Dubbo实现的是可插拔的,而且是针对接口编程,因此不能在框架内采取硬编码的方式,所以Dubbo吸取了JDK中SPI的思想。
1.Dubbo提供了SPI注解
1.1 SPI注解的值就是它的实现类
@Documented @Retention(RetentionPolicy.RUNTIME) @Target({ElementType.TYPE}) public @interface SPI { /** * 缺省扩展点名。 */ String value() default ""; }
1.2 随意抓取的一个示例如下:
@SPI("dubbo") public interface Protocol { /** * 获取缺省端口,当用户没有配置端口时使用。 * * @return 缺省端口 */ int getDefaultPort(); /** * 暴露远程服务:<br> * 1. 协议在接收请求时,应记录请求来源方地址信息:RpcContext.getContext().setRemoteAddress();<br> * 2. export()必须是幂等的,也就是暴露同一个URL的Invoker两次,和暴露一次没有区别。<br> * 3. export()传入的Invoker由框架实现并传入,协议不需要关心。<br> * * @param <T> 服务的类型 * @param invoker 服务的执行体 * @return exporter 暴露服务的引用,用于取消暴露 * @throws RpcException 当暴露服务出错时抛出,比如端口已占用 */ @Adaptive <T> Exporter<T> export(Invoker<T> invoker) throws RpcException; /** * 引用远程服务:<br> * 1. 当用户调用refer()所返回的Invoker对象的invoke()方法时,协议需相应执行同URL远端export()传入的Invoker对象的invoke()方法。<br> * 2. refer()返回的Invoker由协议实现,协议通常需要在此Invoker中发送远程请求。<br> * 3. 当url中有设置check=false时,连接失败不能抛出异常,并内部自动恢复。<br> * * @param <T> 服务的类型 * @param type 服务的类型 * @param url 远程服务的URL地址 * @return invoker 服务的本地代理 * @throws RpcException 当连接服务提供方失败时抛出 */ @Adaptive <T> Invoker<T> refer(Class<T> type, URL url) throws RpcException; /** * 释放协议:<br> * 1. 取消该协议所有已经暴露和引用的服务。<br> * 2. 释放协议所占用的所有资源,比如连接和端口。<br> * 3. 协议在释放后,依然能暴露和引用新的服务。<br> */ void destroy(); }
可以看见在Protocol上配置的SPI注解的值是“dubbo“,也就意味着默认使用的实现类是DubboProtocol,根据我们在JDK SPI的经验,它肯定是有一些映射关系存在,Dubbo的映射关系维护在META-INF/internal下的文件中,示例中的接口对应的文件是com.alibaba.dubbo.rpc.Protocol。在其内部维护了好多<key,value>的值,如下:可以看见dubbo在其内。
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.RedisProtocol2.Dubbo中SPI的使用
通过下面的例子,我们先来感性的认识一下Dubbo中是如何获取到实现类的对象的。
Protocol refprotocol = ExtensionLoader.getExtensionLoader(Protocol.class).getDefaultExtension(); Protocol refprotocol = ExtensionLoader.getExtensionLoader(Protocol.class).getExtension("dubbo");
2.1 ExtensionLoader
首先需要获取ExtensionLoader,这个你可以类比JDK中的ServiceLoader,或者类比它为Classloader。通过它,我们可以进一步获取到实现类。我们称它为扩展加载器。
2.2 getExtension("dubbo")
通过get("dubbo")的方式我们可以获取到它默认的实现类,但是"dubbo"这个是硬编码了,与SPI的初衷肯定是违背的,因此在Dubbo里并不是这样获取到实现类的。Dubbo提供了一个注解@Adaptive。
2.3 @Adaptive
这个注解标志着会实现了一个适配器类,并且它是通过代理动态生成的,通过这个适配器类,Dubbo可以轻易的根据配置文件的键值对来获取到想要的实现类,在此期间还不需要改动任何接口中的编码(比如刚刚的get("dubbo"))。针对这个适配器需要仔细研读源码,才能有体会,它为什么会这么做,,后续会进一步填补自己对于这个注解的认知。
3.Dubbo中SPI使用时的源码跟踪和分析
我们就已上一篇提到的ServiceConfig内的一个静态变量作为入口。我把它再次贴过来一下:
private static final Protocol protocol = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();3.1ExtensionLoader的属性
以下是公共部分,全局变量
//存放SPI文件的三个目录 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/"; private static final Pattern NAME_SEPARATOR = Pattern.compile("\\s*[,]+\\s*"); //某个接口的ExtensionLoader private static final ConcurrentMap<Class<?>, ExtensionLoader<?>> EXTENSION_LOADERS = new ConcurrentHashMap<Class<?>, ExtensionLoader<?>>(); //SPI实现类的对象实例 private static final ConcurrentMap<Class<?>, Object> EXTENSION_INSTANCES = new ConcurrentHashMap<Class<?>, Object>();
以下是私有部分
