JDK 标准的 SPI 会一次性实例化扩展点所有实现，如果有扩展实现初始化很耗时，但如果没用上也加载，会很浪费资源。

如果扩展点加载失败，连扩展点的名称都拿不到了。比如：JDK 标准的 ScriptEngine，通过 getName() 获取脚本类型的名称，但如果 RubyScriptEngine 因为所依赖的 jruby.jar 不存在，导致 RubyScriptEngine 类加载失败，这个失败原因被吃掉了，和 ruby 对应不起来，当用户执行 ruby 脚本时，会报不支持 ruby，而不是真正失败的原因。

增加了对扩展点 IoC 和 AOP 的支持，一个扩展点可以直接 setter 注入其它扩展点。

某肥遇到了一个bug,需要引入多线程,结果引入多线程后,竟然出现了两个bug

public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) {if (type == null)//拓展点类型非空判断throw new IllegalArgumentException("Extension type == null");if(!type.isInterface()) {//拓展点类型只能是接口throw new IllegalArgumentException("Extension type(" + type + ") is not interface!");}if(!withExtensionAnnotation(type)) {//需要添加spi注解,否则抛异常throw new IllegalArgumentException("Extension type(" + type + ") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");}//从缓存EXTENSION_LOADERS中获取,如果不存在则新建后加入缓存//对于每一个拓展,都会有且只有一个ExtensionLoader与其对应ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);if (loader == null) {EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);}return loader;
}private ExtensionLoader(Class<?> type) {this.type = type;//这里会存在递归调用,ExtensionFactory的objectFactory为null,其他的均为AdaptiveExtensionFactory//AdaptiveExtensionFactory的factories中有SpiExtensionFactory,SpringExtensionFactory//getAdaptiveExtension()这个是获取一个拓展装饰类对象.细节在下篇讲解objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
}

public T getExtension(String name) {if (name == null || name.length() == 0)throw new IllegalArgumentException("Extension name == null");if ("true".equals(name)) {return getDefaultExtension();}Holder<Object> holder = cachedInstances.get(name);if (holder == null) {cachedInstances.putIfAbsent(name, new Holder<Object>());holder = cachedInstances.get(name);}Object instance = holder.get();if (instance == null) {synchronized (holder) {instance = holder.get();if (instance == null) {instance = createExtension(name);holder.set(instance);}}}return (T) instance;
}

private final ConcurrentMap<String, Holder<Object>> cachedInstances = new ConcurrentHashMap<String, Holder<Object>>();

public class Holder<T> {private volatile T value;public void set(T value) {this.value = value;}public T get() {return value;}}

@SPI("javassist")
public interface Compiler {//省略...
}

public Class<?> compile(String code, ClassLoader classLoader) {Compiler compiler;ExtensionLoader<Compiler> loader = ExtensionLoader.getExtensionLoader(Compiler.class);String name = DEFAULT_COMPILER; // copy referenceif (name != null && name.length() > 0) {compiler = loader.getExtension(name);} else {compiler = loader.getDefaultExtension();}return compiler.compile(code, classLoader);
}

//com.alibaba.dubbo.common.compiler.Compiler 文件配置如下
adaptive=com.alibaba.dubbo.common.compiler.support.AdaptiveCompiler
jdk=com.alibaba.dubbo.common.compiler.support.JdkCompiler
javassist=com.alibaba.dubbo.common.compiler.support.JavassistCompiler

//返回指定名字的扩展
public T getExtension(String name){}

//@SPI("javassist")代表默认的spi对象,比如Compiler默认使用的是javassist,可通过
ExtensionLoader<Compiler> loader = ExtensionLoader.getExtensionLoader(Compiler.class);
compiler = loader.getDefaultExtension();
//方式获取实现类,根据配置,即为
//com.alibaba.dubbo.common.compiler.support.JavassistCompiler

// 这一步步演进的过程,才是最大的收获
装饰者设计模式->静态代理->JDK、CGLIB动态代理->AOP