Spring Boot Gradle Plugin Reference Guide

1. Introduction

The Spring Boot Gradle Plugin provides Spring Boot support in Gradle. It allows you to package executable jar or war archives, run Spring Boot applications, and use the dependency management provided by spring-boot-dependencies. Spring Boot’s Gradle plugin requires Gradle 6 (6.3 or later). Gradle 5.6 is also supported but this support is deprecated and will be removed in a future release.

In addition to this user guide, API documentation is also available.

2. Getting Started

To get started with the plugin it needs to be applied to your project.

The plugin is published to the Spring milestones repository. Gradle can be configured to use the milestones repository and the plugin can then be applied using the plugins block. To configure Gradle to use the milestones repository, add the following to your settings.gradle (Groovy) or settings.gradle.kts (Kotlin):

Groovy

pluginManagement {
	repositories {
		maven { url 'https://repo.spring.io/milestone' }
		gradlePluginPortal()
	}
}

Kotlin

pluginManagement {
	repositories {
		maven { url = uri("https://repo.spring.io/milestone") }
		gradlePluginPortal()
	}
}

The plugin can then be applied using the plugins block:

Groovy

plugins {
	id 'org.springframework.boot' version '2.4.0-M4'
}

Kotlin

plugins {
	id("org.springframework.boot") version "2.4.0-M4"
}

Applied in isolation the plugin makes few changes to a project. Instead, the plugin detects when certain other plugins are applied and reacts accordingly. For example, when the java plugin is applied a task for building an executable jar is automatically configured. A typical Spring Boot project will apply the groovy, java, or org.jetbrains.kotlin.jvm plugin as a minimum and also use the io.spring.dependency-management plugin or Gradle’s native bom support for dependency management. For example:

Groovy

apply plugin: 'java'
apply plugin: 'io.spring.dependency-management'

Kotlin

plugins {
	java
	id("org.springframework.boot") version "2.4.0-M4"
}

apply(plugin = "io.spring.dependency-management")

To learn more about how the Spring Boot plugin behaves when other plugins are applied please see the section on reacting to other plugins.

3. Managing Dependencies

To manage dependencies in your Spring Boot application, you can either apply the io.spring.dependency-management plugin or, if you are using Gradle 6 or later, use Gradle’s native bom support. The primary benefit of the former is that it offers property-based customization of managed versions, while using the latter will likely result in faster builds.

3.1. Managing Dependencies with the Dependency Management Plugin

When you apply the io.spring.dependency-management plugin, Spring Boot’s plugin will automatically import the spring-boot-dependencies bom from the version of Spring Boot that you are using. This provides a similar dependency management experience to the one that’s enjoyed by Maven users. For example, it allows you to omit version numbers when declaring dependencies that are managed in the bom. To make use of this functionality, declare dependencies in the usual way but omit the version number:

Groovy

dependencies {
	implementation('org.springframework.boot:spring-boot-starter-web')
	implementation('org.springframework.boot:spring-boot-starter-data-jpa')
}

Kotlin

dependencies {
	implementation("org.springframework.boot:spring-boot-starter-web")
	implementation("org.springframework.boot:spring-boot-starter-data-jpa")
}

3.1.1. Customizing Managed Versions

The spring-boot-dependencies bom that is automatically imported when the dependency management plugin is applied uses properties to control the versions of the dependencies that it manages. Browse the Dependency versions Appendix in the Spring Boot reference for a complete list of these properties.

To customize a managed version you set its corresponding property. For example, to customize the version of SLF4J which is controlled by the slf4j.version property:

Groovy

ext['slf4j.version'] = '1.7.20'

Kotlin

extra["slf4j.version"] = "1.7.20"

Each Spring Boot release is designed and tested against a specific set of third-party dependencies. Overriding versions may cause compatibility issues and should be done with care.

3.1.2. Using Spring Boot’s Dependency Management in Isolation

Spring Boot’s dependency management can be used in a project without applying Spring Boot’s plugin to that project. The SpringBootPlugin class provides a BOM_COORDINATES constant that can be used to import the bom without having to know its group ID, artifact ID, or version.

First, configure the project to depend on the Spring Boot plugin but do not apply it:

Groovy

buildscript {
	repositories {
		maven { url 'https://repo.spring.io/libs-milestone' }
	}

	dependencies {
		classpath 'org.springframework.boot:spring-boot-gradle-plugin:2.4.0-M4'
	}
}

Kotlin

plugins {
	id("org.springframework.boot") version "2.4.0-M4" apply false
}

The Spring Boot plugin’s dependency on the dependency management plugin means that you can use the dependency management plugin without having to declare a dependency on it. This also means that you will automatically use the same version of the dependency management plugin as Spring Boot uses.

Apply the dependency management plugin and then configure it to import Spring Boot’s bom:

Groovy

apply plugin: 'io.spring.dependency-management'

dependencyManagement {
	imports {
		mavenBom org.springframework.boot.gradle.plugin.SpringBootPlugin.BOM_COORDINATES
	}
}

Kotlin

apply(plugin = "io.spring.dependency-management")

the<DependencyManagementExtension>().apply {
	imports {
		mavenBom(org.springframework.boot.gradle.plugin.SpringBootPlugin.BOM_COORDINATES)
	}
}

The Kotlin code above is a bit awkward. That’s because we’re using the imperative way of applying the dependency management plugin.

