赵计刚
每天进步一点点
 1 package com.alibaba.dubbo.demo.test;
 2 
 3 import com.alibaba.dubbo.common.extension.ExtensionLoader;
 4 import com.alibaba.dubbo.rpc.Protocol;
 5 
 6 public class TestExtension {
 7     public static void main(String[] args) {
 8         ExtensionLoader<Protocol> loader = ExtensionLoader.getExtensionLoader(Protocol.class);
 9         final Protocol dubboProtocol = loader.getExtension("dubbo");
10         final Protocol adaptiveExtension = loader.getAdaptiveExtension();
11     }
12 }

讲解这三行代码的源码。

一  Protocol接口的定义

 1 package com.alibaba.dubbo.rpc;
 2 
 3 import com.alibaba.dubbo.common.URL;
 4 import com.alibaba.dubbo.common.extension.Adaptive;
 5 import com.alibaba.dubbo.common.extension.SPI;
 6 
 7 /**
 8  * Protocol. (API/SPI, Singleton, ThreadSafe)
 9  */
10 @SPI("dubbo")
11 public interface Protocol {
12 
13     /**
14      * 获取缺省端口,当用户没有配置端口时使用。
15      *
16      * @return 缺省端口
17      */
18     int getDefaultPort();
19 
20     /**
21      * 暴露远程服务:<br>
22      * 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>
23      * 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>
24      * 3. export()传入的Invoker由框架实现并传入,协议不需要关心。<br>
25      *
26      * @param <T>     服务的类型
27      * @param invoker 服务的执行体
28      * @return exporter 暴露服务的引用,用于取消暴露
29      * @throws RpcException 当暴露服务出错时抛出,比如端口已占用
30      */
31     @Adaptive
32     <T> Exporter<T> export(Invoker<T> invoker) throws RpcException;
33 
34     /**
35      * 引用远程服务:<br>
36      * 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>
37      * 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>
38      * 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>
39      *
40      * @param <T>  服务的类型
41      * @param type 服务的类型
42      * @param url  远程服务的URL地址
43      * @return invoker 服务的本地代理
44      * @throws RpcException 当连接服务提供方失败时抛出
45      */
46     @Adaptive
47     <T> Invoker<T> refer(Class<T> type, URL url) throws RpcException;
48 
49     /**
50      * 释放协议:<br>
51      * 1. 取消该协议所有已经暴露和引用的服务。<br>
52      * 2. 释放协议所占用的所有资源,比如连接和端口。<br>
53      * 3. 协议在释放后,依然能暴露和引用新的服务。<br>
54      */
55     void destroy();
56 }

注意:这里有两个核心注解

  • @SPI:指定一个接口为SPI接口(可扩展接口)
    1 @Documented
    2 @Retention(RetentionPolicy.RUNTIME)
    3 @Target({ElementType.TYPE})
    4 public @interface SPI {
    5     /** 缺省扩展点名 */
    6     String value() default "";
    7 }

     

  • @Adaptive该注解可以注解在两个地方:

    • 接口上:例如AdaptiveExtensionFactory(该类不是工厂类,有特殊的逻辑)  AdaptiveCompiler(实际上也是工厂类,但是不能靠动态生成,否则会形成死循环)
    • 接口的方法上:会动态生成相应的动态类(实际上是一个工厂类,工厂设计模式),例如Protocol$Adapter

这个接口极其重要,后续的整个服务暴露和服务调用会用到该接口的两个方法。

 

二 获取ExtensionLoader

1 ExtensionLoader<Protocol> loader = ExtensionLoader.getExtensionLoader(Protocol.class);

ExtensionLoader可以类比为JDK-SPI中的ServiceLoader

首先来看一下ExtensionLoader的类属性:

