dubbo之SPI模式
一 概述
一大早来上班,准备写****博客,发现前面的文章下有人评价,有点欣喜,有点安慰。算是对我这段时间作品的回报和我前进的动力吧!继续写吧.. 前面阅读dubbo源码经常看到
- Protocol protocol = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();
二 什么是SPI技术
感觉自己的语言描述不是很精准,就不再自己创造了,在网上拷贝了一段描述,讲明了什么是SPI技术,为什么要用SPI,用SPI有什么好处。内如下: SPI的全名为Service Provider Interface.大多数开发人员可能不熟悉,因为这个是针对厂商或者插件的。在java.util.ServiceLoader的文档里有比较详细的介绍。简单的总结下java spi机制的思想。我们系统里抽象的各个模块,往往有很多不同的实现方案,比如日志模块的方案,xml解析模块、jdbc模块的方案等。面向的对象的设计里,我们一般推荐模块之间基于接口编程,模块之间不对实现类进行硬编码。一旦代码里涉及具体的实现类,就违反了可拔插的原则,如果需要替换一种实现,就需要修改代码。为了实现在模块装配的时候能不在程序里动态指明,这就需要一种服务发现机制。 java spi就是提供这样的一个机制:为某个接口寻找服务实现的机制。有点类似IOC的思想,就是将装配的控制权移到程序之外,在模块化设计中这个机制尤其重要,java spi的具体约定为:当服务的提供者,提供了服务接口的一种实现之后,在jar包的META-INF/services/目录里同时创建一个以服务接口命名的文件。该文件里就是实现该服务接口的具体实现类。而当外部程序装配这个模块的时候,就能通过该jar包META-INF/services/里的配置文件找到具体的实现类名,并装载实例化,完成模块的注入。 基于这样一个约定就能很好的找到服务接口的实现类,而不需要再代码里制定。jdk提供服务实现查找的一个工具类:java.util.ServiceLoader
三 JDK中的SPI技术
JDK中拥有SPI的支持,主要涉及java.util.ServiceLoader类的使用,我写了一个小的DEMO让我们初步理解下SPI的使用。首先我写了一个接口代码如下:
- package spi;
- /**
- * Created by Administrator on 2017/8/28.
- */
- public interface DubboService {
- public void sayHello();
- }
然后写了这个接口的两个实现类
- package spi;
- /**
- * Created by Administrator on 2017/8/28.
- */
- public class RedService implements DubboService {
- public void sayHello() {
- System.out.println("我是RedService服务实现");
- }
- }
- package spi;
- /**
- * Created by Administrator on 2017/8/28.
- */
- public class YellowService implements DubboService {
- public void sayHello() {
- System.out.println("我是YellowService服务实现");
- }
- }
- package spi;
- import java.util.Iterator;
- import java.util.ServiceLoader;
- /**
- * Created by Administrator on 2017/8/28.
- */
- public class ServiceMain {
- public static void main(String[] args){
- ServiceLoader<DubboService> spiLoader = ServiceLoader.load(DubboService.class);
- Iterator<DubboService> iteratorSpi=spiLoader.iterator();
- while (iteratorSpi.hasNext()){
- DubboService dubboService=iteratorSpi.next();
- dubboService.sayHello();
- }
- }
- }
1.我是RedService服务实现
2.我是YellowService服务实现
分析main函数中的代码,ServiceLoader类是JDK自带的哦!调用load方法即可加载接口参数的所有实现!看到这里是不是在想在META-INF文件夹下创建services/接口全限定名是不是固定写法!好吧,我们跟进到ServiceLoader类中。第一行代码就是private static final String PREFIX = "META-INF/services/"; 是不是立马就明白了ServiceLoader类中已经将配置路径固定好了(如果感兴趣也可以自己写个类来实现ServiceLoader的功能),调用load方法,根据传入的接口参数找到该接口的对应文件,然后一行一行的读取文件中的实现类,使用java反射机制实例化接口的实现对象。到这里SPI技术的原理应该理解了。也可以看看ServiceLoader类的源码能更深入的理解!看看我的目录接口如下图:
新建的spi.DubboService中的内容如下:
四 dubbo中SPI的应用
dubbo框架中大量使用了SPI技术,里面有很多个组件,每个组件在框架中都是以接口的形成抽象出来!具体的实现又分很多种,在程序执行时根据用户的配置来按需取接口的实现。方便了接口的各种实现灵活应用。不过dubbo使用的SPI技术不是源用jdk的实现,但是它们的思想仍然是一样的。我这里仍然以Protocol 协议接口来讲解,还是这段代码
- Protocol protocol = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();
- 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!");
- }
- //根据接口对象取ExtensionLoader类
- ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
- if (loader == null) {
- //如果为空保存接口类对应的 新建的ExtensionLoader对象
- EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
- loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
- }
- return loader;
- }
2.判断Map中根据接口get对象,如果没有就new个ExtensionLoader对象保存进去。并返回该ExtensionLoader对象。
3.注意创建ExtensionLoader对象的构造函数代码,将传入的接口type属性赋值给了ExtensionLoader类的type属性
4.创建ExtensionFactory objectFactory对象。怎么创建的,还是那个熟悉的写法,嘿嘿!出现的好频繁哇!
