Maven: Wrapper
The mvnw file that you see in some repos: what is it, and why do you need it
In some repos that use Maven, you’ll find a file called mvnw that appears to be a shell script. Sometimes there is even an mvnw.bat, that appears
to be a Windows/DOS old-school .bat file designed to run on Windows. It may be entirely unclear why this is there, or why you’d need it.
This article is here to clear up that mystery.
What is mvnw
The mvnw file is a “Maven Wrapper”. Simply put, it allows you to use Maven on systems where Maven has not been installed.
Maven is:
- installed on CSIL
- easily installable on Mac, Windows, Linux
You might therefore conclude that you don’t need the mvnw file. And often that’s true. There is at least
one circumstance, however, where we’ve found it to be vital.
Using mvnw on Heroku with Flyway Database migrations
On Heroku, there are various phases of bringing up an application, for example:
- build: it runs the compile commands for your stack (e.g.
mvn compile) and collect all of your dependencies and executables into what Heroku calls a slug - release: usually, if you don’t specify any actions in this phase, nothing happens.
But if you have any actions in the release phase, the release is not made available until these actions succeed. - startup: this is when Heroku actually starts up the application (for a web app, this means starting up your server to listen for connections on a port.)
When running Flyway migrations, for reasons discussed elsewhere (e.g. in our documentation on Flyway), it is preferable to run these migrations during the release phase.
That leads us back to mvnw. The way in which we have chosen to setup Flyway migrations, we are using Maven commands. However,
the Maven that is built into Heroku is only available during the “build” phase. Adding mvnw to a repo makes Maven also available during
the release phase.
The wrapper downloader
The wrapper downloader (e.g. .mvn/wrapper/MavenWrapperDownloader.java) is a file that helps the mvnw command to work properly. You can see an example in this pull request:
You may also need a file called .mvn/wrapper/maven-wrapper.properties.
Related topics:
- Maven: Hello World—A relatively simple Hello World app with Maven
- Maven: Adding custom jar dependency—Including a jar file that isn't available as a standard Maven dependency
- Maven: Convert Ant to Maven—replacing ant build.xml with maven pom.xml
- Maven: FAQ—Frequently Asked Questions
- Maven: Installing—on Windows, Mac, Linux
- Maven: Javadoc—Generating javadoc when using Maven
- Maven: Managing Versions—How do you get the right version for dependencies, plugins, etc.
- Maven: Pom.xml Order—In what order should the elements of a pom.xml appear
- Maven: Profiles—Making your pom.xml do different things on localhost vs heroku, for example
- Maven: Testing—Doing JUnit testing and other testing using mvn test, mvn verify, etc.
- Maven: Wrapper—The mvnw file that you see in some repos: what is it, and why do you need it
- Maven: Xlint options—For example, what to do when you get `Recompile with -Xlint:unchecked for details`