dubbo extension扩展点 源代码

1.首先我们创建一些类，我们再一步一步的进行分析dubbo的扩展点的源代码

  

public class DefaultExtensionImpl implements MyInterface {    @Override    public String sayHello(String name, String type) {        return this.getClass().getName() + "  " + name + "  " + type;    }}

public class ExtensionTest {@SuppressWarnings("rawtypes")public static void main(String[] args) {System.out.println("!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!");ExtensionLoader extensionLoader = ExtensionLoader.getExtensionLoader(MyInterface.class);MyInterface myFirstExtension = (MyInterface) extensionLoader.getAdaptiveExtension();System.out.println(myFirstExtension.sayHello("xxx",ExtensionType.defaults));}}

public class ExtensionType {public static final String defaults = "defaults";public static final String other = "other";}

@Adaptivepublic class MyAdaptiveExtension implements MyInterface {@SuppressWarnings("rawtypes")@Overridepublic String sayHello(String name, String type) {ExtensionLoader extensionLoader = ExtensionLoader.getExtensionLoader(MyInterface.class);MyInterface extension = (MyInterface) extensionLoader.getDefaultExtension();if (ExtensionType.defaults.equals(type)) {extension = (MyInterface) extensionLoader.getExtension("defaults");}if (ExtensionType.other.equals(type)) {extension = (MyInterface) extensionLoader.getExtension("other");}return extension.sayHello(name, type);}}

@SPI("defaults")public interface MyInterface {public String sayHello(String name, String type);}

public class OtherExtensionImpl implements MyInterface {@Overridepublic String sayHello(String name, String type) {return this.getClass().getName() + "  " + name + "  " + type;}}

com.alibaba.dubbo.demo.extensiontest.MyInterface内容：

defaults=com.alibaba.dubbo.demo.extensiontest.DefaultExtensionImpl
other=com.alibaba.dubbo.demo.extensiontest.OtherExtensionImpl
adaptive=com.alibaba.dubbo.demo.extensiontest.MyAdaptiveExtension

运行的结果

2.现在需要创建一个ExtensionLoader对象 ，ExtensionLoader是个单例，但是提供了一个getExtensionLoader方法

@SuppressWarnings("unchecked")public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) {// 不能为空if (type == null)throw new IllegalArgumentException("Extension type == null");// 接口if (!type.isInterface()) {throw new IllegalArgumentException("Extension type(" + type+ ") is not interface!");}// spi类型if (!withExtensionAnnotation(type)) {throw new IllegalArgumentException("Extension type(" + type+ ") is not extension, because WITHOUT @"+ SPI.class.getSimpleName() + " Annotation!");}// 先从EXTENSION_LOADERS获取ExtensionLoaderExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);// 新建一个ExtensionLoader对象if (loader == null) {EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);}return loader;}

3.创建一个ExtensionLoader对象

private ExtensionLoader(Class<?> type) {        this.type = type;         //如果type是ExtensionFactory.class返回null  如果不是的话 那么就创建一个type为ExtensionFactory.class的对象，之后再调用getAdaptiveExtension方法        objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());}

4.调用getAdaptiveExtension方法

@SuppressWarnings("unchecked")    public T getAdaptiveExtension() {        //先从cachedAdaptiveInstance(自适应的实现类，该类需要有@Adaptive注解)获取，不为null的话那么直接返回        Object instance = cachedAdaptiveInstance.get();         if (instance == null) {            if(createAdaptiveInstanceError == null) {                synchronized (cachedAdaptiveInstance) {                    instance = cachedAdaptiveInstance.get();                    if (instance == null) {                        try {                            //如果为null的话 调用createAdaptiveExtension方法                            instance = createAdaptiveExtension();                            //获取到实例之后，set到cachedAdaptiveInstance里                            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;    }

5.调用createAdaptiveExtension方法

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

6.调用getAdaptiveExtensionClass方法

private Class<?> getAdaptiveExtensionClass() {        getExtensionClasses();                //如果cachedAdaptiveClass不为空 直接返回                if (cachedAdaptiveClass != null) {            return cachedAdaptiveClass;        }        return cachedAdaptiveClass = createAdaptiveExtensionClass();    }

