官网地址：https://dubbo.apache.org/zh/docs/v2.7/dev/source/dubbo-spi/
双亲委托模式的弊端
判断类是否加载的时候,应用类加载器会顺着双亲路径往上判断,直到启动类加载器.但是启动类加载器不会往下询问,这个委托路线是单向的,即顶层的类加载器,无法访问底层的类加载器所加载的类
启动类加载器中的类为系统的核心类,比如,在系统类中,提供了一个接口,并且该接口还提供了一个工厂方法用于创建该接口的实例,但是该接口的实现类在应用层中,接口和工厂方法在启动类加载器中,就会出现工厂方法无法创建由应用类加载器加载的应用实例问题.
拥有这样问题的组件有很多,比如JDBC（DataSource、Driver）、Xml parser等.JDBC本身是java连接数据库的一个标准,是进行数据库连接的抽象层,由java编写的一组类和接口组成，接口的实现由各个数据库厂商来完成
文件内容com.mysql.cj.jdbc.Driver
双亲委托模式的补充
在Java中,把核心类(rt.jar)中提供外部服务,可由应用层自行实现的接口,这种方式成为spi
ExtensionLoader
org.apache.dubbo.common.extension.ExtensionLoader#getExtension源码分析
public T getExtension(String name) {
// 默认包装成Wrapper对象
return getExtension(name, true);
}
public T getExtension(String name, boolean wrap) {
if (StringUtils.isEmpty(name)) {
throw new IllegalArgumentException("Extension name == null");
}
// 返回默认的扩展实现类
if ("true".equals(name)) {
return getDefaultExtension();
}
// volatile + synchronized + double-check 获取或创建扩展点实现类
final Holder<Object> holder = getOrCreateHolder(name);
Object instance = holder.get();
if (instance == null) {
synchronized (holder) {
instance = holder.get();
if (instance == null) {
instance = createExtension(name, wrap);
holder.set(instance);
}
}
}
return (T) instance;
}
createExtension源码分析
private T createExtension(String name, boolean wrap) {
// 从配置文件中加载所有的拓展类，可得到“配置项名称”到“配置类”的映射关系表
Class<?> clazz = getExtensionClasses().get(name);
if (clazz == null) {
// 抛出异常
throw findException(name);
}
try {
T instance = (T) EXTENSION_INSTANCES.get(clazz);
if (instance == null) {
// 通过反射创建实例
EXTENSION_INSTANCES.putIfAbsent(clazz, clazz.newInstance());
instance = (T) EXTENSION_INSTANCES.get(clazz);
}
// 向实例中注入依赖，为变量赋值
injectExtension(instance);
if (wrap) {
List<Class<?>> wrapperClassesList = new ArrayList<>();
if (cachedWrapperClasses != null) {
wrapperClassesList.addAll(cachedWrapperClasses);
// 根据Activate注解中的order属性值排序
wrapperClassesList.sort(WrapperComparator.COMPARATOR);
Collections.reverse(wrapperClassesList);
}
// 循环创建 Wrapper 实例
if (CollectionUtils.isNotEmpty(wrapperClassesList)) {
for (Class<?> wrapperClass : wrapperClassesList) {
Wrapper wrapper = wrapperClass.getAnnotation(Wrapper.class);
if (wrapper == null
|| (ArrayUtils.contains(wrapper.matches(), name) && !ArrayUtils.contains(wrapper.mismatches(), name))) {
// 将当前 instance 作为参数传给 Wrapper 的构造方法，并通过反射创建 Wrapper 实例
// 然后向 Wrapper 实例中注入依赖，最后将 Wrapper 实例再次赋值给 instance 变量
instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance));
}
}
}
}
// 是否实现了 Lifecycle，生命周期的初始化处理
initExtension(instance);
return instance;
} catch (Throwable t) {
throw new IllegalStateException("Extension instance (name: " + name + ", class: " +
type + ") couldn't be instantiated: " + t.getMessage(), t);
}
}
getExtensionClasses源码分析
private Map<String, Class<?>> getExtensionClasses() {
// 从缓存中获取已加载的拓展类
Map<String, Class<?>> classes = cachedClasses.get();
if (classes == null) {
synchronized (cachedClasses) {
classes = cachedClasses.get();
if (classes == null) {
// 加载所有的扩展类
classes = loadExtensionClasses();
cachedClasses.set(classes);
}
}
}
return classes;
}
loadExtensionClasses源码分析
private Map<String, Class<?>> loadExtensionClasses() {
// 对 SPI 注解进行解析，如果存在默认扩展名则获取并缓存
cacheDefaultExtensionName();
Map<String, Class<?>> extensionClasses = new HashMap<>();
// 根据加载策略加载指定文件夹下的配置文件
for (LoadingStrategy strategy : strategies) {
loadDirectory(extensionClasses, strategy.directory(), type.getName(), strategy.preferExtensionClassLoader(), strategy.overridden(), strategy.excludedPackages());
