Gradle Effective Implementations Guide: This comprehensive guide will get you up and running with build automation using Gradle., Second Edition

Gradle uses a Domain Specific Language (DSL) based on Groovy to declare builds. The DSL provides a flexible language that can be extended by us. As the DSL is based on Groovy, we can write Groovy code to describe a build and use the power and expressiveness of the Groovy language. Groovy is a language for the Java Virtual Machine (JVM), such as Java and Scala. Groovy makes it easy to work with collections, has closures, and a lot of useful features. The syntax is closely related to the Java syntax. In fact, we could write a Groovy class file with Java syntax and it will compile. However, using the Groovy syntax makes it easier to express the code intent and we need less boilerplate code than with Java. To get the most out of Gradle, it is best to also learn the basics of the Groovy language, but it is not necessary to start writing Gradle scripts.

Gradle is designed to be a build language and not a rigid framework. The Gradle core itself is written in Java and Groovy. To extend Gradle, we can use Java and Groovy to write our custom code. We can even write our custom code in Scala if we want to.

Gradle provides support for Java, Groovy, Scala, web, and OSGi projects out of the box. These projects have sensible convention-over-configuration settings that we probably already use ourselves. However, we have the flexibility to change these configuration settings if required for our projects.

In this section, we will download and install Gradle before writing our first Gradle build script.

Before we install Gradle, we must make sure that we have a Java Development SE Kit (JDK) installed on our computer. Gradle requires JDK 6 or higher. Gradle will use the JDK found on the path of our computer. We can check this by running the following command on the command line:

$ java -version

Although Gradle uses Groovy, we don't have to install Groovy ourselves. Gradle bundles the Groovy libraries with the distribution and will ignore a Groovy installation that is already available on our computer.

Gradle is available on the Gradle website at http://www.gradle.org/downloads. From this page, we can download the latest release of Gradle. We can also download an older version if we want. We can choose among three different distributions to download. We can download the complete Gradle distribution with binaries, sources, and documentation; or we can only download the binaries; or we can only download the sources.

To get started with Gradle, we will download the standard distribution with the binaries, sources, and documentation. At the time of writing this book, the current release is 2.12.

$ gradle -v
------------------------------------------------------------
Gradle 2.12
------------------------------------------------------------
Build time:   2016-03-14 08:32:03 UTC
Build number: none
Revision:     b29fbb64ad6b068cb3f05f7e40dc670472129bc0
Groovy:       2.4.4
Ant:          Apache Ant(TM) version 1.9.3 compiled on             December23 2013
JVM:          1.8.0_66 (Oracle Corporation 25.66-b17)
OS:           Mac OS X 10.11.3 x86_64

$ curl -s get.sdkman.io | bash

$ sdk install gradle
Downloading: gradle 2.12
% Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
  0   354    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
100 42.6M  100 42.6M    0     0  1982k      0  0:00:22  0:00:22 --:--:-- 3872k
Installing: gradle 2.12
Done installing!
Do you want gradle 2.12 to be set as default? (Y/n): Y
Setting gradle 2.12 as default.

$ sdk use gradle 2.12
Using gradle version 2.12 in this shell.

We now have a running Gradle installation. It is time to create our first Gradle build script. Gradle uses the concept of projects to define a related set of tasks. A Gradle build can have one or more projects. A project is a very broad concept in Gradle, but it is mostly a set of components that we want to build for our application.

A project has one or more tasks. Tasks are a unit of work that need to be executed by the build. Examples of tasks are compiling source code, packaging class files into a JAR file, running tests, and deploying the application.

We now know a task is a part of a project, so to create our first task, we also create our first Gradle project. We use the gradle command to run a build. Gradle will look for a file named build.gradle in the current directory. This file is the build script for our project. We define our tasks that need to be executed in this build script file.

We create a new build.gradle file and open this in a text editor. We type the following code to define our first Gradle task:

task helloWorld << { 
    println 'Hello world.' 
} 

With this code, we will define a helloWorld task. The task will print the words Hello world. to the console. The println is a Groovy method to print text to the console and is basically a shorthand version of the System.out.println Java method.

The code between the brackets is a closure. A closure is a code block that can be assigned to a variable or passed to a method. Java doesn't support closures, but Groovy does. As Gradle uses Groovy to define the build scripts, we can use closures in our build scripts.

