Could not find or load main class org.gradle.wrapper.GradleWrapperMain

In addition to @RaGe's answer may be the situation I faced where i had a global git ignore that was ignoring .jar files and so the gradle wrapper jar was never being committed. Thus I was getting that error on the Jenkins server after an attempted /var/lib/jenkins/my_project/gradlew build. I had to explicitly force an add of the jar and then commit:

git add -f gradle/wrapper/gradle-wrapper.jar

Your gradle wrapper is missing, broken or corrupted.

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What is gradle wrapper:

gradlew is the gradle wrapper executable - batch script on windows and shell script elsewhere. The wrapper script when invoked, downloads the defined gradle version and executes it. By distributing the wrapper with your project, anyone can work with it without needing to install Gradle beforehand. Even better, users of the build are guaranteed to use the version of Gradle that the build was designed to work with.

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Restoring gradle wrapper:

It used to be that you needed to add a wrapper task to your build.gradle to restore gradle wrapper and all its dependencies. For instance:

task wrapper(type: Wrapper) {
    gradleVersion = '4.1'
}

Newer versions of gradle do not require this. It is now a built-in task. Just run:

gradle wrapper

You can also supply additional flags to specify versions etc

gradle wrapper --gradle-version 6.2 --distribution-type all

When you run this task, a gradle wrapper script, and the required jar files are added to your source folders. Properties are stored in gradle/wrapper/gradle-wrapper.properties

(You may need to install gradle locally to run this. brew install gradle on mac for instance. See more detailed instructions here)

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Why was it missing in the first place?

OP seems to have deleted something that gradle wrapper depends on.

But a common reason is that a .gitignore entry prevents wrapper jars from being checked into git. Note that the .gitignore in effect may be in the source folder, or a global one in your user home folder or git global configuration. It is common to have a *.jar entry in .gitignore.

You can add an exception for gradlew's jar files in .gitignore

*.jar
!gradle/wrapper/gradle-wrapper.jar

or force add the wrapper jar into git

git add -f gradle/wrapper/gradle-wrapper.jar

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Ref: Gradle Wrapper