 1     /** 存放SPI文件的三个目录,其中META-INF/services/也是jdk的SPI文件的存放目录 */
 2     private static final String SERVICES_DIRECTORY = "META-INF/services/";
 3     private static final String DUBBO_DIRECTORY = "META-INF/dubbo/";
 4     private static final String DUBBO_INTERNAL_DIRECTORY = DUBBO_DIRECTORY + "internal/";//这个是最终jar包中存放spi文件的位置
 5 
 6     private static final Pattern NAME_SEPARATOR = Pattern.compile("\\s*[,]+\\s*");
 7     /** key: SPI接口Class value: 该接口的ExtensionLoader */
 8     private static final ConcurrentMap<Class<?>, ExtensionLoader<?>> EXTENSION_LOADERS = new ConcurrentHashMap<Class<?>, ExtensionLoader<?>>();
 9     /** key: SPI接口Class value: SPI实现类的对象实例 */
10     private static final ConcurrentMap<Class<?>, Object> EXTENSION_INSTANCES = new ConcurrentHashMap<Class<?>, Object>();

注意:上述的都是类属性,即所有该类的实例都共享。而后边的实例属性就属于每一个类的实例私有

再来看一下ExtensionLoader的实例属性:

 1     /** SPI接口Class */
 2     private final Class<?> type;
 3     /** SPI实现类对象实例的创建工厂 */
 4     private final ExtensionFactory objectFactory;
 5     /** key: ExtensionClass的Class value: SPI实现类的key */
 6     private final ConcurrentMap<Class<?>, String> cachedNames = new ConcurrentHashMap<Class<?>, String>();
 7     /** 存放所有的extensionClass */
 8     private final Holder<Map<String, Class<?>>> cachedClasses = new Holder<Map<String, Class<?>>>();
 9 
10     private final Map<String, Activate> cachedActivates = new ConcurrentHashMap<String, Activate>();
11     /** 缓存创建好的extensionClass实例 */
12     private final ConcurrentMap<String, Holder<Object>> cachedInstances = new ConcurrentHashMap<String, Holder<Object>>();
13     /** 缓存创建好的适配类实例 */
14     private final Holder<Object> cachedAdaptiveInstance = new Holder<Object>();
15     /** 存储类上带有@Adaptive注解的Class */
16     private volatile Class<?> cachedAdaptiveClass = null;
17     /** 默认的SPI文件中的key */
18     private String cachedDefaultName;
19     /** 存储在创建适配类实例这个过程中发生的错误 */
20     private volatile Throwable createAdaptiveInstanceError;
21     /** 存放具有一个type入参的构造器的实现类的Class对象 */
22     private Set<Class<?>> cachedWrapperClasses;
23     /** key :实现类的全类名  value: exception, 防止真正的异常被吞掉 */
24     private Map<String, IllegalStateException> exceptions = new ConcurrentHashMap<String, IllegalStateException>();

来看一下getExtensionLoader(Class<T> type)的源码:

 1     /**
 2      * 1 校验入参type:非空 + 接口 + 含有@SPI注解
 3      * 2 根据type接口从全局缓存EXTENSION_LOADERS中获取ExtensionLoader,如果有直接返回;如果没有,则先创建,之后放入缓存,最后返回
 4      */
 5     public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) {
 6         if (type == null)
 7             throw new IllegalArgumentException("Extension type == null");
 8         if (!type.isInterface()) {
 9             throw new IllegalArgumentException("Extension type(" + type + ") is not interface!");
10         }
11         if (!withExtensionAnnotation(type)) {
12             throw new IllegalArgumentException("Extension type(" + type +
13                     ") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");
14         }
15 
16         ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
17         if (loader == null) {
18             EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
19             loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
20         }
21         return loader;
22     }

创建ExtensionLoader:

1     private ExtensionLoader(Class<?> type) {
2         this.type = type;
3         objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
4     }

当前创建的ExtensionLoader对象(我们取名为ExtensionLoader对象1)的type是com.alibaba.dubbo.rpc.Protocol,所以此时会执行:ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension()。

首先是创建ExtensionFactory,通过上边核心类部分ExtensionFactory接口的源码可以看出,此类也是一个SPI接口类,且没有指定默认的实现类的key。

1 ExtensionLoader.getExtensionLoader(ExtensionFactory.class)

下面的代码与上述的过程相似,只是此时创建的另外一个ExtensionLoader对象(我们取名为ExtensionLoader对象2)的type是com.alibaba.dubbo.common.extension.ExtensionFactory,而objectFactory是null。之后,这个ExtensionLoader对象2被放入EXTENSION_LOADERS缓存。这里给出ExtensionFactory的定义,该类也极其重要。//TODO

