Dubbo系列(二)源碼分析之SPI機制

Dubbo系列(二)源碼分析之SPI機制

在閱讀Dubbo源碼時,常常看到

ExtensionLoader.getExtensionLoader(*.class).getAdaptiveExtension();

ExtensionLoader.getExtensionLoader(*.class).getExtension(“name”);

那麼需要深入瞭解dubbo,瞭解SPI源碼是必不可少的過程

下面根據dubbo 2.5.4進行源碼分析

ps.由於源碼分析過程不便於分段落,很容易混亂,需要根據源碼信息一步一步追蹤。

萬惡之源——SPI入口

    public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) {
        if(type == null) {
            throw new IllegalArgumentException("Extension type == null");
        } else if(!type.isInterface()) {
            throw new IllegalArgumentException("Extension type(" + type + ") is not interface!");
        } else if(!withExtensionAnnotation(type)) {
            throw new IllegalArgumentException("Extension type(" + type + ") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");
        } else {
            ExtensionLoader loader = (ExtensionLoader)EXTENSION_LOADERS.get(type);
            if(loader == null) {
                EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader(type));
                loader = (ExtensionLoader)EXTENSION_LOADERS.get(type);
            }
    
            return loader;
        }
    }

EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader(type));

調用了構造器方法,並置入EXTENSION_LOADERS,那麼EXTENSION_LOADERS可以推測出是一個容器,用來保存ExtensionLoader信息,不必每次都new,而便於調用。

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

那麼就需要明白ExtensionLoader這個類實例化對象是用來幹什麼的,首先我們知道EXTENSION_LOADERS是用來保存type-ExtensionLoader信息的map,type是接口類類型,那麼很容易聯想到ExtensionLoader對象則是用來爲該接口類生產不同用戶所需的實現類的。

開始驗證猜想:type是接口類類型,ExtensionLoader對象是用來爲該接口類生產不同用戶所需的實現類的。

ExtensionLoader.getExtensionLoader(*.class).getAdaptiveExtension();入手,看看是如何獲取真正的實現類的。

getExtensionLoader(type)

    public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) {
        if(type == null) {
            throw new IllegalArgumentException("Extension type == null");
        } else if(!type.isInterface()) {
            throw new IllegalArgumentException("Extension type(" + type + ") is not interface!");
        } else if(!withExtensionAnnotation(type)) {
            throw new IllegalArgumentException("Extension type(" + type + ") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");
        } else {
            ExtensionLoader loader = (ExtensionLoader)EXTENSION_LOADERS.get(type);
            if(loader == null) {
                EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader(type));
                loader = (ExtensionLoader)EXTENSION_LOADERS.get(type);
            }
          
          //最終返回一個EXTENSION_LOADERS已存在或新創建的new ExtensionLoader(type)對象
    
            return loader;
        }
    }

new ExtensionLoader(type)

    private ExtensionLoader(Class<?> type) {
      //實例化對象有兩個屬性,1.type 2.objectFactory
        this.type = type;
      //objectFactory過濾了ExtensionFactory.class,那麼進入:後面的語句
        this.objectFactory = type == ExtensionFactory.class?null:(ExtensionFactory)getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension();
    }

(ExtensionFactory)getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension();

(ExtensionFactory)getExtensionLoader(ExtensionFactory.class)是不是有熟悉的感覺

此時傳入的類型爲ExtensionFactory,調用方式與開場白沒有區別!

那麼再次猜想:ExtensionFactory是用來生產Extension的類,且也是通過SPI的方式獲得的擴展類。

Extension是用來生產擴展類具體實現類的類,可能會有一點繞,不過對比spring中的beanFactoryFactoryBean還是比較好理解的。)

現在終於要進入getAdaptiveExtension()方法了

getAdaptiveExtension()

    public T getAdaptiveExtension() {
      		//從緩存獲取默認實現類
            Object instance = this.cachedAdaptiveInstance.get();
            if(instance == null) {
                if(this.createAdaptiveInstanceError != null) {
                    throw new IllegalStateException("fail to create adaptive instance: " + this.createAdaptiveInstanceError.toString(), this.createAdaptiveInstanceError);
                }
    
