Gradle - no main manifest attribute

Question

I'm building a JAR file with Gradle. When I try to run it I get the following error

no main manifest attribute, in RxJavaDemo.jar

I tried manipulating the manifest property but I think I'm forgetting to add the dependencies or something to it. What exactly am I doing wrong?

apply plugin: 'java'
apply plugin: 'application'
mainClassName = 'demo.MainDashboard'
dependencies {
 compile files ("H:/Processes/Development/libraries/hikari-cp/HikariCP-2.4.1.jar")
 compile files ("H:/Processes/Development/libraries/controls-fx/controlsfx.jar")
 compile files ("H:/Processes/Development/libraries/database_connections/sqlite-jdbc-3.8.6.jar")
 compile files ("H:/Processes/Development/libraries/guava/guava-18.0.jar")
 compile files ("H:/Processes/Development/libraries/rxjava/rxjava-1.0.12.jar")
 compile files ("H:/Processes/Development/libraries/rxjava-extras/rxjava-extras-0.5.15.jar")
 compile files ("H:/Processes/Development/libraries/rxjavafx/RxJavaFX-1.0.0-RC1-SNAPSHOT.jar")
 compile files ("H:/Processes/Development/libraries/rxjavaguava/rxjava-guava-1.0.3.jar")
 compile files ("H:/Processes/Development/libraries/rxjava-jdbc/rxjava-jdbc-0.6.3.jar")
 compile files ("H:/Processes/Development/libraries/slf4j/slf4j-api-1.7.12.jar")
 compile files ("H:/Processes/Development/libraries/tom-commons/tom-commons.jar")
}
sourceSets {
 main.java.srcDir "src/main/java"
 main.resources.srcDir "src/main/resources"
}
jar { 
 manifest {
 attributes(
 "Class-Path": configurations.compile.collect { it.getName() }.join(' '))
 }
 from configurations.compile.collect { entry -> zipTree(entry) }
}

Answer 1

Try to change your manifest attributes like:

jar {
 manifest {
 attributes(
 'Class-Path': configurations.compile.collect { it.getName() }.join(' '),
 'Main-Class': 'hello.HelloWorld'
 )
 }
}

And then just change 'hello.helloWorld' to '<your packagename>.<the name of your Main class>' (where your Main class has a main method). In this case, you make in your manifest an attribute, which point to this class, then a jar is running.

• Daniela: @Stanislav 'Main-Class' value is the main class? What are hello and helloWorld in your example?
• Stanislav: @DanielaMaia it's just a full qualified class name, sure it has to be written as hello.HelloWorld, where hello is the package where the HelloWorld class is located
• AutonomousApps: I needed to remove the collect {} portion to get it to work for me. Your code assumes that all dependencies are in the same folder as your main class.
• Sebastian Nielsen: @AutonomousApps How exactly did you do that?
• dbaltor: The lastest Gradle version has replaced compile. Instead use: 'Class-Path': configurations.runtimeClasspath.files.collect { it.getName() }.join(' ')

Answer 2

To make the jar file executable (so that the java -jar command works), specify the Main-Class attribute in MANIFEST.MF.

In Gradle, you can do it by configuring the jar task.

• for Groovy DSL see these answers ([1], [2])
• for Kotlin DSL you can use the following code snippet:

tasks.withType<Jar> {
 manifest {
 attributes["Main-Class"] = "com.caco3.Main"
 }
}

Why mainClassName does not work as expected?

Or why mainClassName does not specify the attribute in the manifest?

The mainClassName property comes from the application plugin. The plugin:

makes it easy to start the application locally during development, and to package the application as a TAR and/or ZIP including operating system specific start scripts.

So the application plugin does not aim at producing executable jars

When a mainClassName property set, then:

1. $ ./gradlew run will launch the main method in the class specified in the attribute
2. the zip/tar archive built using distZip/distTar tasks will contain a script, which will launch the main method of the specified previously class.

Here is the line of shell script setting the main class:

$ grep Main2 gradletest
eval set -- $DEFAULT_JVM_OPTS $JAVA_OPTS $GRADLETEST_OPTS -classpath "\"$CLASSPATH\"" com.caco3.gradletest.Main2 "$APP_ARGS"

• Manius: I get that the application plugin doesn't have a goal of creating a runnable jar, but it's still annoying that we have to do these extra steps in Gradle all the time for a runnable jar. It should have everything it needs for a manifest...
• Manius: Although I suppose the reason is, your jar will be missing dependencies so it's not usually runnable anyway. :)

Answer 3

To complement Denis Zavedeev answer, here are more ways for Kotlin DSL (build.gradle.kts):

tasks.jar {
 manifest.attributes["Main-Class"] = "com.example.MyMainClass"
}

Another notation:

tasks.jar {
 manifest {
 attributes["Main-Class"] = "com.example.MyMainClass"
 }
}

Side note: to create a runnable fat JAR (also called uber JAR), see this post.

• Yamin Siahmargooei: Starting with Gradle 8.x somehow array access for attributes is unavailable.
• Mahozad: @YaMiN Are you sure? I tried both of the above code snippets right now with Gradle 8.0 and it seemed to work with no problem.
• Yamin Siahmargooei: Yeah I'm sure. I don't know why but after upgrading to Gradle 8.0.2 it gives me No set method providing array access error, however, the project compiles.

Answer 4

FWIW - I used the following jar task to assemble all my compile dependencies into the jar file, and used the above recommendation to get the class-path properly set

apply plugin: 'java-library'
jar {
 manifest {
 attributes(
 'Class-Path': configurations.compile.collect { it.getName() }.join(' '),
 'Main-Class': 'your.main.class.goes.here'
 )
 }
 // You can reference any part of the dependency configurations,
 // and you can have as many from statements as you need
 from configurations.compile 
 // I just copied them into the top of the jar, so it looks like the eclipse exported 
 // runnable jar, but you could designate a lib directory, and reference that in the 
 // classpath as "lib/$it.name" instead of it.getName()
 into '' 
}

Answer 5

Reference this solution effective for Spring 3:

plugins {
 id 'maven-publish'
 id 'java-library'
}
jar {
 enabled = false
}
java {
 withSourcesJar()
 withJavadocJar()
}
publishing {
 publications {
 publication(MavenPublication) {
 artifact bootJar
 from components.java
 }
 }
}

Answer 6

If no solution helps you (you tried all previous answers) - try to simply build your spring boot app and IntelliJ idea place .jar to the temp build folder. Only this solution resolved my problem.

You should create a task in your build.gradle file:

baseName - your future .jar file's name

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p.s. manual starting jar task did not solve my problem in my case