Maven Wrapper

The Maven Wrapper is an easy way to ensure a user of your Maven build has everything necessary to run your Maven build.

Why might this be necessary? Maven to date has been very stable for users, is available on most systems or is easy to procure: but with many of the recent changes in Maven it will be easier for users to have a fully encapsulated build setup provided by the project. With the Maven Wrapper this is very easy to do and it's a great idea borrowed from Gradle.

The easiest way to setup the Maven Wrapper for your project is to use the Takari Maven Plugin with its provided wrapper goal. To add or update all the necessary Maven Wrapper files to your project execute the following command:

mvn -N io.takari:maven:0.7.7:wrapper

Note: The default usage should be mvn -N io.takari:maven:wrapper but for some users this seem to result in usage of an old version of the wrapper and therefore installation of older Maven defaults and so on.

Normally you instruct users to install a specific version of Apache Maven, put it on the PATH and then run the mvn command like the following:

mvn clean install

But now, with a Maven Wrapper setup, you can instruct users to run wrapper scripts:

./mvnw clean install

or on Windows

mvnw.cmd clean install

A normal Maven build will be executed with the one important change that if the user doesn't have the necessary version of Maven specified in .mvn/wrapper/maven-wrapper.properties it will be downloaded for the user first, installed and then used.

Subsequent uses of mvn/mvnw.cmd use the previously downloaded, specific version as needed.

Supported Systems

The wrapper should work on various operating systems including

• Linux (numerous versions, tested on Ubuntu and CentOS)
• OSX / macOS
• Windows (various newer versions)
• Solaris (10 and 11)

For all those *nix operating systems, various shells should work including

• sh
• bash
• dash
• zsh

In terms of Apache Maven versions itself, the wrapper should work with any Maven 3.x version and it defaults to the latest release - currently 3.6.3. We do NOT plan to support the deprecated, EOL'ed Maven 2.x.

The maven-wrapper itself is compiled to work with Java 5. The Takari Maven Plugin for installation however uses Java 7. Once the wrapper is installed with the plugin you should be able to use the wrapper on the project with Java 5 and up. This is however not really tested by the committers.

Changes

Please check out the changelog for more information about our releases.

Verbose Mode

The wrapper supports a verbose mode in which it outputs further information. It is activated by setting the MVNW_VERBOSE environment variable to true.

By default it is off.

Usage without Binary JAR

By default, the Maven Wrapper JAR archive is added to the using project as small binary file .mvn/wrapper/maven-wrapper.jar. It is used to bootstrap the download and invocation of Maven from the wrapper shell scripts.

If your project is not allowed to contain binary files like this, you can configure your version control system to exclude checkin/commit of the wrapper jar.

If the JAR is not found to be available by the scripts they will attempt to download the file from the URL specified in .mvn/wrapper/maven-wrapper.properties under wrapperUrl and put it in place. The download is attempted via curl, wget and, as last resort, by compiling the ./mvn/wrapper/MavenWrapperDownloader.java file and executing the resulting class.

If your Maven repository is password protected you can specify your username via the environment variable MVNW_USERNAME and the password via the environment variable MVNW_PASSWORD.

Using a Different Version of Maven

To switch the version of Maven used to build a project you can initialize it using:

mvn -N io.takari:maven:0.7.7:wrapper -Dmaven=3.5.4

which works for any version except snapshots. Once you have a wrapper you can change its version by setting the distributionUrl in .mvn/wrapper/maven-wrapper.properties, e.g.

distributionUrl=https://repo.maven.apache.org/maven2/org/apache/maven/apache-maven/3.5.4/apache-maven-3.5.4-bin.zip

Using Basic Authentication for Distribution Download

To download Maven from a location that requires Basic Authentication you have 2 options:

1. Set the environment variables MVNW_USERNAME and MVNW_PASSWORD

   or

2. add user and password to the distributionUrl like that: distributionUrl=https://username:password@<yourserver>/maven2/org/apache/maven/apache-maven/3.2.1/apache-maven-3.2.1-bin.zip

Specifying Maven Distribution Base Path

This is a feature of Maven itself and the wrapper just happens to take it into account. Simply set MAVEN_USER_HOME to the desired path and the wrapper uses it as the base of the Maven distro installation.

See https://www.lewuathe.com/maven-wrapper-home.html and takari#17

Using a Maven Repository Manager

When using an internal Maven repository manager you have two options:

1. Just set the correct URL to wrapper jar and Maven distro in maven-wrapper.properties in your project
2. Leave the default URL in the project pointing to Maven Central and set the environment variable MVNW_REPOURL to your repo manager URL such as https://repo.example.com/central-repo-proxy.

If MVNW_REPOURL is set during the wrapper installation with the takari-maven-plugin, the URL is used in the maven-wrapper.properties file.

If not set, but your mirror URL in your settings.xml is configured, it will be used.

Developing and Releasing

To test Maven wrapper usage:

• ensure you are building on a Unix filesystem with correct mounting for executable flag setting
• build the maven-wrapper as snapshot version
• update version in takari-maven-plugin
• build takari-maven-plugin
• use on test project with takari-maven-plugin version

mvn -N -X io.takari:maven:0.7.7-SNAPSHOT:wrapper

For release

• make sure version in mvnw files, MavenWrapperDownloader.java and config files and pom files is new upcoming release
• cut maven release with usual release plugin usage
• update maven plugin to release version
• release it as well

Updating Maven version:

• update URL in maven-wrapper/.mvn/wrapper/maven-wrapper.properties
• update URL in MavenWrapperMain
• updated DEFAULT_MAVEN_VER parameter in takari-maven-plugin WrapperMojo class