Gradle - Could not find or load main class

Question

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.

Does not applying `application` also implicitly applies `java` plug-in? — nabster, Oct 31 '22 at 16:07

score 89 · Accepted Answer · edited Oct 22 '18 at 07:27

89

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"
```

edited Oct 22 '18 at 07:27

John Bentley

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answered Jul 24 '14 at 04:14

kdabir

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1

Where do I change sourceSets? In my build.gradle file? Or is there a configuration file that I need to change? – kibowki Jul 25 '14 at 15:04
2

When I add `sourceSets.main.java.srcDirs = ['src']` I get an error in the jar task – Victor M Perez Jun 08 '19 at 20:49
2

`MissingPropertyException: Could not set unknown property 'mainClassName'` for Gradlew 4.1 – fuat Nov 06 '20 at 08:19
1

@fuat you need to have the application plugin – VFX May 02 '21 at 04:01
1

`Could not set unknown property 'mainClassName' for root project` error. Please show full build.gradle file – parsecer Aug 29 '22 at 00:53

Noumenon · Answer 2 · 2017-02-27T16:35:03.817

35

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'
    }
}

edited Feb 27 '17 at 16:35

answered Sep 10 '15 at 14:34

Noumenon

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hepifish · Answer 3 · 2015-05-04T13:50:50.080

28

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.

edited May 04 '15 at 13:50

answered May 04 '15 at 13:41

hepifish

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score 10 · Answer 4 · answered Apr 17 '16 at 06:09

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'
}

For me, adding your jar { manifest { attributes } } bit helped. Thanks! — Paul, Oct 21 '16 at 20:36
What is Netbeans doing differently that Gradle seems to be ran differently? I am attempting this manually so I can understand Gradle better. Netbeans seems to be really good at Maven, but Gradle seems to be fragile in Netbeans. — cody.tv.weber, Oct 22 '19 at 19:01

score 3 · Answer 5 · answered Dec 21 '15 at 19:42

3

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.

answered Dec 21 '15 at 19:42

Dino Tw

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Also, since no one else is specifying it: the `build.gradle` file has to be located "alongside" the `src/` directory, not along with the .java files in `src/main/java/hello/` – joakimk Jan 25 '16 at 13:45

score 3 · Answer 6 · answered Oct 22 '19 at 19:31

For Netbeans 11 users, this works for me:

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

// This comes out to package + '.' + mainClassName
mainClassName = 'com.hello.JavaApplication1'

Here generally is my tree:

C:\...\NETBEANSPROJECTS\JAVAAPPLICATION1
│   build.gradle
├───src
│   ├───main
│   │   └───java
│   │       └───com
│   │           └───hello
│   │                   JavaApplication1.java
│   │
│   └───test
│       └───java
└───test

score 2 · Answer 7 · answered Jan 28 '19 at 20:50

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

score 1 · Answer 8 · answered Feb 10 '16 at 11:00

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'
}

score 1 · Answer 9 · answered Jan 11 '17 at 14:22

1

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

answered Jan 11 '17 at 14:22

JohnTheBeloved

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build/clean + build/build does build build doesn't fix the runtime issue – MonoThreaded Aug 20 '18 at 21:56

score 0 · Answer 10 · answered Jul 26 '17 at 00:50

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.

score 0 · Answer 11 · answered Aug 09 '18 at 21:09

0

verify if gradle.properties define right one JAVA_HOVE

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

or

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

answered Aug 09 '18 at 21:09

cane

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score 0 · Answer 12 · answered Sep 14 '18 at 11:22

0

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);
}

answered Sep 14 '18 at 11:22

Premakumar

47
5

That's already the entry point for a springboot project to start and the question is about configuration conflicts not how to initiate a project – aya salama Nov 08 '19 at 14:53

score 0 · Answer 13 · edited Aug 15 '19 at 10:54

0

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'

edited Aug 15 '19 at 10:54

B--rian

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answered Aug 15 '19 at 10:18

minato

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Welcome to SO!. Please edit your question and elaborate a bit on why your solution is working, i.e. what exactly you are doing. – B--rian Aug 15 '19 at 10:50
Need full build.gradle file as an example – parsecer Aug 29 '22 at 00:52

score 0 · Answer 14 · answered Dec 01 '20 at 14:30

If you're using Spring Boot, this might be the issue: https://github.com/gradle/gradle/issues/2489.

Basically, the output directories changed in Gradle 4.0, so if you have them hardcoded the execution will fail.

The solution is to replace:

bootRun {
   dependsOn pathingJar
   doFirst {
      classpath = files("$buildDir/classes/main", "$buildDir/resources/main", pathingJar.archivePath)
   }
}

by:

bootRun {
   dependsOn pathingJar
   doFirst {
      classpath = files(sourceSets.main.output.files, pathingJar.archivePath)
   }
}

score 0 · Answer 15 · answered Jul 23 '21 at 09:19

I had forgotten to add String[] args inside main method public static void main()

After changing it to public static void main(String[] args), everything worked.

My build.gradle looks like this

apply plugin: 'java'
apply plugin: 'application'
mainClassName = 'hugo.HelloWorld'

Mahozad · Answer 16 · 2022-05-04T08:04:13.653

0

For Kotlin DSL (build.gradle.kts) use this code:

tasks.jar {
    manifest.attributes["Main-Class"] = "my.package.name.MyClassName"
    // ...
}

Another similar notation:

tasks.jar {
    manifest {
        attributes["Main-Class"] = "my.package.name.MyClassName"
    }
    // ...
}

edited May 04 '22 at 08:04

answered Feb 18 '22 at 09:36

Mahozad

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Gradle - Could not find or load main class

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