Gradle - Could not find or load main class

I'm trying to run a very simple project using Gradle and running into the following error when using the gradlew run command:

could not find or load main class 'hello.HelloWorld'

Here is my file structure:

SpringTest
    -src
        -hello
            -HelloWorld.java
            -Greeter.java
    -build
         -libs
         -tmp
    -gradle
         -wrapper
    -build.gradle
    -gradlew
    -gradlew.bat

I excluded the contents of the libs and tmp folders because I didn't think that would be relevant information for this issue, but I can add it in if need be.

Here is my build.gradle file:

apply plugin: 'java'
apply plugin: 'application'
apply plugin: 'eclipse'

mainClassName = 'hello/HelloWorld'

repositories {
    mavenLocal()
    mavenCentral()
}

dependencies {
    compile "joda-time:joda-time:2.2"
}

jar {
    baseName = "gs-gradle"
    version = "0.1.0"
}

task wrapper(type: Wrapper) {
    gradleVersion = '1.11'
}

Any idea on how to fix this issue? I've tried all sorts of things for the mainClassName attribute but nothing seems to work.

kunal

I see two problems here, one with sourceSet another with mainClassName.

Either move java source files to src/main/java instead of just src. Or set sourceSet properly by adding the following to build.gradle.
```
sourceSets.main.java.srcDirs = ['src']
```
mainClassName should be fully qualified class name, not path.
```
mainClassName = "hello.HelloWorld"
```

Modify build.gradle to put your main class in the manifest:

jar {
    manifest {
        attributes 'Implementation-Title': 'Gradle Quickstart',
                   'Implementation-Version': version,
                   'Main-Class': 'hello.helloWorld'
    }
}

I just ran into this problem and decided to debug it myself since i couldn't find a solution on the internet. All i did is change the mainClassName to it's whole path(with the correct subdirectories in the project ofc)

    mainClassName = 'main.java.hello.HelloWorld'

I know it's been almost one year since the post has been made, but i think someone will find this information useful.

Happy coding.

Just to make it clear for newbies trying to run a gradle project from Netbeans:
To understand this, you need to see what the main class name looks like and what the gradle build looks like:

Main class:

package com.stormtrident;

public class StormTrident {
    public static void main(String[] cmdArgs) {
    }
}

Notice that it is part of the package "com.stormtrident".

Gradle build:

apply plugin: 'java'

defaultTasks 'jar'

jar {
 from {
        (configurations.runtime).collect {
            it.isDirectory() ? it : zipTree(it)
        }
    }    
    manifest {
        attributes 'Main-Class': 'com.stormtrident.StormTrident'
    }
}


sourceCompatibility = '1.8'
[compileJava, compileTestJava]*.options*.encoding = 'UTF-8'

if (!hasProperty('mainClass')) {
    ext.mainClass = 'com.stormtrident.StormTrident'
}

repositories {
    mavenCentral()
}

dependencies {
    //---apache storm
    compile 'org.apache.storm:storm-core:1.0.0'  //compile 
    testCompile group: 'junit', name: 'junit', version: '4.10'
}

Struggled with the same problem for some time. But after creating the directory structure src/main/java and putting the source(from the top of the package), it worked as expected.

The tutorial I tried with. After you execute gradle build, you will have to be able to find classes under build directory.

In my build.gradle, I resolved this issue by creating a task and then specifying the "mainClassName" as follows:

task(runSimpleXYZProgram, group: 'algorithms', description: 'Description of what your program does', dependsOn: 'classes', type: JavaExec) {
    mainClassName = 'your.entire.package.classContainingYourMainMethod'
}

I resolved it by adding below code to my application.

// enter code here this is error I was getting when I run build.gradle. main class name has not been configured and it could not be resolved

public static void main(String[] args) {
    SpringApplication.run(Application.class, args);
}

I fixed this by running a clean of by gradle build (or delete the gradle build folder mannually)

This occurs if you move the main class to a new package and the old main class is still referenced in the claspath

When I had this error, it was because I didn't have my class in a package. Put your HelloWorld.java file in a "package" folder. You may have to create a new package folder:

Right click on the hello folder and select "New" > "Package". Then give it a name (e.g: com.example) and move your HelloWorld.java class into the package.

verify if gradle.properties define right one JAVA_HOVE

org.gradle.java.home=C:\Program Files (x86)\Java\jdk1.8.0_181

if it's not defined be sure if Eclipse know JDK and not JRE

If you decided to write your hello.World class in Kotlin, another issue might be that you have to reference it as mainClassName = "hello.WorldKt".

src/main/java/hello/World.kt:

package hello
fun main(args: Array<String>) {
    ...
}
// class World {} // this line is not necessary

kapsid

For a project structure like

project_name/src/main/java/Main_File.class

in the file build.gradle, add the following line

mainClassName = 'Main_File'

来源：https://stackoverflow.com/questions/24924932/gradle-could-not-find-or-load-main-class

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