This is a java build tool made for packaging project into jar or war file.
I'm doing this to understand how java build tool like Maven and Gradle actually works in building jar and war from project sources.
JAR, which stands for "Java ARchive", is a format that aggregates classes and metadata. It was orignally designed for serving 'Applets', which are small executable programs that run inside web browser with JVM support, through a network. Downloading an applet as a JAR file is much faster than downloading each class file individually. Even though Java Applets are no longer supported, JAR remains a standard way to deploy Java libraries and executable application.
A JAR has special directory called META-INF. Files inside this directory (like MANIFEST.MF) are recognized by the Java Platform at runtime and are used for configuration. This directory is automatically created when using jar command. Beneath the META-INF directory, there are class files compiled from your Java source codes and other JAR files as dependencies, espacially under lib directory.
WAR is a JAR format for web application. It stores Servlets, JSPs, and static assets like HTML and CSS. It also include a special directory called WEB-INF, which have resources for servlet container. Underneath WEB-INF there are classes and lib directory, which stores class files of servlet and other utility, and dependency Jar files respectivily. Basically, any static files outside of META-INF and WEB-INF are served via there URI. For example, when WAR file that contains hello/index.html is deployed to servlet container running on localhost:8080, that HTML can be access at http://localhost:8080/hello/index.html. But static files inside WEB-INF are not accessable via their URI. So template files and other reousrce files that used by servlet but not directly accesable should be stored under WEB-INF, while other static file should store outside of WEB-INF.
In a Maven project structure there are java, resources, and webapp directories. When you package project into WAR, Java files under java are compile and stored inside WEB-INF/classes, while files under resources are simply copied. Files under webapp are copied to the root of WAR file. To store static files that exposed directly via URI or template files under WEB-INF, you should add that files under webapp. Thus, a Maven project structure looks like this.
├── src
│ ├── main
│ │ ├── java
│ │ │ └── Servlet.java
│ │ ├── resources
│ │ │ └── application.properties
│ │ └── webapp
│ │ ├── WEB-INF
│ │ │ └── template.jsp
│ │ └── index.html
│ └── test
│ └── ...
├── pom.xml
If you build this project, what you will get is like this:
├── META-INF
│ └── MANIFEST.MF
├── WEB-INF
│ ├── classes
│ │ ├── Servlet.class
│ │ └── application.properties
│ ├── lib
│ │ └── dependency.jar
│ └── template.jsp
└── index.html
./package <module>./package --main-class <class> <module>./package --war <module>