How to migrate a Maven based web application POM to use Gradle? - maven

Question

Yes, yes, I know! There are hundreds of Maven-to-Gradle related questions here on SO, but none of them, AFAICT addresses the inherent difficulties (of a constantly moving target [Gradle!]) of porting a very simple Maven POM to use Gradle for compilation.
In this particular case, we have an app the original developers are relying on using the GWT plugin for Eclipse, but that many other developers would like to use something more modern and less bloated, i.e. Gradle. All sources simply brush this off to, "you can simply convert pom.xml in Gradle", as is stated in the Gradle documentation, unfortunately this is simply not working for most practical purposes.
The project under consideration is circuitJS1 which was ported to use Maven here. The resulting pom.xml reads:
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.lushprojects.circuitjs1</groupId>
<artifactId>circuitjs</artifactId>
<version>2.1.15</version>
<packaging>gwt-app</packaging>
<dependencyManagement>
<dependencies>
<dependency>
<groupId>com.google.gwt</groupId>
<artifactId>gwt</artifactId>
<version>2.8.2</version>
<type>pom</type>
<scope>import</scope>
</dependency>
</dependencies>
</dependencyManagement>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
<!-- generating dependency report is slow; disable it -->
<dependency.locations.enabled>false</dependency.locations.enabled>
</properties>
<dependencies>
<dependency>
<groupId>com.google.gwt</groupId>
<artifactId>gwt-user</artifactId>
</dependency>
<dependency>
<groupId>com.google.gwt</groupId>
<artifactId>gwt-dev</artifactId>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.7.0</version>
<configuration>
<source>1.8</source>
<target>1.8</target>
</configuration>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-source-plugin</artifactId> <!-- artifact with sources is easier to handle during development -->
<version>3.0.1</version>
<executions>
<execution>
<id>attach-sources</id>
<goals>
<goal>jar</goal>
</goals>
</execution>
</executions>
</plugin>
<!-- declaring only in order to skip during site deployment -->
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-site-plugin</artifactId>
<version>3.7</version>
<executions>
<execution>
<id>deploy</id>
<phase>deploy</phase>
<configuration>
<skip>true</skip>
</configuration>
</execution>
</executions>
<configuration>
<skip>true</skip>
<siteDirectory>site</siteDirectory>
</configuration>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-project-info-reports-plugin</artifactId>
<version>2.9</version>
<configuration><!-- we don't need those reports; disabling speeds up
build -->
<dependencyDetailsEnabled>false</dependencyDetailsEnabled>
<dependencyLocationsEnabled>false</dependencyLocationsEnabled>
</configuration>
</plugin>
<!-- gwt compiler -->
<plugin>
<groupId>net.ltgt.gwt.maven</groupId>
<artifactId>gwt-maven-plugin</artifactId>
<version>1.0-rc-9</version>
<extensions>true</extensions>
<configuration>
<moduleName>com.lushprojects.circuitjs1.circuitjs1</moduleName>
<!-- this is the best setting for a laptop with 2 cores and HT -->
<localWorkers>0.5C</localWorkers>
<warName>circuitjs</warName>
<optimize>9</optimize>
<compilerArgs>
<compilerArg>-style</compilerArg>
<compilerArg>PRETTY</compilerArg>
</compilerArgs>
<codeServerPort>8888</codeServerPort>
</configuration>
</plugin>
<!-- copy a few things around before packaging the website -->
<plugin>
<artifactId>maven-resources-plugin</artifactId>
<version>3.0.2</version>
<executions>
<execution>
<id>copy-resources</id>
<phase>install</phase>
<goals>
<goal>copy-resources</goal>
</goals>
<configuration>
<outputDirectory>${project.build.directory}/site</outputDirectory>
<resources>
<resource>
<directory>war</directory>
</resource>
<resource>
<directory>${project.build.directory}/${project.name}-${project.version}/circuitjs1</directory>
<targetPath>circuitjs1</targetPath>
</resource>
</resources>
</configuration>
</execution>
</executions>
</plugin>
</plugins>
</build>
</project>
Cloning that repository and simply doing: mvn install will give you a working (circuit simulation) website, that you can open with your browser, under the file location:
file:///C:/path/to/circuitjs1/target/site/circuitjs.html
However, using gradle init --type pom to convert to Gradle, yield a build.gradle like this:
/* This file was generated by the Gradle 'init' task. */
plugins {
id 'java'
id 'maven-publish'
}
repositories {
mavenLocal()
maven {
url = 'http://repo.maven.apache.org/maven2'
}
}
dependencies {
compile 'com.google.gwt:gwt-user:2.8.2'
compile 'com.google.gwt:gwt-dev:2.8.2'
}
task sourcesJar(type: Jar) {
classifier = 'sources'
from(sourceSets.main.allJava)
}
group = 'com.lushprojects.circuitjs1'
version = '2.1.15'
sourceCompatibility = '1.8'
publishing {
publications {
maven(MavenPublication) {
from(components.java)
artifact(sourcesJar)
}
}
}
tasks.withType(JavaCompile) {
options.encoding = 'UTF-8'
}
Unfortunately, using that with gradle build, doesn't seem to yield anything that can be ran in the browser. It does generate ./build/ directory but which contents are not of any use at all.
build
├── classes
│   └── java
│   └── main
│   └── com
├── generated
│   └── sources
│   └── annotationProcessor
│   └── java
├── libs
│   └── circuitjs-2.1.15.jar
└── tmp
├── compileJava
└── jar
└── MANIFEST.MF
I have also tried to beat this file into shape by adding things like:
//id 'com.gradle.build-scan' version '2.2.1'
id 'java'
id 'maven'
//id 'gwt-maven'
id 'maven-publish'
id 'war'
//id 'org.metahelicase.site-builder' version '1.1'
//id 'org.jbake.site' version '5.0.0'
//id 'gradle.site' version '0.6'
//id 'com.stehno.gradle.webpreview' version '0.3.0'
But to no avail...
How can I make this build.gradle script work correctly to produce the web files necessary to run the application locally in my browser?
Are there any up-to-date tools available that can do this for you?

Answer 1

The problem was that because the Maven build file (pom.xml) had so many plugins in it, that I was thinking they were needed also in Gradle. That is a big mistake because Gradle's build.gradle file only need minimal plugin info. The rest is downloaded and used automagically.
In my case it only needed the Gradle version of GWT, the gwt-gradle-plugin plugin.
The corrected and hugely simplified build.gradle becomes:
// This must be before plugins!
buildscript {
repositories {
mavenCentral()
}
dependencies {
classpath "org.wisepersist:gwt-gradle-plugin:1.0.8"
}
}
plugins {
id 'java'
}
repositories {
mavenLocal()
maven {
url = 'https://plugins.gradle.org/m2/'
url = 'http://repo.maven.apache.org/maven2'
}
}
dependencies {
implementation 'com.google.gwt:gwt-user:2.8.2'
implementation 'com.google.gwt:gwt-dev:2.8.2'
}
apply plugin: 'gwt-compiler'
...
Now you can just run:
gradle compileGwt --console verbose --info
and all files will be built in the ./build directory.