The << syntax is, technically speaking, an operator shorthand for the leftShift()method, which actually means add to. Therefore, here we are defining that we want to add the closure (with the println 'Hello world' statement) to our task with the helloWorld name.

First, we save build.gradle, and with the gradle helloWorld command, we execute our build:

$ gradle helloWorld
:helloWorld
Hello world.
BUILD SUCCESSFUL
Total time: 2.384 secs
This build could be faster, please consider using the Gradle  Daemon: https://docs.gradle.org/2.12/userguide/gradle_daemon.html

The first line of output shows our line Hello world. Gradle adds some more output such as the fact that the build was successful and the total time of the build. As Gradle runs in the JVM, every time we run a Gradle build, the JVM must be also started. The last line of the output shows a tip that we can use the Gradle daemon to run our builds. We will discuss more about the Gradle daemon later, but it essentially keeps Gradle running in memory so that we don't get the penalty of starting the JVM each time we run Gradle. This drastically speeds up the execution of tasks.

We can run the same build again, but only with the output of our task using the Gradle --quiet or -q command-line option. Gradle will suppress all messages except error messages. When we use the --quiet (or -q) option, we get the following output:

$ gradle --quiet helloWorld
Hello world.

We can create our simple build script with one task. We can ask Gradle to show us the available tasks for our project. Gradle has several built-in tasks that we can execute. We type gradle -q tasks to see the tasks for our project:

$ gradle -q tasks
------------------------------------------------------------
All tasks runnable from root project
------------------------------------------------------------
Build Setup tasks
-----------------
init - Initializes a new Gradle build. [incubating]
wrapper - Generates Gradle wrapper files. [incubating]
Help tasks
----------
components - Displays the components produced by root project        'hello-world'. [incubating]
dependencies - Displays all dependencies declared in root project  'hello-world'.
dependencyInsight - Displays the insight into a specific n      dependency in root project 'hello-world'.
help - Displays a help message.
model - Displays the configuration model of root project   'hello-world'. [incubating]
projects - Displays the sub-projects of root project 'hello-world'.
properties - Displays the properties of root project 'hello-world'.
tasks - Displays the tasks runnable from root project 'hello-world'.
Other tasks
-----------
helloWorld
To see all tasks and more detail, run gradle tasks --all
To see more detail about a task, run gradle help --task <task>

Here, we see our helloWorld task in the Other tasks section. The Gradle built-in tasks are displayed in the Help tasks section. For example, to get some general help information, we execute the help task:

$ gradle -q help
Welcome to Gradle 2.12.
To run a build, run gradle <task> ...
To see a list of available tasks, run gradle tasks
To see a list of command-line options, run gradle --help
To see more detail about a task, run gradle help --task <task>

The properties task is very useful to see the properties available for our project. We haven't defined any property ourselves in the build script, but Gradle provides a lot of built-in properties. The following output shows some of the properties:

$ gradle -q properties
------------------------------------------------------------
Root project
------------------------------------------------------------
allprojects: [root project 'hello-world']
ant: org.gradle.api.internal.project.DefaultAntBuilder@3bd3d05e
antBuilderFactory: org.gradle.api.internal.project.DefaultAntBuilderFactory@6aba5d30
artifacts: org.gradle.api.internal.artifacts.dsl.DefaultArtifactHandler_Decorated@61d34b4
asDynamicObject: org.gradle.api.internal.ExtensibleDynamicObject@588307f7
baseClassLoaderScope: org.gradle.api.internal.initialization.DefaultClassLoaderScope@7df76d99

buildDir: /Users/mrhaki/Projects/gradle-effective-implementation-guide-2/gradle-impl-guide-2/src/docs/asciidoc/Chapter1/Code_Files/hello-world/build
buildFile: /Users/mrhaki/Projects/gradle-effective-implementation-guide-2/gradle-impl-guide-2/src/docs/asciidoc/Chapter1/Code_Files/hello-world/build.gradle
buildScriptSource: org.gradle.groovy.scripts.UriScriptSource@459cfcca
buildscript: org.gradle.api.internal.initialization.DefaultScriptHandler@2acbc859
childProjects: {}
class: class org.gradle.api.internal.project.DefaultProject_Decorated
classLoaderScope: org.gradle.api.internal.initialization.DefaultClassLoaderScope@6ab7ce48
components: []
configurationActions: org.gradle.configuration.project.DefaultProjectConfigurationActionContainer@2c6aed22
...

