Add new integration for Servlet Filters, HttpServlet, and RequestDispatcher

Disabled by default, and only creates a span if existing trace detected.

To enable all of them:
* System Property: `-Ddd.integration.servlet.enabled=true`
* Environment Variable: `DD_INTEGRATION_SERVLET_ENABLED=true`

(They have independent configs as well.  If needed, view the source below.)

dd-java-agent/agent-bootstrap/src/main/java/datadog/trace/instrumentation/api/AgentScope.java

@@ -5,7 +5,7 @@ import java.io.Closeable;
 public interface AgentScope extends Closeable {
   AgentSpan span();
 
-  void setAsyncPropagation(boolean value);
+  AgentScope setAsyncPropagation(boolean value);
 
   @Override
   void close();

dd-java-agent/agent-bootstrap/src/main/java/datadog/trace/instrumentation/api/AgentTracer.java

@@ -208,7 +208,9 @@ public class AgentTracer {
     }
 
     @Override
-    public void setAsyncPropagation(final boolean value) {}
+    public AgentScope setAsyncPropagation(final boolean value) {
+      return this;
+    }
 
     @Override
     public void close() {}

dd-java-agent/agent-tooling/src/main/java/datadog/trace/agent/tooling/OpenTracing32.java

@@ -237,10 +237,11 @@ public final class OpenTracing32 implements TracerAPI {
     }
 
     @Override
-    public void setAsyncPropagation(final boolean value) {
+    public AgentScope setAsyncPropagation(final boolean value) {
       if (scope instanceof TraceScope) {
         ((TraceScope) scope).setAsyncPropagation(value);
       }
+      return this;
     }
 
     @Override

dd-java-agent/instrumentation/servlet/src/main/java/datadog/trace/instrumentation/servlet/ServletRequestSetter.java

@@ -0,0 +1,14 @@
+package datadog.trace.instrumentation.servlet;
+
+import datadog.trace.instrumentation.api.AgentPropagation;
+import javax.servlet.ServletRequest;
+
+/** Inject into request attributes since the request headers can't be modified. */
+public class ServletRequestSetter implements AgentPropagation.Setter<ServletRequest> {
+  public static final ServletRequestSetter SETTER = new ServletRequestSetter();
+
+  @Override
+  public void set(final ServletRequest carrier, final String key, final String value) {
+    carrier.setAttribute(key, value);
+  }
+}

dd-java-agent/instrumentation/servlet/src/main/java/datadog/trace/instrumentation/servlet/dispatcher/RequestDispatcherDecorator.java

@@ -0,0 +1,22 @@
+package datadog.trace.instrumentation.servlet.dispatcher;
+
+import datadog.trace.agent.decorator.BaseDecorator;
+
+public class RequestDispatcherDecorator extends BaseDecorator {
+  public static final RequestDispatcherDecorator DECORATE = new RequestDispatcherDecorator();
+
+  @Override
+  protected String[] instrumentationNames() {
+    return new String[] {"servlet", "servlet-dispatcher"};
+  }
+
+  @Override
+  protected String spanType() {
+    return null;
+  }
+
+  @Override
+  protected String component() {
+    return "java-web-servlet-dispatcher";
+  }
+}

dd-java-agent/instrumentation/servlet/src/main/java/datadog/trace/instrumentation/servlet/dispatcher/RequestDispatcherInstrumentation.java

@@ -0,0 +1,108 @@
+package datadog.trace.instrumentation.servlet.dispatcher;
+
+import static datadog.trace.agent.tooling.ByteBuddyElementMatchers.safeHasSuperType;
+import static datadog.trace.instrumentation.api.AgentTracer.activateSpan;
+import static datadog.trace.instrumentation.api.AgentTracer.activeSpan;
+import static datadog.trace.instrumentation.api.AgentTracer.propagate;
+import static datadog.trace.instrumentation.api.AgentTracer.startSpan;
+import static datadog.trace.instrumentation.servlet.ServletRequestSetter.SETTER;
+import static datadog.trace.instrumentation.servlet.dispatcher.RequestDispatcherDecorator.DECORATE;
+import static java.util.Collections.singletonMap;
+import static net.bytebuddy.matcher.ElementMatchers.isInterface;
+import static net.bytebuddy.matcher.ElementMatchers.isPublic;
+import static net.bytebuddy.matcher.ElementMatchers.named;
+import static net.bytebuddy.matcher.ElementMatchers.not;
+import static net.bytebuddy.matcher.ElementMatchers.takesArgument;
+
+import com.google.auto.service.AutoService;
+import datadog.trace.agent.tooling.Instrumenter;
+import datadog.trace.api.DDTags;
+import datadog.trace.bootstrap.InstrumentationContext;
+import datadog.trace.instrumentation.api.AgentScope;
+import datadog.trace.instrumentation.api.AgentSpan;
+import java.util.Map;
+import javax.servlet.RequestDispatcher;
+import javax.servlet.ServletRequest;
+import net.bytebuddy.asm.Advice;
+import net.bytebuddy.description.method.MethodDescription;
+import net.bytebuddy.description.type.TypeDescription;
+import net.bytebuddy.matcher.ElementMatcher;
+
+@AutoService(Instrumenter.class)
+public final class RequestDispatcherInstrumentation extends Instrumenter.Default {
+  public RequestDispatcherInstrumentation() {
+    super("servlet", "servlet-dispatcher");
+  }
+
+  @Override
+  public boolean defaultEnabled() {
+    return false;
+  }
+
+  @Override
+  public String[] helperClassNames() {
+    return new String[] {
+      "datadog.trace.instrumentation.servlet.ServletRequestSetter",
+      "datadog.trace.agent.decorator.BaseDecorator",
+      packageName + ".RequestDispatcherDecorator",
+    };
+  }
+
+  @Override
+  public ElementMatcher<TypeDescription> typeMatcher() {
+    return not(isInterface()).and(safeHasSuperType(named("javax.servlet.RequestDispatcher")));
+  }
+
+  @Override
+  public Map<String, String> contextStore() {
+    return singletonMap("javax.servlet.RequestDispatcher", String.class.getName());
+  }
+
+  @Override
+  public Map<? extends ElementMatcher<? super MethodDescription>, String> transformers() {
+    return singletonMap(
+        named("forward")
+            .or(named("include"))
+            .and(takesArgument(0, named("javax.servlet.ServletRequest")))
+            .and(takesArgument(1, named("javax.servlet.ServletResponse")))
+            .and(isPublic()),
+        RequestDispatcherAdvice.class.getName());
+  }
+
+  public static class RequestDispatcherAdvice {
+
+    @Advice.OnMethodEnter(suppress = Throwable.class)
+    public static AgentScope start(
+        @Advice.Origin("#m") final String method,
+        @Advice.This final RequestDispatcher dispatcher,
+        @Advice.Argument(0) final ServletRequest request) {
+      if (activeSpan() == null) {
+        // Don't want to generate a new top-level span
+        return null;
+      }
+
+      final AgentSpan span = startSpan("servlet." + method);
+      DECORATE.afterStart(span);
+
+      final String target =
+          InstrumentationContext.get(RequestDispatcher.class, String.class).get(dispatcher);
+      span.setTag(DDTags.RESOURCE_NAME, target);
+
+      // In case we lose context, inject trace into to the request.
+      propagate().inject(span, request, SETTER);
+
+      return activateSpan(span, true).setAsyncPropagation(true);
+    }
+
+    @Advice.OnMethodExit(onThrowable = Throwable.class, suppress = Throwable.class)
+    public static void stop(
+        @Advice.Enter final AgentScope scope, @Advice.Thrown final Throwable throwable) {
+      if (scope == null) {
+        return;
+      }
+      DECORATE.onError(scope, throwable);
+      DECORATE.beforeFinish(scope);
+      scope.close();
+    }
+  }
+}