- private ExtensionLoader(Class<?> type) {
- this.type = type;
- objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
- }
- /**
- * 动态生成接口或者点的代理类
- */
- public T getAdaptiveExtension() {
- //取里面的Value
- Object instance = cachedAdaptiveInstance.get();
- //为null的处理
- if (instance == null) {
- //判断是否有异常处理
- if(createAdaptiveInstanceError == null) {
- //针对cachedAdaptiveInstance加锁处理
- synchronized (cachedAdaptiveInstance) {
- instance = cachedAdaptiveInstance.get();
- if (instance == null) {
- try {
- instance = createAdaptiveExtension();//动态生成入口
- cachedAdaptiveInstance.set(instance);//设置代理对象到value中
- } 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 {
- //(T) getAdaptiveExtensionClass().newInstance()创建class对象实例
- return injectExtension((T) getAdaptiveExtensionClass().newInstance());//接口的代理实现class创建一个实体对象
- } catch (Exception e) {
- throw new IllegalStateException("Can not create adaptive extenstion " + type + ", cause: " + e.getMessage(), e);
- }
- }
- private Class<?> getAdaptiveExtensionClass() {
- getExtensionClasses();//通过SPI加载接口延伸的所有实现到map中保存
- if (cachedAdaptiveClass != null) {
- return cachedAdaptiveClass;
- }
- return cachedAdaptiveClass = createAdaptiveExtensionClass();//动态生成接口的代理实现class对象
- }
- /**
- * 通过SPI加载当前传入接口延伸的所有实现到map中保存
- * @return
- */
- 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;
- }
- //通过SPI加载接口对应的所有实现类
- // 此方法已经getExtensionClasses方法同步过。
- private Map<String, Class<?>> loadExtensionClasses() {
- //解析type接口上的SPI注解
- final SPI defaultAnnotation = type.getAnnotation(SPI.class);
- if(defaultAnnotation != null) {
- //获取注解标记值
- 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;
- }
- 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type接口是clazz类的接口
- 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.");
- }
- //判断接口实现类是否标注了该注解
- 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);
- }
- }
1.loadExtensionClasses方法判断ExtensionLoader类中的传入的type接口是否标注了SPI注解,并获取SPI注解的值,这个值为接口的默认实现标记。
2.loadFile方法用来加载配置路径下的接口的实现类。比如在调用loadFile方法时,传入的参数DUBBO_INTERNAL_DIRECTORY,DUBBO_DIRECTORY,SERVICES_DIRECTORY。他们都描述了接口实现类配置文件路径,看看3个属性的值如下:
- 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/";
是不是感觉跟JDK里面SPI技术路径描述很类型。讲到这里我们来看看dubbo框架的接口实现配置是怎么玩的,这里我就以com.alibaba.dubbo.rpc.Protocol接口来研究!哟!是不是感觉跟JDK里面配置不太一样,它按照key=value的形式来保存的,在分析下loadFile方法中的代码,它也是按照key=value的格式来解析出接口的具体实现,将最终解析的数据保存到了传入的map参数extensionClasses中。大家应该感到好奇为什么要做个key=value的配置。打个比方Protocol协议接口在dubbo框架里实现有hession,http,rmi,webservice,dubbo等好几种实现,在程序运行中我们根据配置来使用具体的协议,比方我要使用rmi协议,那我就配置rmi,我想使用dubbo 我就配置dubbo。配置好以后会根据这个属性配置取找相关的具体协议实现。所以这里的key=value应该就是做这个事情的。我这里贴几张图看看Protocol接口的实现配置,图下几个图:
rmi协议实现:
hession协议实现:
dubbo协议实现,也就是默认的协议:
我们再次回到getAdaptiveExtensionClass方法中。继续看createAdaptiveExtensionClass方法
- private Class<?> createAdaptiveExtensionClass() {
- String code = createAdaptiveExtensionClassCode();//创建接口的代理类实现
- ClassLoader classLoader = findClassLoader();//获取当前使用的类加载器
- com.alibaba.dubbo.common.compiler.Compiler compiler = ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.common.compiler.Compiler.class).getAdaptiveExtension();//获取代码编译器
- return compiler.compile(code, classLoader);//加载代理
- }
- //动态生成的协议代理类
- 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 {
- 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();