1 package com.alibaba.dubbo.common.extension;
2 
3 @SPI
4 public interface ExtensionFactory {
5     <T> T getExtension(Class<T> type, String name);
6 }

之后执行ExtensionLoader对象2的getAdaptiveExtension()方法。

 1     /**
 2      * 首先从cachedAdaptiveInstance缓存中获取AdaptiveExtension实例
 3      * 如果不为null, 直接返回;
 4      * 如果为null, 先创建AdaptiveExtension实例, 之后放入cachedAdaptiveInstance缓存中,最后返回
 5      */
 6     public T getAdaptiveExtension() {
 7         Object instance = cachedAdaptiveInstance.get();
 8         if (instance == null) {
 9             if (createAdaptiveInstanceError == null) {
10                 synchronized (cachedAdaptiveInstance) {
11                     instance = cachedAdaptiveInstance.get();
12                     if (instance == null) {
13                         try {
14                             instance = createAdaptiveExtension();
15                             cachedAdaptiveInstance.set(instance);
16                         } catch (Throwable t) {
17                             createAdaptiveInstanceError = t;
18                             throw new IllegalStateException("fail to create adaptive instance: " + t.toString(), t);
19                         }
20                     }
21                 }
22             } else {
23                 throw new IllegalStateException("fail to create adaptive instance: " + createAdaptiveInstanceError.toString(), createAdaptiveInstanceError);
24             }
25         }
26 
27         return (T) instance;
28     }

来看createAdaptiveExtension()创建AdaptiveExtension的源码:

 1     /**
 2      * createAdaptiveExtension()
 3      * --getAdaptiveExtensionClass()
 4      *   //从dubbo-spi配置文件中获取AdaptiveExtensionClass
 5      *   --getExtensionClasses()
 6      *     --loadExtensionClasses()
 7      *       --loadFile(Map<String, Class<?>> extensionClasses, String dir)
 8      *   //创建动态代理类
 9      *   --createAdaptiveExtensionClass()
10      *
11      * --injectExtension(T instance)  //dubbo-ioc
12      */
13     private T createAdaptiveExtension() {
14         try {
15             return injectExtension((T) getAdaptiveExtensionClass().newInstance());
16         } catch (Exception e) {
17             throw new IllegalStateException("Can not create adaptive extenstion " + type + ", cause: " + e.getMessage(), e);
18         }
19     }

调用层级看注释。injectExtension(T instance)方法只对objectFactory有用,如果objectFactory==null,则直接返回T instance。所以这里返回的是getAdaptiveExtensionClass().newInstance()

来看getAdaptiveExtensionClass()的源码:

 1     /**
 2      * 获取ExtensionClasses和适配类
 3      * 如果实现类上带有@Adaptive注解,直接创建修饰类
 4      * 如果方法上带有@Adaptive注解,动态生成代理类
 5      */
 6     private Class<?> getAdaptiveExtensionClass() {
 7         getExtensionClasses();
 8         if (cachedAdaptiveClass != null) {
 9             return cachedAdaptiveClass;
10         }
11         return cachedAdaptiveClass = createAdaptiveExtensionClass();
12     }

现在来看getExtensionClasses():

 1     /**
 2      * 先从cachedClasses缓存中获取所有的ExtensionClass,如果有,直接返回;
 3      * 如果没有,通过loadExtensionClasses()从SPI文件中去读取,之后写入缓存
 4      */
 5     private Map<String, Class<?>> getExtensionClasses() {
 6         Map<String, Class<?>> classes = cachedClasses.get();
 7         if (classes == null) {
 8             synchronized (cachedClasses) {
 9                 classes = cachedClasses.get();
10                 if (classes == null) {
11                     classes = loadExtensionClasses();
12                     cachedClasses.set(classes);
13                 }
14             }
15         }
16         return classes;
17     }

现在来看loadExtensionClasses()