We can make the code less awkward by applying the plugin from the root parent project, or by using the plugins block as we’re doing for the Spring Boot plugin. A downside of this method is that it forces us to specify the version of the dependency management plugin:

plugins {
	java
	id("org.springframework.boot") version "2.4.0-M4" apply false
	id("io.spring.dependency-management") version "1.0.10.RELEASE"
}

dependencyManagement {
	imports {
		mavenBom(org.springframework.boot.gradle.plugin.SpringBootPlugin.BOM_COORDINATES)
	}
}

3.1.3. Learning More

To learn more about the capabilities of the dependency management plugin, please refer to its documentation.

3.2. Managing Dependencies with Gradle’s Bom Support

Gradle allows a bom to be used to manage a project’s versions by declaring it as a platform or enforcedPlatform dependency. A platform dependency treats the versions in the bom as recommendations and other versions and constraints in the dependency graph may cause a version of a dependency other than that declared in the bom to be used. An enforcedPlatform dependency treats the versions in the bom as requirements and they will override any other version found in the dependency graph.

The SpringBootPlugin class provides a BOM_COORDINATES constant that can be used to declare a dependency upon Spring Boot’s bom without having to know its group ID, artifact ID, or version, as shown in the following example:

Groovy

dependencies {
	implementation platform(org.springframework.boot.gradle.plugin.SpringBootPlugin.BOM_COORDINATES)
}

Kotlin

dependencies {
	implementation(platform(org.springframework.boot.gradle.plugin.SpringBootPlugin.BOM_COORDINATES))
}

A platform or enforced platform will only constrain the versions of the configuration in which it has been declared or that extend from the configuration in which it has been declared. As a result, in may be necessary to declare the same dependency in more than one configuration.

3.2.1. Customizing Managed Versions

When using Gradle’s bom support, you cannot use the properties from spring-boot-dependencies to control the versions of the dependencies that it manages. Instead, you must use one of the mechanisms that Gradle provides. One such mechanism is a resolution strategy. SLF4J’s modules are all in the org.slf4j group so their version can be controlled by configuring every dependency in that group to use a particular version, as shown in the following example:

Groovy

configurations.all {
	resolutionStrategy.eachDependency { DependencyResolveDetails details ->
		if (details.requested.group == 'org.slf4j') {
			details.useVersion '1.7.20'
		}
	}
}

Kotlin

configurations.all {
    resolutionStrategy.eachDependency {
        if (requested.group == "org.slf4j") {
            useVersion("1.7.20")
        }
    }
}

Each Spring Boot release is designed and tested against a specific set of third-party dependencies. Overriding versions may cause compatibility issues and should be done with care.

4. Packaging Executable Archives

The plugin can create executable archives (jar files and war files) that contain all of an application’s dependencies and can then be run with java -jar.

4.1. Packaging Executable Jars

Executable jars can be built using the bootJar task. The task is automatically created when the java plugin is applied and is an instance of BootJar. The assemble task is automatically configured to depend upon the bootJar task so running assemble (or build) will also run the bootJar task.

4.2. Packaging Executable Wars

Executable wars can be built using the bootWar task. The task is automatically created when the war plugin is applied and is an instance of BootWar. The assemble task is automatically configured to depend upon the bootWar task so running assemble (or build) will also run the bootWar task.

4.2.1. Packaging Executable and Deployable Wars

A war file can be packaged such that it can be executed using java -jar and deployed to an external container. To do so, the embedded servlet container dependencies should be added to the providedRuntime configuration, for example:

Groovy

dependencies {
	implementation('org.springframework.boot:spring-boot-starter-web')
	providedRuntime('org.springframework.boot:spring-boot-starter-tomcat')
}

Kotlin

dependencies {
	implementation("org.springframework.boot:spring-boot-starter-web")
	providedRuntime("org.springframework.boot:spring-boot-starter-tomcat")
}

This ensures that they are package in the war file’s WEB-INF/lib-provided directory from where they will not conflict with the external container’s own classes.

providedRuntime is preferred to Gradle’s compileOnly configuration as, among other limitations, compileOnly dependencies are not on the test classpath so any web-based integration tests will fail.

4.3. Packaging Executable and Normal Archives

By default, when the bootJar or bootWar tasks are configured, the jar or war tasks are disabled. A project can be configured to build both an executable archive and a normal archive at the same time by enabling the jar or war task:

Groovy

jar {
	enabled = true
}

Kotlin

tasks.getByName<Jar>("jar") {
	enabled = true
}

To avoid the executable archive and the normal archive from being written to the same location, one or the other should be configured to use a different location. One way to do so is by configuring a classifier:

Groovy

bootJar {
	classifier = 'boot'
}

Kotlin

tasks.getByName<BootJar>("bootJar") {
	classifier = "boot"
}

4.4. Configuring Executable Archive Packaging

The BootJar and BootWar tasks are subclasses of Gradle’s Jar and War tasks respectively. As a result, all of the standard configuration options that are available when packaging a jar or war are also available when packaging an executable jar or war. A number of configuration options that are specific to executable jars and wars are also provided.

