To generate Javadoc documentation, we must use the javadoc task that is of type org.gradle.api.tasks.javadoc.Javadoc. The task generates documentation for the Java source files in the main source set. If we want to generate documentation for the source sets in our project, we must configure the javadoc task or add an extra javadoc task to our project.

Note that, in our project, we have an api and main source set with the Java source files. If we want to generate documentation for both the source sets, we have to configure the javadoc task in our project. The source property of the javadoc task is, by default, set to sourceSets.main.allJava. If we add sourceSets.api.allJava to the source property, our interface file is also processed by the javadoc task:

apply plugin: 'java'
...
javadoc {
    source sourceSets.api.allJava
}
...

Next, we can run the javadoc task, and the documentation is generated and put into the build/docs/javadoc directory:

$ gradle javadoc
:compileApiJava UP-TO-DATE
:processApiResources UP-TO-DATE
:apiClasses UP-TO-DATE
:compileJava UP-TO-DATE
:processResources UP-TO-DATE
:classes UP-TO-DATE
:javadoc

BUILD SUCCESSFUL

Total time: 3.425 secs

We can set more properties on the javadoc task. For example, we can set a title for the generated documentation with the title property. The default value is the name of the project followed by the project version number, if available.

To change the destination directory, we can set the destinationDir property of the javadoc task to the directory we want.

We can also use the options property to define a lot of properties we know from the Java SDK javadoc tool. The following example shows how we can set some of the options for the javadoc task in our project:

apply plugin: 'java'
...
javadoc {
    source sourceSets.api.allJava
    title = 'Gradle Sample Project'
    options.links = ['http://docs.oracle.com/javase/6/docs/api/']
    options.footer = "Generated on ${new Date().format('dd MMM yyyy')}"
    options.header = "Documention for version ${project.version}"
}
...