 1     /**
 2      * 1 从@SPI注解中将默认值解析出来,并缓存到cachedDefaultName中
 3      * 2 从SPI文件中获取extensionClass并存储到extensionClasses中,最后返回extensionClasses
 4      * 注意:此方法已经getExtensionClasses方法同步过。
 5      */
 6     private Map<String, Class<?>> loadExtensionClasses() {
 7         final SPI defaultAnnotation = type.getAnnotation(SPI.class);
 8         if (defaultAnnotation != null) {
 9             String value = defaultAnnotation.value();
10             if (value != null && (value = value.trim()).length() > 0) {
11                 String[] names = NAME_SEPARATOR.split(value);
12                 if (names.length > 1) {
13                     throw new IllegalStateException("more than 1 default extension name on extension " + type.getName()
14                             + ": " + Arrays.toString(names));
15                 }
16                 if (names.length == 1) cachedDefaultName = names[0];
17             }
18         }
19 
20         Map<String, Class<?>> extensionClasses = new HashMap<String, Class<?>>();
21         loadFile(extensionClasses, DUBBO_INTERNAL_DIRECTORY);
22         loadFile(extensionClasses, DUBBO_DIRECTORY);
23         loadFile(extensionClasses, SERVICES_DIRECTORY);
24         return extensionClasses;
25     }