                Holder var2 = this.cachedAdaptiveInstance;
              //使用雙重檢查來實例化
                synchronized(this.cachedAdaptiveInstance) {
                    instance = this.cachedAdaptiveInstance.get();
                    if(instance == null) {
                        try {
                          //實例化的方法
                            instance = this.createAdaptiveExtension();
                            this.cachedAdaptiveInstance.set(instance);
                        } catch (Throwable var5) {
                            this.createAdaptiveInstanceError = var5;
                            throw new IllegalStateException("fail to create adaptive instance: " + var5.toString(), var5);
                        }
                    }
                }
            }
    
            return instance;
        }

繼續進入createAdaptiveExtension()

    private T createAdaptiveExtension() {
        try {
            return this.injectExtension(this.getAdaptiveExtensionClass().newInstance());
        } catch (Exception var2) {
            throw new IllegalStateException("Can not create adaptive extenstion " + this.type + ", cause: " + var2.getMessage(), var2);
        }
    }

繼續進入this.injectExtension(this.getAdaptiveExtensionClass().newInstance());

那麼有兩個過程,1.實例化對象 2.inject依賴注入 ,類似spring的依賴注入過程,畢竟bean都是由spring管理的

newInstance()就不深入看了,簡單介紹就是通過反射的方式獲取到第一個public構造器,並調用instance()方法進行該type 默認實現類的實例化。

爲了驗證是默認實現類,進入getAdaptiveExtensionClass()

    private Class<?> getAdaptiveExtensionClass() {
        this.getExtensionClasses();
        return this.cachedAdaptiveClass != null?this.cachedAdaptiveClass:(this.cachedAdaptiveClass = this.createAdaptiveExtensionClass());
    }

記住return的對象是cachedAdaptiveClasscreateAdaptiveExtensionClass()

繼續getExtensionClasses();

    private Map<String, Class<?>> getExtensionClasses() {
      //這裏緩存的是已加載類的類型
        Map classes = (Map)this.cachedClasses.get();
        if(classes == null) {
            Holder var2 = this.cachedClasses;
            synchronized(this.cachedClasses) {
                classes = (Map)this.cachedClasses.get();
                if(classes == null) {
                  //加載擴展類
                    classes = this.loadExtensionClasses();
                    this.cachedClasses.set(classes);
                }
            }
        }
    
        return classes;
    }

繼續深入loadExtensionClasses();

    private Map<String, Class<?>> loadExtensionClasses() {
      //回憶起猜想:默認
      //獲取SPI註解
        SPI defaultAnnotation = (SPI)this.type.getAnnotation(SPI.class);
        if(defaultAnnotation != null) {
            String extensionClasses = defaultAnnotation.value();
            if(extensionClasses != null && (extensionClasses = extensionClasses.trim()).length() > 0) {
                String[] names = NAME_SEPARATOR.split(extensionClasses);
                if(names.length > 1) {
                    throw new IllegalStateException("more than 1 default extension name on extension " + this.type.getName() + ": " + Arrays.toString(names));
                }
    
                if(names.length == 1) {
                  //緩存從SPI註解中獲取的默認實現類名
                    this.cachedDefaultName = names[0];
                }
            }
        }
    
        HashMap extensionClasses1 = new HashMap();
      //按照dubbo SPI規範加載這三個路徑下的文件,並加入到extensionClasses1這個map中
        this.loadFile(extensionClasses1, "META-INF/dubbo/internal/");
        this.loadFile(extensionClasses1, "META-INF/dubbo/");
        this.loadFile(extensionClasses1, "META-INF/services/");
        return extensionClasses1;
    }

return extensionClasses1回溯後發現,此時這三個路徑下的文件中配置的實現類都加載到cachedClasses中,且使得cachedDefaultName中有了值,作爲默認實現類。

此時進入createAdaptiveExtensionClass()

    private Class<?> createAdaptiveExtensionClass() {
    	//生成代碼
        String code = this.createAdaptiveExtensionClassCode();
        //獲得類加載器
        ClassLoader classLoader = findClassLoader();
        //獲得編譯器
        Compiler compiler = (Compiler)getExtensionLoader(Compiler.class).getAdaptiveExtension();
        //獲得編譯後的類
        return compiler.compile(code, classLoader);
    }

這裏則是實例化實現類的核心方法了

可見,實例化實現類的核心方法首先是通過代碼寫代碼!

