Could not find or load main class org.apache.maven.wrapper.MavenWrapperMain

Question

I have a spring boot project here: https://github.com/jcasbin/jcasbin-springboot-plugin . I encountered the following error in Travis CI :

shell
3.43s$ ./mvnw install -DskipTests=true -Dmaven.javadoc.skip=true -B -V
/home/travis/build/jcasbin/jcasbin-springboot-plugin
Picked up _JAVA_OPTIONS: -Xmx2048m -Xms512m
Error: Could not find or load main class org.apache.maven.wrapper.MavenWrapperMain
The command "eval ./mvnw install -DskipTests=true -Dmaven.javadoc.skip=true -B -V " failed. Retrying, 2 of 3.

It seems that the mvnw command fails. This file is generated by my IDE: IntelliJ IDEA 2018.1. I don't know what it is used for and why does it fail?

I already fired an issue in Spring Boot's github issues here , but Spring project said it's Maven wrapper's issue and pointed me here. I don't quite understand what it means. Is it a Maven bug? How to fix the error then?

Answer 1

Phil's answer is correct. And here is how to create .mvn directory with needed jar inside.
I found the answer here ( https://www.baeldung.com/maven-wrapper )

If you have maven and you want to make maven wrapper working, you need to setup maven wrapper:

mvn -N io.takari:maven:wrapper

It should create .mvn directory and put needed jar in it.

Answer 2

You're missing the .mvn folder in your git repository. You should have a folder called .mvn which contains the files wrapper/maven-wrapper.jar , wrapper/maven-wrapper.properties and jvm.config . Perhaps you missed it because it's a hidden folder.

Try doing git add -f .mvn from the command line then commit and push.

Answer 3

1. Adding to what Oleksii Volynskyi,

use the official maven wrapper

mvn wrapper:wrapper

rather than io.takari:maven:wrapper which is no longer maintained

2. As mentioned by Phil Webb add.mvn folder to the git repository

Answer 4

For anyone using Windows seeing this error, check if the .mvn folder files got converted to CRLF. If so, changing them to LF will likely fix your problem.

Answer 5

In my case, I had the project saved in a folder which route included a character with an accent (in my case "á"). Removing the accent fixed the problem.

Answer 6

«I don't know what it is used for»:

A mvnw is a maven wrapper. Basically it is a script that decouples maven from your local installation and may add checks and features to the actual mvn command - for the complete explanation check the official maven documentation .

To set it up (assuming you had it running solely with mvn ) it should suffice to run:

    mvn wrapper:wrapper 

And that should create at least the following new files:

    .
    ├── .mvn
    │   └── wrapper
    │       ├── maven-wrapper.jar
    │       └── maven-wrapper.properties
    ├── mvnw
    └── mvnw.cmd

Then, by evoking mvnw you'd run everything by that maven-wrapper.jar - completely unattached from you local maven installation.

If the maven-wrapper.jar is not in place after the command, you probably have a network issue - the most common one is a proxy that needs to be configured.

«How to fix»:

What is the problem?

Proxy issue:

Initially, the mvn wrapper:wrapper couldn't fetch the maven-wrapper.jar for me.

.
├── .mvn
│   └── wrapper
│       └── maven-wrapper.properties
├── mvnw
└── mvnw.cmd

If you take the deep dive into the rabbit hole and check the actual wrapper code , you'll notice there is no default proxy - but you can set a proxy for the java virtual machine by creating a .mvn/jvm.config file containing:

    -Dhttp.proxyHost=proxy.domain.name
    -Dhttp.proxyPort=8080
    -Dhttps.proxyHost=proxy.domain.name
    -Dhttps.proxyPort=8080 

With that in place just rerunning the setup command

    mvn wrapper:wrapper

fetched everything by itself:

    .
    ├─ .mvn
    │  ├── jvm.config
    │  └── wrapper
    │      ├── maven-wrapper.jar
    │      └── maven-wrapper.properties
    ├── mvnw
    └── mvnw.cmd

By now, running the ./mvnw clean install , should work ok - it did for me (but only for a few days)

Class Issue:

One nice day, I came into office and the mvnw was returnnig the same error you stated....

For me the issue was actually a line ending (LF <-> CRLF) issue in the jvm.config file.

As the mvnw script expands the options into an actual command the command would get messed up with a CR splitting it before the classpath option. Since that classpath is where the maven-wrapper.jar (containing the MaveWrapperMain) is passed, the java command couldn't actually get the class.

How did I fix it:

I simply re-created the file containing only LFs and no CRLFs.

Bonus:

As I noticed that this CRLF was introduced by a user that was on a windows machine, I checked some git documentation to avoid repeating the issue, ran the following and advised everyone to do the same:

    git config --global core.autocrlf true

---

Hope this centralizes the steps I found scattered and ends up saving time to someone.

Answer 7

我实际上只是做了 mvn clean install 并且它起作用了，我实际上使用的是旧版本的 spring 2.0 来具体