Hot reload in Spring Boot Application

[julienroy50]

Hello, I have an Spring Boot 3 application package in war. We have maven and are using the liberty-maven-plugin. The pom looks like this

<?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 https://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>3.4.0</version>
		<relativePath /> <!-- lookup parent from repository -->
	</parent>
	<groupId>com.ibm.brmt</groupId>
	<artifactId>app</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<name>app</name>
	<packaging>war</packaging>
	<description>APP</description>
	<url />
	<licenses>
		<license />
	</licenses>
	<developers>
		<developer />
	</developers>
	<scm>
		<connection />
		<developerConnection />
		<tag />
		<url />
	</scm>
	<properties>
		<java.version>21</java.version>
	</properties>
	<dependencies>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-actuator</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-web</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-thymeleaf</artifactId>
		</dependency>
		<dependency>
			<groupId>io.micrometer</groupId>
			<artifactId>micrometer-registry-prometheus</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-devtools</artifactId>
			<scope>runtime</scope>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-test</artifactId>
			<scope>test</scope>
		</dependency>
	</dependencies>
	<build>
		<plugins>
			<plugin>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-maven-plugin</artifactId>
			</plugin>
			<plugin>
				<groupId>io.openliberty.tools</groupId>
				<artifactId>liberty-maven-plugin</artifactId>
				<version>3.11.1</version>
				<configuration>
					<serverName>app</serverName>
					<skipInstallFeature>true</skipInstallFeature>
					<skipTests>true</skipTests>
				</configuration>
			</plugin>
		</plugins>
	</build>
</project>

Every times we are modifying the code, we need to restart the server in order to take changes into account. Is it possible to have hot reload? Also the restart takes around 10 sec. Is it possible to tweak this to have a shorter restart time in developement mode?

[scottkurz]

I'm not sure why you need to restart the server after making app changes.

Generally speaking, since you're using a WAR package, it should be possible to use the Liberty Maven Plugin "dev mode" to achieve "hot reload" like any WAR. You do need to use the SpringBootServletInitializer to bootstrap the Spring objects from within the web container.

This is explained in my article Running a Spring Boot 3.x application WAR file on Liberty, which is a bit over-simplified to show the different options using a Spring Boot WAR in Liberty.

The article explains too that you can also, alternatively, use the <springBootApplication> optimized deployment for a Spring Boot WAR, which allows you to use thinning and app args but unfortunately does not at this point in time integrate with the Liberty Maven Plugin "dev mode" to offer a hot reload.

Does that help?

[julienroy50]

I do not know why my hot reload does not work. I did initialize the project with https://start.spring.io/ initially. I attached the projects here. It has no sensible information in it.
gap.zip

[scottkurz]

OK, I see your problem is that you need to delete this from your server.xml.
<applicationMonitor updateTrigger="mbean" />

The mbean trigger is helpful when using certain approaches like some of our older Eclipse IDE tooling to deploy, but the default monitoring is better for using dev mode, so you can just delete this element.

[julienroy50]

It works! Thank you.

[cherylking]

@julienroy50 Great news. Can I close this discussion then?

[julienroy50]

Yes thank you