4.4.1. Configuring the Main Class

By default, the executable archive’s main class will be configured automatically by looking for a class with a public static void main(String[]) method in directories on the task’s classpath.

The main class can also be configured explicitly using the task’s mainClass property:

Groovy

bootJar {
	mainClass = 'com.example.ExampleApplication'
}

Kotlin

tasks.getByName<BootJar>("bootJar") {
	mainClass.set("com.example.ExampleApplication")
}

Alternatively, the main class name can be configured project-wide using the mainClass property of the Spring Boot DSL:

Groovy

springBoot {
	mainClass = 'com.example.ExampleApplication'
}

Kotlin

springBoot {
	mainClass.set("com.example.ExampleApplication")
}

If the application plugin has been applied its mainClassName project property must be configured and can be used for the same purpose:

Groovy

mainClassName = 'com.example.ExampleApplication'

Kotlin

application {
	mainClassName = "com.example.ExampleApplication"
}

Lastly, the Start-Class attribute can be configured on the task’s manifest:

Groovy

bootJar {
	manifest {
		attributes 'Start-Class': 'com.example.ExampleApplication'
	}
}

Kotlin

tasks.getByName<BootJar>("bootJar") {
	manifest {
		attributes("Start-Class" to "com.example.ExampleApplication")
	}
}

If the main class is written in Kotlin, the name of the generated Java class should be used. By default, this is the name of the Kotlin class with the Kt suffix added. For example, ExampleApplication becomes ExampleApplicationKt. If another name is defined using @JvmName then that name should be used.

4.4.2. Including Development-only Dependencies

By default all dependencies declared in the developmentOnly configuration will be excluded from an executable jar or war.

If you want to include dependencies declared in the developmentOnly configuration in your archive, configure the classpath of its task to include the configuration, as shown in the following example for the bootWar task:

Groovy

bootWar {
	classpath configurations.developmentOnly
}

Kotlin

tasks.getByName<BootWar>("bootWar") {
	classpath(configurations["developmentOnly"])
}

4.4.3. Configuring Libraries that Require Unpacking

Most libraries can be used directly when nested in an executable archive, however certain libraries can have problems. For example, JRuby includes its own nested jar support which assumes that jruby-complete.jar is always directly available on the file system.

To deal with any problematic libraries, an executable archive can be configured to unpack specific nested jars to a temporary directory when the executable archive is run. Libraries can be identified as requiring unpacking using Ant-style patterns that match against the absolute path of the source jar file:

Groovy

bootJar {
	requiresUnpack '**/jruby-complete-*.jar'
}

Kotlin

tasks.getByName<BootJar>("bootJar") {
	requiresUnpack("**/jruby-complete-*.jar")
}

For more control a closure can also be used. The closure is passed a FileTreeElement and should return a boolean indicating whether or not unpacking is required.

4.4.4. Making an Archive Fully Executable

Spring Boot provides support for fully executable archives. An archive is made fully executable by prepending a shell script that knows how to launch the application. On Unix-like platforms, this launch script allows the archive to be run directly like any other executable or to be installed as a service.

Currently, some tools do not accept this format so you may not always be able to use this technique. For example, jar -xf may silently fail to extract a jar or war that has been made fully-executable. It is recommended that you only enable this option if you intend to execute it directly, rather than running it with java -jar, deploying it to a servlet container, or including it in an OCI image.

To use this feature, the inclusion of the launch script must be enabled:

Groovy

bootJar {
	launchScript()
}

Kotlin

tasks.getByName<BootJar>("bootJar") {
	launchScript()
}

This will add Spring Boot’s default launch script to the archive. The default launch script includes several properties with sensible default values. The values can be customized using the properties property:

Groovy

bootJar {
	launchScript {
		properties 'logFilename': 'example-app.log'
	}
}

Kotlin

tasks.getByName<BootJar>("bootJar") {
	launchScript {
		properties(mapOf("logFilename" to "example-app.log"))
	}
}

If the default launch script does not meet your needs, the script property can be used to provide a custom launch script:

Groovy

bootJar {
	launchScript {
		script = file('src/custom.script')
	}
}

Kotlin

tasks.getByName<BootJar>("bootJar") {
	launchScript {
		script = file("src/custom.script")
	}
}

4.4.5. Using the PropertiesLauncher

To use the PropertiesLauncher to launch an executable jar or war, configure the task’s manifest to set the Main-Class attribute:

Groovy

bootWar {
	manifest {
		attributes 'Main-Class': 'org.springframework.boot.loader.PropertiesLauncher'
	}
}

Kotlin

tasks.getByName<BootWar>("bootWar") {
	manifest {
		attributes("Main-Class" to "org.springframework.boot.loader.PropertiesLauncher")
	}
}

4.4.6. Packaging Layered Jars

By default, the bootJar task builds an archive that contains the application’s classes and dependencies in BOOT-INF/classes and BOOT-INF/lib respectively. For cases where a docker image needs to be built from the contents of the jar, it’s useful to be able to separate these directories further so that they can be written into distinct layers.