之后来看一下非常重要的一个方法loadFile(Map<String, Class<?>> extensionClasses, String dir)。

  1     /**
  2      * 1 加载dir目录下的指定type名称的文件(例如:dubbo-2.5.5.jar中的/META-INF/dubbo/internal/com.alibaba.dubbo.common.extension.ExtensionFactory)
  3      * 2 遍历该文件中的每一行
  4      * (1)获取实现类key和value, 例如 name=spi, line=com.alibaba.dubbo.common.extension.factory.SpiExtensionFactory
  5      * (2)根据line创建Class对象
  6      * (3)将具有@Adaptive注解的实现类的Class对象放在cachedAdaptiveClass缓存中, 注意该缓存只能存放一个具有@Adaptive注解的实现类的Class对象,如果有两个满足条件,则抛异常
  7      * 下面的都是对不含@Adaptive注解的实现类的Class对象:
  8      * (4)查看是否具有含有一个type入参的构造器, 如果有(就是wrapper类), 将当前的Class对象放置到cachedWrapperClasses缓存中
  9      * (5)如果没有含有一个type入参的构造器, 获取无参构造器. 如果Class对象具有@Active注解, 将该对象以<实现类的key, active>存储起来
 10      * (6)最后,将<Class对象, 实现类的key>存入cachedNames缓存,并将这些Class存入extensionClasses中.
 11      * @param extensionClasses
 12      * @param dir
 13      */
 14     private void loadFile(Map<String, Class<?>> extensionClasses, String dir) {
 15         String fileName = dir + type.getName();
 16         try {
 17             Enumeration<java.net.URL> urls;
 18             ClassLoader classLoader = findClassLoader();
 19             if (classLoader != null) {
 20                 urls = classLoader.getResources(fileName);
 21             } else {
 22                 urls = ClassLoader.getSystemResources(fileName);
 23             }
 24             if (urls != null) {
 25                 while (urls.hasMoreElements()) {
 26                     java.net.URL url = urls.nextElement();
 27                     try {
 28                         BufferedReader reader = new BufferedReader(new InputStreamReader(url.openStream(), "utf-8"));
 29                         try {
 30                             String line = null;
 31                             while ((line = reader.readLine()) != null) {
 32                                 final int ci = line.indexOf('#');
 33                                 if (ci >= 0) line = line.substring(0, ci);
 34                                 line = line.trim();
 35                                 if (line.length() > 0) {
 36                                     try {
 37                                         String name = null;
 38                                         int i = line.indexOf('=');
 39                                         if (i > 0) {
 40                                             name = line.substring(0, i).trim();
 41                                             line = line.substring(i + 1).trim();
 42                                         }
 43                                         if (line.length() > 0) {
 44                                             Class<?> clazz = Class.forName(line, true, classLoader);
 45                                             if (!type.isAssignableFrom(clazz)) {
 46                                                 throw new IllegalStateException("Error when load extension class(interface: " +
 47                                                         type + ", class line: " + clazz.getName() + "), class "
 48                                                         + clazz.getName() + "is not subtype of interface.");
 49                                             }
 50                                             if (clazz.isAnnotationPresent(Adaptive.class)) {
 51                                                 if (cachedAdaptiveClass == null) {
 52                                                     cachedAdaptiveClass = clazz;
 53                                                 } else if (!cachedAdaptiveClass.equals(clazz)) {
 54                                                     throw new IllegalStateException("More than 1 adaptive class found: "
 55                                                             + cachedAdaptiveClass.getClass().getName()
 56                                                             + ", " + clazz.getClass().getName());
 57                                                 }
 58                                             } else {
 59                                                 try {
 60                                                     clazz.getConstructor(type);
 61                                                     Set<Class<?>> wrappers = cachedWrapperClasses;
 62                                                     if (wrappers == null) {
 63                                                         cachedWrapperClasses = new ConcurrentHashSet<Class<?>>();
 64                                                         wrappers = cachedWrapperClasses;
 65                                                     }
 66                                                     wrappers.add(clazz);
 67                                                 } catch (NoSuchMethodException e) {
 68                                                     clazz.getConstructor();
 69                                                     if (name == null || name.length() == 0) {
 70                                                         name = findAnnotationName(clazz);
 71                                                         if (name == null || name.length() == 0) {
 72                                                             if (clazz.getSimpleName().length() > type.getSimpleName().length()
 73                                                                     && clazz.getSimpleName().endsWith(type.getSimpleName())) {
 74                                                                 name = clazz.getSimpleName().substring(0, clazz.getSimpleName().length() - type.getSimpleName().length()).toLowerCase();
 75                                                             } else {
 76                                                                 throw new IllegalStateException("No such extension name for the class " + clazz.getName() + " in the config " + url);
 77                                                             }
 78                                                         }
 79                                                     }
 80                                                     String[] names = NAME_SEPARATOR.split(name);
 81                                                     if (names != null && names.length > 0) {
 82                                                         Activate activate = clazz.getAnnotation(Activate.class);
 83                                                         if (activate != null) {
 84                                                             cachedActivates.put(names[0], activate);
 85                                                         }
 86                                                         for (String n : names) {
 87                                                             if (!cachedNames.containsKey(clazz)) {
 88                                                                 cachedNames.put(clazz, n);
 89                                                             }
 90                                                             Class<?> c = extensionClasses.get(n);
 91                                                             if (c == null) {
 92                                                                 extensionClasses.put(n, clazz);
 93                                                             } else if (c != clazz) {
 94                                                                 throw new IllegalStateException("Duplicate extension " + type.getName() + " name " + n + " on " + c.getName() + " and " + clazz.getName());
 95                                                             }
 96                                                         }
 97                                                     }
 98                                                 }
 99                                             }
100                                         }
101                                     } catch (Throwable t) {
102                                         IllegalStateException e = new IllegalStateException("Failed to load extension class(interface: " + type + ", class line: " + line + ") in " + url + ", cause: " + t.getMessage(), t);
103                                         exceptions.put(line, e);
104                                     }
105                                 }
106                             } // end of while read lines
107                         } finally {
108                             reader.close();
109                         }
110                     } catch (Throwable t) {
111                         logger.error("Exception when load extension class(interface: " +
112                                 type + ", class file: " + url + ") in " + url, t);
113                     }
114                 } // end of while urls
115             }
116         } catch (Throwable t) {
117             logger.error("Exception when load extension class(interface: " +
118                     type + ", description file: " + fileName + ").", t);
119         }
120     }

上述的方法分别从三个目录查找SPI文件并进行加载。在这里只有在META-INF/dubbo/internal/com.alibaba.dubbo.common.extension.ExtensionFactory中有值,内容如下:

1 adaptive=com.alibaba.dubbo.common.extension.factory.AdaptiveExtensionFactory
2 spi=com.alibaba.dubbo.common.extension.factory.SpiExtensionFactory
3 spring=com.alibaba.dubbo.config.spring.extension.SpringExtensionFactory

其中AdaptiveExtensionFactory在类上具有@Adaptive注解,这个类会在后续去讲,这里先略过。

执行过后,看一下:

