Sample Code of Dependency Injection

This repository is a beginner-friendly sample that implements a minimal dependency-injection (DI) container using jakarta.inject annotations (@Named / @Inject).
It is designed so you can follow where objects are created and how dependencies are injected.

What This Sample Shows

• Automatically scanning classes annotated with @Named
• Creating instances with a no-arg constructor
• Injecting dependencies into fields annotated with @Inject
• A simple Controller → Service → Repository flow

Prerequisites

• Java 25 (aligned with pom.xml)
• Maven

Check your Java version:

java -version

Check your Maven version:

mvn -version

Quick Start

1. Resolve dependencies and build

mvn -DskipTests package

2. Run the field-injection sample

mvn -q -DskipTests exec:java -Dexec.mainClass=dev.sobue.sample.di.field.FieldInjectionApplication

If you see logs starting with start app and a result = ... line, the run succeeded.

3. Run the constructor-injection sample

mvn -q -DskipTests exec:java -Dexec.mainClass=dev.sobue.sample.di.constructor.ConstructorInjectionApplication

Run Tests

mvn test

Key Files

• src/main/java/dev/sobue/sample/di/field/FieldInjectionApplication.java
  Entry point. Initializes the DI container and invokes the controller.
• src/main/java/dev/sobue/sample/di/field/container/Context.java
  The DI container implementation: scanning, instantiation, and injection.
• src/main/java/dev/sobue/sample/di/field/controller / service / repository
  Sample layer implementations.
• src/main/java/dev/sobue/sample/di/constructor/ConstructorInjectionApplication.java
  Entry point for constructor injection.
• src/main/java/dev/sobue/sample/di/constructor/container/Context.java
  Constructor-injection container implementation.
• src/main/java/dev/sobue/sample/di/constructor/controller / service / repository
  Constructor-injection sample layers.
• src/test/java/dev/sobue/sample/di/field
  Unit tests.

How the Container Works (High-Level)

• Context.initialize(...) scans the target package for @Named classes and registers instances.
• It then injects dependencies into fields annotated with @Inject, resolved by type.
• If multiple candidates match the same type, an exception is thrown (this is a simple container).

Common Errors

• no default constructor
  The @Named class must have a no-arg constructor.
• not single object
  Multiple implementations for the same type were registered. Adjust the resolution logic in Context.getBean(...).

References (Official Docs)

• JDK 25 Documentation: https://docs.oracle.com/en/java/javase/25/
• Jakarta Dependency Injection: https://jakarta.ee/specifications/dependency-injection/
• Jakarta EE Specifications: https://jakarta.ee/specifications/

CI Status

• SonarCloud
• GitHub Actions verifies Maven on JDK 25 and JDK 26.
• SonarCloud analysis runs on JDK 25 when the SONAR_TOKEN secret is configured.

Update Properties

mvn versions:update-properties -DgenerateBackupPoms=false -Dproperties=jakarta.inject-api.version,commons-codec.version,logback.version,lombok.version,junit.version,mockito.version,maven-clean-plugin.version,maven-enforcer-plugin.version,maven-checkstyle-plugin.version,maven-resources-plugin.version,maven-compiler-plugin.version,maven-surefire-plugin.version,maven-source-plugin.version,maven-site-plugin.version,maven-jxr-plugin.version,maven-project-info-reports-plugin.version,maven-pmd-plugin.version,maven-javadoc-plugin.version,jacoco-maven-plugin.version,clover-maven-plugin.version,spotbugs-maven-plugin.version,sonar-maven-plugin.version,checkstyle.version,pmd.version