@WebListener does not work in a native image without additional reflection hints

[oliveryasuna]

Reproducible with Spring Boot 3.1.0.

1. Define a minimal servlet listener:

@WebListener
public class MyServletContextListener implements ServletContextListener {
}

2. Add @ServletComponentScan to a configuration.

If you run the application from main, everything works as expected. However, if you build a native image with GraalVM, you get an exception:

java.lang.ClassNotFoundException: com.oliveryasuna.MyServletContextListener
        at org.springframework.boot.web.embedded.tomcat.TomcatEmbeddedWebappClassLoader.loadClass(TomcatEmbeddedWebappClassLoader.java:72)
        at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1245)
        at org.apache.catalina.core.DefaultInstanceManager.loadClass(DefaultInstanceManager.java:490)
        at org.apache.catalina.core.DefaultInstanceManager.loadClassMaybePrivileged(DefaultInstanceManager.java:472)
        at org.apache.catalina.core.DefaultInstanceManager.newInstance(DefaultInstanceManager.java:142)
        at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4381)
        at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:4892)
        at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:183)
        at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1332)
        at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1322)
        at java.base@17.0.5/java.util.concurrent.FutureTask.run(FutureTask.java:264)
        at org.apache.tomcat.util.threads.InlineExecutorService.execute(InlineExecutorService.java:75)
        at java.base@17.0.5/java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:145)
        at org.apache.catalina.core.ContainerBase.startInternal(ContainerBase.java:871)
        at org.apache.catalina.core.StandardHost.startInternal(StandardHost.java:846)
        at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:183)
        at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1332)
        at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1322)
        at java.base@17.0.5/java.util.concurrent.FutureTask.run(FutureTask.java:264)
        at org.apache.tomcat.util.threads.InlineExecutorService.execute(InlineExecutorService.java:75)
        at java.base@17.0.5/java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:145)
        at org.apache.catalina.core.ContainerBase.startInternal(ContainerBase.java:871)
        at org.apache.catalina.core.StandardEngine.startInternal(StandardEngine.java:241)
        at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:183)
        at org.apache.catalina.core.StandardService.startInternal(StandardService.java:428)
        at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:183)
        at org.apache.catalina.core.StandardServer.startInternal(StandardServer.java:913)
        at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:183)
        at org.apache.catalina.startup.Tomcat.start(Tomcat.java:485)
        at org.springframework.boot.web.embedded.tomcat.TomcatWebServer.initialize(TomcatWebServer.java:123)
        at org.springframework.boot.web.embedded.tomcat.TomcatWebServer.<init>(TomcatWebServer.java:104)
        at org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory.getTomcatWebServer(TomcatServletWebServerFactory.java:489)
        at org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory.getWebServer(TomcatServletWebServerFactory.java:211)
        at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.createWebServer(ServletWebServerApplicationContext.java:183)
        at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.onRefresh(ServletWebServerApplicationContext.java:161)
        at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:602)
        at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:146)
        at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:733)
        at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:435)
        at org.springframework.boot.SpringApplication.run(SpringApplication.java:311)
        at org.springframework.boot.SpringApplication.run(SpringApplication.java:1305)
        at org.springframework.boot.SpringApplication.run(SpringApplication.java:1294)
        at com.oliveryasuna.Application.main(Application.java:21)

I assume that this issue would present itself on all servlet component types: @WebFilter, @WebServlet, and @WebListener. I tried adding a reflection runtime hint, but this didn't work:

hints.reflection()
          .registerType(
              TypeReference.of(MyServletContextListener.class),
              MemberCategory.INVOKE_DECLARED_CONSTRUCTORS
          );

[bclozel]

This works for me with the following configuration:

@SpringBootApplication
@ServletComponentScan
@ImportRuntimeHints(TomcatnativeApplication.WebListenerHintsRegistrar.class)
public class TomcatnativeApplication {

    public static void main(String[] args) {
        SpringApplication.run(TomcatnativeApplication.class, args);
    }

    static class WebListenerHintsRegistrar implements RuntimeHintsRegistrar {
        @Override
        public void registerHints(RuntimeHints hints, ClassLoader classLoader) {
            hints.reflection().registerType(MyServletContextListener.class, MemberCategory.INVOKE_DECLARED_CONSTRUCTORS);
        }
    }
}

Additional reflection might be required (typically, invocation of public methods) - this depends on how the Servlet container is calling those methods.

Arguably, @ServletComponentScan and the ServletComponentScanRegistrar could automatically register runtime hints for those classes. I'm moving this issue to Spring Boot so this enhancement can be considered.

[oliveryasuna]

@bclozel Thank you. Not sure why wrapping in TypeReference.of didn't work.

[bclozel]

@oliveryasuna most probably your hint was not called at all during the AOT phase and there's a setup error that doesn't register it properly.

[wilkinsona]

I don't think @WebServlet and @WebFilter are affected due to the way that they're registered. Their registration is strongly typed so there's no reflection involved and Graal can discover the classes during image compilation. @WebListener is different as the listeners are handled as string class names. For example, this is how they're registered with Tomcat:

spring-boot/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/tomcat/TomcatServletWebServerFactory.java

Lines 399 to 401 in b645eb3

	for (String webListenerClassName : getWebListenerClassNames()) {
	context.addApplicationListener(webListenerClassName);
	}

It then loads the classes using reflection.

[snicoll]

We could probably rework ServletComponentScanRegistrar to be an AOT contributor. That would prevent the need for a dedicated exclude filter.