my-notes

Apache Maven: Beginner to Guru

DevOps Notes

Some notes by me while taking this course.

Things I want to be able to clarify

First Maven Execution

Noticed that when running some maven tasks for the first time (like clean and package) it downloads artifacts and it takes a lot of time, and the next executions go really fast (no need to download stuff).

Doing this in a pipeline, where the runner is a “fresh” container, results in downloading artifacts allways.

There must be a way to prevent this.

UPDATE: yes, and it’s by using the cache feature of the CI tool (like GitLab CI). Caching dependencies is the way to go.

The very first execution of mvn package for my simple Hello World project took 01:08 min, because maven needed to download a bunch of artifacts from https://repo.maven.apache.org/.

The next executions took less than 1 second!!!

That fact alerted me that probably I could dramatically improve the speed of my GitLab CI pipeline…

Every stage of the pipeline starts a “fresh” container. We have two stages with maven (one to build and another to run a SAST Scanner), I noticed that in both we’re downloading artifacts…

I need to save such artifacts in a cache!

Compiling Java

Creating Java jar files from Command line

This “Hello World”:

public class HelloWorld {
  public  static void main(String[] args) {
    System.out.println("Hello World!");
  }
}

$ # the source is compiled with `javac`
$ javac HelloWorld.java 
$ # it creates the .class file
$ ls
HelloWorld.class  HelloWorld.java
$ # you can run that class with the `java` command:
$ java HelloWorld 
Hello World!
$ # let's build a .jar file with the `jar` command
$ jar cf myjar.jar HelloWorld.class 
$ # let's run the class HelloWorld from that .jar file
$ java -classpath myjar.jar HelloWorld 
Hello World!

Note: the .jar file is actually a zipped file.

In simple terms, what Maven basically does is compiling classes and then package them into a .jar file.

Using 3rd Party Jars with Command Line Java

Note: there’s a commons-lang3-3.8.1.jar file in the zipped “Resources” of the lecture.

# building it
$ javac -classpath ./lib/* HelloWorld.java 

# running it
$ java -classpath ./lib/*:./ HelloWorld 
Hello World!
Hello world

Another thing maven does is managing such dependencies.

Getting Started with Maven

In order to build with Maven we need a pom.xml.

Example for our HelloWorld:

<?xml version="1.0" encoding="UTF-8"?>
<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>guru.springframework</groupId>
    <artifactId>hello-world</artifactId>
    <version>0.0.1-SNAPSHOT</version>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <java.version>11</java.version>
        <maven.compiler.source>${java.version}</maven.compiler.source>
        <maven.compiler.target>${java.version}</maven.compiler.target>
    </properties>

</project>

Pay attention to that <artifactId> and <version>. They’re goint to be used to generate the .jar file.

# clean out the environment (looks similar to `make clean`)
mvn clean

# create a jar file
mvn package

# inside the `target/` folder you'll see a file named
# `hello-world-0.0.1-SNAPSHOT.jar`

# if you run the `clean` again, the target/ directory will be deleted
mvn clean

# put your HelloWorld.java in a subdir
mkdir -p src/main/java
mv HelloWorld.java src/main/java

# you can clean and package in the same command line
mvn clean package

Dependencies

In order to add dependencies to your project, just add the dependency’s coordinates to your pom.xml, like this:

<dependencies>
    <dependency>
        <groupId>org.apache.commons</groupId>
        <artifactId>commons-lang3</artifactId>
        <version>3.8.1</version>
    </dependency>
</dependencies>

In the next run of mvn package it’ll download the dependencies.

Maven Basics

Maven Coordinates

    <groupId>guru.springframework</groupId>
    <artifactId>hello-world</artifactId>
    <version>0.0.1-SNAPSHOT</version>

Maven Repositories

Types:

                      ,-> Central
Project <-> ~/.m2/ <-{
                      '-> Others

Maven Wagon

Maven Wagon is a unified Maven API working as a transport abstraction for artifact and repository handling code.

In some corporate environments you may need to configure Wagon for Proxy Settings.

Maven POM

Maven Dependencies

Dependency is an artifact which your Maven project depends upon. Typically a jar or pom.

Maven does dependency management.

Dependency Mediation: determines what version to use when multiple versions of the same dependency are encountered. Usually the latest wins.

Dependency Scope:

Dependencies are managed by the Maven Dependency Plugin

Important Goals:

Note about mvn syntax: the notation mvn dependency:tree is like mvn ${plugin}:${goal}.

Maven Standard Directory Layout

https://maven.apache.org/guides/introduction/introduction-to-the-standard-directory-layout.html

Basically: src/main/java.

Maven Build Lifecycles

Maven Clean Lifecycle

Maven Clean Lifecycle

Default Lifecycle - High Level

Key concept:

Maven Wrapper

It’s wrapper around maven which allows you to build your project via shell script on a machine without Maven installed.

mvn -N io.takari:maven:wrapper

# specify the maven version:
mvn -N io.takari:maven:wrapper -Dmaven=3.6.0

It creates a hidden folder called .mvn/.

Note: using a wrapper is considered a “best practice”.

Maven Archetypes

Maven Archetypes are project templates.

http://maven.apache.org/archetype/index.html

Example of use:

# from: http://maven.apache.org/archetypes/maven-archetype-simple/
mvn archetype:generate -DarchetypeGroupId=org.apache.maven.archetypes -DarchetypeArtifactId=maven-archetype-simple -DarchetypeVersion=1.4

Common Maven Plugins

Maven Lifecycle Plugin

Maven Clean Plugin

Maven Compiler Plugin

Maven Resources Plugin

Mave Surefire Plugin

Maven jar plugin

Maven Deploy Plugin

Maven Site Plugin

Generating Source with Maven

XSD to Java with Maven

Java Classes from JSON Schema

Generating Java Classes with Project Lombok