//接口Class private final Class<?> type; //SPI实现类对象实例的创建工厂 private final ExtensionFactory objectFactory; //SPI实现类的key private final ConcurrentMap<Class<?>, String> cachedNames = new ConcurrentHashMap<Class<?>, String>(); //存放所有的extensionClass private final Holder<Map<String, Class<?>>> cachedClasses = new Holder<Map<String,Class<?>>>(); private final Map<String, Activate> cachedActivates = new ConcurrentHashMap<String, Activate>(); //存储类上带有@Adaptive注解的Class private volatile Class<?> cachedAdaptiveClass = null; //缓存创建好的extensionClass实例 private final ConcurrentMap<String, Holder<Object>> cachedInstances = new ConcurrentHashMap<String, Holder<Object>>(); //默认的SPI文件中的key private String cachedDefaultName; //缓存创建好的适配类实例 private final Holder<Object> cachedAdaptiveInstance = new Holder<Object>(); private volatile Throwable createAdaptiveInstanceError; //缓存具有一个type入参的构造器的装饰器类在此 private Set<Class<?>> cachedWrapperClasses; private Map<String, IllegalStateException> exceptions = new ConcurrentHashMap<String, IllegalStateException>();
私有部分需要注意三个缓存值:
cachedAdaptiveClass:缓存的是那些有Adaptive注解的类。
cachedWrapperClasses:缓存的是那些存在接口参数构造器的装饰器类。
例如下面这个装饰器。
public class ProtocolFilterWrapper implements Protocol { private final Protocol protocol; public ProtocolFilterWrapper(Protocol protocol){ if (protocol == null) { throw new IllegalArgumentException("protocol == null"); } this.protocol = protocol; }
cachedClasses:存放所有的extensionClass。
3.1ExtensionLoader的获取
3.1.1主要逻辑就是先从缓存中取,没有的话再去构造出实例。
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; }3.1.2 不过在new的时候呢,有点小插曲,就是obejctFactory的获取,在本例子中呢,由于我们本来是想获取Protocol的ExtensionLoader的,所以它的type是com.alibaba.dubbo.rpc.Protocol,所以转而先去获取ExtenSionFactory的适配器类了,在这边呢我们不去追究它了,以主线Protocol的获取为准。
private ExtensionLoader(Class<?> type) { this.type = type; objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension()); }
到此,我们先认为ExtenSionLoader的获取已经完成了,忽略ExtenSionFactory的细节部分。
3.2 接着就是需要获取它的适配器类了,方法是getAdaptiveExtension()。
首先检查的依然是适配器缓存中是不是有这个实例了,没有的需要构造,执行createAdaptiveExtension();
public T getAdaptiveExtension() { Object instance = cachedAdaptiveInstance.get(); if (instance == null) { if(createAdaptiveInstanceError == null) { synchronized (cachedAdaptiveInstance) { instance = cachedAdaptiveInstance.get(); if (instance == null) { try { 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; }
3.3 createAdaptiveExtension()
这个方法由2部分组成:
3.3.1 injectExtension(),该方法主要在做注入的流程。
3.3.2 getAdaptiveExtensionClass(),顾名思义,就是获取扩展类的Class对象,然后根据Class实例化出扩展类实例。
private T createAdaptiveExtension() { try { return injectExtension((T) getAdaptiveExtensionClass().newInstance()); } catch (Exception e) { throw new IllegalStateException("Can not create adaptive extenstion " + type + ", cause: " + e.getMessage(), e); } }
我们需要从3.3.2 的getAdaptiveExtensionClass方法进行向下寻找,所以此段后的都是这个方法的展开
----------------------------------------3.3.2开始------------------------------->
3.3.2.1 getAdaptiveExtensionClass()
先从这个方法去看如何获取的,首先去获取ExtenSionClasses及其扩展类。接着判断缓存cachedAdaptiveClass (缓存带有注解@Adaptive的类),如果有的话直接返回,没有的话就执行createAdaptiveExtensionClass(猜测这个方法就是去创建出方法中带有注解的适配器类)
private Class<?> getAdaptiveExtensionClass() { getExtensionClasses(); if (cachedAdaptiveClass != null) { return cachedAdaptiveClass; } return cachedAdaptiveClass = createAdaptiveExtensionClass(); }3.3.2.2 getExtensionClasses()
首先从缓存cachedClasses(存放了所以的ExtensionClasses)获取所有:
如果有,直接返回。
如果没有,则通过loadExtensionClasses去读取SPI文件,之后再缓存进来
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; }3.3.2.3 loadExtensionClasses()
第一步,解析SPI注解中的值,并缓存到cachedDefaultName中,
第二步,读取SPI文件,获得扩展类,返回,注意loadFile,三次loadFile读取的路径就是公共变量内的三个路径。最后存放的是一个Map,这也是和JDK 的SPI不同之处,JDK中是迭代器,太落后了,使用Map的话通过get能一次读到,但是迭代器需要进行循环。
// 此方法已经getExtensionClasses方法同步过。 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<?>>(); loadFile(extensionClasses, DUBBO_INTERNAL_DIRECTORY); loadFile(extensionClasses, DUBBO_DIRECTORY); loadFile(extensionClasses, SERVICES_DIRECTORY); return extensionClasses; }3.3.2.4 loadFile()
