以加载Protocol扩展点为例：
ExtensionLoader.getExtensionLoader(Protocol.class).getExtension(DubboProtocol.NAME);getExtensionLoader
通过源码可以看到，经过一系列校验后，先尝试从EXTENSION_LOADERS缓存中根据传入的type参数获取对应的loader对象，如果缓存不存在则new一个ExtensionLoader对象并放入缓存，最后返回loader。
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 an interface!");
}
if (!withExtensionAnnotation(type)) {
throw new IllegalArgumentException("Extension type (" + type +
") is not an extension, because it is NOT annotated with @" + SPI.class.getSimpleName() + "!");
}
ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
if (loader == null) {
EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
}
return loader;
}getExtension
如果name是"true"，返回默认扩展点实现。
如果不是默认实现，则先尝试从缓存中获取，如果缓存中没有，再t通过createExtension方法创建实例，最后再放入缓存。
public T getExtension(String name) {
if (StringUtils.isEmpty(name)) {
throw new IllegalArgumentException("Extension name == null");
}
// 如果name是"true"，返回默认扩展点实现
if ("true".equals(name)) {
return getDefaultExtension();
}
final Holder<Object> holder = getOrCreateHolder(name);
Object instance = holder.get();
if (instance == null) {
synchronized (holder) {
instance = holder.get();
if (instance == null) {
instance = createExtension(name);
holder.set(instance);
}
}
}
return (T) instance;
}createExtension
createExtension方法中主要有3个逻辑：
1、从SPI配置文件加载class。
2、根据得到class创建实例并缓存。
3、依赖注入。
private T createExtension(String name) {
// 从SPI配置文件加载class
Class<?> clazz = getExtensionClasses().get(name);
if (clazz == null) {
throw findException(name);
}
try {
T instance = (T) EXTENSION_INSTANCES.get(clazz);
if (instance == null) {
EXTENSION_INSTANCES.putIfAbsent(clazz, clazz.newInstance());
instance = (T) EXTENSION_INSTANCES.get(clazz);
}
// 依赖注入
injectExtension(instance);
Set<Class<?>> wrapperClasses = cachedWrapperClasses;
if (CollectionUtils.isNotEmpty(wrapperClasses)) {
// 一个扩展点有多个wrapper
for (Class<?> wrapperClass : wrapperClasses) {
// 找到构造方法参数类型为type的包装类，为其注入扩展类实例
instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance));
}
}
return instance;
} catch (Throwable t) {
throw new IllegalStateException("Extension instance (name: " + name + ", class: " +
type + ") couldn't be instantiated: " + t.getMessage(), t);
}
}getExtensionClasses
通过源码可以看到，getExtensionClasses方法的主要逻辑就是通过double check方式获取并返回classes集合。
classes的具体加载细节在loadExtensionClasses方法中实现，这里会从三个指定路径（META-INF/services/、META-INF/dubbo/、META-INF/dubbo/internal/）去加载配置文件，按照SPI规范，配置文件名一定为扩展点接口的全限定名，配置内容则是k-v形式的键值对，key与createExtension的入参name对应，value则是扩展点实现类的全限定名。最终会通过反射将实现类加载到容器中。
在最后的loadClass方法中，会将扩展类进行分类缓存，分别是自适应扩展类缓存，wrapper包装类缓存和普通扩展类缓存。
private Map<String, Class<?>> getExtensionClasses() {
Map<String, Class<?>> classes = cachedClasses.get();
if (classes == null) {
synchronized (cachedClasses) {
classes = cachedClasses.get();
if (classes == null) {
classes = loadExtensionClasses();
cachedClasses.set(classes);
}
}
}
return classes;
}
private Map<String, Class<?>> loadExtensionClasses() {
cacheDefaultExtensionName();
Map<String, Class<?>> extensionClasses = new HashMap<>();
loadDirectory(extensionClasses, DUBBO_INTERNAL_DIRECTORY, type.getName());
loadDirectory(extensionClasses, DUBBO_INTERNAL_DIRECTORY, type.getName().replace("org.apache", "com.alibaba"));
loadDirectory(extensionClasses, DUBBO_DIRECTORY, type.getName());
loadDirectory(extensionClasses, DUBBO_DIRECTORY, type.getName().replace("org.apache", "com.alibaba"));
loadDirectory(extensionClasses, SERVICES_DIRECTORY, type.getName());
loadDirectory(extensionClasses, SERVICES_DIRECTORY, type.getName().replace("org.apache", "com.alibaba"));
