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How does exec.mainClass work without exec-maven-plugin?

I have a project that uses the MojoHaus Exec Maven plugin to run some Java code. Here's the pom.xml file:
<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>io.happycoding</groupId>
<artifactId>google-cloud-vision-hello-world-standalone</artifactId>
<version>1</version>
<properties>
<mainClass>io.happycoding.vision.CloudVisionHelloWorld</mainClass>
<exec.cleanupDaemonThreads>false</exec.cleanupDaemonThreads>
<maven.compiler.source>11</maven.compiler.source>
<maven.compiler.target>11</maven.compiler.target>
</properties>
<dependencies>
<dependency>
<groupId>com.google.cloud</groupId>
<artifactId>google-cloud-vision</artifactId>
<version>1.100.0</version>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>exec-maven-plugin</artifactId>
<version>3.0.0</version>
<executions>
<execution>
<goals>
<goal>java</goal>
</goals>
</execution>
</executions>
<configuration>
<mainClass>${mainClass}</mainClass>
</configuration>
</plugin>
</plugins>
</build>
</project>
This works fine, and I can run my code using this command:
mvn clean package exec:java
I understand that the exec-maven-plugin plugin, which is specified in the plugins tag, runs the code using the mainClass property.
I was surprised to find that this pom.xml also works:
<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>io.happycoding</groupId>
<artifactId>google-cloud-vision-hello-world-standalone</artifactId>
<version>1</version>
<properties>
<exec.mainClass>io.happycoding.vision.CloudVisionHelloWorld</exec.mainClass>
<exec.cleanupDaemonThreads>false</exec.cleanupDaemonThreads>
<maven.compiler.source>11</maven.compiler.source>
<maven.compiler.target>11</maven.compiler.target>
</properties>
<dependencies>
<dependency>
<groupId>com.google.cloud</groupId>
<artifactId>google-cloud-vision</artifactId>
<version>1.100.0</version>
</dependency>
</dependencies>
</project>
This file specifies the exec.mainClass property, but does not specify any plugins. However, I can still run my code just fine with this same command:
mvn clean package exec:java
But I don't understand how Maven knows to run this command without any plugins specified.
Is the Exec Maven plugin somehow automatically installed in Maven by default? Or is exec.mainClass somehow setting a property used by a different default tool within Maven?
I tried reading the official documentation, but I didn't see anything that mentions whether the plugin is included by default.
I've found that I can also pass the exec.mainClass property in as a command line argument, but I still don't understand how Maven knows what to do with that without the plugin being explicitly defined.
I much prefer the shorter file, but I want to make sure I understand how it's working and that I'm not missing anything that's going to bite me later.

When you specify exec:java, you are specifying a plugin, specifically exec-maven-plugin (along with the goal java). Other common plugins that are used by being explicitly identified on the command line rather than being attached to phases such as clean or package include versions, dependency, and archetype (this last of which doesn't even require a POM to be present, since it generally creates new ones).
Note that in your POM you don't attach exec to any phases (that plugin usually isn't); therefore your plugin entry serves only to provide configuration settings in the case you run the plugin explicitly from the command line, in your specific case equivalent to the exec.mainClass property.

Is it possible to have Maven "WAR" pom.xml package up my classes up in a JAR and put the JAR in the /WEB-INF/lib folder?