loadDirectory(extensionClasses, strategy.directory(), type.getName().replace("org.apache", "com.alibaba"), strategy.preferExtensionClassLoader(), strategy.overridden(), strategy.excludedPackages());
}
return extensionClasses;
}
private void cacheDefaultExtensionName() {
// 获取 SPI 注解，这里的 type 变量是在调用 getExtensionLoader 方法时传入的
final SPI defaultAnnotation = type.getAnnotation(SPI.class);
if (defaultAnnotation == null) {
return;
}
String value = defaultAnnotation.value();
if ((value = value.trim()).length() > 0) {
// 对 SPI 注解内容进行切分
String[] names = NAME_SEPARATOR.split(value);
// 检测 SPI 注解内容是否合法，不合法则抛出异常
if (names.length > 1) {
throw new IllegalStateException("More than 1 default extension name on extension " + type.getName()
+ ": " + Arrays.toString(names));
}
// 设置默认名称，参考 getDefaultExtension 方法
if (names.length == 1) {
cachedDefaultName = names[0];
}
}
}
loadDirectory源码分析
private void loadDirectory(Map<String, Class<?>> extensionClasses, String dir, String type,
boolean extensionLoaderClassLoaderFirst, boolean overridden, String... excludedPackages) {
// fileName = 文件夹路径 + type 全限定名
String fileName = dir + type;
try {
Enumeration<java.net.URL> urls = null;
ClassLoader classLoader = findClassLoader();
// try to load from ExtensionLoader's ClassLoader first
if (extensionLoaderClassLoaderFirst) {
ClassLoader extensionLoaderClassLoader = ExtensionLoader.class.getClassLoader();
if (ClassLoader.getSystemClassLoader() != extensionLoaderClassLoader) {
urls = extensionLoaderClassLoader.getResources(fileName);
}
}
// 根据文件名加载所有的同名文件
if (urls == null || !urls.hasMoreElements()) {
if (classLoader != null) {
urls = classLoader.getResources(fileName);
} else {
urls = ClassLoader.getSystemResources(fileName);
}
}
if (urls != null) {
while (urls.hasMoreElements()) {
java.net.URL resourceURL = urls.nextElement();
// 加载资源
loadResource(extensionClasses, classLoader, resourceURL, overridden, excludedPackages);
}
}
} catch (Throwable t) {
logger.error("Exception occurred when loading extension class (interface: " +
type + ", description file: " + fileName + ").", t);
}
}
loadResource源码分析
private void loadResource(Map<String, Class<?>> extensionClasses, ClassLoader classLoader,
java.net.URL resourceURL, boolean overridden, String... excludedPackages) {
try {
try (BufferedReader reader = new BufferedReader(new InputStreamReader(resourceURL.openStream(), StandardCharsets.UTF_8))) {
String line;
// 按行读取配置内容
while ((line = reader.readLine()) != null) {
// 定位 # 字符
final int ci = line.indexOf('#');
if (ci >= 0) {
// 截取 # 之前的字符串，# 之后的内容为注释，需要忽略
line = line.substring(0, ci);
}
line = line.trim();
if (line.length() > 0) {
try {
String name = null;
int i = line.indexOf('=');
if (i > 0) {
// 以等于号 = 为界，截取键与值
name = line.substring(0, i).trim();
line = line.substring(i + 1).trim();
}
if (line.length() > 0 && !isExcluded(line, excludedPackages)) {
// 加载类，并通过 loadClass 方法对类进行缓存
loadClass(extensionClasses, resourceURL, Class.forName(line, true, classLoader), name, overridden);
}
} catch (Throwable t) {
IllegalStateException e = new IllegalStateException("Failed to load extension class (interface: " + type + ", class line: " + line + ") in " + resourceURL + ", cause: " + t.getMessage(), t);
exceptions.put(line, e);
}
}
}
}
} catch (Throwable t) {
logger.error("Exception occurred when loading extension class (interface: " +
type + ", class file: " + resourceURL + ") in " + resourceURL, t);
}
}
loadClass源码分析