dd-java-agent/instrumentation/servlet/src/main/java/datadog/trace/instrumentation/servlet/dispatcher/ServletContextInstrumentation.java

@@ -0,0 +1,62 @@
+package datadog.trace.instrumentation.servlet.dispatcher;
+
+import static datadog.trace.agent.tooling.ByteBuddyElementMatchers.safeHasSuperType;
+import static java.util.Collections.singletonMap;
+import static net.bytebuddy.matcher.ElementMatchers.isInterface;
+import static net.bytebuddy.matcher.ElementMatchers.isPublic;
+import static net.bytebuddy.matcher.ElementMatchers.named;
+import static net.bytebuddy.matcher.ElementMatchers.not;
+import static net.bytebuddy.matcher.ElementMatchers.returns;
+import static net.bytebuddy.matcher.ElementMatchers.takesArgument;
+
+import com.google.auto.service.AutoService;
+import datadog.trace.agent.tooling.Instrumenter;
+import datadog.trace.bootstrap.InstrumentationContext;
+import java.util.Map;
+import javax.servlet.RequestDispatcher;
+import net.bytebuddy.asm.Advice;
+import net.bytebuddy.description.method.MethodDescription;
+import net.bytebuddy.description.type.TypeDescription;
+import net.bytebuddy.matcher.ElementMatcher;
+
+@AutoService(Instrumenter.class)
+public final class ServletContextInstrumentation extends Instrumenter.Default {
+  public ServletContextInstrumentation() {
+    super("servlet", "servlet-dispatcher");
+  }
+
+  @Override
+  public boolean defaultEnabled() {
+    return false;
+  }
+
+  @Override
+  public ElementMatcher<TypeDescription> typeMatcher() {
+    return not(isInterface()).and(safeHasSuperType(named("javax.servlet.ServletContext")));
+  }
+
+  @Override
+  public Map<String, String> contextStore() {
+    return singletonMap("javax.servlet.RequestDispatcher", String.class.getName());
+  }
+
+  @Override
+  public Map<? extends ElementMatcher<? super MethodDescription>, String> transformers() {
+    return singletonMap(
+        returns(named("javax.servlet.RequestDispatcher"))
+            .and(takesArgument(0, String.class))
+            // javax.servlet.ServletContext.getRequestDispatcher
+            // javax.servlet.ServletContext.getNamedDispatcher
+            .and(isPublic()),
+        RequestDispatcherTargetAdvice.class.getName());
+  }
+
+  public static class RequestDispatcherTargetAdvice {
+    @Advice.OnMethodExit(suppress = Throwable.class)
+    public static void saveTarget(
+        @Advice.Argument(0) final String target,
+        @Advice.Return final RequestDispatcher dispatcher) {
+      InstrumentationContext.get(RequestDispatcher.class, String.class).put(dispatcher, target);
+    }
+  }
+}

dd-java-agent/instrumentation/servlet/src/main/java/datadog/trace/instrumentation/servlet/filter/FilterDecorator.java

@@ -0,0 +1,22 @@
+package datadog.trace.instrumentation.servlet.filter;
+
+import datadog.trace.agent.decorator.BaseDecorator;
+
+public class FilterDecorator extends BaseDecorator {
+  public static final FilterDecorator DECORATE = new FilterDecorator();
+
+  @Override
+  protected String[] instrumentationNames() {
+    return new String[] {"servlet", "servlet-filter"};
+  }
+
+  @Override
+  protected String spanType() {
+    return null;
+  }
+
+  @Override
+  protected String component() {
+    return "java-web-servlet-filter";
+  }
+}

dd-java-agent/instrumentation/servlet/src/main/java/datadog/trace/instrumentation/servlet/filter/FilterInstrumentation.java

