How to Create a Multi-Module Spring Boot Application using Java Module

Java is one of the most mature and persistent development languages that exists. Recently, it shifted to a 6-month release schedule, enabling it to deliver more frequent updates to the language. One of the changes introduced in Java 9 was the modular system.

The Java Platform Module System (JPMS) adds two fundamental capabilities when building Java apps:

Reliable configuration - replacing the brittle, error-prone class-path mechanism with a means for program components to declare explicit dependencies upon one another.
Strong encapsulation - allowing a component to declare which of its public types are accessible to other components and which are not.

Packages may be grouped into modules that serve as building blocks in the construction of very large programs. The declaration of a module specifies which other modules (packages, classes, and interfaces) are required to compile and run code.

While we have the visibility modifiers—public, private, protected, and default—they are not enough to provide external visibility. A common example used to illustrate the problem is a “Util” class, this class may be used throughout a library from various packages within a JAR file but is NOT meant to be used outside of the library. JPMS can help with this type of situation.

Table Content

Introduction
Install a Java 9+ JDK
Project Structure
- How to Structure a Modular Project with Maven
Build an Application Without Java Modules
- Create the Persistence Module
- Create the Web Application Module
Secure Your Web Application
- Register an Application on Okta
- Configure the App with Okta Information
Using Java Modules
- Modularize the persistence Library
- Modularize the application Project
Running the Application
Learning More About Java Modular System

Introduction

When Java 9 was created, the JDK went under a major refactoring to modularize its content. It created various modules to organize the contents. Some examples include: java.base, java.sql, and java.xml(along with many others). To date, there are a total of 60 modules in Java 14 JDK.

java.base has fundamental classes like Object, String, Integer, Double, etc. While java.sql has classes related to accessing the JDBC API like ResultSet, Connection and others. Additionally, java.xml has classes related to XML manipulation like XMLStreamReader, XMLStreamWriter and similar classes in that vein.

The modularization enabled the possibility of reducing the Java runtime to include just the java.base if your application only depends on this module. By using the jlink tool that is bundled with the JDK, you can create a micro runtime with only the JDK modules you need.

For the rest of this article, you should have at least some basic understanding of Spring Boot, Maven, and REST web services principles as well as Docker installed on your machine.

Install a Java 9+ JDK

First, you’ll need a Java 9+ JDK in order to use modules. If you have been using Java 8, you’ll likely have to download a separate JDK with a version of 9 or later to be used for this tutorial. This project is set up to use JDK 11 in this tutorial. You can download the JDKs from AdoptOpenJDK. Just make sure your JAVA_HOME environment variable is pointing to that JDK.

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