Java 9 Experiments

Modules

The project consists of two modules:

• provider - it has an interface in api.MyApi with some methods. It also has an implementation in its implementation package.
• consumer - it wants to use an implementation of the interface api.MyApi from the provider. It also has a main method that you will need to execute when fiddling about with the module-info.java files.

The project itself happens to be a Maven Multi module project.

Things to notice in the provider module

The two classes in the helper package are public. Also the implementation of the api.MyApi found in the implementation package is public.

The module-info.java file:

• exports the api package - if removed the api.MyApi class would be hidden to other modules meaning other modules would not compile.
• provides the api.MyApi with the class implementation.MyApiImpl - if removed other the implementing class MyApiImpl would not be "available" to other modules meaning the MyApi.class would fail to load.

I write "available" in quotes because though the MyApi can now be resolved by other modules using the ServiceLoader but doing a new new MyApiImpl() in the consumer wont work. So the implementation of the interface is hidden from consumers - though it is public in the module where it is declared.

What happens if I remove the exports api in the module-info.java?

The provider compiles file but the consumer fails to compile with these errors:

This relates to the module-info.java in the consumer module.

[ERROR] <PATH>/module-info.java:[3,10] package api is not visible
[ERROR] (package api is declared in module provider, which does not export it)

And an error in all files that use the api package (in my case MyService:)

[ERROR] <PATH>/MyService.java:[5,8] package api is not visible
[ERROR] (package api is declared in module provider, which does not export it)

What happens if I remove the provides api.MyApi with implementation.MyApiImpl; in the module-info.java?

All things compile, but the ServiceLoader in MyService in the consumer module will not be able to load MyApi.class as no modules provide an implementation service. This will be discovered at runtime when the consumer tries to load the class.

Things to notice in the consumer module

The module-info.java file:

• requires the provider module.
• uses the api.MyApi.

What happens if I remove the requires provider in the module-info.java?

When I recompile I'll be met with one error relating to module-info.java file:

[ERROR] <PATH>/module-info.java:[2,10] package api is not visible
[ERROR] (package api is declared in module provider, but module consumer does not read it)

And an error in all files that use the api package (in my case MyService:)

[ERROR] <PATH>/MyService.java:[5,8] package api is not visible
[ERROR] (package api is declared in module provider, but module consumer does not read it)

What happens if I remove the uses api.MyApi in the module-info.java?

Now the module compiles, hurray. But I'll be met by this runtime exception:

Exception in thread "main" java.util.ServiceConfigurationError: api.MyApi: module consumer does not declare `uses`
	at java.base/java.util.ServiceLoader.fail(ServiceLoader.java:588)
	at java.base/java.util.ServiceLoader.checkCaller(ServiceLoader.java:574)
	at java.base/java.util.ServiceLoader.<init>(ServiceLoader.java:503)
	at java.base/java.util.ServiceLoader.load(ServiceLoader.java:1684)
	at consumer/service.MyService.main(MyService.java:11)

Prerequisites

Obviously you need Java 9. You also new a newer version of Maven. I use 3.5.0.