那麼生成的類是什麼樣的呢?

以protocol爲例,最終生成Protocol$Adaptive類:

    public class Protocol$Adaptive 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.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])");
            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);
        }
    
        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();
          
          //SPI中配置項爲"dubbo",所以這裏編譯出來的默認值爲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])");
         
            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);
        }
    }

難受的是,我們發現裏面依然嵌套了一層extension的調用模式,說明這個類依然是一個代理類!

com.alibaba.dubbo.rpc.Protocol extension = (com.alibaba.dubbo.rpc.Protocol) ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.rpc.Protocol.class).getExtension(extName);

不過不一樣的是,這裏調用的方法是getExtension(extName),不要着急,終於到了最終實例化的時候了:

再次回到ExtensionLoader.getExtension(name)方法

    public T getExtension(String name) {
        if(name != null && name.length() != 0) {
            if("true".equals(name)) {
                return this.getDefaultExtension();
            } else {
              //又是各種緩存
                Holder holder = (Holder)this.cachedInstances.get(name);
                if(holder == null) {
                    this.cachedInstances.putIfAbsent(name, new Holder());
                    holder = (Holder)this.cachedInstances.get(name);
                }
    
                Object instance = holder.get();
                if(instance == null) {
                    synchronized(holder) {
                        instance = holder.get();
                        if(instance == null) {
                          //終於找到你了,真正的實例化方法!
                            instance = this.createExtension(name);
                            holder.set(instance);
                        }
                    }
                }
    
                return instance;
            }
        } else {
            throw new IllegalArgumentException("Extension name == null");
        }
    }

進入最終的實例化方法createExtension(String name)

    private T createExtension(String name) {
      //刷新緩存的已知實現類
        Class clazz = (Class)this.getExtensionClasses().get(name);
        if(clazz == null) {
            throw this.findException(name);
        } else {
            try {
              //從已經實例化過的緩存map中獲取
                Object t = EXTENSION_INSTANCES.get(clazz);
                if(t == null) {
                  //實例化並放入緩存
                    EXTENSION_INSTANCES.putIfAbsent(clazz, clazz.newInstance());
                    t = EXTENSION_INSTANCES.get(clazz);
                }
              
              //爲t進行依賴注入
                this.injectExtension(t);
                Set wrapperClasses = this.cachedWrapperClasses;
                Class wrapperClass;
              
              //注入給該t的wrapepr
                if(wrapperClasses != null && wrapperClasses.size() > 0) {
                    for(Iterator var5 = wrapperClasses.iterator(); var5.hasNext(); t = this.injectExtension(wrapperClass.getConstructor(new Class[]{this.type}).newInstance(new Object[]{t}))) {
                        wrapperClass = (Class)var5.next();
                    }
                }
    
                return t;
            } catch (Throwable var7) {
                throw new IllegalStateException("Extension instance(name: " + name + ", class: " + this.type + ")  could not be instantiated: " + var7.getMessage(), var7);
            }
        }
    }

最終返回一個可能進行wrapper包裝過的對象!

終於實例化結束,跳出實例化的過程,回到injectExtension()

是不是覺得奇怪,前面在實例化的過程中已經進行了injectExtension()的操作

其實,這裏進行注入的對象應該是Protocol$Adaptive,這個動態代理對象!

injectExtension()注入過程

    private T injectExtension(T instance) {
        try {
            if(this.objectFactory != null) {
                Method[] e = instance.getClass().getMethods();
                int var3 = e.length;
    
                for(int var4 = 0; var4 < var3; ++var4) {
                    Method method = e[var4];
                  //啥也別看了,看到set了,可以猜想到通過調用set方法進行注入
                    if(method.getName().startsWith("set") && method.getParameterTypes().length == 1 && Modifier.isPublic(method.getModifiers())) {
                      //獲取set方法的第一個參數類型
                        Class pt = method.getParameterTypes()[0];
    