@@ -0,0 +1,89 @@
+package datadog.trace.instrumentation.servlet.filter;
+
+import static datadog.trace.agent.tooling.ByteBuddyElementMatchers.safeHasSuperType;
+import static datadog.trace.instrumentation.api.AgentTracer.activateSpan;
+import static datadog.trace.instrumentation.api.AgentTracer.activeSpan;
+import static datadog.trace.instrumentation.api.AgentTracer.startSpan;
+import static datadog.trace.instrumentation.servlet.filter.FilterDecorator.DECORATE;
+import static java.util.Collections.singletonMap;
+import static net.bytebuddy.matcher.ElementMatchers.isInterface;
+import static net.bytebuddy.matcher.ElementMatchers.isPublic;
+import static net.bytebuddy.matcher.ElementMatchers.named;
+import static net.bytebuddy.matcher.ElementMatchers.not;
+import static net.bytebuddy.matcher.ElementMatchers.takesArgument;
+
+import com.google.auto.service.AutoService;
+import datadog.trace.agent.tooling.Instrumenter;
+import datadog.trace.api.DDTags;
+import datadog.trace.instrumentation.api.AgentScope;
+import datadog.trace.instrumentation.api.AgentSpan;
+import java.util.Map;
+import javax.servlet.Filter;
+import net.bytebuddy.asm.Advice;
+import net.bytebuddy.description.method.MethodDescription;
+import net.bytebuddy.description.type.TypeDescription;
+import net.bytebuddy.matcher.ElementMatcher;
+
+@AutoService(Instrumenter.class)
+public final class FilterInstrumentation extends Instrumenter.Default {
+  public FilterInstrumentation() {
+    super("servlet", "servlet-filter");
+  }
+
+  @Override
+  public boolean defaultEnabled() {
+    return false;
+  }
+
+  @Override
+  public String[] helperClassNames() {
+    return new String[] {
+      "datadog.trace.agent.decorator.BaseDecorator", packageName + ".FilterDecorator",
+    };
+  }
+
+  @Override
+  public ElementMatcher<TypeDescription> typeMatcher() {
+    return not(isInterface()).and(safeHasSuperType(named("javax.servlet.Filter")));
+  }
+
+  @Override
+  public Map<? extends ElementMatcher<? super MethodDescription>, String> transformers() {
+    return singletonMap(
+        named("doFilter")
+            .and(takesArgument(0, named("javax.servlet.ServletRequest")))
+            .and(takesArgument(1, named("javax.servlet.ServletResponse")))
+            .and(isPublic()),
+        FilterAdvice.class.getName());
+  }
+
+  public static class FilterAdvice {
+
+    @Advice.OnMethodEnter(suppress = Throwable.class)
+    public static AgentScope start(@Advice.This final Filter filter) {
+      if (activeSpan() == null) {
+        // Don't want to generate a new top-level span
+        return null;
+      }
+
+      final AgentSpan span = startSpan("servlet.filter");
+      DECORATE.afterStart(span);
+
+      // Here we use "this" instead of "the method target" to distinguish abstract filter instances.
+      span.setTag(DDTags.RESOURCE_NAME, filter.getClass().getSimpleName() + ".doFilter");
+
+      return activateSpan(span, true).setAsyncPropagation(true);
+    }
+
+    @Advice.OnMethodExit(onThrowable = Throwable.class, suppress = Throwable.class)
+    public static void stopSpan(
+        @Advice.Enter final AgentScope scope, @Advice.Thrown final Throwable throwable) {
+      if (scope == null) {
+        return;
+      }
+      DECORATE.onError(scope, throwable);
+      DECORATE.beforeFinish(scope);
+      scope.close();
+    }
+  }
+}

dd-java-agent/instrumentation/servlet/src/main/java/datadog/trace/instrumentation/servlet/http/HttpServletDecorator.java

@@ -0,0 +1,22 @@
+package datadog.trace.instrumentation.servlet.http;
+
+import datadog.trace.agent.decorator.BaseDecorator;
+
+public class HttpServletDecorator extends BaseDecorator {
+  public static final HttpServletDecorator DECORATE = new HttpServletDecorator();
+
+  @Override
+  protected String[] instrumentationNames() {
+    return new String[] {"servlet", "servlet-service"};
+  }
+
+  @Override
+  protected String spanType() {
+    return null;
+  }
+
+  @Override
+  protected String component() {
+    return "java-web-servlet-service";
+  }
+}

dd-java-agent/instrumentation/servlet/src/main/java/datadog/trace/instrumentation/servlet/http/HttpServletInstrumentation.java

