Jetty does not load any servlets - servlets

Question

I'm using OpenJRE 11 and Jetty 9 as installed on Ubuntu Server LTS. With this source:
$ find .
./build.gradle
./gradlew
./src
./src/main
./src/main/java
./src/main/java/com.example
./src/main/java/com.example/servlet
./src/main/java/com.example/servlet/ExampleServlet.java
./src/main/webapp
./src/main/webapp/WEB-INF
./src/main/webapp/WEB-INF/jetty-web.xml
./src/main/webapp/WEB-INF/web.xml
build.gradle:
plugins {
id 'java'
id 'war'
}
repositories {
jcenter()
}
group 'com.example'
version '0.1'
// For Ubuntu's openjdk-11-jre-headless
sourceCompatibility = 1.11
dependencies {
def jsp_dep = 'org.eclipse.jetty:apache-jsp:9.4.15.v20190215'
if (gradle.startParameter.taskNames.contains('war')) {
// Bundled in Ubuntu's libjetty9-java so not embedded
compileOnly jsp_dep
}
// This should not need to be 'implementation'.
// When hosted in Jetty, this alone is enough to support JSTL. This is a thin reference to
// org.apache.taglibs:taglibs-standard-{spec,impl}:1.2.5
implementation 'org.eclipse.jetty:apache-jstl:9.4.15.v20190215'
}
ExampleServlet:
package com.example.servlet;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
#WebServlet("/example")
public class ExampleServlet extends HttpServlet {
#Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) {
}
}
jetty-web.xml:
<?xml version="1.0" encoding="UTF-8"?>
<!-- configure-9_4 is missing -->
<!DOCTYPE Configure PUBLIC "-//Jetty//Configure//EN" "http://www.eclipse.org/jetty/configure_9_3.dtd">
<Configure class="org.eclipse.jetty.webapp.WebAppContext">
</Configure>
web.xml:
<?xml version="1.0" encoding="UTF-8"?>
<!-- https://download.oracle.com/otn-pub/jcp/servlet-4-final-eval-spec/servlet-4_0_FINAL.pdf
-->
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
version="4.0">
</web-app>
Compilation and installation:
$ ./gradlew war
BUILD SUCCESSFUL in 1s
2 actionable tasks: 2 executed
$ unzip -l build/libs/jetty-example-0.1.war
Archive: build/libs/jetty-example-0.1.war
Length Date Time Name
--------- ---------- ----- ----
0 2019-06-26 14:08 META-INF/
25 2019-06-26 14:08 META-INF/MANIFEST.MF
0 2019-06-26 14:08 WEB-INF/
0 2019-06-26 14:08 WEB-INF/classes/
0 2019-06-26 14:08 WEB-INF/classes/com/
0 2019-06-26 14:08 WEB-INF/classes/com/example/
0 2019-06-26 14:08 WEB-INF/classes/com/example/servlet/
696 2019-06-26 14:08 WEB-INF/classes/com/example/servlet/ExampleServlet.class
0 2019-06-26 14:08 WEB-INF/lib/
12585 2019-05-04 18:20 WEB-INF/lib/apache-jstl-9.4.15.v20190215.jar
40153 2019-04-15 11:56 WEB-INF/lib/taglibs-standard-spec-1.2.5.jar
206430 2019-04-15 11:56 WEB-INF/lib/taglibs-standard-impl-1.2.5.jar
255 2019-06-26 13:55 WEB-INF/jetty-web.xml
444 2019-06-26 14:03 WEB-INF/web.xml
--------- -------
260588 14 files
$ mv jetty-example-0.1.war root.war
$ chmod 644 root.war
# chown jetty:adm root.war
# mv root.war /var/lib/jetty9/webapps/
# ls -la /tmp/systemd-private-*jetty9.service*/tmp/*root.war*
total 12
drwxr-xr-x 3 jetty jetty 4096 Jun 26 14:08 .
drwxrwxrwt 6 root root 4096 Jun 26 14:08 ..
drwxr-xr-x 2 jetty jetty 4096 Jun 26 14:08 jsp
When then accessing 127.0.0.1:8080/example, I get Jetty's default 404 page, leading me to believe that package scanning is not happening by default. In /var/log/syslog I get these messages:
Jun 26 14:07:07 web jetty9[475]: 2019-06-26 14:07:07.980:INFO:oejshC.root:Scanner-0: Warning: No org.apache.tomcat.JarScanner set in ServletContext. Falling back to default JarScanner implementation.
Jun 26 14:07:08 web jetty9[475]: 2019-06-26 14:07:08.162:INFO:oajs.TldScanner:Scanner-0: At least one JAR was scanned for TLDs yet contained no TLDs. Enable debug logging for this logger for a complete list of JARs that were scanned but no TLDs were found in them. Skipping unneeded JARs during scanning can improve startup time and JSP compilation time.
Jun 26 14:07:08 web jetty9[475]: 2019-06-26 14:07:08.204:INFO:oejsh.ContextHandler:Scanner-0: Started o.e.j.w.WebAppContext#156a87be{root,/,file:///var/lib/jetty9/webapps/root/,AVAILABLE}{/root}
Jun 26 14:07:08 web jetty9[475]: 2019-06-26 14:07:08.280:INFO:oejsh.ContextHandler:Scanner-0: Stopped o.e.j.w.WebAppContext#7f3b84b8{root,/,null,UNAVAILABLE}{/root.war}
Jun 26 14:08:29 web jetty9[475]: 2019-06-26 14:08:29.532:INFO:oeja.AnnotationConfiguration:Scanner-0: Scanning elapsed time=28ms
Jun 26 14:08:29 web jetty9[475]: 2019-06-26 14:08:29.564:INFO:oejshC.root:Scanner-0: Warning: No org.apache.tomcat.JarScanner set in ServletContext. Falling back to default JarScanner implementation.
Jun 26 14:08:29 web jetty9[475]: 2019-06-26 14:08:29.725:INFO:oajs.TldScanner:Scanner-0: At least one JAR was scanned for TLDs yet contained no TLDs. Enable debug logging for this logger for a complete list of JARs that were scanned but no TLDs were found in them. Skipping unneeded JARs during scanning can improve startup time and JSP compilation time.
Jun 26 14:08:29 web jetty9[475]: 2019-06-26 14:08:29.782:INFO:oejsh.ContextHandler:Scanner-0: Started o.e.j.w.WebAppContext#42e87462{root,/,file:///var/lib/jetty9/webapps/root/,AVAILABLE}{/root.war}
Why is default jar scanning not working, and how can I fix it?

Answer 1

As it turns out: it isn't package scanning that's broken; it was a conflicting context. Somehow, Jetty thinks that two overlapping contexts should fail silently. In my particular case, there was a default root directory containing a sample web application in Jetty. Deleting that fixed the problem.