[core] Support custom extensions

[aalmiray]

Define a mechanism to load, configure, and execute custom extensions.

Packaging

Extensions may be packaged in 3 ways:

• single JAR file. Either the extension has no dependencies or it's an uberjar.
• zip file. Contains a set of JAR files. This includes the extension plus its dependencies.
• set of JAR files. This includes the extension plus its dependencies.

Location

Extensions should be placed at ${JRELEASER_HOME}/extensions/<extension-name>/. However, this location may be overridden by configuring an explicit value via the DSL

DSL

Here's how it may look like

extensions:
  foo:
    providers:
      - type: com.github.foo.MustacheFunctionProvider
        properties:
          maxLength: 20
  bar:
    directory: /path/to/the/jars
  qux:
    enabled: false

Extension foo will be loaded from ${JRELEASER_HOME}/extensions/foo. It may be packaged as foo-x.y.z.jar, foo.-x.y.z.zip, or as a set of JAR files inside that directory.
It lists an explicit type com.github.foo.MustacheFunctionProvider which requires additional settings upon initialization. These settings are defined using the properties map. There may be more than one provider type hence a list.

Extension bar will be loaded from /path/to/the/jars. It may be packaged as bar-x.y.z.jar, bar.-x.y.z.zip, or as a set of JAR files inside that directory.

Loading Mechanism

Each extension will be loaded and initialized upon bootstrap after resolving the model. Extensions will be treated as singletons per each invocation session. JReleaser will use the ServiceLoader SPI interface to locate, load, and initialize extensions.

[aalmiray]

Also consider an option for declaring GAV coordinates to download an extension and its transitive dependencies.

[aalmiray]

Released in v1.3.0 -> https://github.com/jreleaser/jreleaser/releases/tag/v1.3.0