Gradle for Java

This post is a quick Gradle tutorial for Java developers. It shows how to use Gradle to build, test and run Java desktop and web application.

Table of Content

What is Gradle?

• Gradle is an open-source build tool that helps developers automating building tasks.
• Gradle allows you to write scripts in Groovy or Kotlin to define tasks to be executed on your code.
• Tasks such as compiling, testing or creating jar file can be automated using Gradle.
• Gradle can be used to define dependencies or jar files in case of Java to run your code.
• Gradle has a core Java plugin that includes many tasks needed for Java development.
• In the case of web application, Gradle can run your web application using plugin server.
• Gradle is your best friend!

Gradle Installation

• Gradle can be installed on Linux, macOS, or Windows.
• Gradle requires only a Java JDK version 7 or higher.
• Download the latest release. 4.9 is the latest as of now!
• Follow installation steps.
• Verifying installation by running gradle -v in command line window.

C:\>gradle -v

------------------------------------------------------------
Gradle 4.9
------------------------------------------------------------

Build time:   2018-07-16 08:14:03 UTC
Revision:     efcf8c1cf533b03c70f394f270f46a174c738efc

Kotlin DSL:   0.18.4
Kotlin:       1.2.41
Groovy:       2.4.12
Ant:          Apache Ant(TM) version 1.9.11 compiled on March 23 2018
JVM:          1.8.0_71 (Oracle Corporation 25.71-b15)
OS:           Windows 8.1 6.3 amd64

First Gradle Project

• Create a new directory e.g. C:\Gradle>first-gradle-project
• Open command line window and go to the dirctory created in the previous step.
• Type gradle init

C:\Gradle\first-gradle-project>gradle init
Starting a Gradle Daemon, 1 incompatible and 1 stopped Daemons could not be reused, 
use --status for details

BUILD SUCCESSFUL in 11s
2 actionable tasks: 2 executed

• Gradle will create new files and sub directories within the parent directory.

• build.gradle this is the main file where we write scripts in Groovy or Kotlin for Gradle to execute.
• Open build.gralde file in a text editor and type the following script.

task hello{
	println "Hello, World!"
}

• Open command prompt, go to the project location and type gradle hello

C:\Gradle\first-gradle-project>gradle hello
Hello, World!

Gradle Fundamentals

• Everything in Gradle sits on top of two basic concepts: projects and tasks.
• Every Gradle build is made up of one or more projects.
• A project represents a thing to be built e.g. JAR file or to be done e.g. deploying to production.
• A Task represents a single atomic piece of work for a build, such as compiling classes or generating javadoc.
• Gradle describes its build using build file build.gradle.
• Gradle build file is located in the root folder of the project.
• Build file defines projects, tasks, plugins, dependencies…ect.
• Gradle uses plugins to extend its core functionality.
• For example, the ability to compile Java code is added by a plugin.
• Gradle has built-in support for dependency management for declaring, resolving and using dependencies required by the project.

Gradle & Java Projects

• While Gradle can be used with other languages we will focus on using Gradle with Java projects.
• Gradle is a build tool automating the creation of Jar/War build.
• Gradle helps automating a wide variety of tasks such as compiling Java classes, packaging binary code & running Junit tests.
• Gradle allows you to declare dependencies and repositories of Jar files needed for your Java project to compile or run.

Building Java Application

1. Create new directory

• Create a new directory e.g. gradle-java-app
• Go to the directory

C:\gradle-projects>mkdir gradle-java-app

C:\gradle-projects>cd gradle-java-app

C:\gradle-projects\gradle-java-app>

2. init java-application

• To create new Java application call gradle init --type java-application

C:\gradle-projects\gradle-java-app>gradle init --type java-application

Starting a Gradle Daemon

BUILD SUCCESSFUL in 8s
2 actionable tasks: 2 executed

• Gradle will create src folder with two sub-folders main and test
• build.gradle will apply java and application plugin, point to guava lib in dependencies and to jcenter() repository.

plugins {
    id 'java'
    id 'application'
}

mainClassName = 'App'

dependencies {
    compile 'com.google.guava:guava:23.0'
    testCompile 'junit:junit:4.12'
}

repositories {
    jcenter()
}

• App.java has main method that print Hello, World! when executed.

public class App {
    public String getGreeting() {
        return "Hello world.";
    }

    public static void main(String[] args) {
        System.out.println(new App().getGreeting());
    }
}

3. build Java Applicaiton

• To build Java application call gradle build task.
• build task depends on 6 other tasks such as compileJava task.

C:\gradle-projects\gradle-java-app>gradle build

BUILD SUCCESSFUL in 0s
7 actionable tasks: 7 up-to-date

• To see all executed tasks call --console=plain build

4. run Java Application

• build.gradle is pointing to the main class mainClassName = 'App'.
• To run main class call gradle run

C:\gradle-projects\gradle-java-app>gradle run

> Task :run
Hello world.

BUILD SUCCESSFUL in 0s
2 actionable tasks: 1 executed, 1 up-to-date

Building Java Web Application

1. Create new directory

• Create a new directory e.g. gradle-java-web-app
• Go to the directory

C:\gradle-projects>mkdir gradle-java-web-app

C:\gradle-projects>cd gradle-java-web-app

C:\gradle-projects\gradle-java-web-app>

2. init java-application

• So far, there is no init type for java web application, so we will use java-appliation as a starter.
• While gradle init --type java-application will not create java web application, it will help creating src and build.gradle file.

C:\gradle-projects\gradle-java-web-app>gradle init --type java-application

BUILD SUCCESSFUL in 0s
2 actionable tasks: 2 executed

• Change the content of build.gradle to be as following
  • use the war plugin
  • and add servlet-api to dependencies

plugins {
    id 'war'  
}

repositories {
    jcenter()
}

dependencies {
    providedCompile 'javax.servlet:javax.servlet-api:3.1.0' 
    testCompile 'junit:junit:4.12'
}

3. Create a servlet class

• Under /scr/main/java/com/hmkcode/ create a Java class HelloServlet.java

package com.hmkcode;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet(name = "HelloServlet", urlPatterns = {"hello"}, loadOnStartup = 1) 
public class HelloServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
        response.getWriter().print("Hello, World!");  
    }

}

4. Add .html file

• Add an index page to the root of the application by creating the file index.html.

<html>
<head>
  <title>Java Web App</title>
</head>
<body>
<p>Say<a href="hello">Hello</a></p> 

</body>
</html>

5. Add the gretty plugin and run the app

• Gretty plugin makes it easy to run or test webapps

build.gradle

plugins {
    id 'war'
    id 'org.gretty' version '2.2.0' 
}

6. Run the web app

• Use gradle appRun to run the web app.

C:\gradle-projects\gradle-java-web-app>gradle appRun

• Open your web browser and type http://localhost:8080/gradle-java-web-app/