private void loadClass(Map<String, Class<?>> extensionClasses, java.net.URL resourceURL, Class<?> clazz, String name,
boolean overridden) throws NoSuchMethodException {
if (!type.isAssignableFrom(clazz)) {
throw new IllegalStateException("Error occurred when loading extension class (interface: " +
type + ", class line: " + clazz.getName() + "), class "
+ clazz.getName() + " is not subtype of interface.");
}
// 检测目标类上是否有 Adaptive 注解
if (clazz.isAnnotationPresent(Adaptive.class)) {
// 设置 cachedAdaptiveClass缓存
cacheAdaptiveClass(clazz, overridden);
}
// 检测 clazz 是否是 Wrapper 类型，判断依据是是否有当前 type 的有参构造器
else if (isWrapperClass(clazz)) {
// 存储 clazz 到 cachedWrapperClasses 缓存中
cacheWrapperClass(clazz);
}
// 程序进入此分支，表明 clazz 是一个普通的拓展类
else {
// 检测 clazz 是否有默认的构造方法，如果没有，则抛出异常
clazz.getConstructor();
if (StringUtils.isEmpty(name)) {
// 如果 name 为空，则尝试从 Extension 注解中获取 name，或使用小写的类名作为 name
name = findAnnotationName(clazz);
if (name.length() == 0) {
throw new IllegalStateException("No such extension name for the class " + clazz.getName() + " in the config " + resourceURL);
}
}
// 切分 name
String[] names = NAME_SEPARATOR.split(name);
if (ArrayUtils.isNotEmpty(names)) {
// 如果类上有 Activate 注解，则使用 names 数组的第一个元素作为键，
// 存储 name 到 Activate 注解对象的映射关系
cacheActivateClass(clazz, names[0]);
for (String n : names) {
// 存储 Class 到名称的映射关系
cacheName(clazz, n);
// 存储名称到 Class 的映射关系
saveInExtensionClass(extensionClasses, clazz, n, overridden);
}
}
}
}
IOC
通过 setter 方法注入依赖，createExtension方法中执行完getExtensionClasses方法，从配置文件中加载完所有的拓展类后，获得了目标扩展实例，进行依赖注入
injectExtension源码分析
private T injectExtension(T instance) {
if (objectFactory == null) {
return instance;
}
try {
// 遍历目标类的所有方法
for (Method method : instance.getClass().getMethods()) {
// 检测方法是否以 set 开头，且方法仅有一个参数，且方法访问级别为 public
if (!isSetter(method)) {
continue;
}
/**
* Check {@link DisableInject} to see if we need auto injection for this property
*/
if (method.getAnnotation(DisableInject.class) != null) {
continue;
}
// 获取 setter 方法参数类型
Class<?> pt = method.getParameterTypes()[0];
if (ReflectUtils.isPrimitives(pt)) {
continue;
}
try {
// 获取属性名，比如 setName 方法对应属性名 name
String property = getSetterProperty(method);
// 从 ObjectFactory 中获取依赖对象，SPI方式，Spring方式
Object object = objectFactory.getExtension(pt, property);
if (object != null) {
// 通过反射调用 setter 方法设置依赖
method.invoke(instance, object);
}
} catch (Exception e) {
logger.error("Failed to inject via method " + method.getName()
+ " of interface " + type.getName() + ": " + e.getMessage(), e);
}
}
} catch (Exception e) {
logger.error(e.getMessage(), e);
}
return instance;
}
AOP
createExtension方法中
if (wrap) {
List<Class<?>> wrapperClassesList = new ArrayList<>();
if (cachedWrapperClasses != null) {
wrapperClassesList.addAll(cachedWrapperClasses);
// 根据Activate注解中的order属性值排序
wrapperClassesList.sort(WrapperComparator.COMPARATOR);
Collections.reverse(wrapperClassesList);
}
// 循环创建 Wrapper 实例
if (CollectionUtils.isNotEmpty(wrapperClassesList)) {
for (Class<?> wrapperClass : wrapperClassesList) {
Wrapper wrapper = wrapperClass.getAnnotation(Wrapper.class);
// Wrapper注解为空，或者不为空但是符合匹配条件
if (wrapper == null
|| (ArrayUtils.contains(wrapper.matches(), name) && !ArrayUtils.contains(wrapper.mismatches(), name))) {
// 将当前 instance 作为参数传给 Wrapper 的构造方法，并通过反射创建 Wrapper 实例，实现了AOP
// 然后向 Wrapper 实例中注入依赖，最后将 Wrapper 实例再次赋值给 instance 变量
instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance));
}
}
}
}
Adaptive
getAdaptiveExtension源码分析
public T getAdaptiveExtension() {
