官方文檔.png
在官方文檔中，提到擴(kuò)展點(diǎn)加載機(jī)制，不妨看看是如何實(shí)現(xiàn)的。
擴(kuò)展點(diǎn)加載機(jī)制，最重要的類就是
ExtensionLoader。
getExtensionLoader的實(shí)現(xiàn)原理
在獲取擴(kuò)展加載器的時(shí)候，首先會判斷是否有注解SPI，沒有則拋異常，有SPI注解的前提下，先從緩存取，取不到，根據(jù)class類型新建一個(gè)新的擴(kuò)展點(diǎn)加載器。
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!");
}
//是否有SPI注解
if (!withExtensionAnnotation(type)) {
throw new IllegalArgumentException("Extension type(" + type +
") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");
}
//從緩存中加載
ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
if (loader == null) {
根據(jù)類型新增一個(gè)擴(kuò)展點(diǎn)加載器
EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
}
return loader;
}
在新增加載點(diǎn)擴(kuò)展器的時(shí)候，會根據(jù)class類型獲取一個(gè)運(yùn)行期自適應(yīng)的擴(kuò)展類型
private ExtensionLoader(Class<?> type) {
this.type = type;
objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
}
我們看到，如果類型不是ExtensionFactory，則從ExtensionFactory的擴(kuò)展加載器中獲取一個(gè)可擴(kuò)展的的加載器。這里先不看ExtensionFactory，先看一下獲取可擴(kuò)展加載器方法。
getAdaptiveExtension獲取可調(diào)整的擴(kuò)展加載器
在獲取可擴(kuò)展加載器的時(shí)候，先從緩存獲取，若緩存不存在，創(chuàng)建新的擴(kuò)展加載器。在創(chuàng)建新的擴(kuò)展加載器的時(shí)候，首先判斷配置文件中（META-INF/dubbo/，META-INF/services/,META-INF/dubbo/internal）配置的class是否有被Adaptive注解的類，如果有，則直接返回被注解Adaptive的可擴(kuò)展加載類，如果沒有，則生成一個(gè)擴(kuò)展類。
public T getAdaptiveExtension() {
//先從緩存中獲取
Object instance = cachedAdaptiveInstance.get();
if (instance == null) {
if (createAdaptiveInstanceError == null) {
synchronized (cachedAdaptiveInstance) {
instance = cachedAdaptiveInstance.get();
if (instance == null) {
try {
//創(chuàng)建一個(gè)新的可擴(kuò)展加載器
instance = createAdaptiveExtension();
cachedAdaptiveInstance.set(instance);
} 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() {
//根據(jù)可擴(kuò)展實(shí)例做擴(kuò)展
try {
return injectExtension((T) getAdaptiveExtensionClass().newInstance());
} catch (Exception e) {
throw new IllegalStateException("Can not create adaptive extension " + type + ", cause: " + e.getMessage(), e);
}
}
//獲取可擴(kuò)展實(shí)例
private Class<?> getAdaptiveExtensionClass() {
// 加載當(dāng)前Extension的所有實(shí)現(xiàn),如果有@Adaptive類型，則會賦值為cachedAdaptiveClass屬性緩存起來
getExtensionClasses();
if (cachedAdaptiveClass != null) {
//如果緩存有可擴(kuò)展類，則直接返回
return cachedAdaptiveClass;
}
//緩存中不存在創(chuàng)建一個(gè)
return cachedAdaptiveClass = createAdaptiveExtensionClass();
}
獲取可擴(kuò)展類
private Map<String, Class<?>> getExtensionClasses() {
Map<String, Class<?>> classes = cachedClasses.get();
if (classes == null) {
synchronized (cachedClasses) {
classes = cachedClasses.get();
if (classes == null) {
//如果沒有，則從文件中加載
classes = loadExtensionClasses();
cachedClasses.set(classes);
}
}
}
return classes;
}
從配置文件中加載
private Map<String, Class<?>> loadExtensionClasses() {
final SPI defaultAnnotation = type.getAnnotation(SPI.class);
//忽略1w行代碼
// 文件路徑：
loadFile(extensionClasses, DUBBO_INTERNAL_DIRECTORY);
//文件路徑
loadFile(extensionClasses, DUBBO_DIRECTORY);
//文件路徑
loadFile(extensionClasses, SERVICES_DIRECTORY);
return extensionClasses;
}
//根據(jù)文件名（包路徑）查找所有文件，將每一行的配置讀取出來，判斷是否是實(shí)現(xiàn)該接口的類，如果接口中有注解`Adaptive`，則更新`cachedAdaptiveClass`的值。
private void loadFile(Map<String, Class<?>> extensionClasses, String dir) {
