The jigsaw-module-vs-legacy_service-loader from schuettec

jigsaw-module-vs-legacy_service-loader's Introduction

# Java 9 and the module system

Since Java 9 introduced the new module system, I wondered how module-code can access legacy libraries that are not maintained anymore. This repository contains a working show case that demonstrates how a modular application can access legacy code using a service interface and the service loader feature.

For more information about the service loader facility refer to [Service Loader documentation](https://docs.oracle.com/en/java/javase/14/docs/api/java.base/java/util/ServiceLoader.html).

## Project structure

This project is a modular Maven project. The following modules exist:
```
service-loadeer                   -   The main project folder
\-- service-api                   -   A Java 9 module that exports the service interface 'IncementService'. 
 \-- service-impl                 -   A Java 8 legacy implementation of the service interface. 
  \                                   This code will not reside in a module. 
   \                                  It will be available through the legacy class path.
    \-- service-runner            -   A Java 9 module that declares a dependency to 'service-api' since the runner 
     \                                is used to run the service.
      \                               This module defines the 'requires' directive to 'service-api'.
       \                              This module defines the 'uses' directive to 'IncrementService'.
        \-- service-runner-legacy -   A Java 8 legacy project to run the whole example in the legacy class path (Not used).                  
``` 
## The show case

This project is a working example of a modular Java 9 application, that uses a Java 9 compliant module as service interface declaration. At runtime a legacy JAR is used to provide an instance of the service interface. The application then runs the legacy code "through" the Java 9 service interface.

This way it is possible to isolate legacy libraries and provide their services to Java 9 modular applications.

## The build

This project builds on Travis CI. The proof that it works, is a simple line printed to `System.err` that shows "OK.".

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