May 10, 2018

Gradle Goodness: Command Line Options For Custom Tasks

Gradle added an incubation feature to Gradle 4.6 to add command line options for custom tasks. This means we can run a task using Gradle and add command line options to pass information to the task. Without this feature we would have to use project properties passed via the -P or --project-property. The good thing about the new feature is that the Gradle help task displays extra information about the command line options supported by a custom task.

To add a command line option we simply use the @Option annotation on the setter method of a task property. We must make sure the argument for the setter method is either a boolean, Boolean, String, enum, List<String> or List<enum>. The @Option annotation requires an option argument with the name of the option as it must be entered by the user. Optionally we can add a description property with a description about the option. It is good to add the description, because the help task of Gradle displays this information and helps the user of our custom task.

Let's start with a custom task that opens a file in the default application associated for the type of file. The task has own property file of type File. Remember we cannot create an command line option for all property types. To add a command line option we must overload the setFile method to accept a String value. This setter method is used to expose the file property as option.

// File: buildSrc/src/main/groovy/mrhaki/gradle/OpenFile.groovy
package mrhaki.gradle

import org.gradle.api.DefaultTask
import org.gradle.api.GradleException
import org.gradle.api.tasks.Input
import org.gradle.api.tasks.TaskAction
import org.gradle.api.tasks.options.Option

import groovy.transform.CompileStatic
import java.awt.Desktop

/**
 * Open a file or URI with the associated application.
 */
@CompileStatic
class OpenFile extends DefaultTask {

    /**
     * File to open.
     */
    @Input
    File file

    /**
     * Set description and group for task.
     */
    OpenFile() {
        description = 'Opens file with the associated application.'
        group = 'Help'
    }

    /**
     * Overload the setter method to support a String parameter. Now
     * we can add the @Option annotation to expose our file property
     * as command line option.
     *
     * @param path The object to resolve as a {@link File} for {@link #file} property.
     */
    @Option(option = 'file', description = 'Set the filename of the file to be opened.')
    void setFile(final String path) {
        this.file = project.file(path)
    }

    /**
     * Check if {@link Desktop} is supported. If not throw exception with message.
     * Otherwise open file with application associated to file format on the
     * runtime platform.
     *
     * @throws GradleException If {@link Desktop} is not supported.
     */
    @TaskAction
    void openFile() {
        if (Desktop.isDesktopSupported()) {
            Desktop.desktop.browse(new URI("file://${file.absolutePath}"))
        } else {
            throw new GradleException('Native desktop not supported on this platform. Cannot open file.')
        }
    }

}

We use the task in the following build file. We create a task openReadme and set the file property value in the build script. We also create the task open, but don't set the file property. We will use the command line option file for this task:

task open(type: mrhaki.gradle.OpenFile)

task openReadme(type: mrhaki.gradle.OpenFile) {
    file = project.file('README')
}

To run the open task and set the file command line option we use a double-dash as prefix:

$ gradle open --file=README

We can more information about the supported options for our task by invoking the help task. We define the task name with the option task:

$ gradle help --task=open

> Task :help
Detailed task information for open

Path
     :open

Type
     OpenFile (mrhaki.gradle.OpenFile)

Options
     --file     Set the filename of the file to be opened.

Description
     Opens file with the associated application.

Group
     Help

BUILD SUCCESSFUL in 0s
1 actionable task: 1 executed

To define a set of valid values for an option we must add a method to our class that returns a list of valid values annotated with the @OptionValues annotation. We must set the task property name as argument for the annotation. Then when we invoke Gradle's help task we see a list of valid values. Validation of the property value is not done with the annotation, it is only for informational purposes. So validation must be added to the task code by ourselves.

We rewrite our OpenFile task and add the requirement that only files in the project directory can be opened and the filename must be in upper case. The method availableFiles return all files in the project directory with their name in upper case. The annotation @OptionValues is added to the method. In the task action method openFile we check if the file is valid to be opened

package mrhaki.gradle

import org.gradle.api.DefaultTask
import org.gradle.api.GradleException
import org.gradle.api.tasks.Input
import org.gradle.api.tasks.TaskAction
import org.gradle.api.tasks.options.Option
import org.gradle.api.tasks.options.OptionValues

import groovy.transform.CompileStatic
import java.awt.Desktop

/**
 * Open a file or URI with the associated application.
 */
@CompileStatic
class OpenFile extends DefaultTask {

    /**
     * File to open.
     */
    @Input
    File file

    /**
     * Set description and group for task.
     */
    OpenFile() {
        description = 'Opens file with the associated application.'
        group = 'Help'
    }

    /**
     * Check if {@link Desktop} is supported. If not throw exception with message.
     * Otherwise open file with application associated to file format on the
     * runtime platform.
     *
     * @throws GradleException If {@link Desktop} is not supported or
     *                         if filename not all uppercase in project dir.
     */
    @TaskAction
    void openFile() {
        if (!fileAllUpperCaseInProjectDir) {
            throw new GradleException('Only all uppercase filenames in project directory are supported.')
        }

        if (Desktop.isDesktopSupported()) {
            Desktop.desktop.browse(new URI("file://${file.absolutePath}"))
        } else {
            throw new GradleException('Native desktop not supported on this platform. Cannot open file.')
        }
    }

    private boolean isFileAllUpperCaseInProjectDir() {
        getAllUpperCase()(file.name) && project.projectDir == file.parentFile
    }

    /**
     * Overload the setter method to support a String parameter. Now
     * we can add the @Option annotation to expose our file property
     * as command line option.
     *
     * @param path The object to resolve as a {@link File} for {@link #file} property.
     */
    @Option(option = 'file', description = 'Set the filename of the file to be opened.')
    void setFile(final String path) {
        this.file = project.file(path)
    }

    /**
     * Show all files with filename in all uppercase in the project directory.
     *
     * @return All uppercase filenames.
     */
    @OptionValues('file')
    List<String> availableFiles() {
        project.projectDir.listFiles()*.name.findAll(allUpperCase)
    }

    private Closure getAllUpperCase() {
        { String word -> word == word.toUpperCase() }
    }
}

Let's run the help task and see that available values are shown this time:

$ gradle help --task=open

> Task :help
Detailed task information for open

Path
     :open

Type
     OpenFile (mrhaki.gradle.OpenFile)

Options
     --file     Set the filename of the file to be opened.
                Available values are:
                     README

Description
     Opens file with the associated application.

Group
     Help

BUILD SUCCESSFUL in 2s
1 actionable task: 1 executed

Written with Gradle 4.7.