Layered jars use the same layout as regular boot packaged jars, but include an additional meta-data file that describes each layer.

By default, the following layers are defined:

dependencies for any non-project dependency whose version does not contain SNAPSHOT.
spring-boot-loader for the jar loader classes.
snapshot-dependencies for any non-project dependency whose version contains SNAPSHOT.
application for project dependencies, application classes, and resources.

The layers order is important as it determines how likely previous layers can be cached when part of the application changes. The default order is dependencies, spring-boot-loader, snapshot-dependencies, application. Content that is least likely to change should be added first, followed by layers that are more likely to change.

To disable this feature, you can do so in the following manner:

Groovy

bootJar {
	layered {
		enabled = false
	}
}

Kotlin

tasks.getByName<BootJar>("bootJar") {
	layered {
		isEnabled = false
	}
}

When a layered jar is created, the spring-boot-jarmode-layertools jar will be added as a dependency to your jar. With this jar on the classpath, you can launch your application in a special mode which allows the bootstrap code to run something entirely different from your application, for example, something that extracts the layers. If you wish to exclude this dependency, you can do so in the following manner:

Groovy

bootJar {
	layered {
		includeLayerTools = false
	}
}

Kotlin

tasks.getByName<BootJar>("bootJar") {
	layered {
		isIncludeLayerTools = false
	}
}

Custom Layers Configuration

Depending on your application, you may want to tune how layers are created and add new ones.

This can be done using configuration that describes how the jar can be separated into layers, and the order of those layers. The following example shows how the default ordering described above can be defined explicitly:

Groovy

bootJar {
	layered {
		application {
			intoLayer("spring-boot-loader") {
				include "org/springframework/boot/loader/**"
			}
			intoLayer("application")
		}
		dependencies {
			intoLayer("application") {
				includeProjectDependencies()
			}
			intoLayer("snapshot-dependencies") {
				include "*:*:*SNAPSHOT"
			}
			intoLayer("dependencies")
		}
		layerOrder = ["dependencies", "spring-boot-loader", "snapshot-dependencies", "application"]
	}
}

Kotlin

tasks.getByName<BootJar>("bootJar") {
	layered {
		application {
			intoLayer("spring-boot-loader") {
				include("org/springframework/boot/loader/**")
			}
			intoLayer("application")
		}
		dependencies {
			intoLayer("snapshot-dependencies") {
				include("*:*:*SNAPSHOT")
			}
			intoLayer("dependencies")
		}
		layerOrder = listOf("dependencies", "spring-boot-loader", "snapshot-dependencies", "application")
	}
}

The layered DSL is defined using three parts:

The application closure defines how the application classes and resources should be layered.
The dependencies closure defines how dependencies should be layered.
The layerOrder method defines the order that the layers should be written.

Nested intoLayer closures are used within application and dependencies sections to claim content for a layer. These closures are evaluated in the order that they are defined, from top to bottom. Any content not claimed by an earlier intoLayer closure remains available for subsequent ones to consider.

The intoLayer closure claims content using nested include and exclude calls. The application closure uses Ant-style patch matching for include/exclude parameters. The dependencies section uses group:artifact[:version] patterns. It also provides includeProjectDependencies() and excludeProjectDependencies() methods that can be used to include or exclude project dependencies.

If no include call is made, then all content (not claimed by an earlier closure) is considered.

If no exclude call is made, then no exclusions are applied.

Looking at the dependencies closure in the example above, we can see that the first intoLayer will claim all project dependencies for the application layer. The next intoLayer will claim all SNAPSHOT dependencies for the snapshot-dependencies layer. The third and final intoLayer will claim anything left (in this case, any dependency that is not a project dependency or a SNAPSHOT) for the dependencies layer.

The application closure has similar rules. First claiming org/springframework/boot/loader/** content for the spring-boot-loader layer. Then claiming any remaining classes and resources for the application layer.

The order that intoLayer closures are added is often different from the order that the layers are written. For this reason the layerOrder method must always be called and must cover all layers referenced by the intoLayer calls.

5. Packaging OCI Images

The plugin can create an OCI image from executable jars using Cloud Native Buildpacks (CNB). Images can be built using the bootBuildImage task.

For security reasons, images build and run as non-root users. See the CNB specification for more details.

The task is automatically created when the java plugin is applied and is an instance of BootBuildImage.

The bootBuildImage task can not be used with a fully executable Spring Boot archive that includes a launch script. Disable launch script configuration in the bootJar task when building a jar file that is intended to be used with bootBuildImage.

5.1. Docker Daemon

The bootBuildImage task requires access to a Docker daemon. By default, it will communicate with a Docker daemon over a local connection. This works with Docker Engine on all supported platforms without configuration.