@@ -0,0 +1,112 @@
+package datadog.trace.instrumentation.servlet.http;
+
+import static datadog.trace.agent.tooling.ByteBuddyElementMatchers.safeHasSuperType;
+import static datadog.trace.instrumentation.api.AgentTracer.activateSpan;
+import static datadog.trace.instrumentation.api.AgentTracer.activeSpan;
+import static datadog.trace.instrumentation.api.AgentTracer.startSpan;
+import static datadog.trace.instrumentation.servlet.http.HttpServletDecorator.DECORATE;
+import static java.util.Collections.singletonMap;
+import static net.bytebuddy.matcher.ElementMatchers.isInterface;
+import static net.bytebuddy.matcher.ElementMatchers.isProtected;
+import static net.bytebuddy.matcher.ElementMatchers.isPublic;
+import static net.bytebuddy.matcher.ElementMatchers.nameStartsWith;
+import static net.bytebuddy.matcher.ElementMatchers.named;
+import static net.bytebuddy.matcher.ElementMatchers.not;
+import static net.bytebuddy.matcher.ElementMatchers.takesArgument;
+
+import com.google.auto.service.AutoService;
+import datadog.trace.agent.tooling.Instrumenter;
+import datadog.trace.api.DDTags;
+import datadog.trace.bootstrap.InstrumentationContext;
+import datadog.trace.instrumentation.api.AgentScope;
+import datadog.trace.instrumentation.api.AgentSpan;
+import java.lang.reflect.Method;
+import java.util.Map;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+import net.bytebuddy.asm.Advice;
+import net.bytebuddy.description.method.MethodDescription;
+import net.bytebuddy.description.type.TypeDescription;
+import net.bytebuddy.matcher.ElementMatcher;
+
+@AutoService(Instrumenter.class)
+public final class HttpServletInstrumentation extends Instrumenter.Default {
+  public HttpServletInstrumentation() {
+    super("servlet", "servlet-service");
+  }
+
+  @Override
+  public boolean defaultEnabled() {
+    return false;
+  }
+
+  @Override
+  public String[] helperClassNames() {
+    return new String[] {
+      "datadog.trace.agent.decorator.BaseDecorator", packageName + ".HttpServletDecorator",
+    };
+  }
+
+  @Override
+  public ElementMatcher<TypeDescription> typeMatcher() {
+    return not(isInterface()).and(safeHasSuperType(named("javax.servlet.http.HttpServlet")));
+  }
+
+  /**
+   * Here we are instrumenting the protected method for HttpServlet. This should ensure that this
+   * advice is always called after Servlet3Instrumentation which is instrumenting the public method.
+   */
+  @Override
+  public Map<? extends ElementMatcher<? super MethodDescription>, String> transformers() {
+    return singletonMap(
+        named("service")
+            .or(nameStartsWith("do")) // doGet, doPost, etc
+            .and(takesArgument(0, named("javax.servlet.http.HttpServletRequest")))
+            .and(takesArgument(1, named("javax.servlet.http.HttpServletResponse")))
+            .and(isProtected().or(isPublic())),
+        HttpServletAdvice.class.getName());
+  }
+
+  @Override
+  public Map<String, String> contextStore() {
+    return singletonMap(
+        "javax.servlet.http.HttpServletResponse", "javax.servlet.http.HttpServletRequest");
+  }
+
+  public static class HttpServletAdvice {
+
+    @Advice.OnMethodEnter(suppress = Throwable.class)
+    public static AgentScope start(
+        @Advice.Origin final Method method,
+        @Advice.Argument(0) final HttpServletRequest request,
+        @Advice.Argument(1) final HttpServletResponse response) {
+      // For use by HttpServletResponseInstrumentation:
+      InstrumentationContext.get(HttpServletResponse.class, HttpServletRequest.class)
+          .put(response, request);
+
+      if (activeSpan() == null) {
+        // Don't want to generate a new top-level span
+        return null;
+      }
+
+      final AgentSpan span = startSpan("servlet." + method.getName());
+      DECORATE.afterStart(span);
+
+      // Here we use the Method instead of "this.class.name" to distinguish calls to "super".
+      span.setTag(DDTags.RESOURCE_NAME, DECORATE.spanNameForMethod(method));
+
+      return activateSpan(span, true).setAsyncPropagation(true);
+    }
+
+    @Advice.OnMethodExit(onThrowable = Throwable.class, suppress = Throwable.class)
+    public static void stopSpan(
+        @Advice.Enter final AgentScope scope, @Advice.Thrown final Throwable throwable) {
+      if (scope == null) {
+        return;
+      }
+      DECORATE.onError(scope, throwable);
+      DECORATE.beforeFinish(scope);
+      scope.close();
+    }
+  }
+}

dd-java-agent/instrumentation/servlet/src/main/java/datadog/trace/instrumentation/servlet/http/HttpServletResponseDecorator.java

@@ -0,0 +1,22 @@
+package datadog.trace.instrumentation.servlet.http;
+
+import datadog.trace.agent.decorator.BaseDecorator;
+
+public class HttpServletResponseDecorator extends BaseDecorator {
+  public static final HttpServletResponseDecorator DECORATE = new HttpServletResponseDecorator();
+
+  @Override
+  protected String[] instrumentationNames() {
+    return new String[] {"servlet", "servlet-response"};
+  }
+
+  @Override
+  protected String spanType() {
+    return null;
+  }
+
+  @Override
+  protected String component() {
+    return "java-web-servlet-response";
+  }
+}

dd-java-agent/instrumentation/servlet/src/main/java/datadog/trace/instrumentation/servlet/http/HttpServletResponseInstrumentation.java