The dependencies task will show dependencies (if any) for our project. Our first project doesn't have any dependencies, as the output shows when we run the task:

$ gradle -q dependencies
------------------------------------------------------------
Root project
------------------------------------------------------------
No configurations

The projects tasks will display subprojects (if any) for a root project. Our project doesn't have any subprojects. Therefore, when we run the projects task, the output shows us our project has no subprojects:

$ gradle -q projects
------------------------------------------------------------
Root project
------------------------------------------------------------
Root project 'hello-world'
No sub-projects
To see a list of the tasks of a project, run gradle <project-path>:tasks
For example, try running gradle :tasks

The model tasks displays information about the model that Gradle builds internally from our project build file. This feature is incubating, which means that the functionality can change in future versions of Gradle:

$ gradle -q model
------------------------------------------------------------
Root project
------------------------------------------------------------
+ model
+ tasks
     | Type:       org.gradle.model.ModelMap<org.gradle.api.Task>
        | Creator:     Project.<init>.tasks()
        + components
              | Type:       org.gradle.api.reporting.components.ComponentReport
              | Value:      task ':components'
              | Creator:     tasks.addPlaceholderAction(components)
              | Rules:
                ⤷ copyToTaskContainer
        + dependencies
              | Type:       org.gradle.api.tasks.diagnostics.DependencyReportTask
              | Value:      task ':dependencies'
              | Creator:     tasks.addPlaceholderAction(dependencies)
              | Rules:
...

We will discuss more about these and the other tasks in this book.

Before we look at more Gradle command-line options, it is good to discuss a real-time save feature of Gradle: task name abbreviation. With task name abbreviation, we don't have to type the complete task name on the command line. We only have to type enough of the name to make it unique within the build.

In our first build, we only have one task, so the gradle h command should work just fine. However, we didn't take the built-in help task into account. So, to uniquely identify our helloWorld task, we use the hello abbreviation:

$ gradle -q hello
Hello world.

We can also abbreviate each word in a CamelCase task name. For example, our helloWorld task name can be abbreviated to hW:

$ gradle -q hW
Hello world.

This feature saves us the time spent in typing the complete task name and can speed up the execution of our tasks.

With just a simple build script, we already discussed that we have a couple of default tasks besides our own task that we can execute. To execute multiple tasks, we only have to add each task name to the command line. Let's execute our helloWorld custom task and built-in tasks task:

$ gradle helloWorld tasks
:helloWorld
Hello world.
:tasks
------------------------------------------------------------
All tasks runnable from root project
------------------------------------------------------------
Build Setup tasks
-----------------
init - Initializes a new Gradle build. [incubating]
wrapper - Generates Gradle wrapper files. [incubating]
Help tasks
----------
 components - Displays the components produced by root project 'hello-world'. [incubating]
 dependencies - Displays all dependencies declared in root project 'hello-world'.
dependencyInsight - Displays the insight into a specific  dependency in root project 'hello-world'.
help - Displays a help message.
model - Displays the configuration model of root project 'hello-world'. [incubating]
projects - Displays the sub-projects of root project 'hello-world'.
properties - Displays the properties of root project 'hello-world'.
tasks - Displays the tasks runnable from root project 'hello-world'.
Other tasks
-----------
helloWorld
To see all tasks and more detail, run gradle tasks --all
To see more detail about a task, run gradle help --task <task>
BUILD SUCCESSFUL
Total time: 2.028 secs
This build could be faster, please consider using the Gradle Daemon: https://docs.gradle.org/2.12/userguide/gradle_daemon.html

We see the output of both tasks. First, helloWorld is executed, followed by tasks. In the output, we see the task names prepended  with a colon (:) and the output is in the next lines.

Gradle executes the tasks in the same order as they are defined in the command line. Gradle will only execute a task once during the build. So even if we define the same task multiple times, it will only be executed once. This rule also applies when tasks have dependencies on other tasks. Gradle will optimize the task execution for us and we don't have to worry about that.

