Apache Dubbo的SPI机制是如何实现的

package com.vivo.study
public interface Car {
void getPrice();
}

package com.vivo.study.impl
 

public class AudiCar implements Car {
    @Override
    public void getPrice() {
        System.out.println("Audi A6L's price is  500000 RMB.");
    }
}
 
package com.vivo.study.impl

public class BydCar implements Car {
    @Override
    public void getPrice() {
        System.out.println("BYD han's price is 220000 RMB.");
    }
}

com.vivo.study.impl.AudiCar
com.vivo.study.impl.BydCar

public class SpiDemo {
public static void main(String[] args) {
        ServiceLoader<Car> load = ServiceLoader.load(Car.class);
        Iterator<Car> iterator = load.iterator();
while (iterator.hasNext()) {
            Car car = iterator.next();
            car.getPrice();
        }
    }
}

public
 
    // ConcurrentHashMap缓存,key -> Class value -> ExtensionLoader实例
    private static final ConcurrentMap<Class<?>, ExtensionLoader<?>> EXTENSION_LOADERS = new ConcurrentHashMap<>(64);
 
    private ExtensionLoader(Class<?> type) {
        this.type = type;
        objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
    }
 
    public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) {
        if (type == null) {
            throw new IllegalArgumentException("Extension type == null");
        }
        // 检查是否是接口，如果不是则抛出异常
        if (!type.isInterface()) {
            throw new IllegalArgumentException("Extension type (" + type + ") is not an interface!");
        }
 
        // 检查接口是否是被@SPI注解修饰，如果没有则抛出异常
        if (!withExtensionAnnotation(type)) {
            throw new IllegalArgumentException("Extension type (" + type +
                    ") is not an extension, because it is NOT annotated with @" + SPI.class.getSimpleName() + "!");
        }
 
        // 从缓存里取，没有则初始化一个并放入缓存EXTENSION_LOADERS中
        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;
    }
}

getExtensionClasses
  |->loadExtensionClasses
    |->cacheDefaultExtensionName
    |->loadDirectory
      |->loadResource
        |->loadClass

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE, ElementType.METHOD})
public @interface Adaptive {
String[] value() default {};
}

@SPI("impl1")
public interface SimpleExt {
    @Adaptive({"key1", "key2"})
    String yell(URL url, String s);
}

public class ExtensionLoader<T> {
// ...
private Class<?> getAdaptiveExtensionClass() {
// 根据对应的SPI文件加载扩展类并缓存，细节此处不展开
        getExtensionClasses();
// 如果存在被@Adaptive修饰的类则直接返回此类
if (cachedAdaptiveClass != null) {
return cachedAdaptiveClass;
        }
// 动态生成Xxxx$Adaptive类
return cachedAdaptiveClass = createAdaptiveExtensionClass();
    }
private Class<?> createAdaptiveExtensionClass() {
// 生成Xxxx$Adaptive类代码，可自行加日志或断点查看生成的代码
        String code = new AdaptiveClassCodeGenerator(type, cachedDefaultName).generate();
        ClassLoader classLoader = findClassLoader();
// 获取动态编译器，默认为javassist
        org.apache.dubbo.common.compiler.Compiler compiler = ExtensionLoader.getExtensionLoader(org.apache.dubbo.common.compiler.Compiler.class).getAdaptiveExtension();
return compiler.compile(code, classLoader);
    }
}

public class AdaptiveClassCodeGenerator {
// ...