  • cachedAdaptiveClass=class com.alibaba.dubbo.common.extension.factory.AdaptiveExtensionFactory
  • extensionClasses=[{"spring","class com.alibaba.dubbo.config.spring.extension.SpringExtensionFactory"}, {"spi", "class com.alibaba.dubbo.common.extension.factory.SpiExtensionFactory"}],后续会这个集合存储在cachedClasses缓存中。

上边一直在讲解getAdaptiveExtensionClass().newInstance()这句代码中的getAdaptiveExtensionClass(),此方法返回一个com.alibaba.dubbo.common.extension.factory.AdaptiveExtensionFactory类,之后来看一下其newInstance()代码,调用这个方法,默认会执行AdaptiveExtensionFactory的无参构造器。这里给出AdaptiveExtensionFactory的完整代码:

 1 package com.alibaba.dubbo.common.extension.factory;
 2 
 3 import com.alibaba.dubbo.common.extension.Adaptive;
 4 import com.alibaba.dubbo.common.extension.ExtensionFactory;
 5 import com.alibaba.dubbo.common.extension.ExtensionLoader;
 6 
 7 import java.util.ArrayList;
 8 import java.util.Collections;
 9 import java.util.List;
10 
11 /**
12  * AdaptiveExtensionFactory
13  */
14 @Adaptive
15 public class AdaptiveExtensionFactory implements ExtensionFactory {
16     private final List<ExtensionFactory> factories;
17 
18     /**
19      * 遍历cachedClasses中缓存的extensionClasses的key,之后根据key来实例化对应的实现类,最后放置到EXTENSION_INSTANCES缓存中
20      */
21     public AdaptiveExtensionFactory() {
22         ExtensionLoader<ExtensionFactory> loader = ExtensionLoader.getExtensionLoader(ExtensionFactory.class);
23         List<ExtensionFactory> list = new ArrayList<ExtensionFactory>();
24         for (String name : loader.getSupportedExtensions()) {
25             list.add(loader.getExtension(name));
26         }
27         factories = Collections.unmodifiableList(list);
28     }
29 
30     public <T> T getExtension(Class<T> type, String name) {
31         for (ExtensionFactory factory : factories) {
32             T extension = factory.getExtension(type, name);
33             if (extension != null) {
34                 return extension;
35             }
36         }
37         return null;
38     }
39 }

从上可以看出,这个装饰类只是实例化好了各个ExtensionFactory(这里是SpiExtensionFactory和SpringExtensionFactory),后续通过工厂获取实现类实例都是由具体工厂来完成。

来看一下实例化代码的地方,即loader.getExtension(name):

 1     /**
 2      * 从cachedInstances缓存中获取name对应的实例,如果没有,通过createExtension(name)创建,之后放入缓存
 3      * getExtension(String name)
 4      * --createExtension(String name)
 5      * ----injectExtension(T instance)
 6      */
 7     public T getExtension(String name) {
 8         if (name == null || name.length() == 0)
 9             throw new IllegalArgumentException("Extension name == null");
10         if ("true".equals(name)) {
11             return getDefaultExtension();
12         }
13         Holder<Object> holder = cachedInstances.get(name);
14         if (holder == null) {
15             cachedInstances.putIfAbsent(name, new Holder<Object>());
16             holder = cachedInstances.get(name);
17         }
18         Object instance = holder.get();
19         if (instance == null) {
20             synchronized (holder) {
21                 instance = holder.get();
22                 if (instance == null) {
23                     instance = createExtension(name);
24                     holder.set(instance);
25                 }
26             }
27         }
28         return (T) instance;
29     }

来看一下创建createExtension(name):

