Begins work to bridge from alibaba to apache dubbo

Starting with the 2.7.3 all of our main code can compile with
org.apache.dubbo:dubbo and also com.alibaba:dubbo

There is some strange work here because the spring integrations were not
backported. Until such time as it is, we need reflection to actually
load the extensions. As this is fragile, this change shouldn't merge
until a maven invoker test is setup to prove the tests also pass with
com.alibaba:dubbo still. Also, we need to cleanup documentation around
spring as it will mention types that don't exist.

Fixes #867

instrumentation/dubbo-rpc/pom.xml

@@ -30,12 +30,12 @@
     <main.signature.artifact>java16</main.signature.artifact>
     <!-- TODO: not compile compatible with apache dubbo until 2.7.2
          See https://github.com/apache/incubator-zipkin-brave/issues/867 -->
-    <dubbo.version>2.6.6</dubbo.version>
+    <dubbo.version>2.7.3-SNAPSHOT</dubbo.version>
   </properties>
 
   <dependencies>
     <dependency>
-      <groupId>com.alibaba</groupId>
+      <groupId>org.apache.dubbo</groupId>
       <artifactId>dubbo</artifactId>
       <version>${dubbo.version}</version>
     </dependency>

instrumentation/dubbo-rpc/src/main/java/brave/dubbo/rpc/TracingFilter.java

@@ -23,8 +23,6 @@
 import brave.propagation.TraceContextOrSamplingFlags;
 import com.alibaba.dubbo.common.Constants;
 import com.alibaba.dubbo.common.extension.Activate;
-import com.alibaba.dubbo.common.extension.ExtensionLoader;
-import com.alibaba.dubbo.config.spring.extension.SpringExtensionFactory;
 import com.alibaba.dubbo.remoting.exchange.ResponseCallback;
 import com.alibaba.dubbo.rpc.Filter;
 import com.alibaba.dubbo.rpc.Invocation;
@@ -48,10 +46,14 @@ public final class TracingFilter implements Filter {
   TraceContext.Injector<Map<String, String>> injector;
 
   /**
-   * {@link ExtensionLoader} supplies the tracing implementation which must be named "tracing". For
-   * example, if using the {@link SpringExtensionFactory}, only a bean named "tracing" will be
-   * injected.
+   * <h3>Notes for com.alibaba:dubbo</h3>
+   *
+   * {@code com.alibaba.dubbo.common.extension.ExtensionLoader} supplies the tracing implementation
+   * which must be named "tracing". For example, if using the
+   * {@code com.alibaba.dubbo.config.spring.extension.SpringExtensionFactory}, only a bean named
+   * "tracing" will be injected.
    */
+  // TODO: port the above comment for org.apache.dubbo:dubbo
   public void setTracing(Tracing tracing) {
     tracer = tracing.tracer();
     extractor = tracing.propagation().extractor(GETTER);

instrumentation/dubbo-rpc/src/test/java/brave/dubbo/rpc/ITTracingFilter.java

@@ -17,9 +17,10 @@
 import brave.propagation.StrictScopeDecorator;
 import brave.propagation.ThreadLocalCurrentTraceContext;
 import brave.sampler.Sampler;
-import com.alibaba.dubbo.common.extension.ExtensionLoader;
 import com.alibaba.dubbo.config.ReferenceConfig;
 import com.alibaba.dubbo.rpc.Filter;
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
 import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.LinkedBlockingQueue;
 import java.util.concurrent.TimeUnit;
@@ -70,10 +71,35 @@ Tracing.Builder tracingBuilder(Sampler sampler) {
         .sampler(sampler);
   }
 
+  // Reflection because com.alibaba.dubbo.common.extension.ExtensionLoader was not backported
   void setTracing(Tracing tracing) {
-    ((TracingFilter) ExtensionLoader.getExtensionLoader(Filter.class)
-        .getExtension("tracing"))
+    Class extensionLoaderClass;
+    try {
+      extensionLoaderClass = Class.forName("com.alibaba.dubbo.common.extension.ExtensionLoader");
+    } catch (ClassNotFoundException e) {
+      try {
+        extensionLoaderClass = Class.forName("org.apache.dubbo.common.extension.ExtensionLoader");
+      } catch (ClassNotFoundException ex) {
+        throw new AssertionError(ex);
+      }
+    }
+
+    Method getExtensionLoader;
+    Method getExtension;
+    try {
+      getExtensionLoader = extensionLoaderClass.getMethod("getExtensionLoader", Class.class);
+      getExtension = extensionLoaderClass.getMethod("getExtension", String.class);
+    } catch (NoSuchMethodException e) {
+      throw new AssertionError(e);
+    }
+
+    try {
+      Object extensionLoader = getExtensionLoader.invoke(null, Filter.class);
+      ((TracingFilter) getExtension.invoke(extensionLoader, "tracing"))
         .setTracing(tracing);
+    } catch (IllegalAccessException | InvocationTargetException e) {
+      throw new AssertionError(e);
+    }
     this.tracing = tracing;
   }
 

instrumentation/dubbo-rpc/src/test/java/brave/dubbo/rpc/ITTracingFilter_Provider.java

@@ -14,10 +14,10 @@
 package brave.dubbo.rpc;
 
 import brave.sampler.Sampler;
-import com.alibaba.dubbo.common.beanutil.JavaBeanDescriptor;
 import com.alibaba.dubbo.config.ApplicationConfig;
 import com.alibaba.dubbo.config.ReferenceConfig;
 import com.alibaba.dubbo.rpc.RpcContext;
+import java.util.Map;
 import org.junit.Before;
 import org.junit.Test;
 import zipkin2.Span;
@@ -32,14 +32,18 @@ public class ITTracingFilter_Provider extends ITTracingFilter {
   @Before public void setup() {
     server.service.setFilter("tracing");
     server.service.setRef((method, parameterTypes, args) -> {
-      JavaBeanDescriptor arg = (JavaBeanDescriptor) args[0];
-      if (arg.getProperty("value").equals("bad")) {
-        throw new IllegalArgumentException();
-      }
-      String value = tracing != null && tracing.currentTraceContext().get() != null
+      // com.alibaba.dubbo.common.beanutil.JavaBeanDescriptor not in org.apache.dubbo:dubbo
+      Iterable<Map.Entry<Object, Object>> arg = (Iterable<Map.Entry<Object, Object>>) args[0];
+      arg.forEach(entry -> {
+        if (!entry.getKey().equals("value")) return;
+        if (entry.getValue().equals("bad")) {
+          throw new IllegalArgumentException();
+        }
+        String value = tracing != null && tracing.currentTraceContext().get() != null
           ? tracing.currentTraceContext().get().traceIdString()
           : "";
-      arg.setProperty("value", value);
+        entry.setValue(value);
+      });
       return args[0];
     });
     server.start();