JDK的SPI有什么缺陷？dubbo做了什么改进？
JDK的SPI机制的缺点⽂件中的所有类都会被加载且被实例化。这样也就导致获取某个实现类的方式不够灵活，只能通过 Iterator 形式获取，不能根据某个参数来获取对应的实现类。如果不想用某些实现类，或者某些类实例化很耗时，它也被载入并实例化了，没有办法指定某⼀个类来加载和实例化，这就造成了浪费。
此时dubbo的SPI可以解决
dubbo的SPI机制
dubbo⾃⼰实现了⼀套SPI机制来解决Java的SPI机制存在的问题。
dubbo中则采用了类似kv对的样式，在具体使用的时候则通过相关想法即可获取，而且获取的文件路径也不一致
ExtensionLoader 类文件
private static final String SERVICES_DIRECTORY = "META-INF/services/";
private static final String DUBBO_DIRECTORY = "META-INF/dubbo/";
private static final String DUBBO_INTERNAL_DIRECTORY = DUBBO_DIRECTORY + "internal/";如上述代码片段可知，dubbo是支持从META-INF/dubbo/,META-INF/dubbo/internal/以及META-INF/services/三个文件夹的路径去获取spi配置
例如com.alibaba.dubbo.rpc.Protocol 文件内容
registry=com.alibaba.dubbo.registry.integration.RegistryProtocol
filter=com.alibaba.dubbo.rpc.protocol.ProtocolFilterWrapper
listener=com.alibaba.dubbo.rpc.protocol.ProtocolListenerWrapper
mock=com.alibaba.dubbo.rpc.support.MockProtocol
injvm=com.alibaba.dubbo.rpc.protocol.injvm.InjvmProtocol
dubbo=com.alibaba.dubbo.rpc.protocol.dubbo.DubboProtocol
rmi=com.alibaba.dubbo.rpc.protocol.rmi.RmiProtocol
hessian=com.alibaba.dubbo.rpc.protocol.hessian.HessianProtocol
com.alibaba.dubbo.rpc.protocol.http.HttpProtocol
com.alibaba.dubbo.rpc.protocol.webservice.WebServiceProtocol
thrift=com.alibaba.dubbo.rpc.protocol.thrift.ThriftProtocol
memcached=memcom.alibaba.dubbo.rpc.protocol.memcached.MemcachedProtocol
redis=com.alibaba.dubbo.rpc.protocol.redis.RedisProtocol不过观察上述文件会发现，HttpProtocol是没有对应的k值，那就是说无法通过kv对获取到其协议实现类。后面通过源码可以发现，如果没有对应的name的时候，dubbo会通过findAnnotationName方法获取一个可用的name
Dubbo SPI 使用 & 源码学习
通过获取协议的代码来分析下具体的操作过程
Protocol 获取
Protocol protocol = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();
public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) {
// 静态方法，意味着可以直接通过类调用
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);
// 从map中获取该类型的ExtensionLoader数据
if (loader == null) {
EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
// 如果没有则，创建一个新的ExtensionLoader对象，并且以该类型存储
loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
// 再从map中获取
// 这里这样做的原因就是为了防止并发的问题，而且map本是也是个ConcurrentHashMap
}
return loader;
}private ExtensionLoader(Class<?> type) {
this.type = type;
objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
}
// 传入的type是Protocol.class，所以需要获取ExtensionFactory.class最合适的实现类public T getAdaptiveExtension() {
Object instance = cachedAdaptiveInstance.get();
if (instance == null) {
if(createAdaptiveInstanceError == null) {
synchronized (cachedAdaptiveInstance) {
instance = cachedAdaptiveInstance.get();
if (instance == null) {
try {
instance = createAdaptiveExtension();
// 创建对象，也是需要关注的函数
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() {
try {
return injectExtension((T) getAdaptiveExtensionClass().newInstance());
// (T) getAdaptiveExtensionClass().newInstance() 创建一个具体的实例对象
// getAdaptiveExtensionClass() 生成相关的class
// injectExtension 往该对象中注入数据
} catch (Exception e) {
throw new IllegalStateException("Can not create adaptive extenstion " + type + ", cause: " + e.getMessage(), e);
}
}private Class<?> getAdaptiveExtensionClass() {
getExtensionClasses();
// 通过加载SPI配置，获取到需要的所有的实现类存储到map中
// 通过可能会去修改cachedAdaptiveClass数据，具体原因在spi配置文件解析中分析
if (cachedAdaptiveClass != null) {
return cachedAdaptiveClass;
}
return cachedAdaptiveClass = createAdaptiveExtensionClass();
}private Class<?> createAdaptiveExtensionClass() {
String code = createAdaptiveExtensionClassCode();
// 动态生成需要的代码内容字符串
ClassLoader classLoader = findClassLoader();
com.alibaba.dubbo.common.compiler.Compiler compiler = ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.common.compiler.Compiler.class).getAdaptiveExtension();
return compiler.compile(code, classLoader);
// 编译，生成相应的类
}如下代码Protocol$Adpative 整个的类就是通过createAdaptiveExtensionClassCode()方法生成的一个大字符串
