SPI机制中的SPI实现类的读取和预处理是由ExtensionLoader类的loadFile方法来完成的
loadFile方法的作用是读取dubbo的某个SPI接口的spi描述文件，然后进行缓存，缓存，缓存（很重要，说三遍） ，读取的位置是在jar包下的META-INFdubbointernal目录下
一. 先对loadFile方法进行下粗的了解：
- loadFile方法方法被调用的顺序如下：
getExtensionClasses() ——> loadExtensionClasses()——>loadFile()
2.loadFile方法的签名如下：
private void loadFile(Map<String, Class<?>> extensionClasses, String dir)
3. 方法的参数 extensionClasses
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; } 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; }
extensionClasses这个参数一看就知道会被loadFile方法在执行过程中填充，通过看调用链上的前两个方法的代码就能看出其作用 ，其实extensionClasses只是个临时变量，最终会保存在ExtensionLoader类的实例属性cachedClasses中 ，cachedClasses的定义和相关代码如下：
private final Holder<Map<String, Class<?>>> cachedClasses = new Holder<Map<String,Class<?>>>(); /** * Helper Class for hold a value. * * @author william.liangf */ public class Holder<T> { private volatile T value; public void set(T value) { this.value = value; } public T get() { return value; } }
二. 详细分析loadFile方法：
//通过传入type参数，即某个SPI接口的类路径全名，结合dubbo的spi机制定义的目录来确定要读取的spi描述文件
String fileName = dir + type.getName();
//读取文件中的每一行
BufferedReader reader = new BufferedReader(new InputStreamReader(url.openStream(), "utf-8"));
line = reader.readLine()
//在JDK中加载每一行读到的类
Class<?> clazz = Class.forName(line, true, classLoader);
//要确保读到的每个类都是SPI接口的实现类
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.");
}
// 如果实现类上有Adaptive注解，则将读到的类赋值给ExtensionLoader类的实例属性cachedAdaptiveClass
if (clazz.isAnnotationPresent(Adaptive.class)) {
cachedAdaptiveClass = clazz;
}
else { //如果实现类上没有Adaptive注解，会做如下处理：
1. clazz.getConstructor(type); //如果这个带参的构造函数存在 , 则说明当前的读到的Class是有包装类的 ，则先将Class的类对象clazz放入ExtensionLoader类的实例属性 cachedWrapperClasses中
Set<Class<?>> wrappers = cachedWrapperClasses;
if (wrappers == null) {
cachedWrapperClasses = new ConcurrentHashSet<Class<?>>();
wrappers = cachedWrapperClasses;
}
wrappers.add(clazz);
//如果带参的构造函数不存在，则执行以下逻辑
2. clazz.getConstructor(); //获取不带参的构造函数
Activate activate = clazz.getAnnotation(Activate.class); //判断类上是否有Activate注解
if (activate != null) {
cachedActivates.put(names[0], activate); //如果Activate注解存在则放入到ExtensionLoader类的实例属性cachedActivates中
}
//以clazz为key，clazz的name为value缓存到ExtensionLoader类的实例属性cachedNames中
if (! cachedNames.containsKey(clazz)) {
cachedNames.put(clazz, n);
}
//以clazz的name为key，以clazz为value缓存到传入的参数extensionClasses中
Class<?> c = extensionClasses.get(n);
if (c == null) {
extensionClasses.put(n, clazz); //间接缓存到cachedClasses中
}
}
其实看源码分析的文章是有前提的，你必需自己要先去尽力去看过了。
总结：loadFile方法是ExtensionLoader类的私有方法，由于它不是static方法，所以调用时一定会有一个ExtensionLoader实例来调用这个方法的。所以它的作用就是每执行一次，它会去jar包下的META-INFdubbointernal目录下读取当前ExtensionLoader实例里的type属性对应的SPI接口的spi接口描述文件，也即是加载这个SPI接口对应的各个实现类。这个方法在执行过程中根据一些条件会做一些缓存的事。具体会影响到ExtensionLoader的下列属性：
cachedAdaptiveClass // private volatile Class<?> cachedAdaptiveClass = null;
cachedWrapperClasses //private Set<Class<?>> cachedWrapperClasses;
cachedActivates //private final Map<String, Activate> cachedActivates = new ConcurrentHashMap<String, Activate>();
cachedNames //private final ConcurrentMap<Class<?>, String> cachedNames = new ConcurrentHashMap<Class<?>, String>();
并间接影响了以下属性：
cachedClasses //private final Holder<Map<String, Class<?>>> cachedClasses = new Holder<Map<String,Class<?>>>();
以下是 loadFile方法的源码，也一并列上：
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) { 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); } }
总结：
1. loadFile方法的作用是读取各个SPI描述文件中的内容并作预处理
2. 缓存预处理的结果，缓存原则如下：
（1） 带Adaptive注解的实现类会被缓存在cachedAdaptiveClass上
（2） 其他的会被缓存在cachedClasses 上