The gradle command is used to execute a build. This command accepts several command-line options. We know the --quiet (or -q) option to reduce the output of a build. If we use the --help (or -h or -?) option, we see the complete list of options, as follows:

$ gradle --help
USAGE: gradle [option...] [task...]
-?, -h, --help          Shows this help message.
-a, --no-rebuild        Do not rebuild project dependencies.
-b, --build-file        Specifies the build file.
-c, --settings-file     Specifies the settings file.
--configure-on-demand   Only relevant projects are configured in this build run. This means faster build for large multi-project builds. [incubating]
--console               Specifies which type of console output to generate. Values are 'plain', 'auto' (default) or 'rich'.
--continue              Continues task execution after a task failure.
-D, --system-prop       Set system property of the JVM (e.g. -Dmyprop=myvalue).
-d, --debug             Log in debug mode (includes normal stacktrace).
--daemon                Uses the Gradle daemon to run the build. Starts the daemon if not running.
--foreground            Starts the Gradle daemon in the foreground. [incubating]
-g, --gradle-user-home  Specifies the gradle user home directory.
--gui                   Launches the Gradle GUI.
-I, --init-script       Specifies an initialization script.
-i, --info              Set log level to info.
-m, --dry-run           Runs the builds with all task actions disabled.
--max-workers           Configure the number of concurrent workers Gradle is allowed to use. [incubating]
--no-color              Do not use color in the console output. [deprecated - use --console=plain instead]
--no-daemon             Do not use the Gradle daemon to run the build.
--offline               The build should operate without accessing network resources.
-P, --project-prop      Set project property for the build script (e.g. -Pmyprop=myvalue).
-p, --project-dir       Specifies the start directory for Gradle. Defaults to current directory.
--parallel              Build projects in parallel. Gradle will attempt to determine the optimal number of executor threads to use. [incubating]
--parallel-threads      Build projects in parallel, using the specified number of executor threads. [deprecated - Please use --parallel, optionally in conjunction with --max-workers.] [incubating]
--profile               Profiles build execution time and generates a report in the <build_dir>/reports/profile directory.
--project-cache-dir     Specifies the project-specific cache directory. Defaults to .gradle in the root project directory.
-q, --quiet             Log errors only.
--recompile-scripts     Force build script recompiling.
--refresh-dependencies  Refresh the state of dependencies.
--rerun-tasks           Ignore previously cached task results.
-S, --full-stacktrace   Print out the full (very verbose) stacktrace for all exceptions.
-s, --stacktrace        Print out the stacktrace for all exceptions.
--stop                  Stops the Gradle daemon if it is running.
-t, --continuous        Enables continuous build. Gradle does not exit and will re-execute tasks when task file inputs change. [incubating]
-u, --no-search-upward  Don't search in parent folders for a settings.gradle file.
-v, --version           Print version info.
-x, --exclude-task      Specify a task to be excluded from execution.

$ gradle --info helloworld
Starting Build
Settings evaluated using settings file '/master/settings.gradle'.
Projects loaded. Root project using build file '/Users/mrhaki/Projects/gradle-effective-implementation-guide-2/gradle-impl-guide-2/src/docs/asciidoc/Chapter1/Code_Files/hello-world/build.gradle'.
Included projects: [root project 'hello-world']
Evaluating root project 'hello-world' using build file '/Users/mrhaki/Projects/gradle-effective-implementation-guide-2/gradle-impl-guide-2/src/docs/asciidoc/Chapter1/Code_Files/hello-world/build.gradle'.
All projects evaluated.
Selected primary task 'helloWorld' from project :
Tasks to be executed: [task ':helloWorld']
:helloWorld (Thread[main,5,main]) started.
:helloWorld
Executing task ':helloWorld' (up-to-date check took 0.001 secs) due to:
  Task has not declared any outputs.
Hello world.
:helloWorld (Thread[main,5,main]) completed. Took 0.021 secs.
BUILD SUCCESSFUL
Total time: 1.325 secs
This build could be faster, please consider using the Gradle Daemon: https://docs.gradle.org/2.12/userguide/gradle_daemon.html

$ cd ..
$ gradle --project-dir hello-world -q helloWorld
Hello world.

$ mv build.gradle hello.build
$ gradle --build-file hello.build -q helloWorld
Hello world.

$ gradle --dry-run helloWorld
:helloWorld SKIPPED
BUILD SUCCESSFUL
Total time: 1.307 secs
This build could be faster, please consider using the Gradle Daemon: https://docs.gradle.org/2.12/userguide/gradle_daemon.html

$ gradle --daemon helloWorld
Starting a new Gradle Daemon for this build (subsequent builds will be faster).
:helloWorld
Hello world.
BUILD SUCCESSFUL
Total time: 2.136 secs
$ gradle helloWorld
:helloWorld
Hello world.
BUILD SUCCESSFUL
Total time: 0.594 secs

$ gradle --no-daemon helloWorld
:helloWorld
Hello world.
BUILD SUCCESSFUL
Total time: 1.325 secs

$ gradle --stop
Stopping daemon(s).
Gradle daemon stopped.