private String generateMethodContent(Method method) {
// 获取方法上的@Adaptive注解
        Adaptive adaptiveAnnotation = method.getAnnotation(Adaptive.class);
        StringBuilder code = new StringBuilder(512);
if (adaptiveAnnotation == null) {
// 方法时没有@Adaptive注解，生成不支持的代码
return generateUnsupported(method);
        } else {
// 方法参数里URL是第几个参数，不存在则为-1
int urlTypeIndex = getUrlTypeIndex(method);
// found parameter in URL type
if (urlTypeIndex != -1) {
// Null Point check
                code.append(generateUrlNullCheck(urlTypeIndex));
            } else {
// did not find parameter in URL type
                code.append(generateUrlAssignmentIndirectly(method));
            }
// 获取方法上@Adaptive配置的value
// 比如 @Adaptive({"key1","key2"}),则会返回String数组{"key1","key2"}
// 如果@Adaptive没有配置value，则会根据简写接口名按驼峰用.分割，比如SimpleExt对应simple.ext
            String[] value = getMethodAdaptiveValue(adaptiveAnnotation);
// 参数里是否存在org.apache.dubbo.rpc.Invocation
            boolean hasInvocation = hasInvocationArgument(method);
            code.append(generateInvocationArgumentNullCheck(method));
// 生成String extName = xxx;的代码 ，extName用于获取具体的Extension实例
            code.append(generateExtNameAssignment(value, hasInvocation));
// check extName == null?
            code.append(generateExtNameNullCheck(value));
            code.append(generateExtensionAssignment());
// return statement
            code.append(generateReturnAndInvocation(method));
        }
return code.toString();
    }
}

@SPI("impl1")
public interface SimpleExt {
@Adaptive
String echo(URL url, String s);
}

String extName = url.getParameter("simple.ext", "impl1")

@SPI("impl1")
public interface SimpleExt {
@Adaptive({"key1", "key2"})
String yell(URL url, String s);
}

String extName = url.getParameter( "simple.ext")

@SPI
public interface SimpleExt {
@Adaptive({"key1", "key2"})
String yell(URL url, String s);
}

String extName = url.getParameter("key1", url.getParameter("key2", "impl1"));

@SPI
public interface SimpleExt {
@Adaptive({"key1", "key2"})
String yell(URL url, String s);
}

String extName = url.getParameter("key1", url.getParameter("key2"));

// export:key-> service.filter ; group-> provider
private static <T> Invoker<T> buildInvokerChain(final Invoker<T> invoker, String key, String group) {
    // 在Provider暴露服务服务export时，会根据获取Url中的service.filter对应的值和group=provider来获取激活对应的Filter
    List<Filter> filters = ExtensionLoader.getExtensionLoader(Filter.class).getActivateExtension(invoker.getUrl(), key, group);
}

@SPI("impl1")
public interface SimpleExt {
    // @Adaptive example, do not specify a explicit key.
    @Adaptive
    String echo(URL url, String s);
 
    @Adaptive({"key1", "key2"})
    String yell(URL url, String s);
 
    // no @Adaptive
    String bang(URL url, int i);
}

package org.apache.dubbo.common.extension.ext1;
  
import org.apache.dubbo.common.extension.ExtensionLoader;
  
public class SimpleExt$Adaptive implements org.apache.dubbo.common.extension.ext1.SimpleExt {
  
    public java.lang.String yell(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("key1", url.getParameter("key2", "impl1"));
        if (extName == null)
            throw new IllegalStateException("Failed to get extension (org.apache.dubbo.common.extension.ext1.SimpleExt) name from url (" + url.toString() + ") use keys([key1, key2])");
        org.apache.dubbo.common.extension.ext1.SimpleExt extension = (org.apache.dubbo.common.extension.ext1.SimpleExt)ExtensionLoader.getExtensionLoader(org.apache.dubbo.common.extension.ext1.SimpleExt.class).getExtension(extName);
        return extension.yell(arg0, arg1);
    }
  
    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("simple.ext", "impl1");
        if (extName == null)
            throw new IllegalStateException("Failed to get extension (org.apache.dubbo.common.extension.ext1.SimpleExt) name from url (" + url.toString() + ") use keys([simple.ext])");
        org.apache.dubbo.common.extension.ext1.SimpleExt extension = (org.apache.dubbo.common.extension.ext1.SimpleExt) ExtensionLoader.getExtensionLoader(org.apache.dubbo.common.extension.ext1.SimpleExt.class).getExtension(extName);
        return extension.echo(arg0, arg1);
    }
  
    public java.lang.String bang(org.apache.dubbo.common.URL arg0, int arg1) {
        throw new UnsupportedOperationException("The method public abstract java.lang.String org.apache.dubbo.common.extension.ext1.SimpleExt.bang(org.apache.dubbo.common.URL,int) of interface org.apache.dubbo.common.extension.ext1.SimpleExt is not adaptive method!");
    }
  
}