7.调用getExtensionClasses方法

private Map<String, Class<?>> getExtensionClasses() {                //从cachedClasses获取                Map<String, Class<?>> classes = cachedClasses.get();         if (classes == null) {            synchronized (cachedClasses) {                classes = cachedClasses.get();                if (classes == null) {                    //如果为null的话，开始进行读取配置文件                    classes = loadExtensionClasses();                    //放入到cachedClasses对象里                    cachedClasses.set(classes);                }            }        }        return classes;}

8.调用loadExtensionClasses方法

private Map<String, Class<?>> loadExtensionClasses() {        final SPI defaultAnnotation = type.getAnnotation(SPI.class);        if(defaultAnnotation != null) {            String value = defaultAnnotation.value();                //如果value不为空的话 赋值cachedDefaultName                if(value != null && (value = value.trim()).length() > 0) {                String[] names = NAME_SEPARATOR.split(value);                if(names.length > 1) {                    throw new IllegalStateException("more than 1 default extension name on extension " + type.getName()                            + ": " + Arrays.toString(names));                }                if(names.length == 1) cachedDefaultName = names[0];            }        }                Map<String, Class<?>> extensionClasses = new HashMap<String, Class<?>>();        //读取配置文件META-INF/dubbo/internal/        loadFile(extensionClasses, DUBBO_INTERNAL_DIRECTORY);        //读取配置文件META-INF/dubbo/        loadFile(extensionClasses, DUBBO_DIRECTORY);        //读取配置文件META-INF/services/        loadFile(extensionClasses, SERVICES_DIRECTORY);        return extensionClasses;    }

9.调用loadFile方法

private void loadFile(Map<String, Class<?>> extensionClasses, String dir) {        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);            }            //如果文件路径不为空的话            if (urls != null) {                                //进行遍历                                while (urls.hasMoreElements()) {                     java.net.URL url = urls.nextElement();                    try {                        BufferedReader reader = new BufferedReader(new InputStreamReader(url.openStream(), "utf-8"));                        try {                            String line = null;                            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) {                                            //获取类的名称                                            name = line.substring(0, i).trim();                                            line = line.substring(i + 1).trim();                                        }                                        if (line.length() > 0) {                                            //获取类                                            Class<?> clazz = Class.forName(line, true, classLoader);                                            //判断该类是不是实现该接口                                            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.");                                            }                                            //判断该类是否是一个自适应的adaptive的类                                            if (clazz.isAnnotationPresent(Adaptive.class)) {                                                if(cachedAdaptiveClass == null) {                                                    cachedAdaptiveClass = clazz;                                                } else if (! cachedAdaptiveClass.equals(clazz)) {                                                    throw new IllegalStateException("More than 1 adaptive class found: "                                                            + cachedAdaptiveClass.getClass().getName()                                                            + ", " + clazz.getClass().getName());                                                }                                            } else {                                                try {                                                    //判断是不是装饰类                                                    clazz.getConstructor(type);                                                    Set<Class<?>> wrappers = cachedWrapperClasses;                                                    if (wrappers == null) {                                                        cachedWrapperClasses = new ConcurrentHashSet<Class<?>>();                                                        wrappers = cachedWrapperClasses;                                                    }                                                    wrappers.add(clazz);                                                } catch (NoSuchMethodException e) {                                                    //普通的实现类                                                    clazz.getConstructor();                                                    if (name == null || name.length() == 0) {                                                        name = findAnnotationName(clazz);                                                        if (name == null || name.length() == 0) {                                                            if (clazz.getSimpleName().length() > type.getSimpleName().length()                                                                    && clazz.getSimpleName().endsWith(type.getSimpleName())) {                                                                name = clazz.getSimpleName().substring(0, clazz.getSimpleName().length() - type.getSimpleName().length()).toLowerCase();                                                            } else {                                                                throw new IllegalStateException("No such extension name for the class " + clazz.getName() + " in the config " + url);                                                            }                                                        }                                                    }                                                    String[] names = NAME_SEPARATOR.split(name);                                                    if (names != null && names.length > 0) {                                                        Activate activate = clazz.getAnnotation(Activate.class);                                                        if (activate != null) {                                                            cachedActivates.put(names[0], activate);                                                        }                                                        for (String n : names) {                                                            if (! cachedNames.containsKey(clazz)) {                                                                //进行赋值                                                                cachedNames.put(clazz, n);                                                            }                                                            Class<?> c = extensionClasses.get(n);                                                            if (c == null) {                                                                //进行赋值                                                                extensionClasses.put(n, clazz);                                                            } else if (c != clazz) {                                                                throw new IllegalStateException("Duplicate extension " + type.getName() + " name " + n + " on " + c.getName() + " and " + clazz.getName());                                                            }                                                        }                                                    }                                                }                                            }                                        }                                    } catch (Throwable t) {                                        IllegalStateException e = new IllegalStateException("Failed to load extension class(interface: " + type + ", class line: " + line + ") in " + url + ", cause: " + t.getMessage(), t);                                        exceptions.put(line, e);                                    }                                }                            } // end of while read lines                        } finally {                            reader.close();                        }                    } catch (Throwable t) {                        logger.error("Exception when load extension class(interface: " +                                            type + ", class file: " + url + ") in " + url, t);                    }                } // end of while urls            }        } catch (Throwable t) {            logger.error("Exception when load extension class(interface: " +                    type + ", description file: " + fileName + ").", t);        }    }

