motan源碼分析二：使用spi機制進行類加載

在motan的源碼中使用了很多的spi機制進行對象的創建，下面我們來具體分析一下它的實現方法。

1.在實際的jar包的\META-INF\services目錄中引入相關的文件，例如下圖中，我解壓了core的jar文件後，獲得到的相應文件列表：

2.以第一節中的ConfigHandler為例來分析，打開上圖中的com.weibo.api.motan.config.handler.ConfigHandler文件，文件內容標識著ConfigHandler接口的實現類為：com.weibo.api.motan.config.handler.SimpleConfigHandler

#
# Copyright 2009-2016 Weibo, Inc.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#
com.weibo.api.motan.config.handler.SimpleConfigHandler

3.在第一節中，創建ConfigHandler對象的代碼是這樣的：

ConfigHandler configHandler = ExtensionLoader.getExtensionLoader(ConfigHandler.class).getExtension(MotanConstants.DEFAULT_VALUE);

4.開始進入到實際的加載代碼核心部分，首先來看一下類加載器的具體實現：

public static  ExtensionLoader getExtensionLoader(Class type) {
 checkInterfaceType(type);//基礎性檢查
 ExtensionLoader 
 loader = (ExtensionLoader) extensionLoaders.get(type);//之前是否已經加載過此加載器
 if (loader == null) {
 loader = initExtensionLoader(type);//第一次加載
 }
 return loader;
 }
 private static  void checkInterfaceType(Class clz) {
 if (clz == null) {
 failThrows(clz, "Error extension type is null");
 }
 if (!clz.isInterface()) {
 failThrows(clz, "Error extension type is not interface");
 }
 if (!isSpiType(clz)) {
 failThrows(clz, "Error extension type without @Spi annotation");
 }
 }
 public static synchronized  ExtensionLoader initExtensionLoader(Class type) {
 ExtensionLoader loader = (ExtensionLoader) extensionLoaders.get(type);
 if (loader == null) {
 loader = new ExtensionLoader(type);//新創建一個加載器
 extensionLoaders.putIfAbsent(type, loader);
 loader = (ExtensionLoader) extensionLoaders.get(type);
 }
 return loader;
 }
 

5.下面我們將進入到加載器的內部，分析具體的實現：

private ExtensionLoader(Class type) {
 this(type, Thread.currentThread().getContextClassLoader());//使用當前線程的類加載器做為加載器，type為ConfigHandler接口
}
public T getExtension(String name) {
 checkInit();//檢查是否初始化
 if (name == null) {
 return null;
 }
 try {
 Spi spi = type.getAnnotation(Spi.class);
 if (spi.scope() == Scope.SINGLETON) {
 return getSingletonInstance(name);//返回唯一的對象
 } else {
 Class clz = extensionClasses.get(name);
 if (clz == null) {
 return null;
 }
 return clz.newInstance();//重新創建對象
 }
 } catch (Exception e) {
 failThrows(type, "Error when getExtension " + name, e);
 }
 return null;
}
private void checkInit() {
 if (!init) {
 loadExtensionClasses();
 }
}
private synchronized void loadExtensionClasses() {
 if (init) {
 return;
 }
 extensionClasses = loadExtensionClasses(PREFIX);//加載相關的類 

 singletonInstances = new ConcurrentHashMap<string>();
 init = true;
}
private ConcurrentMap<string>> loadExtensionClasses(String prefix) {
 String fullName = prefix + type.getName();
 //全名為:jar包名+\META-INF\services\com.weibo.api.motan.config.handler.ConfigHandler文件裡的類
 List<string> classNames = new ArrayList<string>();
 try {
 Enumeration urls;
 if (classLoader == null) {
 urls = ClassLoader.getSystemResources(fullName);
 } else {
 urls = classLoader.getResources(fullName);
 }
 if (urls == null || !urls.hasMoreElements()) {
 return new ConcurrentHashMap<string>>();
 }
 while (urls.hasMoreElements()) {
 URL url = urls.nextElement();
 parseUrl(type, url, classNames);
 }
 } catch (Exception e) {
 throw new MotanFrameworkException(
 "ExtensionLoader loadExtensionClasses error, prefix: " + prefix + " type: " + type.getClass(), e);
 }
 return loadClass(classNames);
}
/<string>/<string>/<string>/<string>/<string>

6.在parseUrl方法中進行文件的內容讀取，並在loadClass中完成類的加載

private void parseUrl(Class type, URL url, List<string> classNames) throws ServiceConfigurationError {
 InputStream inputStream = null;
 BufferedReader reader = null;
 try {
 inputStream = url.openStream();
 reader = new BufferedReader(new InputStreamReader(inputStream, MotanConstants.DEFAULT_CHARACTER));
 String line = null;
 int indexNumber = 0;
 while ((line = reader.readLine()) != null) { 

 indexNumber++;
 parseLine(type, url, line, indexNumber, classNames);//讀取到類的名稱：com.weibo.api.motan.config.handler.SimpleConfigHandler
 }
 } catch (Exception x) {
 failLog(type, "Error reading spi configuration file", x);
 } finally {
 try {
 if (reader != null) {
 reader.close();
 }
 if (inputStream != null) {
 inputStream.close();
 }
 } catch (IOException y) {
 failLog(type, "Error closing spi configuration file", y);
 }
 }
}
private ConcurrentMap<string>> loadClass(List<string> classNames) {
 ConcurrentMap<string>> map = new ConcurrentHashMap<string>>();
 for (String className : classNames) {
 try {
 Class clz;
 if (classLoader == null) {
 clz = (Class) Class.forName(className);//裝載類：com.weibo.api.motan.config.handler.SimpleConfigHandler
 } else {
 clz = (Class) Class.forName(className, true, classLoader);
 }
 checkExtensionType(clz);
 String spiName = getSpiName(clz);
 if (map.containsKey(spiName)) {
 failThrows(clz, ":Error spiName already exist " + spiName);
 } else {
 map.put(spiName, clz);
 }
 } catch (Exception e) {
 failLog(type, "Error load spi class", e);
 }
 }
 return map;
}
/<string>/<string>/<string>/<string>/<string> 

motan類加載的知識點總結：

1.使用jdk的spi規範，在\META-INF\services中添加實際的使用類描述，從而實現類與類之間的完全解耦；

2.類加載器使用的是當前線程的類加載器；

3.motan的類加載器可以支持單例和多例兩種模式；

4.motan中大量使用了spi的類加載方式。

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關鍵字: 軟件 源碼 Apache