[FIXED] Spring Boot implementation as Java9/Jigsaw Module

Issue

I have created a simple Spring Boot Interceptor class with a Config class to add that interceptor to the list of interceptors. This is inside a module of a project.

This is managed through Maven and the strufture is similar to:

.
├── interceptors
│   ├── pom.xml
│   ├── src
│   │   ├── main
│   │   │   ├── java
│   │   │   │   ├── com
│   │   │   │   │   └── mypackage
│   │   │   │   │      ├── config
│   │   │   │   │      │   ├── MyConfig.java
│   │   │   │   │      │   └── package-info.java
│   │   │   │   │      └── interceptors
│   │   │   │   │          ├── MyInterceptor.java
│   │   │   │   │          └── package-info.java
│   │   │   │   └── module-info.java
│   │   └── test
│   └── target
├── pom.xml
└── target

The project uses Maven and has some external Spring dependencies.

I have the below imports on my classes:

MyInterceptor:

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.util.UUID;

import org.slf4j.MDC;
import org.springframework.lang.NonNull;
import org.springframework.stereotype.Component;
import org.springframework.web.servlet.HandlerInterceptor;

MyConfig:

import org.springframework.context.annotation.Configuration;
import org.springframework.web.servlet.config.annotation.InterceptorRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;

import com.mypackage.interceptors.MyInterceptor;

The code works and gets compiled without issues both in the IDE and using Maven.

I have added a module-info.java file and imports aren't working anymore. Consequently, the project won't compile.

To fix the imports, I have added the below to the module-info.java file:

module interceptors {
    requires slf4j.api;
    requires spring.core;
    requires spring.webmvc;
    requires spring.context;
    requires org.apache.tomcat.embed.core;

    exports com.mypackage.config;
    exports com.mypackage.interceptors;
}

After doing this, the imports won't show up as red anymore, but when I do a maven clean install, I get these errors:

module not found: slf4j.api
module not found: spring.core
module not found: spring.webmvc
module not found: spring.context
module not found: org.apache.tomcat.embed.core

If I add them as maven dependencies, the behavior is still the same.

Can someone please explain to me in plain English what am I doing wrong and how to fix this? I am new to this project Jigsaw.

Use case: This module will later be added to another Spring Boot project and used as dependency there for the auto-configuration of beans with external configuration source. So, I am writing this interceptor class so that every Spring project will use it instead of copy-pasting the implementation. I assume I need this module-info.java to expose it to other packages. Please correct me if I'm wrong.

Solution

This module will later be added to another Spring Boot project and used as dependency there using modules in a spring boot app does usually make no sense because you end up with a single jar file. module-info are typically needed/used for app which comprise of several jar files or jar's which are used as a library.

All spring/spring boot jars are not yet java modules (which means they don't have any module-info.class file...but they have an automatic-module-name entry in MANIFEST.MF) and tomcat-embed does not even contain an automatic-module-name.

Adding a maven dependency does not mean you can use a dependency as a java module...it's needed having at least an automatic-module-name etc

Answered By - khmarbaise
Answer Checked By - Marilyn (JavaFixing Volunteer)