//文件路徑為基本路徑+包路徑
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 {
//讀文件
BufferedReader reader = new BufferedReader(new InputStreamReader(url.openStream(), "utf-8"));
//讀每一行數(shù)據(jù)，判斷=之前為bean的ID，后面為bean的class類型
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)) {
//如果類不是實(shí)現(xiàn)該接口的實(shí)例，則啟動報(bào)錯(cuò)
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)) {
//如果該類注解了Adaptive注解，則更新cachedAdaptiveClass為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);
}
如果沒有被注解Adaptive注解的，則直接創(chuàng)建一個(gè)新的可擴(kuò)展加載類。
private Class<?> createAdaptiveExtensionClass() {
//創(chuàng)建可擴(kuò)展加載器的代碼。。
String code = createAdaptiveExtensionClassCode();
//獲取類加載器
ClassLoader classLoader = findClassLoader();
//獲取編輯器
com.alibaba.dubbo.common.compiler.Compiler compiler = ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.common.compiler.Compiler.class).getAdaptiveExtension();
//將代碼編譯為可擴(kuò)展類
return compiler.compile(code, classLoader);
}
在對可擴(kuò)展實(shí)例做擴(kuò)展，會根據(jù)代理工廠創(chuàng)建一個(gè)實(shí)例,在構(gòu)造方法中，使用的是ExtensionFactory的擴(kuò)展
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) : "";
//獲取擴(kuò)展實(shí)例
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;
}
看一下 ExtensionFactory的定義，和實(shí)現(xiàn)
@SPI
public interface ExtensionFactory {
<T> T getExtension(Class<T> type, String name);
}
@Adaptive
public class AdaptiveExtensionFactory implements ExtensionFactory {
private final List<ExtensionFactory> factories;
public AdaptiveExtensionFactory() {
ExtensionLoader<ExtensionFactory> loader = ExtensionLoader.getExtensionLoader(ExtensionFactory.class);
List<ExtensionFactory> list = new ArrayList<ExtensionFactory>();
for (String name : loader.getSupportedExtensions()) {
list.add(loader.getExtension(name));
}
factories = Collections.unmodifiableList(list);
}
public <T> T getExtension(Class<T> type, String name) {
for (ExtensionFactory factory : factories) {
T extension = factory.getExtension(type, name);
if (extension != null) {
return extension;
}
}
return null;
}
}
public class SpiExtensionFactory implements ExtensionFactory {
public <T> T getExtension(Class<T> type, String name) {
if (type.isInterface() && type.isAnnotationPresent(SPI.class)) {
ExtensionLoader<T> loader = ExtensionLoader.getExtensionLoader(type);
if (loader.getSupportedExtensions().size() > 0) {
return loader.getAdaptiveExtension();
}
}
return null;
}
}
public class SpringExtensionFactory implements ExtensionFactory {
private static final Set<ApplicationContext> contexts = new ConcurrentHashSet<ApplicationContext>();
public static void addApplicationContext(ApplicationContext context) {
contexts.add(context);
}
public static void removeApplicationContext(ApplicationContext context) {
contexts.remove(context);
}
@SuppressWarnings("unchecked")
public <T> T getExtension(Class<T> type, String name) {
for (ApplicationContext context : contexts) {
if (context.containsBean(name)) {
Object bean = context.getBean(name);
if (type.isInstance(bean)) {
return (T) bean;
}
}
}
return null;
}
}
很顯然，在這里用到的是AdaptiveExtensionFactory，因?yàn)樗蛔⒔?code>@Adaptive注解了，然后從ExtensionFactory中獲取擴(kuò)展加載類。