Environment variables can be set to configure the bootBuildImage task to use the Docker daemon provided by minikube. The following table shows the environment variables and their values:

Environment variable Description

Environment variable	Description
DOCKER_HOST	URL containing the host and port for the Docker daemon - e.g. `tcp://192.168.99.100:2376`
DOCKER_TLS_VERIFY	Enable secure HTTPS protocol when set to `1` (optional)
DOCKER_CERT_PATH	Path to certificate and key files for HTTPS (required if `DOCKER_TLS_VERIFY=1`, ignored otherwise)

DOCKER_HOST

URL containing the host and port for the Docker daemon - e.g. tcp://192.168.99.100:2376

DOCKER_TLS_VERIFY

Enable secure HTTPS protocol when set to 1 (optional)

DOCKER_CERT_PATH

Path to certificate and key files for HTTPS (required if DOCKER_TLS_VERIFY=1, ignored otherwise)

On Linux and macOS, these environment variables can be set using the command eval $(minikube docker-env) after minikube has been started.

Docker daemon connection information can also be provided using docker properties in the plugin configuration. The following table summarizes the available properties:

Property Description

Property	Description
`host`	URL containing the host and port for the Docker daemon - e.g. `tcp://192.168.99.100:2376`
`tlsVerify`	Enable secure HTTPS protocol when set to `true` (optional)
`certPath`	Path to certificate and key files for HTTPS (required if `tlsVerify` is `true`, ignored otherwise)

host

URL containing the host and port for the Docker daemon - e.g. tcp://192.168.99.100:2376

tlsVerify

Enable secure HTTPS protocol when set to true (optional)

certPath

Path to certificate and key files for HTTPS (required if tlsVerify is true, ignored otherwise)

For more details, see also examples.

5.2. Docker Registry

If the Docker images specified by the builder or runImage properties are stored in a private Docker image registry that requires authentication, the authentication credentials can be provided using docker.builderRegistry properties.

If the generated Docker image is to be published to a Docker image registry, the authentication credentials can be provided using docker.publishRegistry properties.

Properties are provided for user authentication or identity token authentication. Consult the documentation for the Docker registry being used to store images for further information on supported authentication methods.

The following table summarizes the available properties for docker.builderRegistry and docker.publishRegistry:

Property Description

Property	Description
`username`	Username for the Docker image registry user. Required for user authentication.
`password`	Password for the Docker image registry user. Required for user authentication.
`url`	Address of the Docker image registry. Optional for user authentication.
`email`	E-mail address for the Docker image registry user. Optional for user authentication.
`token`	Identity token for the Docker image registry user. Required for token authentication.

username

Username for the Docker image registry user. Required for user authentication.

password

Password for the Docker image registry user. Required for user authentication.

url

Address of the Docker image registry. Optional for user authentication.

email

E-mail address for the Docker image registry user. Optional for user authentication.

token

Identity token for the Docker image registry user. Required for token authentication.

For more details, see also examples.

5.3. Image Customizations

The plugin invokes a builder to orchestrate the generation of an image. The builder includes multiple buildpacks that can inspect the application to influence the generated image. By default, the plugin chooses a builder image. The name of the generated image is deduced from project properties.

Task properties can be used to configure how the builder should operate on the project. The following table summarizes the available properties and their default values:

Property Command-line option Description Default value

Property	Command-line option	Description	Default value
`builder`	`--builder`	Name of the Builder image to use.	`paketobuildpacks/builder:base`
`runImage`	`--runImage`	Name of the run image to use.	No default value, indicating the run image specified in Builder metadata should be used.
`imageName`	`--imageName`	Image name for the generated image.	`docker.io/library/${project.name}:${project.version}`
`pullPolicy`	`--pullPolicy`	Policy used to determine when to pull the builder and run images from the registry. Acceptable values are `ALWAYS`, `NEVER`, and `IF_NOT_PRESENT`.	`ALWAYS`
`environment`		Environment variables that should be passed to the builder.
`cleanCache`		Whether to clean the cache before building.	`false`
`verboseLogging`		Enables verbose logging of builder operations.	`false`
`publish`	`--publishImage`	Whether to publish the generated image to a Docker registry.	`false`

builder

--builder

Name of the Builder image to use.

paketobuildpacks/builder:base

runImage

--runImage

Name of the run image to use.

No default value, indicating the run image specified in Builder metadata should be used.

imageName

--imageName

Image name for the generated image.

docker.io/library/${project.name}:${project.version}

pullPolicy

--pullPolicy

Policy used to determine when to pull the builder and run images from the registry. Acceptable values are ALWAYS, NEVER, and IF_NOT_PRESENT.

ALWAYS

environment

Environment variables that should be passed to the builder.

cleanCache

Whether to clean the cache before building.

false

verboseLogging

Enables verbose logging of builder operations.

false

publish

--publishImage

Whether to publish the generated image to a Docker registry.

false

The plugin detects the target Java compatibility of the project using the JavaPlugin’s targetCompatibility property. When using the default Paketo builder and buildpacks, the plugin instructs the buildpacks to install the same Java version. You can override this behaviour as shown in the builder configuration examples.

