官方文档.png
在官方文档中，提到扩展点加载机制，不妨看看是如何实现的。
扩展点加载机制，最重要的类就是
ExtensionLoader。
getExtensionLoader的实现原理
在获取扩展加载器的时候，首先会判断是否有注解SPI，没有则抛异常，有SPI注解的前提下，先从缓存取，取不到，根据class类型新建一个新的扩展点加载器。
public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) {
if (type == null)
throw new IllegalArgumentException("Extension type == null");
if (!type.isInterface()) {
throw new IllegalArgumentException("Extension type(" + type + ") is not interface!");
}
//是否有SPI注解
if (!withExtensionAnnotation(type)) {
throw new IllegalArgumentException("Extension type(" + type +
") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");
}
//从缓存中加载
ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
if (loader == null) {
根据类型新增一个扩展点加载器
EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
}
return loader;
}
在新增加载点扩展器的时候，会根据class类型获取一个运行期自适应的扩展类型
private ExtensionLoader(Class<?> type) {
this.type = type;
objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
}
我们看到，如果类型不是ExtensionFactory，则从ExtensionFactory的扩展加载器中获取一个可扩展的的加载器。这里先不看ExtensionFactory，先看一下获取可扩展加载器方法。
getAdaptiveExtension获取可调整的扩展加载器
在获取可扩展加载器的时候，先从缓存获取，若缓存不存在，创建新的扩展加载器。在创建新的扩展加载器的时候，首先判断配置文件中（META-INF/dubbo/，META-INF/services/,META-INF/dubbo/internal）配置的class是否有被Adaptive注解的类，如果有，则直接返回被注解Adaptive的可扩展加载类，如果没有，则生成一个扩展类。
public T getAdaptiveExtension() {
//先从缓存中获取
Object instance = cachedAdaptiveInstance.get();
if (instance == null) {
if (createAdaptiveInstanceError == null) {
synchronized (cachedAdaptiveInstance) {
instance = cachedAdaptiveInstance.get();
if (instance == null) {
try {
//创建一个新的可扩展加载器
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;
}
private T createAdaptiveExtension() {
//根据可扩展实例做扩展
try {
return injectExtension((T) getAdaptiveExtensionClass().newInstance());
} catch (Exception e) {
throw new IllegalStateException("Can not create adaptive extension " + type + ", cause: " + e.getMessage(), e);
}
}
//获取可扩展实例
private Class<?> getAdaptiveExtensionClass() {
// 加载当前Extension的所有实现,如果有@Adaptive类型，则会赋值为cachedAdaptiveClass属性缓存起来
getExtensionClasses();
if (cachedAdaptiveClass != null) {
//如果缓存有可扩展类，则直接返回
return cachedAdaptiveClass;
}
//缓存中不存在创建一个
return cachedAdaptiveClass = createAdaptiveExtensionClass();
}
获取可扩展类
private Map<String, Class<?>> getExtensionClasses() {
Map<String, Class<?>> classes = cachedClasses.get();
if (classes == null) {
synchronized (cachedClasses) {
classes = cachedClasses.get();
if (classes == null) {
//如果没有，则从文件中加载
classes = loadExtensionClasses();
cachedClasses.set(classes);
}
}
}
return classes;
}
从配置文件中加载
private Map<String, Class<?>> loadExtensionClasses() {
final SPI defaultAnnotation = type.getAnnotation(SPI.class);
//忽略1w行代码
// 文件路径：
loadFile(extensionClasses, DUBBO_INTERNAL_DIRECTORY);
//文件路径
loadFile(extensionClasses, DUBBO_DIRECTORY);
//文件路径
loadFile(extensionClasses, SERVICES_DIRECTORY);
return extensionClasses;
}
//根据文件名（包路径）查找所有文件，将每一行的配置读取出来，判断是否是实现该接口的类，如果接口中有注解`Adaptive`，则更新`cachedAdaptiveClass`的值。
private void loadFile(Map<String, Class<?>> extensionClasses, String dir) {
//文件路径为基本路径+包路径
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 url = urls.nextElement();
try {
//读文件
BufferedReader reader = new BufferedReader(new InputStreamReader(url.openStream(), "utf-8"));
//读每一行数据，判断=之前为bean的ID，后面为bean的class类型
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) {
name = line.substring(0, i).trim();