@@ -0,0 +1,105 @@
+package datadog.trace.instrumentation.servlet.http;
+
+import static datadog.trace.agent.tooling.ByteBuddyElementMatchers.safeHasSuperType;
+import static datadog.trace.instrumentation.api.AgentTracer.activateSpan;
+import static datadog.trace.instrumentation.api.AgentTracer.activeSpan;
+import static datadog.trace.instrumentation.api.AgentTracer.propagate;
+import static datadog.trace.instrumentation.api.AgentTracer.startSpan;
+import static datadog.trace.instrumentation.servlet.ServletRequestSetter.SETTER;
+import static datadog.trace.instrumentation.servlet.http.HttpServletResponseDecorator.DECORATE;
+import static java.util.Collections.singletonMap;
+import static net.bytebuddy.matcher.ElementMatchers.isInterface;
+import static net.bytebuddy.matcher.ElementMatchers.named;
+import static net.bytebuddy.matcher.ElementMatchers.not;
+
+import com.google.auto.service.AutoService;
+import datadog.trace.agent.tooling.Instrumenter;
+import datadog.trace.api.DDTags;
+import datadog.trace.bootstrap.InstrumentationContext;
+import datadog.trace.instrumentation.api.AgentScope;
+import datadog.trace.instrumentation.api.AgentSpan;
+import java.util.Map;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+import net.bytebuddy.asm.Advice;
+import net.bytebuddy.description.method.MethodDescription;
+import net.bytebuddy.description.type.TypeDescription;
+import net.bytebuddy.matcher.ElementMatcher;
+
+@AutoService(Instrumenter.class)
+public final class HttpServletResponseInstrumentation extends Instrumenter.Default {
+  public HttpServletResponseInstrumentation() {
+    super("servlet", "servlet-response");
+  }
+
+  @Override
+  public boolean defaultEnabled() {
+    return false;
+  }
+
+  @Override
+  public String[] helperClassNames() {
+    return new String[] {
+      "datadog.trace.instrumentation.servlet.ServletRequestSetter",
+      "datadog.trace.agent.decorator.BaseDecorator",
+      packageName + ".HttpServletResponseDecorator",
+    };
+  }
+
+  @Override
+  public ElementMatcher<TypeDescription> typeMatcher() {
+    return not(isInterface())
+        .and(safeHasSuperType(named("javax.servlet.http.HttpServletResponse")));
+  }
+
+  @Override
+  public Map<? extends ElementMatcher<? super MethodDescription>, String> transformers() {
+    return singletonMap(named("sendError").or(named("sendRedirect")), SendAdvice.class.getName());
+  }
+
+  @Override
+  public Map<String, String> contextStore() {
+    return singletonMap(
+        "javax.servlet.http.HttpServletResponse", "javax.servlet.http.HttpServletRequest");
+  }
+
+  public static class SendAdvice {
+
+    @Advice.OnMethodEnter(suppress = Throwable.class)
+    public static AgentScope start(
+        @Advice.Origin("#m") final String method, @Advice.This final HttpServletResponse resp) {
+      if (activeSpan() == null) {
+        // Don't want to generate a new top-level span
+        return null;
+      }
+
+      final HttpServletRequest req =
+          InstrumentationContext.get(HttpServletResponse.class, HttpServletRequest.class).get(resp);
+      if (req == null) {
+        // Missing the response->request linking... probably in a wrapped instance.
+        return null;
+      }
+
+      final AgentSpan span = startSpan("servlet.response");
+      DECORATE.afterStart(span);
+
+      span.setTag(DDTags.RESOURCE_NAME, "HttpServletResponse." + method);
+
+      // In case we lose context, inject trace into to the request.
+      propagate().inject(span, req, SETTER);
+
+      return activateSpan(span, true).setAsyncPropagation(true);
+    }
+
+    @Advice.OnMethodExit(onThrowable = Throwable.class, suppress = Throwable.class)
+    public static void stopSpan(
+        @Advice.Enter final AgentScope scope, @Advice.Thrown final Throwable throwable) {
+      if (scope == null) {
+        return;
+      }
+      DECORATE.onError(scope, throwable);
+      DECORATE.beforeFinish(scope);
+      scope.close();
+    }
+  }
+}

dd-java-agent/instrumentation/servlet/src/test/groovy/FilterTest.groovy

@@ -0,0 +1,105 @@
+import datadog.trace.agent.test.AgentTestRunner
+import javax.servlet.Filter
+import javax.servlet.FilterChain
+import javax.servlet.FilterConfig
+import javax.servlet.ServletException
+import javax.servlet.ServletRequest
+import javax.servlet.ServletResponse
+
+import static datadog.trace.agent.test.utils.TraceUtils.basicSpan
+import static datadog.trace.agent.test.utils.TraceUtils.runUnderTrace
+
+class FilterTest extends AgentTestRunner {
+  static {
+    System.setProperty("dd.integration.servlet.enabled", "true")
+  }
+
+  def "test doFilter no-parent"() {
+    when:
+    filter.doFilter(null, null, null)
+
+    then:
+    assertTraces(0) {}
+
+    where:
+    filter = new TestFilter()
+  }
+
+  def "test doFilter with parent"() {
+    when:
+    runUnderTrace("parent") {
+      filter.doFilter(null, null, null)
+    }
+
+    then:
+    assertTraces(1) {
+      trace(0, 2) {
+        basicSpan(it, 0, "parent")
+        span(1) {
+          operationName "servlet.filter"
+          resourceName "${filter.class.simpleName}.doFilter"
+          childOf span(0)
+          tags {
+            "component" "java-web-servlet-filter"
+            defaultTags()
+          }
+        }
+      }
+    }
+
+    where:
+    filter << [new TestFilter(), new TestFilter() {}]
+  }
+
+  def "test doFilter exception"() {
+    setup:
+    def ex = new Exception("some error")
+    def filter = new TestFilter() {
+      @Override
+      void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) {
+        throw ex
+      }
+    }
+
+    when:
+    runUnderTrace("parent") {
+      filter.doFilter(null, null, null)
+    }
+
+    then:
+    def th = thrown(Exception)
+    th == ex
+
+    assertTraces(1) {
+      trace(0, 2) {
+        basicSpan(it, 0, "parent", null, ex)
+        span(1) {
+          operationName "servlet.filter"
+          resourceName "${filter.class.simpleName}.doFilter"
+          childOf span(0)
+          errored true
+          tags {
+            "component" "java-web-servlet-filter"
+            defaultTags()
+            errorTags(ex.class, ex.message)
+          }
+        }
+      }
+    }
+  }
+
+  static class TestFilter implements Filter {
+
+    @Override
+    void init(FilterConfig filterConfig) throws ServletException {
+    }
+
+    @Override
+    void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
+    }
+
+    @Override
+    void destroy() {
+    }
+  }
+}

dd-java-agent/instrumentation/servlet/src/test/groovy/HttpServletResponseTest.groovy