I have a simple Java Spring MVC web app. I build the WAR file using a Maven pom.xml file.
When I do a "maven install", it produces a WAR file with my compiled Java classes in the /WEB-INFO/classes folder.
My maven pom.xml looks like this (dependencies commented out) ...
<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/maven-v4_0_0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>mycompany.com</groupId>
<artifactId>myapp</artifactId>
<packaging>war</packaging>
<version>0.0.1-SNAPSHOT</version>
<name>My Application</name>
<url>http://maven.apache.org</url>
<dependencies>
<!-- .... -->
</dependencies>
<build>
<finalName>myapp</finalName>
</build>
</project>
Next I want to obfuscate my Java classes (I'll be using Allatori obfuscater), so I'm thinking the easiest thing would be if my Java classes were all put into their own JAR file and stored in the /WEB-INF/lib folder with the rest of the JARs.
Is there a way to modify my Maven pom.xml file so it will package of my classes up in a JAR and put the JAR in the /WEB-INF/lib folder?
UPDATE:
Adding this to the 'build' section (as suggested by "JF Meier" worked) ...
<build>
<finalName>myapp</finalName>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-war-plugin</artifactId>
<version>3.3.0</version>
<configuration>
<archiveClasses>true</archiveClasses>
</configuration>
</plugin>
</plugins>
</build>

You can use the <archiveClasses> configuration (set it to true) to generate an additional jar with the classes.
Also see:
https://maven.apache.org/plugins/maven-war-plugin/war-mojo.html

Move your Java classes in a separate Maven module and add that as a dependency to your WAR project. Then you can do whatever is necessary creating the jar in that module.

spring-boot Maven: How to create executable jar with main class?

Recently, I wrote a Spring-Boot project and I wanted that Maven will be able to create a jar file that I'll be able to run by the command "java -jar ".
This is the pom.xml I wrote:
<?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>SpringBootGame</groupId>
<artifactId>SpringBootGame</artifactId>
<version>1.0-SNAPSHOT</version>
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
<version>2.0.3.RELEASE</version>
</dependency>
</dependencies>
<properties>
<java.version>1.8</java.version>
</properties>
<build>
<plugins>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
<version>2.0.3.RELEASE</version>
<configuration>
<mainClass>com.game.Main</mainClass>
</configuration>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<configuration>
<source>1.8</source>
<target>1.8</target>
</configuration>
</plugin>
</plugins>
</build>
</project>
In order to build the jar file, I had to run the command: "mvn clean package spring-boot:repackage".
My questions are:
Why must I add spring-boot-maven-plugin? I already have spring-boot-starter-web dependency that adds spring-boot as a dependency and maven-compiler-plugin that builds a jar file from the code?
Can you think about a way to configure pom.xml file that I'll be able to get the jar file using the "native" command "mvn clean package" and not the cumbersome "mvn clean package spring-boot:repackage"?
Thanks

Why must I add spring-boot-maven-plugin? I already have spring-boot-starter-web dependency that adds spring-boot as a dependency and maven-compiler-plugin that builds a jar file from the code?
Because the plugin is what adds Maven support for Spring Boot
Can you think about a way to configure pom.xml file that I'll be able to get the jar file using the "native" command "mvn clean package" and not the cumbersome "mvn clean package spring-boot:repackage"?
It looks like you are missing the <packaging>jar</packaging> element in between your <project> </project> element.
The reason you had to run that lengthy command, is because you did not include the <executions></executions> element when including the plugin. Please see section 71.1 of the following docs:
https://docs.spring.io/spring-boot/docs/current/reference/html/build-tool-plugins-maven-plugin.html#build-tool-plugins-include-maven-plugin
71.2 elaborates on the <packaging> element.

In the configuration of the maven plugin you have already in place, you need to add the executable block like :
<configuration>
<executable>true</executable>
</configuration>
this will create in the target folder the runnable jar that can be run by using java -jar command.