10.创建AdaptiveExtensionFactory实例

    

@Adaptivepublic class AdaptiveExtensionFactory implements ExtensionFactory {        private final List<ExtensionFactory> factories;        public AdaptiveExtensionFactory() {        ExtensionLoader<ExtensionFactory> loader = ExtensionLoader.getExtensionLoader(ExtensionFactory.class);        List<ExtensionFactory> list = new ArrayList<ExtensionFactory>();        //loader.getSupportedExtensions() 获取extensions的实现类的key        for (String name : loader.getSupportedExtensions()) {             //获取extension的实现类             list.add(loader.getExtension(name));        }        factories = Collections.unmodifiableList(list);    }    public <T> T getExtension(Class<T> type, String name) {        for (ExtensionFactory factory : factories) {            T extension = factory.getExtension(type, name);            if (extension != null) {                return extension;            }        }        return null;    }}

11.调用getExtension方法

@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();}Holder<Object> holder = cachedInstances.get(name);                //cachedInstances通过name获取对象                if (holder == null) {                    //创建个新对象                    cachedInstances.putIfAbsent(name, new Holder<Object>());                    //放入cachedInstances对象里                    holder = cachedInstances.get(name);}Object instance = holder.get();if (instance == null) {    synchronized (holder) {            instance = holder.get();                    //如果holder的value里没有数据的话                    if (instance == null) {                instance = createExtension(name);                holder.set(instance);            }        }}return (T) instance;}

12.调用createExtension方法

@SuppressWarnings("unchecked")    private T createExtension(String name) {        Class<?> clazz = getExtensionClasses().get(name);        if (clazz == null) {            throw findException(name);        }        try {            //EXTENSION_INSTANCES里是否有实现类            T instance = (T) EXTENSION_INSTANCES.get(clazz);            //没有的话创建对象 赋值到EXTENSION_INSTANCES            if (instance == null) {                EXTENSION_INSTANCES.putIfAbsent(clazz, (T) clazz.newInstance());                instance = (T) EXTENSION_INSTANCES.get(clazz);            }            //注入            injectExtension(instance);            //进行包装            Set<Class<?>> wrapperClasses = cachedWrapperClasses;            if (wrapperClasses != null && wrapperClasses.size() > 0) {                for (Class<?> wrapperClass : wrapperClasses) {                    instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance));                }            }            return instance;        } catch (Throwable t) {            throw new IllegalStateException("Extension instance(name: " + name + ", class: " +                    type + ")  could not be instantiated: " + t.getMessage(), t);        }    }

到此ExtensionLoader已经创建完成。