@@ -0,0 +1,275 @@
+import datadog.trace.agent.test.AgentTestRunner
+import groovy.servlet.AbstractHttpServlet
+import javax.servlet.ServletOutputStream
+import javax.servlet.http.Cookie
+import javax.servlet.http.HttpServletRequest
+import javax.servlet.http.HttpServletResponse
+import spock.lang.Subject
+
+import static datadog.trace.agent.test.utils.TraceUtils.basicSpan
+import static datadog.trace.agent.test.utils.TraceUtils.runUnderTrace
+
+class HttpServletResponseTest extends AgentTestRunner {
+  static {
+    System.setProperty("dd.integration.servlet.enabled", "true")
+  }
+
+  @Subject
+  def response = new TestResponse()
+  def request = Mock(HttpServletRequest) {
+    getMethod() >> "GET"
+    getProtocol() >> "TEST"
+  }
+
+  def setup() {
+    def servlet = new AbstractHttpServlet() {}
+    // We need to call service so HttpServletAdvice can link the request to the response.
+    servlet.service(request, response)
+  }
+
+  def "test send no-parent"() {
+    when:
+    response.sendError(0)
+    response.sendError(0, "")
+    response.sendRedirect("")
+
+    then:
+    assertTraces(0) {}
+  }
+
+  def "test send with parent"() {
+    when:
+    runUnderTrace("parent") {
+      response.sendError(0)
+      response.sendError(0, "")
+      response.sendRedirect("")
+    }
+
+    then:
+    assertTraces(1) {
+      trace(0, 4) {
+        basicSpan(it, 0, "parent")
+        span(1) {
+          operationName "servlet.response"
+          resourceName "HttpServletResponse.sendRedirect"
+          childOf span(0)
+          tags {
+            "component" "java-web-servlet-response"
+            defaultTags()
+          }
+        }
+        span(2) {
+          operationName "servlet.response"
+          resourceName "HttpServletResponse.sendError"
+          childOf span(0)
+          tags {
+            "component" "java-web-servlet-response"
+            defaultTags()
+          }
+        }
+        span(3) {
+          operationName "servlet.response"
+          resourceName "HttpServletResponse.sendError"
+          childOf span(0)
+          tags {
+            "component" "java-web-servlet-response"
+            defaultTags()
+          }
+        }
+      }
+    }
+  }
+
+  def "test send with exception"() {
+    setup:
+    def ex = new Exception("some error")
+    def response = new TestResponse() {
+      @Override
+      void sendRedirect(String s) {
+        throw ex
+      }
+    }
+    def servlet = new AbstractHttpServlet() {}
+    // We need to call service so HttpServletAdvice can link the request to the response.
+    servlet.service(request, response)
+
+    when:
+    runUnderTrace("parent") {
+      response.sendRedirect("")
+    }
+
+    then:
+    def th = thrown(Exception)
+    th == ex
+
+    assertTraces(1) {
+      trace(0, 2) {
+        basicSpan(it, 0, "parent", null, ex)
+        span(1) {
+          operationName "servlet.response"
+          resourceName "HttpServletResponse.sendRedirect"
+          childOf span(0)
+          errored true
+          tags {
+            "component" "java-web-servlet-response"
+            defaultTags()
+            errorTags(ex.class, ex.message)
+          }
+        }
+      }
+    }
+  }
+
+  static class TestResponse implements HttpServletResponse {
+
+    @Override
+    void addCookie(Cookie cookie) {
+
+    }
+
+    @Override
+    boolean containsHeader(String s) {
+      return false
+    }
+
+    @Override
+    String encodeURL(String s) {
+      return null
+    }
+
+    @Override
+    String encodeRedirectURL(String s) {
+      return null
+    }
+
+    @Override
+    String encodeUrl(String s) {
+      return null
+    }
+
+    @Override
+    String encodeRedirectUrl(String s) {
+      return null
+    }
+
+    @Override
+    void sendError(int i, String s) throws IOException {
+
+    }
+
+    @Override
+    void sendError(int i) throws IOException {
+
+    }
+
+    @Override
+    void sendRedirect(String s) throws IOException {
+
+    }
+
+    @Override
+    void setDateHeader(String s, long l) {
+
+    }
+
+    @Override
+    void addDateHeader(String s, long l) {
+
+    }
+
+    @Override
+    void setHeader(String s, String s1) {
+
+    }
+
+    @Override
+    void addHeader(String s, String s1) {
+
+    }
+
+    @Override
+    void setIntHeader(String s, int i) {
+
+    }
+
+    @Override
+    void addIntHeader(String s, int i) {
+
+    }
+
+    @Override
+    void setStatus(int i) {
+
+    }
+
+    @Override
+    void setStatus(int i, String s) {
+
+    }
+
+    @Override
+    String getCharacterEncoding() {
+      return null
+    }
+
+    @Override
+    ServletOutputStream getOutputStream() throws IOException {
+      return null
+    }
+
+    @Override
+    PrintWriter getWriter() throws IOException {
+      return null
+    }
+
+    @Override
+    void setContentLength(int i) {
+
+    }
+
+    @Override
+    void setContentType(String s) {
+
+    }
+
+    @Override
+    void setBufferSize(int i) {
+
+    }
+
+    @Override
+    int getBufferSize() {
+      return 0
+    }
+
+    @Override
+    void flushBuffer() throws IOException {
+
+    }
+
+    @Override
+    void resetBuffer() {
+
+    }
+
+    @Override
+    boolean isCommitted() {
+      return false
+    }
+
+    @Override
+    void reset() {
+
+    }
+
+    @Override
+    void setLocale(Locale locale) {
+
+    }
+
+    @Override
+    Locale getLocale() {
+      return null
+    }
+  }
+}

