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问题:

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.

回答1:

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"
```

回答2:

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'
    }
}

回答3:

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.

回答4:

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'
}

回答5:

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.

回答6:

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'
}

回答7:

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);
}

回答8:

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

回答9:

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.