1. 遍历每一行的数据name是key ,line是后面具体的内容。
2. 具有@Adaptive注解的的实现类存放如对于的缓存。
3.如果是装饰器类,缓存到装饰器缓存中。
4.如果具有@Active注解,将该类以<实现类key,active>存储
5.最后extensionClasses返回的是<key,实现类>
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); 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."); } 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) { logger.warn("==========> Override extension point: " + this.type + " with " + clazz); extensionClasses.put(n, 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); } }
粗略看下deBug的图:
3.3.2.5 loadFile操作之后返回到获取扩展类代码处,为了便利,此处直接贴出来
下面如果存在注解类的缓存,直接返回,如果不存在,则需要执行createAdaptiveExtensionClass
private Class<?> getAdaptiveExtensionClass() { getExtensionClasses(); if (cachedAdaptiveClass != null) { return cachedAdaptiveClass; } return cachedAdaptiveClass = createAdaptiveExtensionClass(); }
3.3.2.6 createAdaptiveExtensionClass()
第一步去生成代理类的code,然后进行编译操作,,,歪日,,内部原来这么复杂啊。
private Class<?> createAdaptiveExtensionClass() { String code = createAdaptiveExtensionClassCode(); 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); }3.3.2.6.1 欣赏一下生成的代理类,骚的不行,然后去进行编译返回,进行3.3.2.1 的装饰
package com.alibaba.dubbo.rpc; import com.alibaba.dubbo.common.extension.ExtensionLoader; 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.Invoker { 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() ); if(extName == null) throw new IllegalStateException("Fail to get extension(com.alibaba.dubbo.rpc.Protocol) name from url(" + url.toString() + ") use keys([protocol])"); 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 java.lang.Class { 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([protocol])"); 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); } }
----------------------------------------3.3.2至此结束------------------------------->
至于3.3.2.1的injectExtension过程暂且不考虑,知道一下是做注入操作。到此我们可以来看看最开始一句话到底最终是个什么结果。
private static final Protocol protocol = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();
就是protocol = Protocol$Adpative,至于为什么会这样呢,因为实现类不固定啊,作为框架开发者来说,他不知道你是怎么样的一个实现类,只能通过代理的方式来进行调用。我们来分析下它是怎么实现调用的。贴一段export的代码,这个在下一节服务暴露时所执行的的方法。
public com.alibaba.dubbo.rpc.Exporter export(com.alibaba.dubbo.rpc.Invoker arg0) throws com.alibaba.dubbo.rpc.Invoker { 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(); //获取具体实现类名称,默认是dubbo 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([protocol])"); //获取实现类实例,进行调用,是不是和开头说的如果想调用dubbo的实现类的话,硬编码不合适,但是dubbo利用了SPI机制实现了动态调用 //ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.rpc.Protocol.class).getExtension(extName) //ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.rpc.Protocol.class).getExtension("dubbo") 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); }
最后补充一下关于它的调用。
final Protocol dubboProtocol = loader.getExtension("dubbo");
也是先从缓存中获取,缓存没有时通过createExtension()方式创建出
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); } 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()方法,这个方法里面有两个地方需要注意一下,一个是injectExtension(IOC),一个是wrapperClasses(AOP),其实它们的作用也就是我括号里标注的,一个是set属性,一个是装饰,增强作用。
private T createExtension(String name) { 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, (T) clazz.newInstance()); instance = (T) EXTENSION_INSTANCES.get(clazz); } injectExtension(instance); Set<Class<?>> wrapperClasses = cachedWrapperClasses; if (wrapperClasses != null && wrapperClasses.size() > 0) { 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 + ") could not be instantiated: " + t.getMessage(), t); } }