dubbo的SPI机制
前面提到了java的SPI机制，现在我们看看dubbo的SPI机制。
HelloSpiService接口，要加上 @SPI 注解
@SPI
public interface HelloSpiService {
void sayHello();
}
实现类HelloSpiServiceImpl同java的SPI机制。
在资源META-INF/ dubbo 文件下创建一个文件com.learn.dubbo.spi.HelloSpiService，文件内容不同与java的SPI的是用键值对。这样的好处是只加载需要的扩展点，节约资源。
helloSpiService=com.learn.dubbo.spi.impl.HelloSpiServiceImpl
测试代码：
public class DubboTest {
public static void main(String[] args) {
ExtensionLoader<HelloSpiService> extensionLoader =
ExtensionLoader.getExtensionLoader(HelloSpiService.class);
HelloSpiService helloSpiService = extensionLoader.getExtension("helloSpiService");
helloSpiService.sayHello();
}
}
运行结果如下：
源码分析
getExtensionLoader
主要做的事情：
- 对type判断，比如是否为空，是否是接口，是否是SPI注解
- 返回一个ExtensionLoader对象
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;
}
private ExtensionLoader(Class<?> type) {
this.type = type;
objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
}
getExtension
获取扩展实例，没有则创建
public T getExtension(String name) {
if (StringUtils.isEmpty(name)) {
throw new IllegalArgumentException("Extension name == null");
}
if ("true".equals(name)) {
// 获取默认的扩展点
return getDefaultExtension();
}
// 获取或创建Holder对象
final Holder<Object> holder = getOrCreateHolder(name);
// holder对象没有值，则创建拓展实例
Object instance = holder.get();
if (instance == null) {
synchronized (holder) {
instance = holder.get();
if (instance == null) {
instance = createExtension(name);
holder.set(instance);
}
}
}
// 返回拓展实例
return (T) instance;
}
private Holder<Object> getOrCreateHolder(String name) {
// 缓存没有，则创建完放入缓存
Holder<Object> holder = cachedInstances.get(name);
if (holder == null) {
// putIfAbsent防止一个name对应多个Holder
cachedInstances.putIfAbsent(name, new Holder<>());
holder = cachedInstances.get(name);
}
return holder;
}
createExtension
从配置信息获取类信息，判断是否实例化过，没有则实例化对象放入缓存
private T createExtension(String name) {
// 从配置文件获取扩展类
Class<?> clazz = getExtensionClasses().get(name);
if (clazz == null) {
throw findException(name);
}
try {
// 如果缓存没有实例化，则实例化放入缓存
T instance = (T) EXTENSION_INSTANCES.get(clazz);
if (instance == null) {
EXTENSION_INSTANCES.putIfAbsent(clazz, clazz.newInstance());
instance = (T) EXTENSION_INSTANCES.get(clazz);
}
// 注入依赖
injectExtension(instance);
Set<Class<?>> wrapperClasses = cachedWrapperClasses;
if (CollectionUtils.isNotEmpty(wrapperClasses)) {
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 + ") couldn't be instantiated: " + t.getMessage(), t);
}
}
getExtensionClasses
先从缓存获取，缓存获取不到，再调用loadExtensionClasses从配置文件获取信息实例化
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;
}
loadExtensionClasses
验证SPI注解内容，再从文件夹读取配置文件，我们配置的 META-INF/dubbo/ 就是这里读取的。
private Map<String, Class<?>> loadExtensionClasses() {
// 验证SPI注解内容
cacheDefaultExtensionName();
// 加载指定文件夹下的配置文件
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;
}
loadDirectory
通过文件路径获取资源路径
private void loadDirectory(Map<String, Class<?>> extensionClasses, String dir, String type) {
// 获取文件路径
String fileName = dir + type;
try {
Enumeration<java.net.URL> urls;
// 获取类加载器
ClassLoader classLoader = findClassLoader();
// 获取资源路径
if (classLoader != null) {
urls = classLoader.getResources(fileName);
} else {
urls = ClassLoader.getSystemResources(fileName);
}
if (urls != null) {
while (urls.hasMoreElements()) {
// 获取资源路径
java.net.URL resourceURL = urls.nextElement();
// 通过资源路径加载资源
loadResource(extensionClasses, classLoader, resourceURL);
}
}
} catch (Throwable t) {
logger.error("Exception occurred when loading extension class (interface: " +
type + ", description file: " + fileName + ").", t);
}
}
loadResource
读取文件内容，解析后，实例化对象并缓存。
private void loadResource(Map<String, Class<?>> extensionClasses, ClassLoader classLoader, java.net.URL resourceURL) {
try {
// 读取文件内容
try (BufferedReader reader = new BufferedReader(new InputStreamReader(resourceURL.openStream(), StandardCharsets.UTF_8))) {
String line;
while ((line = reader.readLine()) != null) {
// 如果有#号，只读取前面部分，#后面是注释
final int ci = line.indexOf('#');
if (ci >= 0) {
line = line.substring(0, ci);
}
line = line.trim();
if (line.length() > 0) {
try {
// 获取key和value
String name = null;
int i = line.indexOf('=');
if (i > 0) {
name = line.substring(0, i).trim();
line = line.substring(i + 1).trim();
}
// 实例化并存入缓存
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);
}
}
loadClass
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.");
}
// Adaptive注解
if (clazz.isAnnotationPresent(Adaptive.class)) {
cacheAdaptiveClass(clazz);
// 是否Wrapper类型
} else if (isWrapperClass(clazz)) {
cacheWrapperClass(clazz);
} else {
// 没有默认构造函数，抛异常
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);
}
}
// 缓存实例化对象
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);
}
}
}
}
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