5.4. Examples

5.4.1. Custom Image Builder and Run Image

If you need to customize the builder used to create the image or the run image used to launch the built image, configure the task as shown in the following example:

Groovy

bootBuildImage {
	builder = "mine/java-cnb-builder"
	runImage = "mine/java-cnb-run"
}

Kotlin

tasks.getByName<BootBuildImage>("bootBuildImage") {
	builder = "mine/java-cnb-builder"
	runImage = "mine/java-cnb-run"
}

This configuration will use a builder image with the name mine/java-cnb-builder and the tag latest, and the run image named mine/java-cnb-run and the tag latest.

The builder and run image can be specified on the command line as well, as shown in this example:

$ gradle bootBuildImage --builder=mine/java-cnb-builder --runImage=mine/java-cnb-run

5.4.2. Builder Configuration

If the builder exposes configuration options, those can be set using the environment property.

The following is an example of configuring the JVM version used by the Paketo Java buildpacks at build time:

Groovy

bootBuildImage {
	environment = ["BP_JVM_VERSION" : "8.*"]
}

Kotlin

tasks.getByName<BootBuildImage>("bootBuildImage") {
	environment = mapOf("BP_JVM_VERSION" to "8.*")
}

In a similar way, Paketo Java buildpacks support configuring JVM runtime behavior. Refer to the Paketo documentation for additional configuration options supported by Paketo Java buildpacks.

If there is a network proxy between the Docker daemon the builder runs in and network locations that buildpacks download artifacts from, you will need to configure the builder to use the proxy. When using the Paketo builder, this can be accomplished by setting the HTTPS_PROXY and/or HTTP_PROXY environment variables as show in the following example:

Groovy

bootBuildImage {
	environment = [
			"HTTP_PROXY" : "http://proxy.example.com",
			"HTTPS_PROXY": "https://proxy.example.com"
	]
}

Kotlin

tasks.getByName<BootBuildImage>("bootBuildImage") {
	environment = mapOf("HTTP_PROXY" to "http://proxy.example.com",
						"HTTPS_PROXY" to "https://proxy.example.com")
}

5.4.3. Custom Image Name

By default, the image name is inferred from the name and the version of the project, something like docker.io/library/${project.name}:${project.version}. You can take control over the name by setting task properties, as shown in the following example:

Groovy

bootBuildImage {
	imageName = "example.com/library/${project.name}"
}

Kotlin

tasks.getByName<BootBuildImage>("bootBuildImage") {
	imageName = "example.com/library/${project.name}"
}

Note that this configuration does not provide an explicit tag so latest is used. It is possible to specify a tag as well, either using ${project.version}, any property available in the build or a hardcoded version.

The image name can be specified on the command line as well, as shown in this example:

$ gradle bootBuildImage --imageName=example.com/library/my-app:v1

5.4.4. Image Publishing

The generated image can be published to a Docker registry by enabling a publish option and configuring authentication for the registry using docker.publishRegistry properties.

Groovy

bootBuildImage {
	imageName = "docker.example.com/library/${project.name}"
	publish = true
	docker {
		publishRegistry {
			username = "user"
			password = "secret"
			url = "https://docker.example.com/v1/"
			email = "[email protected]"
		}
	}
}

Kotlin

tasks.getByName<BootBuildImage>("bootBuildImage") {
	imageName = "docker.example.com/library/${project.name}"
	publish = true
	docker {
		publishRegistry {
			username = "user"
			password = "secret"
			url = "https://docker.example.com/v1/"
			email = "[email protected]"
		}
	}
}

The publish option can be specified on the command line as well, as shown in this example:

$ gradle bootBuildImage --imageName=docker.example.com/library/my-app:v1 --publishImage

5.4.5. Docker Configuration

If you need the plugin to communicate with the Docker daemon using a remote connection instead of the default local connection, the connection details can be provided using docker properties as shown in the following example:

Groovy

bootBuildImage {
	docker {
		host = "tcp://192.168.99.100:2376"
		tlsVerify = true
		certPath = "/home/users/.minikube/certs"
	}
}

Kotlin

tasks.getByName<BootBuildImage>("bootBuildImage") {
	docker {
		host = "tcp://192.168.99.100:2376"
		tlsVerify = true
		certPath = "/home/users/.minikube/certs"
	}
}

If the builder or run image are stored in a private Docker registry that supports user authentication, authentication details can be provided using docker.buiderRegistry properties as shown in the following example:

Groovy

bootBuildImage {
	docker {
		builderRegistry {
			username = "user"
			password = "secret"
			url = "https://docker.example.com/v1/"
			email = "[email protected]"
		}
	}
}

Kotlin

tasks.getByName<BootBuildImage>("bootBuildImage") {
	docker {
		builderRegistry {
			username = "user"
			password = "secret"
			url = "https://docker.example.com/v1/"
			email = "[email protected]"
		}
	}
}

If the builder or run image is stored in a private Docker registry that supports token authentication, the token value can be provided using docker.builderRegistry as shown in the following example:

Groovy

bootBuildImage {
	docker {
		builderRegistry {
			token = "9cbaf023786cd7..."
		}
	}
}

Kotlin

tasks.getByName<BootBuildImage>("bootBuildImage") {
	docker {
		builderRegistry {
			token = "9cbaf023786cd7..."
		}
	}
}