Object instance = cachedAdaptiveInstance.get();
if (instance == null) {
// 并发情况下，如果有error直接报错，不去竞争锁了
if (createAdaptiveInstanceError != null) {
throw new IllegalStateException("Failed to create adaptive instance: " +
createAdaptiveInstanceError.toString(),
createAdaptiveInstanceError);
}
synchronized (cachedAdaptiveInstance) {
instance = cachedAdaptiveInstance.get();
if (instance == null) {
try {
// 创建Adaptive实例
instance = createAdaptiveExtension();
cachedAdaptiveInstance.set(instance);
} catch (Throwable t) {
createAdaptiveInstanceError = t;
throw new IllegalStateException("Failed to create adaptive instance: " + t.toString(), t);
}
}
}
}
return (T) instance;
}
private T createAdaptiveExtension() {
try {
// 获取Adaptive扩展类字节码，反射获取实例
return injectExtension((T) getAdaptiveExtensionClass().newInstance());
} catch (Exception e) {
throw new IllegalStateException("Can't create adaptive extension " + type + ", cause: " + e.getMessage(), e);
}
}
private Class<?> getAdaptiveExtensionClass() {
getExtensionClasses();
if (cachedAdaptiveClass != null) {
return cachedAdaptiveClass;
}
// Javassist方式或JDK方式创建Adaptive扩展类字节码
return cachedAdaptiveClass = createAdaptiveExtensionClass();
}
createAdaptiveExtensionClass源码分析
private Class<?> createAdaptiveExtensionClass() {
// 创建Adaptive类java代码
String code = new AdaptiveClassCodeGenerator(type, cachedDefaultName).generate();
ClassLoader classLoader = findClassLoader();
org.apache.dubbo.common.compiler.Compiler compiler = ExtensionLoader.getExtensionLoader(org.apache.dubbo.common.compiler.Compiler.class).getAdaptiveExtension();
// 编译Adaptive类java代码，创建Adaptive类字节码获取Adaptive类Class对象
return compiler.compile(code, classLoader);
}
AdaptiveClassCodeGenerator#generate源码分析
public String generate() {
// no need to generate adaptive class since there's no adaptive method found.
if (!hasAdaptiveMethod()) {
throw new IllegalStateException("No adaptive method exist on extension " + type.getName() + ", refuse to create the adaptive class!");
}
StringBuilder code = new StringBuilder();
// package信息
code.append(generatePackageInfo());
// import信息
code.append(generateImports());
// 类声明信息 public class %s$Adaptive implements %s
code.append(generateClassDeclaration());
// 方法信息
Method[] methods = type.getMethods();
for (Method method : methods) {
code.append(generateMethod(method));
}
code.append("}");
if (logger.isDebugEnabled()) {
logger.debug(code.toString());
}
return code.toString();
}
java代码结果示例
package org.apache.dubbo.common.extension.adaptive;
import org.apache.dubbo.common.extension.ExtensionLoader;
public class HasAdaptiveExt$Adaptive implements org.apache.dubbo.common.extension.adaptive.HasAdaptiveExt {
public java.lang.String echo(org.apache.dubbo.common.URL arg0, java.lang.String arg1) {
if (arg0 == null) throw new IllegalArgumentException("url == null");
org.apache.dubbo.common.URL url = arg0;
String extName = url.getParameter("has.adaptive.ext", "adaptive");
if(extName == null) throw new IllegalStateException("Failed to get extension (org.apache.dubbo.common.extension.adaptive.HasAdaptiveExt) name from url (" + url.toString() + ") use keys([has.adaptive.ext])");
org.apache.dubbo.common.extension.adaptive.HasAdaptiveExt extension = (org.apache.dubbo.common.extension.adaptive.HasAdaptiveExt)ExtensionLoader.getExtensionLoader(org.apache.dubbo.common.extension.adaptive.HasAdaptiveExt.class).getExtension(extName);
return extension.echo(arg0, arg1);
}
}
Compiler
利用Javassist、JDK将java代码编译为Class对象