return extensionClasses;
}
private void cacheDefaultExtensionName() {
final SPI defaultAnnotation = type.getAnnotation(SPI.class);
if (defaultAnnotation != null) {
String value = defaultAnnotation.value();
if ((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];
}
}
}
}
private void loadDirectory(Map<String, Class<?>> extensionClasses, String dir, String type) {
String fileName = dir + type;
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 occurred when loading extension class (interface: " +
type + ", description file: " + fileName + ").", t);
}
}
private void loadResource(Map<String, Class<?>> extensionClasses, ClassLoader classLoader, java.net.URL resourceURL) {
try {
try (BufferedReader reader = new BufferedReader(new InputStreamReader(resourceURL.openStream(), StandardCharsets.UTF_8))) {
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);
}
}
}
}
} catch (Throwable t) {
logger.error("Exception occurred when loading 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 occurred when loading extension class (interface: " +
type + ", class line: " + clazz.getName() + "), class "
+ clazz.getName() + " is not subtype of interface.");
}
// 如果是自适应类则缓存，且只能有一个
if (clazz.isAnnotationPresent(Adaptive.class)) {
cacheAdaptiveClass(clazz);
} else if (isWrapperClass(clazz)) {
// 如果是包装类，则加入包装类的Set集合
// 是否是wrapper类的条件是，以扩展点对象作为构造函数的入参
cacheWrapperClass(clazz);
} else {
// 缓存普通扩展类
clazz.getConstructor();
if (StringUtils.isEmpty(name)) {
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 (ArrayUtils.isNotEmpty(names)) {
cacheActivateClass(clazz, names[0]);
for (String n : names) {
cacheName(clazz, n);
saveInExtensionClass(extensionClasses, clazz, n);
}
}
}
}injectExtension
最后再来分析一下依赖注入的逻辑：
objectFactory是扩展对象工厂，在ExtensionLoader实例化时，通过自适应扩展类AdaptiveExtensionFactory实现初始化。
在injectExtension方法中，若objectFactory不为空，则遍历目标对象的所有方法，当有方法满足isSetter(method) == true等条件时，通过objectFactory的getExtension方法获取要注入的对象，最后通过反射将其注入目标对象。
private final ExtensionFactory objectFactory;
private ExtensionLoader(Class<?> type) {
this.type = type;
objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
}
...
private T injectExtension(T instance) {
try {
if (objectFactory != null) {
for (Method method : instance.getClass().getMethods()) {
if (isSetter(method)) {
/**
* Check {@link DisableInject} to see if we need auto injection for this property
*/
if (method.getAnnotation(DisableInject.class) != null) {
continue;
}
Class<?> pt = method.getParameterTypes()[0];
if (ReflectUtils.isPrimitives(pt)) {
continue;
}
try {
String property = getSetterProperty(method);
Object object = objectFactory.getExtension(pt, property);
if (object != null) {
method.invoke(instance, object);
}
} catch (Exception e) {
logger.error("Failed to inject via method " + method.getName()
+ " of interface " + type.getName() + ": " + e.getMessage(), e);
}
}
}
}
} catch (Exception e) {
logger.error(e.getMessage(), e);
}
return instance;
}
/**
* get properties name for setter, for instance: setVersion, return "version"
* <p>
* return "", if setter name with length less than 3
*/
private String getSetterProperty(Method method) {
return method.getName().length() > 3 ? method.getName().substring(3, 4).toLowerCase() + method.getName().substring(4) : "";
}
/**
* return true if and only if:
* <p>
* 1, public
* <p>
* 2, name starts with "set"
* <p>
* 3, only has one parameter
*/
private boolean isSetter(Method method) {
return method.getName().startsWith("set")
&& method.getParameterTypes().length == 1
&& Modifier.isPublic(method.getModifiers());
}