Skip storing the dispatcher target when the servlet context returns no dispatcher (quick fix)

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

@@ -55,6 +55,10 @@ public static class RequestDispatcherTargetAdvice {
     public static void saveTarget(
         @Advice.Argument(0) final String target,
         @Advice.Return final RequestDispatcher dispatcher) {
+      if (dispatcher == null) {
+        // containers return no dispatcher for an unknown path or servlet name
+        return;
+      }
       InstrumentationContext.get(RequestDispatcher.class, String.class).put(dispatcher, target);
     }
   }

dd-java-agent/instrumentation/servlet/javax-servlet/javax-servlet-common/src/test/java/ServletContextNullDispatcherTest.java

@@ -0,0 +1,142 @@
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+import datadog.trace.agent.test.AbstractInstrumentationTest;
+import java.io.InputStream;
+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 org.junit.jupiter.api.Test;
+
+/**
+ * Servlet containers return a {@code null} dispatcher for unknown paths or servlet names (e.g.
+ * Tomcat's {@code ApplicationContext}); the instrumentation must not try to store context against a
+ * {@code null} key. Instrumentation errors fail the test via {@link AbstractInstrumentationTest}.
+ */
+class ServletContextNullDispatcherTest extends AbstractInstrumentationTest {
+
+  @Test
+  void getRequestDispatcherReturningNull() {
+    assertNull(new NullDispatcherContext().getRequestDispatcher("unknown"));
+  }
+
+  @Test
+  void getNamedDispatcherReturningNull() {
+    assertNull(new NullDispatcherContext().getNamedDispatcher("unknown"));
+  }
+
+  /** {@link ServletContext} that has no dispatchers, like Tomcat for an unknown target. */
+  static class NullDispatcherContext implements ServletContext {
+    @Override
+    public RequestDispatcher getRequestDispatcher(String path) {
+      return null;
+    }
+
+    @Override
+    public RequestDispatcher getNamedDispatcher(String name) {
+      return null;
+    }
+
+    @Override
+    public ServletContext getContext(String uripath) {
+      return null;
+    }
+
+    @Override
+    public int getMajorVersion() {
+      return 2;
+    }
+
+    @Override
+    public int getMinorVersion() {
+      return 3;
+    }
+
+    @Override
+    public String getMimeType(String file) {
+      return null;
+    }
+
+    @Override
+    public Set getResourcePaths(String path) {
+      return null;
+    }
+
+    @Override
+    public URL getResource(String path) {
+      return null;
+    }
+
+    @Override
+    public InputStream getResourceAsStream(String path) {
+      return null;
+    }
+
+    @Override
+    public Servlet getServlet(String name) {
+      return null;
+    }
+
+    @Override
+    public Enumeration getServlets() {
+      return null;
+    }
+
+    @Override
+    public Enumeration getServletNames() {
+      return null;
+    }
+
+    @Override
+    public void log(String msg) {}
+
+    @Override
+    public void log(Exception exception, String msg) {}
+
+    @Override
+    public void log(String message, Throwable throwable) {}
+
+    @Override
+    public String getRealPath(String path) {
+      return null;
+    }
+
+    @Override
+    public String getServerInfo() {
+      return null;
+    }
+
+    @Override
+    public String getInitParameter(String name) {
+      return null;
+    }
+
+    @Override
+    public Enumeration getInitParameterNames() {
+      return null;
+    }
+
+    @Override
+    public Object getAttribute(String name) {
+      return null;
+    }
+
+    @Override
+    public Enumeration getAttributeNames() {
+      return null;
+    }
+
+    @Override
+    public void setAttribute(String name, Object object) {}
+
+    @Override
+    public void removeAttribute(String name) {}
+
+    @Override
+    public String getServletContextName() {
+      return null;
+    }
+  }
+}