Merge pull request #259 from DataDog/ark/type_hunting

Check parent classloaders when resource location fails.

dd-java-agent/agent-tooling/src/main/java/datadog/trace/agent/tooling/AgentInstaller.java

@@ -41,6 +41,7 @@ public class AgentInstaller {
             .disableClassFormatChanges()
             .with(AgentBuilder.RedefinitionStrategy.RETRANSFORMATION)
             .with(new LoggingListener())
+            .with(new DDLocationStrategy())
             .ignore(nameStartsWith("datadog.trace."))
             .or(nameStartsWith("datadog.opentracing."))
             .or(nameStartsWith("datadog.slf4j."))

dd-java-agent/agent-tooling/src/main/java/datadog/trace/agent/tooling/DDLocationStrategy.java

@@ -0,0 +1,27 @@
+package datadog.trace.agent.tooling;
+
+import java.util.ArrayList;
+import java.util.List;
+import net.bytebuddy.agent.builder.AgentBuilder;
+import net.bytebuddy.dynamic.ClassFileLocator;
+import net.bytebuddy.utility.JavaModule;
+
+/**
+ * Locate resources with the loading classloader. If the loading classloader cannot find the desired
+ * resource, check up the classloader hierarchy until a resource is found or the bootstrap loader is
+ * reached.
+ */
+public class DDLocationStrategy implements AgentBuilder.LocationStrategy {
+  private static final ClassLoader BOOTSTRAP_RESOURCE_LOCATOR = new ClassLoader(null) {};
+
+  @Override
+  public ClassFileLocator classFileLocator(ClassLoader classLoader, JavaModule javaModule) {
+    List<ClassFileLocator> locators = new ArrayList<ClassFileLocator>();
+    while (classLoader != null) {
+      locators.add(ClassFileLocator.ForClassLoader.of(classLoader));
+      classLoader = classLoader.getParent();
+    }
+    locators.add(ClassFileLocator.ForClassLoader.of(BOOTSTRAP_RESOURCE_LOCATOR));
+    return new ClassFileLocator.Compound(locators.toArray(new ClassFileLocator[0]));
+  }
+}

dd-java-agent/agent-tooling/src/test/groovy/datadog/trace/agent/test/ResourceLocatingTest.groovy

@@ -0,0 +1,32 @@
+package datadog.trace.agent.test
+
+import datadog.trace.agent.tooling.DDLocationStrategy
+import net.bytebuddy.agent.builder.AgentBuilder
+import net.bytebuddy.dynamic.ClassFileLocator
+import spock.lang.Specification
+
+class ResourceLocatingTest extends Specification {
+  def "finds resources from parent classloader"() {
+    setup:
+    final String[] lastLookup = new String[1]
+    ClassLoader childLoader = new ClassLoader(this.getClass().getClassLoader()) {
+      @Override
+      URL getResource(String name) {
+        lastLookup[0] = name
+        // do not delegate resource lookup
+        return findResource(name)
+      }
+    }
+    ClassFileLocator locator = new DDLocationStrategy().classFileLocator(childLoader, null)
+    ClassFileLocator defaultLocator = AgentBuilder.LocationStrategy.ForClassLoader.STRONG.classFileLocator(childLoader, null)
+
+    expect:
+    locator.locate("java/lang/Object").isResolved()
+    // lastLookup ensures childLoader was checked before parent for the resource
+    lastLookup[0] == "java/lang/Object.class"
+    (lastLookup[0] = null) == null
+
+    !defaultLocator.locate("java/lang/Object").isResolved()
+    lastLookup[0] == "java/lang/Object.class"
+  }
+}