This patch fixes NPE in jaxrs instrument which occurs when we use interfaces with default method and its impl class.

- reproduce NPE error with JaxRs class and interfaces in JavaInterfaces
- fix the logic target class of method with @Path annotation to avoid NPE
- setting java 1.8 for jaxrs's test

dd-java-agent/instrumentation/jax-rs-annotations-2/jax-rs-annotations-2.gradle

@@ -11,6 +11,10 @@ muzzle {
   }
 }
 
+ext {
+  minJavaVersionForTests = JavaVersion.VERSION_1_8
+}
+
 apply from: "${rootDir}/gradle/java.gradle"
 
 //apply plugin: 'org.unbroken-dome.test-sets'
@@ -34,4 +38,5 @@ dependencies {
 
   // Anything 1.0+ fails with a java.lang.NoClassDefFoundError: org/eclipse/jetty/server/RequestLog
 //  latestDepTestCompile group: 'io.dropwizard', name: 'dropwizard-testing', version: '1.+'
+
 }

dd-java-agent/instrumentation/jax-rs-annotations-2/src/main/java/datadog/trace/instrumentation/jaxrs2/JaxRsAnnotationsDecorator.java

@@ -104,7 +104,7 @@ public class JaxRsAnnotationsDecorator extends BaseDecorator {
   private LinkedList<Path> gatherPaths(final Method method) {
     Class<?> target = method.getDeclaringClass();
     final LinkedList<Path> paths = new LinkedList<>();
-    while (target != Object.class) {
+    while (target != null && target != Object.class) {
       final Path annotation = target.getAnnotation(Path.class);
       if (annotation != null) {
         paths.push(annotation);

dd-java-agent/instrumentation/jax-rs-annotations-2/src/test/groovy/JavaInterfaces.java

@@ -0,0 +1,59 @@
+import javax.ws.rs.GET;
+import javax.ws.rs.Path;
+
+public class JavaInterfaces {
+
+  interface Jax {
+
+    void call();
+  }
+
+  @Path("interface")
+  interface InterfaceWithClassMethodPath extends Jax {
+
+    @GET
+    @Path("invoke")
+    void call();
+  }
+
+  @Path("abstract")
+  abstract class AbstractClassOnInterfaceWithClassPath implements InterfaceWithClassMethodPath {
+
+    @GET
+    @Path("call")
+    @Override
+    public void call() {
+    }
+
+    abstract void actual();
+  }
+
+  @Path("child")
+  class ChildClassOnInterface extends AbstractClassOnInterfaceWithClassPath {
+
+    @Override
+    void actual() {
+    }
+  }
+
+  @Path("interface")
+  interface DefaultInterfaceWithClassMethodPath extends Jax {
+
+    @GET
+    @Path("invoke")
+    default void call() {
+      actual();
+    }
+
+    void actual();
+  }
+
+  @Path("child")
+  class DefaultChildClassOnInterface implements DefaultInterfaceWithClassMethodPath {
+
+    @Override
+    public void actual() {
+    }
+  }
+
+}

dd-java-agent/instrumentation/jax-rs-annotations-2/src/test/groovy/JaxRsAnnotationsInstrumentationTest.groovy

@@ -130,6 +130,15 @@ class JaxRsAnnotationsInstrumentationTest extends AgentTestRunner {
     }
     "POST /abstract/child/call" | new ChildClassWithPath()
 
+    /*
+     * should be "GET /child/call" with ChildClassOnInterface.call
+     */
+    // "GET /abstract/call" | new JavaInterfaces.ChildClassOnInterface()
+    /*
+     * should be "GET /child/invoke" with DefaultChildClassOnInterface.call
+     */
+    // "GET /interface/invoke" | new JavaInterfaces.DefaultChildClassOnInterface()
+
     className = getName(obj.class)
   }