Gradle Build Tool

1) What is Gradle? Why do we really need it?

Gradle's purpose is to solve the problem of building and deploying the application once we've written it.

Gradle helps us do that in a consistent and automated way that's going to run on our server or mobile devices.

The particular things that Gradle reallly help us are:

• Builds deployment artifacts. Artifact - files, Java source and class files, XML configuration files, assets
• Manages dependencies

While ANT and Maven use XML to describe the build file, Gradle use GROOVY, which is a variant of Java.

2) Plugin

A Plugin is a set of tasks that allows us to build a Java project. Without the plugin, Gradle does not know how to build the project.

We can think Gradle as a worker but he needs guidelines to build a house. In this sense, these guidelines are plugins. And a guidelines can contain many actions, which can be thought that a plugin can contain many tasks.

Furthermore, the worker can share the guidelines to other workers. Hence, a Gradle plugin can be used across many projects.

Below, I show some common Gradle tasks. More tasks are shown later.

• Clean: clean up the Java byte-code that the Gradle writes out
• Build: compile the Java code and generate byte-code, JAR artifact, assets
• Test: run our unit tests

I will introduce more about Plugin in Section 5.3. It is OK that we can skip it for a while before coming back.

3) Add Gradle to an existing project (IntelliJ IDEA)

We need to create a 'build.gradle' file in the same directory level with 'src'.

Right click in build.gradle file, select Import Gradle Project

Inside build.gradle, we declare 4 things like this:

After that, we open a Terminal (cmd) inside the project folder, and run:

gradle build

Gradle now executes all tasks inside 'java' plugin, and prints out the result of the build: BUILD SUCCESSFUL

Before moving on, let's explain line 10 of the build file above.

compile: 'com.google.code.gson:gson:2.8.0'

• compile - configuration name (to be discussed)
• com.google.code.gson:gson:2.8.0 - Library specification, containing 3 parts separated by semicolons
  • com.google.code.gson: company or organisation which develops this library
  • gson: unique library name in this company
  • 2.8.0: version of library

4) Gradle Wrapper - Gradlew

What does Gradle Wrapper do?

• It checks if the requested version of Gradle is installed. If not, it will install it automatically
• Passes the command onto the real Gradle

Why Gradle Wrapper is important? Because the build instruction may vary across Gradle versions.

• If someone builds this project, he will build it under the same version of your Gradle.
• CI/CD: if you're publishing your project to a continuous integration server, such as Jenkins, when the build server runs your scripts again, it will be using the exact same version of Gradle, so the build will be exactly the same.

How can you know if you have Gradle Wrapper? If you have all these things:

+gradlew (Linux) or gradlew.bat (Windows)

+gradle folder, containing

  +wrapper

    +gradle-wrapper.jar, and

    +gradle-wrapper.properties

If you have none of them, you can create the wrapper by running:

gradle wrapper --gradle-version 5.4.1 (or different versions)

Note: the requested version of wrapper is specified in the file gradle-wrapper.properties, in the value of distributionUrl

Project object

Everything inside build.gradle is relavant to Project object. For example,

sourceCompatibility=1.8 is actually an assigning operation of project.sourceCompatibility=1.8, similar to dependencies, repository, etc...

settings.gradle : for declaring the rootProject, and include sub projects (SubModules)

// settings.gradle

rootProject.name = "java-project"

include "sub-project-1"

include "sub-project-2"

5) Task and Plugin

5.1) Task executing order

After executing a task, in console, it lists all tasks which are executed just before running the requested task.

For example, if you execute gradle assemble --console=verbose, it shows

> Task :compileJava UP-TO-DATE

> Task :processResources NO-SOURCE

> Task :classes UP-TO-DATE

> Task :jar UP-TO-DATE

> Task :assemble UP-TO-DATE (the last requested task)

Another example: gradle build --console=verbose

> Task :compileJava UP-TO-DATE

> Task :processResources NO-SOURCE

> Task :classes UP-TO-DATE

> Task :jar UP-TO-DATE

> Task :assemble UP-TO-DATE

> Task :compileTestJava NO-SOURCE

> Task :processTestResources NO-SOURCE

> Task :testClasses UP-TO-DATE

> Task :test NO-SOURCE

> Task :check UP-TO-DATE

> Task :build UP-TO-DATE

5.2) Create a task

In build.gradle, create a new task to print out the current date, like this

task showDate{

    doLast {

        println "Current Date" + new Date()

    }

    dependsOn(assemble) 

    group = "My tasks"

    description = "Show current date" 

}

In which,

doLast { ... } : perform particular actions.

dependsOn(assemble): if you run in terminal 'gradle showDate', it will run assemble task before executing showDate

group: group whose this task is in

5.3) Plugin

As I mention earlier in Section 2, a plugin contains a set of tasks.

Plugin is a JAR file. And it is also included to the project. At this point, you may think that Plugin is same as Dependency.

It is both True and False.

It is true because it is a library that something rely on.

However, it is false because it is not a dependency on the project source code, this is a dependency for the build script itself