Clarify documentation on using a Spring Boot app as a dependency

Closes gh-6812

Author: wilkinsona

spring-boot-docs/src/main/asciidoc/build-tool-plugins.adoc

@@ -373,10 +373,10 @@ want the other Boot features but not this one)
 
 |`classifier`
 |A file name segment (before the extension) to add to the archive, so that the original is
- preserved in its original location. Defaults to null in which case the archive is
+ preserved in its original location. Defaults to `null` in which case the archive is
  repackaged in place. The default is convenient for many purposes, but if you want to use
- the original jar as a dependency in another project, it's best to use an extension to
- define the executable archive.
+ the original jar as a dependency in another project you must use a classifier to define
+ the executable archive.
 
 |`withJarTask`
 |The name or value of the `Jar` task (defaults to all tasks of type `Jar`) which is used

spring-boot-docs/src/main/asciidoc/howto.adoc

@@ -2643,14 +2643,24 @@ details.
 
 
 [[howto-create-an-additional-executable-jar]]
-=== Create an additional executable JAR
-If you want to use your project as a library jar for other projects to depend on, and in
-addition have an executable (e.g. demo) version of it, you will want to configure the
-build in a slightly different way.
+=== Use a Spring Boot application as a dependency
+Like a war file, a Spring Boot application is not intended to be used as a dependency. If
+your application contains classes that you want to share with other projects, the
+recommended approach is to move that code into a separate module. The separate module can
+then be depended upon by your application and other projects.
 
-For Maven the normal JAR plugin and the Spring Boot plugin both have a '`classifier`'
-configuration that you can add to create an additional JAR. Example (using the Spring
-Boot Starter Parent to manage the plugin versions and other configuration defaults):
+If you cannot rearrange your code as recommended above, Spring Boot's Maven and Gradle
+plugins must be configured to produce a separate artifact that is suitable for use as a
+dependency. The executable archive cannot be used as a dependency as the
+<<appendix-executable-jar-format.adoc#executable-jar-jar-file-structure,exectuable jar
+format>> packages application classes in `BOOT-INF/classes`. This means
+that they cannot be found when the executable jar is used as a dependency.
+
+To produce the two artifacts, one that can be used as a dependency and one that is
+executable, a classifier must be specified. This classifier is applied to the name of the
+executable archive, leaving the default archive for use as dependency.
+
+To configure a classifier of `exec` in Maven, the following configuration can be used:
 
 [source,xml,indent=0,subs="verbatim,quotes,attributes"]
 ----
@@ -2667,10 +2677,7 @@ Boot Starter Parent to manage the plugin versions and other configuration defaul
 	</build>
 ----
 
-Two jars are produced, the default one, and an executable one using the Boot plugin with
-classifier '`exec`'.
-
-For Gradle users the steps are similar. Example:
+And when using Gradle, the following configuration can be used:
 
 [source,groovy,indent=0,subs="verbatim,attributes"]
 ----