                        try {
                            String e1 = method.getName().length() > 3?method.getName().substring(3, 4).toLowerCase() + method.getName().substring(4):"";
                          //從objectFactory獲取擴展類(依賴的某個接口類)
                          //objectFactory前面瞭解到,是ExtensionFactory的擴展類,用來生產擴展類
                            Object object = this.objectFactory.getExtension(pt, e1);
                            if(object != null) {
                              //注入
                                method.invoke(instance, new Object[]{object});
                            }
                        } catch (Exception var9) {
                            logger.error("fail to inject via method " + method.getName() + " of interface " + this.type.getName() + ": " + var9.getMessage(), var9);
                        }
                    }
                }
            }
        } catch (Exception var10) {
            logger.error(var10.getMessage(), var10);
        }
    
        return instance;
    }

return instance,回溯到getAdaptiveExtension(),最終getAdaptiveExtension()返回一個由objectFactory生產的實例化且進行依賴注入了的默認擴展類。

稍微整理一下調用鏈路:

前文中的兩個調用鏈最終生產的對象應該一目瞭然了:

  1. ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();
    最終獲得的對象爲Protocol$Adaptive,且該對象是一個代理對象,最終調用的對象依然是ProtocolFilterWrapper(ProtocolListenerWrapper({extName}Protocol)),extName默認值爲SPI註解中的值,這裏是dubbo。
  2. ExtensionLoader.getExtensionLoader(Protocol.class).getExtension(“registry”);
    最終獲得的對象爲ProtocolFilterWrapper(ProtocolListenerWrapper(RegistryProtocol))

那麼把前文連接起來,猜想:

   	1. ExtensionFactory是用來生產Extension的擴展類
   	2. ExtensionLoader是用來生產實現類的擴展類
   	3. getAdaptiveExtension()是獲取該Extension的默認實現類

猜想1.

由ExtensionFactory可以找到com.alibaba.dubbo.common.extension.ExtensionFactory,裏面有三種實現spi、spring、adaptive.那麼它的調用模式會根據此時獲取的extension的類型進行選擇性的調用三種實現類的方法。

例如SpiExtensionFactory:

    public class SpiExtensionFactory implements ExtensionFactory {
        public SpiExtensionFactory() {
        }
    
        public <T> T getExtension(Class<T> type, String name) {
          //如果該接口標註了@SPI
            if(type.isInterface() && type.isAnnotationPresent(SPI.class)) {
              //調用ExtensionLoader.getExtensionLoader(type).getAdaptiveExtension()
              //獲取默認實現類
                ExtensionLoader loader = ExtensionLoader.getExtensionLoader(type);
                if(loader.getSupportedExtensions().size() > 0) {
                    return loader.getAdaptiveExtension();
                }
            }
    
            return null;
        }
    }

猜想驗證,且應該是ExtensionFactory是用來生產或獲取Extension的擴展類,例如從spring容器獲取(實例化工作是由spring來完成的)

猜想2.

通過追蹤調用鏈找到createExtension(String name),真正進行了實例化

猜想驗證

猜想3.

通過追蹤調用鏈找到實例化對象爲Protocol$Adaptive(以Protocol爲例)

發現在方法調用過程中有這樣的一句話:

    //SPI中配置項爲"dubbo",所以這裏編譯出來的默認值爲dubbo
    String extName = (url.getProtocol() == null ? "dubbo" : url.getProtocol());

且該方法上標註了@Adaptive的註解

可見,@SPI中的註解表示該接口的默認實現類而在方法上標註@Adaptive註解,則是在該方法上增加可選項,允許通過傳入不同的extName(這個例子是通過getProtocol的方式獲得此時應該適應的對象)來調用自適應對象的方法。

通俗的理解就是,

當接口類上標註了@SPI註解,說明了該接口類的默認實現類

當接口類上標註了@SPI註解,且方法上標註了@Adaptive,則說明了該接口類的該方法需要根據情況調用不同的實現類

當接口類上標註了@Adaptive,則說明是根據需要直接實例化某實現類

詳情可以查閱@SPI和@Adaptive註解說明。

雖然猜想3不能完成驗證,但至少通過跟蹤分析了SPI的調用鏈路讓我們有比較充分的理由這樣推測。

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