public class Protocol$Adpative implements com.alibaba.dubbo.rpc.Protocol {
public void destroy() {throw new UnsupportedOperationException("method public abstract void com.alibaba.dubbo.rpc.Protocol.destroy() of interface com.alibaba.dubbo.rpc.Protocol is not adaptive method!");
}
public int getDefaultPort() {throw new UnsupportedOperationException("method public abstract int com.alibaba.dubbo.rpc.Protocol.getDefaultPort() of interface com.alibaba.dubbo.rpc.Protocol is not adaptive method!");
}
public com.alibaba.dubbo.rpc.Exporter export(com.alibaba.dubbo.rpc.Invoker arg0) throws com.alibaba.dubbo.rpc.RpcException {
// 暴露远程服务，传入的参数是invoke对象
if (arg0 == null) throw new IllegalArgumentException("com.alibaba.dubbo.rpc.Invoker argument == null");
if (arg0.getUrl() == null) throw new IllegalArgumentException("com.alibaba.dubbo.rpc.Invoker argument getUrl() == null");
com.alibaba.dubbo.common.URL url = arg0.getUrl();
String extName = ( url.getProtocol() == null ? "dubbo" : url.getProtocol() );
// 如果没有具体协议，则使用dubbo协议
if(extName == null) throw new IllegalStateException("Fail to get extension(com.alibaba.dubbo.rpc.Protocol) name from url(" + url.toString() + ") use keys()");
com.alibaba.dubbo.rpc.Protocol extension = (com.alibaba.dubbo.rpc.Protocol)ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.rpc.Protocol.class).getExtension(extName);
return extension.export(arg0);
}
public com.alibaba.dubbo.rpc.Invoker refer(java.lang.Class arg0, com.alibaba.dubbo.common.URL arg1) throws com.alibaba.dubbo.rpc.RpcException {
// 引用远程对象，生成相对应的远程invoke对象
if (arg1 == null) throw new IllegalArgumentException("url == null");
com.alibaba.dubbo.common.URL url = arg1;
String extName = ( url.getProtocol() == null ? "dubbo" : url.getProtocol() );
if(extName == null) throw new IllegalStateException("Fail to get extension(com.alibaba.dubbo.rpc.Protocol) name from url(" + url.toString() + ") use keys()");
com.alibaba.dubbo.rpc.Protocol extension = (com.alibaba.dubbo.rpc.Protocol)ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.rpc.Protocol.class).getExtension(extName);
return extension.refer(arg0, arg1);
}
}到现在可以认为是最上面的获取protocol的方法Protocol protocol = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension() 返回了一个代码拼接而成然后编译操作的实现类Protocol$Adpative
可是得到具体的实现呢？在com.alibaba.dubbo.rpc.Protocol extension = (com.alibaba.dubbo.rpc.Protocol)ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.rpc.Protocol.class).getExtension(extName)这个代码中，当然这个是有在具体的暴露服务或者引用远程服务才被调用执行的。
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);
if (holder == null) {
cachedInstances.putIfAbsent(name, new Holder<Object>());
holder = cachedInstances.get(name);
}
// 无论是否真有数据，在cachedInstances存储的是一个具体的Holder对象
Object instance = holder.get();
if (instance == null) {
synchronized (holder) {
instance = holder.get();
if (instance == null) {
instance = createExtension(name);
// 创建对象
holder.set(instance);
}
}
}
return (T) instance;
}private T createExtension(String name) { Class clazz = getExtensionClasses().get(name); // 获取具体的实现类的类 if (clazz == null) { throw findException(name); } try { T instance = (T) EXTENSION_INSTANCES.get(clazz); if (instance == null) { EXTENSION_INSTANCES.putIfAbsent(clazz, (T) clazz.newInstance()); // clazz.newInstance 才是真正创建对象的操作 instance = (T) EXTENSION_INSTANCES.get(clazz); } injectExtension(instance); // 往实例中反射注入参数 Set> loadExtensionClasses() { final SPI defaultAnnotation = type.getAnnotation(SPI.class); // 查看该类是否存在SPI注解信息 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; // 设置默认的名称，如果注解的值经过切割，发现超过1个的数据，则同样会认为错误 } } Map>(); loadFile(extensionClasses, DUBBO_INTERNAL_DIRECTORY); // 加载文件 loadFile(extensionClasses, DUBBO_DIRECTORY); loadFile(extensionClasses, SERVICES_DIRECTORY); return extensionClasses;} private void loadFile(Map
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