Spring Boot - Packaging

Spring Boot provides an easy and consistent way to package applications for deployment. The main goal is to create a self-contained, executable JAR or WAR that includes everything necessary to run the application, including embedded server, application code, and other dependencies.

Let's understand the packaging in Spring Boot:

1. Packaging Types

There are mainly two types of packaging:

JAR (Java ARchive): This is the most common packaging in Spring Boot applications. It produces a single, executable JAR that contains all the required dependencies. It's suitable for microservice architectures and cloud deployments.
WAR (Web ARchive): Used when you want to deploy your application to an external servlet container like Tomcat, JBoss, etc.

2. JAR Packaging

By default, Spring Boot packages your application as an executable JAR. The generated JAR contains an embedded Tomcat, Jetty, or Undertow servlet container, which means you can run it from the command line without needing an external server.

For JAR packaging, use the following in your pom.xml:

<packaging>jar</packaging>

To build and run:

mvn clean package
java -jar target/your-artifactId-version.jar

3. WAR Packaging

If you want to deploy your application to an external servlet container, you can package it as a WAR.

First, change the packaging type in your pom.xml:

<packaging>war</packaging>

Then, you'll need to mark the embedded servlet container as provided (so it won't be included in the WAR):

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-tomcat</artifactId>
    <scope>provided</scope>
</dependency>

After packaging the application with mvn clean package, you can deploy the resulting WAR to any servlet container.

4. Spring Boot Maven Plugin

The spring-boot-maven-plugin facilitates packaging and running Spring Boot applications. It repackages your JAR/WAR to be executable and provides the spring-boot:run goal to run the application.

<build>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
        </plugin>
    </plugins>
</build>

By including this plugin, you can run:

mvn spring-boot:run

This will start your application using the embedded server.

5. Layers (Since Spring Boot 2.3.0+)

Spring Boot 2.3.0 introduced support for packaging applications into layers, allowing for optimizations when building Docker images. This is beneficial in scenarios where the application's dependencies change less frequently than its own code.

To enable layering, add the following configuration to the spring-boot-maven-plugin:

<configuration>
    <layers>
        <enabled>true</enabled>
    </layers>
</configuration>

Conclusion:

Packaging in Spring Boot is streamlined to simplify deployment scenarios. With the flexibility to package applications as either JARs or WARs and the additional support for layers in Docker environments, Spring Boot addresses various deployment needs.

Executable JAR vs. WAR in Spring Boot:
- Description: Spring Boot supports both JAR (Java Archive) and WAR (Web Archive) packaging formats. JARs are executable and self-contained, while WARs are typically deployed in servlet containers.
- Code: Configure the packaging format in the pom.xml or build.gradle file:
```

<packaging>jar</packaging>


<packaging>war</packaging>
```

Customizing the packaging process in Spring Boot:

Description: Customize the packaging process by modifying the build configuration. For example, exclude unnecessary files or specify additional resources.

Code: Customize the build configuration in the pom.xml or build.gradle file:

<!-- Maven example -->
<build>
    <resources>
        <resource>
            <directory>src/main/resources</directory>
            <filtering>true</filtering>
            <excludes>
                <exclude>unwanted-file.txt</exclude>
            </excludes>
        </resource>
    </resources>
</build>

Building and packaging with Maven in Spring Boot:
- Description: Maven is a popular build tool for packaging Spring Boot applications. Use the mvn package command to build and package the application.
- Code: Run the following command in the project directory:
```
mvn clean package
```
Containerization of Spring Boot applications (Docker):
- Description: Docker enables containerization, making it easy to package and distribute Spring Boot applications with their dependencies.
- Code: Create a Dockerfile to describe the container image:
```
FROM openjdk:11
COPY target/myapp.jar /app.jar
ENTRYPOINT ["java", "-jar", "/app.jar"]
```
Packaging Spring Boot applications with Gradle:
- Description: Gradle is an alternative build tool to Maven. It is highly customizable and provides efficient dependency management.
- Code: Configure the packaging in the build.gradle file:
```
// For JAR packaging
apply plugin: 'java'

// For WAR packaging
apply plugin: 'war'
```
Creating a self-contained executable with Spring Boot:
- Description: Spring Boot provides options for creating self-contained executables, making it easy to run applications without needing an external Java installation.
- Code: Use the spring-boot-maven-plugin or spring-boot-gradle-plugin:
```

<build>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
        </plugin>
    </plugins>
</build>
```