6. Publishing your Application

6.1. Publishing with the Maven Plugin

When the maven plugin is applied, an Upload task for the bootArchives configuration named uploadBootArchives is automatically created. By default, the bootArchives configuration contains the archive produced by the bootJar or bootWar task. The uploadBootArchives task can be configured to publish the archive to a Maven repository:

Groovy

uploadBootArchives {
	repositories {
		mavenDeployer {
			repository url: 'https://repo.example.com'
		}
	}
}

Kotlin

tasks.getByName<Upload>("uploadBootArchives") {
	repositories.withGroovyBuilder {
		"mavenDeployer" {
			"repository"("url" to "https://repo.example.com")
		}
	}
}

6.2. Publishing with the Maven-publish Plugin

To publish your Spring Boot jar or war, add it to the publication using the artifact method on MavenPublication. Pass the task that produces that artifact that you wish to publish to the artifact method. For example, to publish the artifact produced by the default bootJar task:

Groovy

publishing {
	publications {
		bootJava(MavenPublication) {
			artifact bootJar
		}
	}
	repositories {
		maven {
			url 'https://repo.example.com'
		}
	}
}

Kotlin

publishing {
	publications {
		create<MavenPublication>("bootJava") {
			artifact(tasks.getByName("bootJar"))
		}
	}
	repositories {
		maven {
			url = uri("https://repo.example.com")
		}
	}
}

6.3. Distributing with the Application Plugin

When the application plugin is applied a distribution named boot is created. This distribution contains the archive produced by the bootJar or bootWar task and scripts to launch it on Unix-like platforms and Windows. Zip and tar distributions can be built by the bootDistZip and bootDistTar tasks respectively. To use the application plugin, its mainClassName property must be configured with the name of your application’s main class.

7. Running your Application with Gradle

To run your application without first building an archive use the bootRun task:

$ ./gradlew bootRun

The bootRun task is an instance of BootRun which is a JavaExec subclass. As such, all of the usual configuration options for executing a Java process in Gradle are available to you. The task is automatically configured to use the runtime classpath of the main source set.

By default, the main class will be configured automatically by looking for a class with a public static void main(String[]) method in directories on the task’s classpath.

The main class can also be configured explicitly using the task’s main property:

Groovy

bootRun {
	main = 'com.example.ExampleApplication'
}

Kotlin

tasks.getByName<BootRun>("bootRun") {
	main = "com.example.ExampleApplication"
}

Alternatively, the main class name can be configured project-wide using the mainClass property of the Spring Boot DSL:

Groovy

springBoot {
	mainClass = 'com.example.ExampleApplication'
}

Kotlin

springBoot {
	mainClass.set("com.example.ExampleApplication")
}

By default, bootRun will configure the JVM to optimize its launch for faster startup during development. This behavior can be disabled by using the optimizedLaunch property, as shown in the following example:

Groovy

bootRun {
	optimizedLaunch = false
}

Kotlin

tasks.getByName<BootRun>("bootRun") {
	isOptimizedLaunch = false
}

If the application plugin has been applied, its mainClassName property must be configured and can be used for the same purpose:

Groovy

application {
	mainClassName = 'com.example.ExampleApplication'
}

Kotlin

application {
	mainClassName = "com.example.ExampleApplication"
}

7.1. Passing Arguments to your Application

Like all JavaExec tasks, arguments can be passed into bootRun from the command line using --args='<arguments>' when using Gradle 4.9 or later. For example, to run your application with a profile named dev active the following command can be used:

$ ./gradlew bootRun --args='--spring.profiles.active=dev'

See the javadoc for JavaExec.setArgsString for further details.

7.2. Passing System properties to your application

Since bootRun is a standard JavaExec task, system properties can be passed to the application’s JVM by specifying them in the build script. To make that value of a system property to be configurable set its value using a project property. To allow a project property to be optional, reference it using findProperty. Doing so also allows a default value to be provided using the ?: Elvis operator, as shown in the following example:

Groovy

bootRun {
	systemProperty 'com.example.property', findProperty('example') ?: 'default'
}

Kotlin

tasks.getByName<BootRun>("bootRun") {
	systemProperty("com.example.property", findProperty("example") ?: "default")
}

The preceding example sets that com.example.property system property to the value of the example project property. If the example project property has not been set, the value of the system property will be default.

Gradle allows project properties to be set in a variety of ways, including on the command line using the -P flag, as shown in the following example:

$ ./gradlew -Pexample=custom

The preceding example sets the value of the example project property to custom. bootRun will then use this as the value of the com.example.property system property.

7.3. Reloading Resources

If devtools has been added to your project it will automatically monitor your application for changes. Alternatively, you can configure bootRun such that your application’s static resources are loaded from their source location:

Groovy

bootRun {
	sourceResources sourceSets.main
}

Kotlin

tasks.getByName<BootRun>("bootRun") {
	sourceResources(sourceSets["main"])
}

This makes them reloadable in the live application which can be helpful at development time.

