准备
dubbo版本：2.5.4
Dubbo SPI源码解析过程
Dubbo 在服务发布过程中默认会加载自适应的协议扩展，在类 ServiceConfig 中存在以下初始化代码，下面以此进行SPI过程分析。
private static final Protocol protocol = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();
相关注解
-
@SPI：表示当前这个接口是一个扩展点，可以实现自己的扩展实现，例如Protocol接口默认扩展点实现是DubboProtocol。 -
@Adaptive：表示一个自适应扩展点，在方法级别上，会动态生成一个适配器类。
getExtensionLoader
该方法需要一个 Class 类型的参数，该参数表示希望加载的扩展点类型，该参数必须是接口，且该接口必须被 @SPI 注解注释，否则拒绝处理。检查通过之后首先会检查 ExtensionLoader 缓存中是否已经存在该扩展对应的 ExtensionLoader ，如果有则直接返回，否则创建一个新的 ExtensionLoader 负责加载该扩展实现，同时将其缓存起来。可以看到对于每一个扩展，dubbo 中只会有一个对应的 ExtensionLoader 实例
// ExtensionLoader
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!");
}
if(!withExtensionAnnotation(type)) {
throw new IllegalArgumentException("Extension type(" + type +
") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");
}
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;
}
getAdaptiveExtension
// ExtensionLoader
public T getAdaptiveExtension() {
// 从缓存中获取自适应的扩展点实例对象
Object instance = cachedAdaptiveInstance.get();
// 第一次加载，缓存为空，则通过双重检验锁创建一个适配器扩展点
if (instance == null) {
if(createAdaptiveInstanceError == null) {
synchronized (cachedAdaptiveInstance) {
instance = cachedAdaptiveInstance.get();
if (instance == null) {
try {
// 创建自适应扩展点>>
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
// ExtensionLoader
private T createAdaptiveExtension() {
try {
// 1.首先通过getAdaptiveExtensionClass获取适配器扩展点Class并创建实例对象
// 2.通过injectExtension实现扩展点注入
return injectExtension((T) getAdaptiveExtensionClass().newInstance());
} catch (Exception e) {
throw new IllegalStateException("Can not create adaptive extenstion " + type + ", cause: " + e.getMessage(), e);
}
}
getAdaptiveExtensionClass
// ExtensionLoader
private Class<?> getAdaptiveExtensionClass() {
// 加载所有路径下的扩展点>>
getExtensionClasses();
// 判断当前扩展点(Protocol)是否存在自定义的适配器，有则直接返回，没有则动态创建
if (cachedAdaptiveClass != null) {
return cachedAdaptiveClass;
}
// 动态生成扩展点适配器类>>
return cachedAdaptiveClass = createAdaptiveExtensionClass();
}
getExtensionClasses
private Map<String, Class<?>> getExtensionClasses() {
Map<String, Class<?>> classes = cachedClasses.get();
if (classes == null) {
synchronized (cachedClasses) {
classes = cachedClasses.get();
if (classes == null) {
// 加载扩展点实现类>>
classes = loadExtensionClasses();
cachedClasses.set(classes);
}
}
}
return classes;
}
loadExtensionClasses
private Map<String, Class<?>> loadExtensionClasses() {
final SPI defaultAnnotation = type.getAnnotation(SPI.class);
if(defaultAnnotation != null) {
String value = defaultAnnotation.value();
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<?>>();
// 加载META-INF/dubbo、META-INF/dubbo/internal、META-INF/services 三个目录下的扩展点实现类
loadFile(extensionClasses, DUBBO_INTERNAL_DIRECTORY);
loadFile(extensionClasses, DUBBO_DIRECTORY);
loadFile(extensionClasses, SERVICES_DIRECTORY);
return extensionClasses;
}
loadFile：解析指定路径下文件获取对应扩展点，通过反射机制实例化
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);
}
if (urls != null) {
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) {
name = line.substring(0, i).trim();
line = line.substring(i + 1).trim();
}
if (line.length() > 0) {
Class<?> clazz = Class.forName(line, true, classLoader);
// 判断当前实现类是否是加载扩展点的实现类
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.");
}
/**
* 判断是否有自定义适配器类
* Adaptive注解加在类上，说明当前类就是自适应扩展点
* 加在方法上，则需要动态创建一个自适应扩展点(Protocol则对应Protocol$Adaptive)
*/
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 {
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);
}
}
回到 getAdaptiveExtensionClass 中的 createAdaptiveExtensionClass方法
// ExtensionLoader
private Class<?> createAdaptiveExtensionClass() {
// 生成扩展点适配器类字节码
String code = createAdaptiveExtensionClassCode();
// 获取类加载器
ClassLoader classLoader = findClassLoader();
com.alibaba.dubbo.common.compiler.Compiler compiler = ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.common.compiler.Compiler.class).getAdaptiveExtension();
// 动态编译字节码
return compiler.compile(code, classLoader);
}
回到 createAdaptiveExtension 中的 injectExtension 扩展点注入方法
private T injectExtension(T instance) {
try {
if (this.objectFactory != null) {
Method[] var2 = instance.getClass().getMethods();
int var3 = var2.length;
for(int var4 = 0; var4 < var3; ++var4) {
Method method = var2[var4];
// 判断是否以set开头，通过setter进行动态注入
if (method.getName().startsWith("set") && method.getParameterTypes().length == 1 && Modifier.isPublic(method.getModifiers())) {
// 获取setter方法的参数类型
Class pt = method.getParameterTypes()[0];
try {
// 获取属性名称
String property = method.getName().length() > 3 ? method.getName().substring(3, 4).toLowerCase() + method.getName().substring(4) : "";
// 根绝类型及名称获取对应的扩展点
Object object = this.objectFactory.getExtension(pt, property);
if (object != null) {
method.invoke(instance, object);
}
} catch (Exception var9) {
logger.error("fail to inject via method " + method.getName() + " of interface " + this.type.getName() + ": " + var9.getMessage(), var9);
}
}
}
}
} catch (Exception var10) {
logger.error(var10.getMessage(), var10);
}
return instance;
}
至此，整个扩展点加载过程完成。