line = line.substring(i + 1).trim();
}
if (line.length() > 0) {
Class<?> clazz = Class.forName(line, true, classLoader);
if (!type.isAssignableFrom(clazz)) {
//如果类不是实现该接口的实例，则启动报错
throw new IllegalStateException("Error when load extension class(interface: " +
type + ", class line: " + clazz.getName() + "), class "
+ clazz.getName() + "is not subtype of interface.");
}
if (clazz.isAnnotationPresent(Adaptive.class)) {
//如果该类注解了Adaptive注解，则更新cachedAdaptiveClass为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 {
clazz.getConstructor(type);
Set<Class<?>> wrappers = cachedWrapperClasses;
if (wrappers == null) {
cachedWrapperClasses = new ConcurrentHashSet<Class<?>>();
wrappers = cachedWrapperClasses;
}
wrappers.add(clazz);
} catch (NoSuchMethodException e) {
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)) {
cachedNames.put(clazz, n);
}
Class<?> c = extensionClasses.get(n);
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());
}
}
}
}
}
}
} 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);
}
如果没有被注解Adaptive注解的，则直接创建一个新的可扩展加载类。
private Class<?> createAdaptiveExtensionClass() {
//创建可扩展加载器的代码。。
String code = createAdaptiveExtensionClassCode();
//获取类加载器
ClassLoader classLoader = findClassLoader();
//获取编辑器
com.alibaba.dubbo.common.compiler.Compiler compiler = ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.common.compiler.Compiler.class).getAdaptiveExtension();
//将代码编译为可扩展类
return compiler.compile(code, classLoader);
}
在对可扩展实例做扩展，会根据代理工厂创建一个实例,在构造方法中，使用的是ExtensionFactory的扩展
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())) {
Class<?> pt = method.getParameterTypes()[0];
try {
String property = method.getName().length() > 3 ? method.getName().substring(3, 4).toLowerCase() + method.getName().substring(4) : "";
//获取扩展实例
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;
}
看一下 ExtensionFactory的定义，和实现
@SPI
public interface ExtensionFactory {
<T> T getExtension(Class<T> type, String name);
}
@Adaptive
public class AdaptiveExtensionFactory implements ExtensionFactory {
private final List<ExtensionFactory> factories;
public AdaptiveExtensionFactory() {
ExtensionLoader<ExtensionFactory> loader = ExtensionLoader.getExtensionLoader(ExtensionFactory.class);
List<ExtensionFactory> list = new ArrayList<ExtensionFactory>();
for (String name : loader.getSupportedExtensions()) {
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;
}
}
public class SpiExtensionFactory implements ExtensionFactory {
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().size() > 0) {
return loader.getAdaptiveExtension();
}
}
return null;
}
}
public class SpringExtensionFactory implements ExtensionFactory {
private static final Set<ApplicationContext> contexts = new ConcurrentHashSet<ApplicationContext>();
public static void addApplicationContext(ApplicationContext context) {
contexts.add(context);
}
public static void removeApplicationContext(ApplicationContext context) {
contexts.remove(context);
}
@SuppressWarnings("unchecked")
public <T> T getExtension(Class<T> type, String name) {
for (ApplicationContext context : contexts) {
if (context.containsBean(name)) {
Object bean = context.getBean(name);
if (type.isInstance(bean)) {
return (T) bean;
}
}
}
return null;
}
}
很显然，在这里用到的是AdaptiveExtensionFactory，因为它被注解@Adaptive注解了，然后从ExtensionFactory中获取扩展加载类。
