Apache Dubbo的SPI機制是如何實現的

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

package com.vivo.study.impl
 
/**
 * 實現一
 *
 */
public class AudiCar implements Car {
    @Override
    public void getPrice() {
        System.out.println("Audi A6L's price is  500000 RMB.");
    }
}
 
package com.vivo.study.impl
/**
 * 實現二
 *
 */
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緩存,key -> Class value -> ExtensionLoader實例
    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");
        }
        // 檢查是否是接口，如果不是則拋出異常
        if (!type.isInterface()) {
            throw new IllegalArgumentException("Extension type (" + type + ") is not an interface!");
        }
 
        // 檢查接口是否是被@SPI註解修飾，如果沒有則拋出異常
        if (!withExtensionAnnotation(type)) {
            throw new IllegalArgumentException("Extension type (" + type +
                    ") is not an extension, because it is NOT annotated with @" + SPI.class.getSimpleName() + "!");
        }
 
        // 從緩存裡取，沒有則初始化一個並放入緩存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 {};
}

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

public class ExtensionLoader<T> {
// ...
private Class<?> getAdaptiveExtensionClass() {
// 根據對應的SPI文件加載擴展類並緩存，細節此處不展開
        getExtensionClasses();
// 如果存在被@Adaptive修飾的類則直接返回此類
if (cachedAdaptiveClass != null) {
return cachedAdaptiveClass;
        }
// 動態生成Xxxx$Adaptive類
return cachedAdaptiveClass = createAdaptiveExtensionClass();
    }
private Class<?> createAdaptiveExtensionClass() {
// 生成Xxxx$Adaptive類代碼，可自行加日志或斷點查看生成的代碼
        String code = new AdaptiveClassCodeGenerator(type, cachedDefaultName).generate();
        ClassLoader classLoader = findClassLoader();
// 獲取動態編譯器，默認為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 {
// ...
/**
     * generate method content
     */
private String generateMethodContent(Method method) {
// 獲取方法上的@Adaptive註解
        Adaptive adaptiveAnnotation = method.getAnnotation(Adaptive.class);
        StringBuilder code = new StringBuilder(512);
if (adaptiveAnnotation == null) {
// 方法時沒有@Adaptive註解，生成不支持的代碼
return generateUnsupported(method);
        } else {
// 方法參數裡URL是第幾個參數，不存在則為-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));
            }
// 獲取方法上@Adaptive配置的value
// 比如 @Adaptive({"key1","key2"}),則會返回String數組{"key1","key2"}
// 如果@Adaptive沒有配置value，則會根據簡寫接口名按駝峰用.分割，比如SimpleExt對應simple.ext
            String[] value = getMethodAdaptiveValue(adaptiveAnnotation);
// 參數裡是否存在org.apache.dubbo.rpc.Invocation
            boolean hasInvocation = hasInvocationArgument(method);
            code.append(generateInvocationArgumentNullCheck(method));
// 生成String extName = xxx;的代碼 ，extName用於獲取具體的Extension實例
            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暴露服務服務export時，會根據獲取Url中的service.filter對應的值和group=provider來獲取激活對應的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!");
    }
  
}