[[Implicit_module_dependencies_for_deployments]]
= Implicit module dependencies for deployments

As explained in the <<Class_Loading_in_WildFly,Class Loading in WildFly>> article,
WildFly {wildflyVersion} is based on module classloading. A class within a module B
isn't visible to a class within a module A, unless module B adds a
dependency on module A. Module dependencies can be explicitly (as
explained in that classloading article) or can be "implicit". This
article will explain what implicit module dependencies mean and how,
when and which modules are added as implicit dependencies.

[[whats-an-implicit-module-dependency]]
== What's an implicit module dependency?

Consider an application deployment which contains Jakarta Enterprise Beans. Jakarta Enterprise Beans typically
need access to classes from the javax.ejb.* package and other Jakarta EE
API packages. The jars containing these packages are already shipped in
WildFly and are available as "modules". The module which contains the
javax.ejb.* classes has a specific name and so does the module which
contains all the Jakarta EE API classes. For an application to be able to
use these classes, it has to add a dependency on the relevant modules.
Forcing the application developers to add module dependencies like these
(i.e. dependencies which can be "inferred") isn't a productive approach.
Hence, whenever an application is being deployed, the deployers within
the server, which are processing this deployment "implicitly" add these
module dependencies to the deployment so that these classes are visible
to the deployment at runtime. This way the application developer doesn't
have to worry about adding them explicitly. How and when these implicit
dependencies are added is explained in the next section.

[[how-and-when-is-an-implicit-module-dependency-added]]
== How and when is an implicit module dependency added?

When a deployment is being processed by the server, it goes through a
chain of "deployment processors". Each of these processors will have a
way to check if the deployment meets a certain criteria and if it does,
the deployment processor adds an implicit module dependency to that
deployment. Let's take an example - Consider (again) an Jakarta Enterprise Beans 3 deployment
which has the following class:

.MySuperDuperBean.java

[source,java,options="nowrap"]
----
@Stateless
public class MySuperDuperBean {
 
...
 
}
----

As can be seen, we have a simple @Stateless Jakarta Enterprise Beans. When the deployment
containing this class is being processed, the Jakarta Enterprise Beans deployment processor
will see that the deployment contains a class with the @Stateless
annotation and thus identifies this as an Jakarta Enterprise Beans deployment. *This is just
one of the several ways, various deployment processors can identify a
deployment of some specific type.* The Jakarta Enterprise Beans deployment processor will
then add an implicit dependency on the Jakarta EE API module, so that all
the Jakarta EE API classes are visible to the deployment.

Some subsystems will always add API classes, even if the trigger
condition is not met. These are listed separately below.

In the next section, we'll list down the implicit module dependencies
that are added to a deployment, by various deployers within WildFly.

[[which-are-the-implicit-module-dependencies]]
== Which are the implicit module dependencies?

[cols=",,,",,options="header"]
|=======================================================================
|Subsystem responsible for adding the implicit dependency |Dependencies
that are always added |Dependencies that are added if a trigger
condition is met |Trigger which leads to the implicit module dependency
being added

|Core Server |javax.api sun.jdk org.jboss.vfs |  | 

|Batch Subsystem |javax.batch.api |  | implicit

|EE Subsystem |javaee.api |  | 

|Jakarta Enterprise Beans 3 subsystem |  |javaee.api |The presence of ejb-jar.xml (in valid
locations in the deployment, as specified by spec) or the presence of
annotation based Jakarta Enterprise Beans (ex: @Stateless, @Stateful, @MessageDriven etc)

|Jakarta RESTful Web Services (Resteasy) subsystem |javax.xml.bind.api
|org.jboss.resteasy.resteasy-atom-provider
org.jboss.resteasy.resteasy-cdi org.jboss.resteasy.resteasy-jaxrs
org.jboss.resteasy.resteasy-jaxb-provider
org.jboss.resteasy.resteasy-jackson2-provider
org.jboss.resteasy.resteasy-jsapi
org.jboss.resteasy.resteasy-multipart-provider
org.jboss.resteasy.async-http-servlet-30 |The presence of Jakarta RESTful Web Services
annotations in the deployment

|Jakarta Connectors subsystem |javax.resource.api |javax.jms.api javax.validation.api
org.jboss.logging org.jboss.ironjacamar.api org.jboss.ironjacamar.impl
org.hibernate.validator |If the deployment is a resource adaptor (RAR)
deployment.

|Jakarta Persistence (Hibernate) subsystem |javax.persistence.api |javaee.api
org.jboss.as.jpa org.hibernate |The presence of an @PersistenceUnit or
@PersistenceContext annotation, or a <persistence-unit-ref> or
<persistence-context-ref> in a deployment descriptor..

|Logging Subsystem |org.jboss.logging org.apache.commons.logging
org.apache.log4j org.apache.logging.log4j.api org.slf4j org.jboss.logging.jul-to-slf4j-stub |  | 

|SAR Subsystem |  |org.jboss.logging org.jboss.modules |The deployment
is a SAR archive

|Security Subsystem |org.picketbox |  | 

|Web Subsystem |  |javaee.api com.sun.jsf-impl org.hibernate.validator
org.jboss.as.web org.jboss.logging |The deployment is a WAR archive. Jakarta Server Faces
is only added if used. Multiple version options exist for mojarra.

|Web Services Subsystem |org.jboss.ws.api org.jboss.ws.spi |  | 

|Weld (CDI) Subsystem |  |javax.persistence.api javaee.api org.javassist
org.jboss.interceptor org.jboss.as.weld org.jboss.logging
org.jboss.weld.core org.jboss.weld.api org.jboss.weld.spi |If a
beans.xml file is detected in the deployment
|=======================================================================