dd-java-agent/instrumentation/servlet/src/test/groovy/HttpServletTest.groovy

@@ -0,0 +1,125 @@
+import datadog.trace.agent.test.AgentTestRunner
+import groovy.servlet.AbstractHttpServlet
+import javax.servlet.http.HttpServletRequest
+import javax.servlet.http.HttpServletResponse
+
+import static datadog.trace.agent.test.utils.TraceUtils.basicSpan
+import static datadog.trace.agent.test.utils.TraceUtils.runUnderTrace
+
+class HttpServletTest extends AgentTestRunner {
+  static {
+    System.setProperty("dd.integration.servlet.enabled", "true")
+  }
+
+  def req = Mock(HttpServletRequest) {
+    getMethod() >> "GET"
+    getProtocol() >> "TEST"
+  }
+  def resp = Mock(HttpServletResponse)
+
+  def "test service no-parent"() {
+    when:
+    servlet.service(req, resp)
+
+    then:
+    assertTraces(0) {}
+
+    where:
+    servlet = new TestServlet()
+  }
+
+  def "test service with parent"() {
+    when:
+    runUnderTrace("parent") {
+      servlet.service(req, resp)
+    }
+
+    then:
+    assertTraces(1) {
+      trace(0, 3) {
+        basicSpan(it, 0, "parent")
+        span(1) {
+          operationName "servlet.service"
+          resourceName "HttpServlet.service"
+          childOf span(0)
+          tags {
+            "component" "java-web-servlet-service"
+            defaultTags()
+          }
+        }
+        span(2) {
+          operationName "servlet.doGet"
+          resourceName "${expectedResourceName}.doGet"
+          childOf span(1)
+          tags {
+            "component" "java-web-servlet-service"
+            defaultTags()
+          }
+        }
+      }
+    }
+
+    where:
+    servlet << [new TestServlet(), new TestServlet() {
+      @Override
+      protected void doGet(HttpServletRequest req, HttpServletResponse resp) {
+      }
+    }]
+
+    expectedResourceName = servlet.class.anonymousClass ? servlet.class.name : servlet.class.simpleName
+  }
+
+  def "test service exception"() {
+    setup:
+    def ex = new Exception("some error")
+    def servlet = new TestServlet() {
+      @Override
+      protected void doGet(HttpServletRequest req, HttpServletResponse resp) {
+        throw ex
+      }
+    }
+
+    when:
+    runUnderTrace("parent") {
+      servlet.service(req, resp)
+    }
+
+    then:
+    def th = thrown(Exception)
+    th == ex
+
+    assertTraces(1) {
+      trace(0, 3) {
+        basicSpan(it, 0, "parent", null, ex)
+        span(1) {
+          operationName "servlet.service"
+          resourceName "HttpServlet.service"
+          childOf span(0)
+          errored true
+          tags {
+            "component" "java-web-servlet-service"
+            defaultTags()
+            errorTags(ex.class, ex.message)
+          }
+        }
+        span(2) {
+          operationName "servlet.doGet"
+          resourceName "${servlet.class.name}.doGet"
+          childOf span(1)
+          errored true
+          tags {
+            "component" "java-web-servlet-service"
+            defaultTags()
+            errorTags(ex.class, ex.message)
+          }
+        }
+      }
+    }
+  }
+
+  static class TestServlet extends AbstractHttpServlet {
+    @Override
+    protected void doGet(HttpServletRequest req, HttpServletResponse resp) {
+    }
+  }
+}

dd-java-agent/instrumentation/servlet/src/test/groovy/RequestDispatcherTest.groovy

@@ -0,0 +1,97 @@
+import datadog.trace.agent.test.AgentTestRunner
+import javax.servlet.ServletException
+import javax.servlet.http.HttpServletRequest
+import javax.servlet.http.HttpServletResponse
+
+import static datadog.trace.agent.test.utils.TraceUtils.basicSpan
+import static datadog.trace.agent.test.utils.TraceUtils.runUnderTrace
+
+class RequestDispatcherTest extends AgentTestRunner {
+  static {
+    System.setProperty("dd.integration.servlet.enabled", "true")
+  }
+
+  def dispatcher = new RequestDispatcherUtils(Mock(HttpServletRequest), Mock(HttpServletResponse))
+
+  def "test dispatch no-parent"() {
+    when:
+    dispatcher.forward("")
+    dispatcher.include("")
+
+    then:
+    assertTraces(0) {}
+  }
+
+  def "test dispatcher #method with parent"() {
+    when:
+    runUnderTrace("parent") {
+      dispatcher."$method"(target)
+    }
+
+    then:
+    assertTraces(1) {
+      trace(0, 2) {
+        basicSpan(it, 0, "parent")
+        span(1) {
+          operationName "servlet.$operation"
+          resourceName target
+          childOf span(0)
+          tags {
+            "component" "java-web-servlet-dispatcher"
+            defaultTags()
+          }
+        }
+      }
+    }
+
+    where:
+    operation | method
+    "forward" | "forward"
+    "forward" | "forwardNamed"
+    "include" | "include"
+    "include" | "includeNamed"
+
+    target = "test-$method"
+  }
+
+  def "test dispatcher #method exception"() {
+    setup:
+    def ex = new ServletException("some error")
+    def dispatcher = new RequestDispatcherUtils(Mock(HttpServletRequest), Mock(HttpServletResponse), ex)
+
+    when:
+    runUnderTrace("parent") {
+      dispatcher."$method"(target)
+    }
+
+    then:
+    def th = thrown(ServletException)
+    th == ex
+
+    assertTraces(1) {
+      trace(0, 2) {
+        basicSpan(it, 0, "parent", null, ex)
+        span(1) {
+          operationName "servlet.$operation"
+          resourceName target
+          childOf span(0)
+          errored true
+          tags {
+            "component" "java-web-servlet-dispatcher"
+            defaultTags()
+            errorTags(ex.class, ex.message)
+          }
+        }
+      }
+    }
+
+    where:
+    operation | method
+    "forward" | "forward"
+    "forward" | "forwardNamed"
+    "include" | "include"
+    "include" | "includeNamed"
+
+    target = "test-$method"
+  }
+}