$ alias gradled='gradle --daemon'
$ gradled helloWorld
:helloWorld
Hello world.
BUILD SUCCESSFUL
Total time: 0.572 secs

$ export GRADLE_OPTS="-Dorg.gradle.daemon=true"
$ gradle helloWorld
:helloWorld
Hello world.
BUILD SUCCESSFUL
Total time: 0.575 secs

org.gradle.daemon=true 

$ gradle helloWorld
:helloWorld
Hello world.
BUILD SUCCESSFUL
Total time: 0.58 secs

Profiling

Gradle also provides the --profile command-line option. This option records the time that certain tasks take to complete. The data is saved in an HTML file in the build/reports/profile directory. We can open this file in a web browser and check the time taken for several phases in the build process. The following image shows the HTML contents of the profile report:

HTML page with profiling information

Finally, we take a look at the --gui command-line option. With this option, we start a graphical shell for our Gradle builds. Until now, we used the command line to start a task. With the Gradle GUI, we have a graphical overview of the tasks in a project and we can execute them by simply clicking on the mouse.

To start the GUI, we invoke the following command:

$ gradle --gui

A window is opened with a graphical overview of our task tree. We only have one task that one is shown in the task tree, as we can seen in the following screenshot:

Overview of tasks in the Gradle GUI

The output of running a task is shown at the bottom of the window. When we start the GUI for the first time, the tasks task is executed and we see the output in the window.

Task tree

The Task Tree tab shows projects and tasks found in our build project. We can execute a task by double-clicking on the task name.

By default, all the tasks are shown, but we can apply a filter to show or hide certain projects and tasks. The Edit filter button opens a new dialog window where we can define the tasks and properties that are a part of the filter. The Toggle filter button makes the filter active or inactive.

We can also right-click on the project and task names. This opens a context menu where we can choose to execute the task, add it to the favorites, hide it (adds it to the filter), or edit the build file. If we have associated the .gradle extension to a text editor in our operating system, then the editor is opened with the content of the build script. These options can be seen in the following screenshot:

Available actions for a task in the Gradle GUI

Favorites

The Favorites tab stores tasks we want to execute regularly. We can add a task by right-clicking on the task in the Task Tree tab and selecting the Add To Favorites menu option, or if we have opened the Favorites tab, we can select the Add button and manually enter the project and task name that we want to add to our favorites list. We can see the Add Favorite dialog window in the following screenshot:

Add favorites in the Gradle GUI

Command line

On the Command Line tab, we can enter any Gradle command that we normally would enter on the command prompt. The command can be added to Favorites as well. We see the Command Line tab contents in the following image:

Execute task with command-line optins

Setup

The last tab is the Setup tab. Here, we can change the project directory, which is set to the current directory by default.

We discussed about the different logging levels as command-line options previously in this chapter. With the GUI, we can select the logging level from the Log Level select box with the different log levels. We can choose debug, info, Lifecycle, and error as log levels. The error log level only shows errors and is the least verbose, while debug is the most verbose log level. The lifecycle log level is the default log level.

Here, we also can set how detailed the exeption stack trace information should be. In the Stack Trace Output section, we can choose from the following three options:

• Exceptions Only: This is for only showing the exceptions when they occur, which is the default value

• Standard Stack Trace (-s): This is for showing more stack trace information for the exceptions

• -S): This is for the most verbose stack trace information for exceptions

If we enable the Only Show Output When Error Occurs option, then we only get output from the build process if the build fails. Otherwise, we don't get any output.

Finally, we can define a different way to start Gradle for the build with the Use Custom Gradle Executor option. For example, we can define a different batch or script file with extra setup information to run the build process. The following screenshot shows the Setup tab page and all the options that we can set:

Setup Gradle options in the Gradle GUI

So, now we have discussed how to install Gradle on our computers. We have written our first Gradle build script with a simple task.

We have also seen how we use the built-in tasks of Gradle to get more information about a project. We discussed how to use the command-line options to help us run the build scripts. We have looked at the Gradle GUI and how we can use it to run Gradle build scripts.

In the next chapter, we will take a further look at tasks. We will discuss how to add actions to a task. We write more complex tasks, where the tasks will depend on other tasks. We will also discuss how Gradle builds up a task graph internally and how to use this in our projects.