 1     private T createExtension(String name) {
 2         /** 从cachedClasses缓存中获取所有的实现类map,之后通过name获取到对应的实现类的Class对象 */
 3         Class<?> clazz = getExtensionClasses().get(name);
 4         if (clazz == null) {
 5             throw findException(name);
 6         }
 7         try {
 8             /** 从EXTENSION_INSTANCES缓存中获取对应的实现类的Class对象,如果没有,直接创建,之后放入缓存 */
 9             T instance = (T) EXTENSION_INSTANCES.get(clazz);
10             if (instance == null) {
11                 EXTENSION_INSTANCES.putIfAbsent(clazz, (T) clazz.newInstance());
12                 instance = (T) EXTENSION_INSTANCES.get(clazz);
13             }
14             injectExtension(instance);//ioc
15             Set<Class<?>> wrapperClasses = cachedWrapperClasses;
16             if (wrapperClasses != null && wrapperClasses.size() > 0) {
17                 for (Class<?> wrapperClass : wrapperClasses) {
18                     instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance));
19                 }
20             }
21             return instance;
22         } catch (Throwable t) {
23             throw new IllegalStateException("Extension instance(name: " + name + ", class: " +
24                     type + ")  could not be instantiated: " + t.getMessage(), t);
25         }
26     }

这里,就体现出来了dubbo-SPI比JDK-SPI的好处:dubbo-SPI不需要遍历所有的实现类来获取想要的实现类,可以直接通过name来获取。

injectExtension(instance)和wrapper包装功能后续再说。

 

到此为止,ExtensionLoader<Protocol> loader = ExtensionLoader.getExtensionLoader(Protocol.class);这行代码的整个源码就讲述完成了。最后来看一下整个代码的执行结果。

类变量

  • ConcurrentMap<Class<?>, ExtensionLoader<?>> EXTENSION_LOADERS
    • "interface com.alibaba.dubbo.rpc.Protocol" -> "com.alibaba.dubbo.common.extension.ExtensionLoader[com.alibaba.dubbo.rpc.Protocol]"
    • "interface com.alibaba.dubbo.common.extension.ExtensionFactory" -> "com.alibaba.dubbo.common.extension.ExtensionLoader[com.alibaba.dubbo.common.extension.ExtensionFactory]"
  • ConcurrentMap<Class<?>, Object> EXTENSION_INSTANCES
    • "class com.alibaba.dubbo.config.spring.extension.SpringExtensionFactory" -> SpringExtensionFactory实例
    • "class com.alibaba.dubbo.common.extension.factory.SpiExtensionFactory" -> SpiExtensionFactory实例

ExtensionLoader<Protocol> loader的实例变量

  • Class<?> type = interface com.alibaba.dubbo.rpc.Protocol
  • ExtensionFactory objectFactory = AdaptiveExtensionFactory(适配类)
    • factories = [SpringExtensionFactory实例, SpiExtensionFactory实例]

总结:

第一点:ExtensionLoader<T> loader = ExtensionLoader.getExtensionLoader(Class<T> type)最终得到的实例变量是:

  • Class<?> type = interface T
  • ExtensionFactory objectFactory = AdaptiveExtensionFactory(适配类)
    • factories = [SpringExtensionFactory实例, SpiExtensionFactory实例]

第二点:ExtensionLoader<T>.getAdaptiveExtension()的调用层级

1 ExtensionLoader<T>.getAdaptiveExtension()
2 --createAdaptiveExtension()
3 ----injectExtension(getAdaptiveExtensionClass())
4 ------getAdaptiveExtensionClass()
5 --------getExtensionClasses()//从spi文件中查找实现类上具有@Adaptive注解的类
6 ----------loadExtensionClasses()
7 ------------loadFile(Map<String, Class<?>> extensionClasses, String dir)
8 --------createAdaptiveExtensionClass()//如果从spi文件中没有找到实现类上具有@Adaptive注解的类,则动态创建类

最终返回的是创建好的Adaptive类,例如AdaptiveExtensionFactory实例。

第三点:ExtensionLoader<T>.getExtension()的调用层级

1 ExtensionLoader<T>.getExtension()
2 --createExtension(String name)
3 ----getExtensionClasses().get(name)//获取扩展类
4 ----injectExtension(instance);//ioc
5 ----wrapper包装;//aop

最终返回的是创建好的具体实现类,例如SpringExtensionFactory实例。 

 

至此,dubbo-spi源码解析完成!!!

posted on 2017-10-01 17:53  赵计刚  阅读(2397)  评论(0编辑  收藏  举报