dd-java-agent/instrumentation/servlet/src/test/groovy/RequestDispatcherUtils.java

@@ -0,0 +1,181 @@
+import java.io.IOException;
+import java.io.InputStream;
+import java.net.MalformedURLException;
+import java.net.URL;
+import java.util.Enumeration;
+import java.util.Set;
+import javax.servlet.RequestDispatcher;
+import javax.servlet.Servlet;
+import javax.servlet.ServletContext;
+import javax.servlet.ServletException;
+import javax.servlet.ServletRequest;
+import javax.servlet.ServletResponse;
+
+public class RequestDispatcherUtils {
+  private final ServletRequest req;
+  private final ServletResponse resp;
+  private final ServletException toThrow;
+
+  public RequestDispatcherUtils(final ServletRequest req, final ServletResponse resp) {
+    this.req = req;
+    this.resp = resp;
+    toThrow = null;
+  }
+
+  public RequestDispatcherUtils(
+      final ServletRequest req, final ServletResponse resp, final ServletException toThrow) {
+    this.req = req;
+    this.resp = resp;
+    this.toThrow = toThrow;
+  }
+
+  /* RequestDispatcher can't be visible to groovy otherwise things break, so everything is
+   * encapsulated in here where groovy doesn't need to access it.
+   */
+
+  void forward(final String target) throws ServletException, IOException {
+    new TestContext().getRequestDispatcher(target).forward(req, resp);
+  }
+
+  void include(final String target) throws ServletException, IOException {
+    new TestContext().getRequestDispatcher(target).include(req, resp);
+  }
+
+  void forwardNamed(final String target) throws ServletException, IOException {
+    new TestContext().getNamedDispatcher(target).forward(req, resp);
+  }
+
+  void includeNamed(final String target) throws ServletException, IOException {
+    new TestContext().getNamedDispatcher(target).include(req, resp);
+  }
+
+  class TestContext implements ServletContext {
+    @Override
+    public ServletContext getContext(final String s) {
+      return null;
+    }
+
+    @Override
+    public int getMajorVersion() {
+      return 0;
+    }
+
+    @Override
+    public int getMinorVersion() {
+      return 0;
+    }
+
+    @Override
+    public String getMimeType(final String s) {
+      return null;
+    }
+
+    @Override
+    public Set getResourcePaths(final String s) {
+      return null;
+    }
+
+    @Override
+    public URL getResource(final String s) throws MalformedURLException {
+      return null;
+    }
+
+    @Override
+    public InputStream getResourceAsStream(final String s) {
+      return null;
+    }
+
+    @Override
+    public RequestDispatcher getRequestDispatcher(final String s) {
+      return new TestDispatcher();
+    }
+
+    @Override
+    public RequestDispatcher getNamedDispatcher(final String s) {
+      return new TestDispatcher();
+    }
+
+    @Override
+    public Servlet getServlet(final String s) throws ServletException {
+      return null;
+    }
+
+    @Override
+    public Enumeration getServlets() {
+      return null;
+    }
+
+    @Override
+    public Enumeration getServletNames() {
+      return null;
+    }
+
+    @Override
+    public void log(final String s) {}
+
+    @Override
+    public void log(final Exception e, final String s) {}
+
+    @Override
+    public void log(final String s, final Throwable throwable) {}
+
+    @Override
+    public String getRealPath(final String s) {
+      return null;
+    }
+
+    @Override
+    public String getServerInfo() {
+      return null;
+    }
+
+    @Override
+    public String getInitParameter(final String s) {
+      return null;
+    }
+
+    @Override
+    public Enumeration getInitParameterNames() {
+      return null;
+    }
+
+    @Override
+    public Object getAttribute(final String s) {
+      return null;
+    }
+
+    @Override
+    public Enumeration getAttributeNames() {
+      return null;
+    }
+
+    @Override
+    public void setAttribute(final String s, final Object o) {}
+
+    @Override
+    public void removeAttribute(final String s) {}
+
+    @Override
+    public String getServletContextName() {
+      return null;
+    }
+  }
+
+  class TestDispatcher implements RequestDispatcher {
+    @Override
+    public void forward(final ServletRequest servletRequest, final ServletResponse servletResponse)
+        throws ServletException, IOException {
+      if (toThrow != null) {
+        throw toThrow;
+      }
+    }
+
+    @Override
+    public void include(final ServletRequest servletRequest, final ServletResponse servletResponse)
+        throws ServletException, IOException {
+      if (toThrow != null) {
+        throw toThrow;
+      }
+    }
+  }
+}