8. Integrating with Actuator

8.1. Generating Build Information

Spring Boot Actuator’s info endpoint automatically publishes information about your build in the presence of a META-INF/build-info.properties file. A BuildInfo task is provided to generate this file. The easiest way to use the task is via the plugin’s DSL:

Groovy

springBoot {
	buildInfo()
}

Kotlin

springBoot {
	buildInfo()
}

This will configure a BuildInfo task named bootBuildInfo and, if it exists, make the Java plugin’s classes task depend upon it. The task’s destination directory will be META-INF in the output directory of the main source set’s resources (typically build/resources/main).

By default, the generated build information is derived from the project:

Property Default value

Property	Default value
`build.artifact`	The base name of the `bootJar` or `bootWar` task, or `unspecified` if no such task exists
`build.group`	The group of the project
`build.name`	The name of the project
`build.version`	The version of the project
`build.time`	The time at which the project is being built

build.artifact

The base name of the bootJar or bootWar task, or unspecified if no such task exists

build.group

The group of the project

build.name

The name of the project

build.version

The version of the project

build.time

The time at which the project is being built

The properties can be customized using the DSL:

Groovy

springBoot {
	buildInfo {
		properties {
			artifact = 'example-app'
			version = '1.2.3'
			group = 'com.example'
			name = 'Example application'
		}
	}
}

Kotlin

springBoot {
	buildInfo {
		properties {
			artifact = "example-app"
			version = "1.2.3"
			group = "com.example"
			name = "Example application"
		}
	}
}

The default value for build.time is the instant at which the project is being built. A side-effect of this is that the task will never be up-to-date. As a result, builds will take longer as more tasks, including the project’s tests, will have to be executed. Another side-effect is that the task’s output will always change and, therefore, the build will not be truly repeatable. If you value build performance or repeatability more highly than the accuracy of the build.time property, set time to null or a fixed value.

Additional properties can also be added to the build information:

Groovy

springBoot {
	buildInfo {
		properties {
			additional = [
				'a': 'alpha',
				'b': 'bravo'
			]
		}
	}
}

Kotlin

springBoot {
	buildInfo {
		properties {
			additional = mapOf(
				"a" to "alpha",
				"b" to "bravo"
			)
		}
	}
}

9. Reacting to Other Plugins

When another plugin is applied the Spring Boot plugin reacts by making various changes to the project’s configuration. This section describes those changes.

9.1. Reacting to the Java Plugin

When Gradle’s java plugin is applied to a project, the Spring Boot plugin:

Creates a BootJar task named bootJar that will create an executable, fat jar for the project. The jar will contain everything on the runtime classpath of the main source set; classes are packaged in BOOT-INF/classes and jars are packaged in BOOT-INF/lib
Configures the assemble task to depend on the bootJar task.
Disables the jar task.
Creates a BootBuildImage task named bootBuildImage that will create a OCI image using a buildpack.
Creates a BootRun task named bootRun that can be used to run your application.
Creates a configuration named bootArchives that contains the artifact produced by the bootJar task.
Creates a configuration named developmentOnly for dependencies that are only required at development time, such as Spring Boot’s Devtools, and should not be packaged in executable jars and wars.
Configures any JavaCompile tasks with no configured encoding to use UTF-8.
Configures any JavaCompile tasks to use the -parameters compiler argument.

9.2. Reacting to the Kotlin Plugin

When Kotlin’s Gradle plugin is applied to a project, the Spring Boot plugin:

Aligns the Kotlin version used in Spring Boot’s dependency management with the version of the plugin. This is achieved by setting the kotlin.version property with a value that matches the version of the Kotlin plugin.
Configures any KotlinCompile tasks to use the -java-parameters compiler argument.

9.3. Reacting to the War Plugin

When Gradle’s war plugin is applied to a project, the Spring Boot plugin:

Creates a BootWar task named bootWar that will create an executable, fat war for the project. In addition to the standard packaging, everything in the providedRuntime configuration will be packaged in WEB-INF/lib-provided.
Configures the assemble task to depend on the bootWar task.
Disables the war task.
Configures the bootArchives configuration to contain the artifact produced by the bootWar task.

9.4. Reacting to the Dependency Management Plugin

When the io.spring.dependency-management plugin is applied to a project, the Spring Boot plugin will automatically import the spring-boot-dependencies bom.

9.5. Reacting to the Application Plugin

When Gradle’s application plugin is applied to a project, the Spring Boot plugin:

Creates a CreateStartScripts task named bootStartScripts that will create scripts that launch the artifact in the bootArchives configuration using java -jar. The task is configured to use the applicationDefaultJvmArgs property as a convention for its defaultJvmOpts property.
Creates a new distribution named boot and configures it to contain the artifact in the bootArchives configuration in its lib directory and the start scripts in its bin directory.
Configures the bootRun task to use the mainClassName property as a convention for its main property.
Configures the bootRun task to use the applicationDefaultJvmArgs property as a convention for its jvmArgs property.
Configures the bootJar task to use the mainClassName property as a convention for the Start-Class entry in its manifest.
Configures the bootWar task to use the mainClassName property as a convention for the Start-Class entry in its manifest.

9.6. Reacting to the Maven plugin

When Gradle’s maven plugin is applied to a project, the Spring Boot plugin will configure the uploadBootArchives Upload task to ensure that no dependencies are declared in the pom that it generates.