mvn process-resources doesn't pull down uber jar created with shade plugin

Goal: Create an executable uber jar with maven shade plugin that can be executed during the mvn compile of another pom.
Repro steps:
Create a pom.xml for the "publisher" component using below pom.
Use a Jenkins build to mvn deploy it (mvn install will work as well)
Add dependency to pom.xml for "consumer" (pom below)
mvn compile the consumer
Expected behavior:
Uber jar for publisher is downloaded somewhere in consumer/target directories
Actual:
Uber jar does not appear in consumer directory
Component 1: Publisher
<?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>com.mec.experiment</groupId>
<artifactId>publisher</artifactId>
<version>1.0-SNAPSHOT</version>
<packaging>jar</packaging>
<dependencies>
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-core</artifactId>
<version>5.2.6.Final</version>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-jar-plugin</artifactId>
<configuration>
<archive>
<manifestFile>src/main/resources/META-INF/MANIFEST.mf</manifestFile>
</archive>
</configuration>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-shade-plugin</artifactId>
<version>2.3</version>
<executions>
<execution>
<phase>package</phase>
<goals>
<goal>shade</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</build>
Component 2: Consumer
<?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>com.mec.experiment</groupId>
<artifactId>consumer</artifactId>
<version>1.0-SNAPSHOT</version>
<dependencies>
<dependency>
<groupId>com.mec.experiment</groupId>
<artifactId>publisher</artifactId>
<version>1.0-SNAPSHOT</version>
</dependency>
</dependencies>

The answer from the possible duplicate I linked to in the comments has a link to a dead example page. Here's a supplement for you. The plugin configuration would belong inside the consumer pom.
exec:java
Full name:
org.codehaus.mojo:exec-maven-plugin:1.5.0:java
Description:
Executes the supplied java class in the current VM with the enclosing project's dependencies as classpath.
Attributes:
Requires a Maven project to be executed.
Requires dependency resolution of artifacts in scope: test.
The goal is thread-safe and supports parallel builds.
Since version: 1.0.
See especially executableDependency for your use case. That looks like it will allow you to reference producer according to its group id and artifact instead of hard-coding a path.

In Maven how to exclude src files in compilation when having custom src path?

I was trying to convert old java projects into maven build, but facing difficulties excluding source files:
when I set goal to install for the parent project to compile and build jar files for all the projects, it still tries to compile the mentioned excluded java file. Below is my pom.xml for that project:
<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.app</groupId>
<artifactId>CCAPS.Impl</artifactId>
<version>1.0.0</version>
<parent>
<groupId>com.app</groupId>
<artifactId>parent</artifactId>
<version>1.0</version>
<relativePath>../Maven.Convertion/parent</relativePath>
</parent>
<properties>
...
</properties>
<dependencies>
...
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<configuration>
<excludes>
<exclude>**/adjudication/mapper/BusinessAddendumItemDBOMapper.java</exclude>
<exclude>**/adjudication/mapper/ContactInfoDBOMapper.java</exclude>
<exclude>**/adjudication/mapper/IncomeDBOMapper.java</exclude>
<exclude>**/adjudication/mapper/IncomeItemDBOMapper.java</exclude>
<exclude>**/adjudication/mapper/InternationalAddressDBOMapper.java</exclude>
<exclude>**/adjudication/mapper/ReferralSourceDBOMapper.java</exclude>
<exclude>**/adjudication/interceptor/*.java</exclude>
</excludes>
</configuration>
</plugin>
</plugins>
<sourceDirectory>${project.basedir}/src/net/ccapsws/ds/adjudication</sourceDirectory>
</build>
</project>
Last line for exclude is where the error throws when compiling:
[ERROR] \Maven Convertion\CCAPSAdjudication\DataSource.CCAPS.Impl\src\net\gc\ccapsws\ds\adjudication\interceptor\CCAPSDTOAuditInterceptor.java:[8,46] error: package net.ccapsws.validation.dictionary does not exist
The thing is I tried with the exclude declaration but since the src path is custom (${project.basedir}/src/net/ccapsws/ds/adjudicationm, for example, is the src path), I suspect maven doesn't recognize the path? Anyone can help with this?
NOTE that I'm not asking resource files, I want to compile files inside src folder but also willing to exclude specific java files during compilation.
EDIT: There's one another project referencing this project, but in the parent POM I'm putting that project after this project in the reactor sequence, so I don't think anywhere else is referencing this project.

The path issue looks similar to the problem answered by these links: https://stackoverflow.com/a/25262893/4055837 and https://stackoverflow.com/a/39450549/4055837