- //默认选择dubbo协议,否则根据url中带的协议属性来选择对应的协议处理对象,这样可以动态选择不同的协议
- 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])");
- //根据拿到的协议key从缓存的map中取协议对象
- 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 {
- 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])");
- //根据拿到的协议key从缓存的map中取协议对象
- 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);
- }
- }
- injectExtension((T) getAdaptiveExtensionClass().newInstance());
- private T injectExtension(T instance) {
- try {
- if (objectFactory != null) {
- for (Method method : instance.getClass().getMethods()) {
- if (method.getName().startsWith("set")
- && method.getParameterTypes().length == 1
- && Modifier.isPublic(method.getModifiers())) {
- Class<?> pt = method.getParameterTypes()[0];
- try {
- String property = method.getName().length() > 3 ? method.getName().substring(3, 4).toLowerCase() + method.getName().substring(4) : "";
- Object object = objectFactory.getExtension(pt, property);
- if (object != null) {
- method.invoke(instance, object);
- }
- } catch (Exception e) {
- logger.error("fail to inject via method " + method.getName()
- + " of interface " + type.getName() + ": " + e.getMessage(), e);
- }
- }
- }
- }
- } catch (Exception e) {
- logger.error(e.getMessage(), e);
- }
- return instance;
- }
貌似没做什么太多的操作,只是利用反射机制判断接口代理类中是否有需要注入的属性。然后就结束啦!我们再次回退到getAdaptiveExtension方法中,最终反馈的就是该接口的代理实现。是不是有点疑惑,看了这么半天
- Protocol protocol = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();
代码只是反馈了接口的代理,那我在实际使用中怎么得到接口的具体实现呢?
额! 在仔细看看Protocol接口代理的具体实现,在使用接口代理中的方法时,都会根据URL来确定接口的具体实现,因为URL中携带了用户大部分的参数配置,根据里面的属性来获取。里面关键代码- com.alibaba.dubbo.rpc.Protocol extension = (com.alibaba.dubbo.rpc.Protocol)ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.rpc.Protocol.class).getExtension(extName);
- /**
- * 返回指定名字的扩展。如果指定名字的扩展不存在,则抛异常 {@link IllegalStateException}.
- *
- * @param name
- * @return
- */
- @SuppressWarnings("unchecked")
- public T getExtension(String name) {
- if (name == null || name.length() == 0)
- throw new IllegalArgumentException("Extension name == null");
- if ("true".equals(name)) {
- return getDefaultExtension();
- }
- //根据传入的name参数确定接口的具体实现类
- Holder<Object> holder = cachedInstances.get(name);
- if (holder == null) {
- cachedInstances.putIfAbsent(name, new Holder<Object>());
- holder = cachedInstances.get(name);
- }
- //判断接口实现类是否存在
- Object instance = holder.get();
- if (instance == null) {
- synchronized (holder) {
- instance = holder.get();
- if (instance == null) {
- //不存在那么创建一个接口实现类
- instance = createExtension(name);
- holder.set(instance);
- }
- }
- }
- return (T) instance;
- }
- private T createExtension(String name) {
- //根据参数获取接口的Class对象
- Class<?> clazz = getExtensionClasses().get(name);
- if (clazz == null) {
- throw findException(name);
- }
- try {
- //判断Map中是否存在改Class的实例
- T instance = (T) EXTENSION_INSTANCES.get(clazz);
- if (instance == null) {
- //创建一个实例并保存到map中
- EXTENSION_INSTANCES.putIfAbsent(clazz, (T) clazz.newInstance());
- instance = (T) EXTENSION_INSTANCES.get(clazz);
- }
- //注入属性到实例中
- 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);
- }
- }
看到这里思路应该比较清晰了!所有的接口代理中,并没有给定具体的实现,全部根据用户的参数配置来动态创建接口的具体实现。这样做让程序非常的灵活,让接口的实现插拔更加方便。如果想增加一个接口的实现,只需要按照SPI的配置方式增加配置文件,xml标签配置指定新接口实现的标记即可。