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 and initial implementation borrowed from Gradle.
The easiest way to setup the Maven Wrapper for your project is to use the Maven Wrapper Plugin with its provided wrapper
goal. To add or update all the necessary Maven Wrapper files to your project execute the following command:
mvn wrapper:wrapper
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 mvnw
/mvnw.cmd
use the previously downloaded, specific version as needed.
The wrapper should work on various operating systems including
A POSIX-compatible Bourne shell is required to run the wrapper script.
In terms of Apache Maven versions itself, the wrapper should work with any Maven 3.x version and it defaults to the release used when setting up the wrapper. We do NOT plan to support the deprecated, EOL'ed Maven 2.x.
The maven-wrapper.jar
(to download and install target Maven version required by mvnw
) and the Maven Wrapper Plugin (to inject Wrapper into a project) use Java 7.
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.
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 use the source distribution of the maven wrapper which adds a file .mvn/wrapper/MavenWrapperDownloader.java
file instead:
# defaults to bin mvn wrapper:wrapper -Dtype=source
You can also chose to opt out of all additional resources except the wrapper scripts:
mvn wrapper:wrapper -Dtype=script
Another type is the lite implementation of mvnw
/mvnw.cmd
scripts which download the maven directly via wget or curl on *nix, or PowerShell on Windows, then call the original mvn
/mvn.cmd
scripts of the downloaded maven distribution. This type does not use maven-wrapper.jar
nor MavenWrapperDownloader.java
, only the wrapper scripts are required.
mvn wrapper:wrapper -Dtype=only-script
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
.
To switch the version of Maven used to build a project, you can initialize it using:
mvn wrapper: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
To download Maven from a location that requires Basic Authentication you have 2 options:
Set the environment variables MVNW_USERNAME
and MVNW_PASSWORD
or
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
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 https://github.com/takari/maven-wrapper/issues/17
When using an internal Maven repository manager, you have two options:
maven-wrapper.properties
in your projectMVNW_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 maven-wrapper-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.
To avoid supply-chain-attacks by downloading a corrupted artifact, it is possible to specify checksums for both the maven-wrapper.jar and the downloaded distribution. To apply verification, add the expected file's SHA-256 sum in hex notation, using only small caps, to maven-wrapper.properties
. The property for validating the maven-wrapper.jar file is named wrapperSha256Sum
whereas the distribution file property is named distributionSha256Sum
.
Maven Wrapper is composed of 3 pieces:
maven-wrapper, providing the maven-wrapper.jar
that downloads, installs and runs target Maven distribution,
maven-wrapper-distribution, that provides mvnw
/mvnw.cmd
scripts distributions,
maven-wrapper-plugin, the wrapper
plugin to ease installing Wrapper into a project.