @Override
    public <T> T getExtension(Class<T> type, String name) { 
        if (type.isInterface() && type.isAnnotationPresent(SPI.class)) { 
            ExtensionLoader<T> loader = ExtensionLoader.getExtensionLoader(type);
            if (!loader.getSupportedExtensions().isEmpty()) { 
                return loader.getAdaptiveExtension();
            }
        }
        return null;
    }
}