[[build-tool-plugins]]
= Build tool plugins

[partintro]
--
Spring Boot provides build tool plugins for Maven and Gradle. The plugins offer a
variety of features, including the packaging of executable jars. This section provides
more details on both plugins, as well as some help should you need to extend an
unsupported build system. If you are just getting started, you might want to read
"`<<using-spring-boot.adoc#using-boot-build-systems>>`" from the
<<using-spring-boot.adoc#using-boot>> section first.
--



[[build-tool-plugins-maven-plugin]]
== Spring Boot Maven plugin
The {spring-boot-maven-plugin-site}/[Spring Boot Maven Plugin] provides Spring Boot
support in Maven, allowing you to package executable jar or war archives and run an
application "`in-place`". To use it you must be using Maven 3.2 (or better).

NOTE: Refer to the {spring-boot-maven-plugin-site}/[Spring Boot Maven Plugin Site]
for complete plugin documentation.



[[build-tool-plugins-include-maven-plugin]]
=== Including the plugin
To use the Spring Boot Maven Plugin simply include the appropriate XML in the `plugins`
section of your `pom.xml`

[source,xml,indent=0,subs="verbatim,attributes"]
----
	<?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>
		<!-- ... -->
		<build>
			<plugins>
				<plugin>
					<groupId>org.springframework.boot</groupId>
					<artifactId>spring-boot-maven-plugin</artifactId>
					<version>{spring-boot-version}</version>
					<executions>
						<execution>
							<goals>
								<goal>repackage</goal>
							</goals>
						</execution>
					</executions>
				</plugin>
			</plugins>
		</build>
	</project>
----

This configuration will repackage a jar or war that is built during the `package` phase of
the Maven lifecycle. The following example shows both the repackaged jar, as well as the
original jar, in the `target` directory:

[indent=0]
----
	$ mvn package
	$ ls target/*.jar
	target/myproject-1.0.0.jar target/myproject-1.0.0.jar.original
----


If you don't include the `<execution/>` configuration as above, you can run the plugin on
its own (but only if the package goal is used as well). For example:

[indent=0]
----
	$ mvn package spring-boot:repackage
	$ ls target/*.jar
	target/myproject-1.0.0.jar target/myproject-1.0.0.jar.original
----

If you are using a milestone or snapshot release you will also need to add appropriate
`pluginRepository` elements:

[source,xml,indent=0,subs="verbatim,attributes"]
----
	<pluginRepositories>
		<pluginRepository>
			<id>spring-snapshots</id>
			<url>https://repo.spring.io/snapshot</url>
		</pluginRepository>
		<pluginRepository>
			<id>spring-milestones</id>
			<url>https://repo.spring.io/milestone</url>
		</pluginRepository>
	</pluginRepositories>
----



[[build-tool-plugins-maven-packaging]]
=== Packaging executable jar and war files
Once `spring-boot-maven-plugin` has been included in your `pom.xml` it will automatically
attempt to rewrite archives to make them executable using the `spring-boot:repackage`
goal. You should configure your project to build a jar or war (as appropriate) using the
usual `packaging` element:

[source,xml,indent=0,subs="verbatim,attributes"]
----
	<?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">
		<!-- ... -->
		<packaging>jar</packaging>
		<!-- ... -->
	</project>
----

Your existing archive will be enhanced by Spring Boot during the `package` phase. The
main class that you want to launch can either be specified using a configuration option,
or by adding a `Main-Class` attribute to the manifest in the usual way. If you don't
specify a main class the plugin will search for a class with a
`public static void main(String[] args)` method.

To build and run a project artifact, you can type the following:

[indent=0]
----
	$ mvn package
	$ java -jar target/mymodule-0.0.1-SNAPSHOT.jar
----

To build a war file that is both executable and deployable into an external container you
need to mark the embedded container dependencies as "`provided`", e.g:

[source,xml,indent=0,subs="verbatim,attributes"]
----
	<?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">
		<!-- ... -->
		<packaging>war</packaging>
		<!-- ... -->
		<dependencies>
			<dependency>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-starter-web</artifactId>
			</dependency>
			<dependency>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-starter-tomcat</artifactId>
				<scope>provided</scope>
			</dependency>
			<!-- ... -->
		</dependencies>
	</project>
----

TIP: See the "`<<howto-create-a-deployable-war-file>>`" section for more details on
how to create a deployable war file.

Advanced configuration options and examples are available in the
{spring-boot-maven-plugin-site}/[plugin info page].



[[build-tool-plugins-gradle-plugin]]
== Spring Boot Gradle plugin
The Spring Boot Gradle Plugin provides Spring Boot support in Gradle, allowing you to
package executable jar or war archives, run Spring Boot applications and use the
dependency management provided by `spring-boot-dependencies`.



[[build-tool-plugins-including-the-gradle-plugin]]
=== Including the plugin
ifeval::["{spring-boot-repo}" == "release"]
To use the Spring Boot Gradle Plugin configure it using the `plugins` block:
[source,groovy,indent=0,subs="verbatim,attributes"]
----
	plugins {
		id 'org.springframework.boot' version '{spring-boot-version}'
	}
----
endif::[]
ifeval::["{spring-boot-repo}" != "release"]
To use the Spring Boot Gradle Plugin simply include a `buildscript` dependency and apply
the `spring-boot` plugin:

[source,groovy,indent=0,subs="verbatim,attributes"]
----
	buildscript {
		repositories {
			maven { url 'https://repo.spring.io/snapshot' }
			maven { url 'https://repo.spring.io/milestone' }
		}
		dependencies {
			classpath 'org.springframework.boot:spring-boot-gradle-plugin:{spring-boot-version}''
		}
	}
	apply plugin: 'org.springframework.boot'
----
endif::[]



[[build-tool-plugins-gradle-dependency-management]]
=== Gradle dependency management
The `spring-boot` plugin automatically applies the
{dependency-management-plugin}/[Dependency Management Plugin] and configures it to import
the `spring-boot-starter-parent` bom. This provides a similar dependency management
experience to the one that is 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, simply declare dependencies in the usual way, but leave the version
number empty:

[source,groovy,indent=0,subs="verbatim,attributes"]
----
	dependencies {
		compile("org.springframework.boot:spring-boot-starter-web")
		compile("org.thymeleaf:thymeleaf-spring4")
		compile("nz.net.ultraq.thymeleaf:thymeleaf-layout-dialect")
	}
----

NOTE: The version of the `spring-boot` gradle plugin that you declare determines the
version of the `spring-boot-starter-parent` bom that is imported (this ensures that builds
are always repeatable). You should always set the version of the `spring-boot` gradle
plugin to the actual Spring Boot version that you wish to use. Details of the versions
that are provided can be found in the <<appendix-dependency-versions, appendix>>.

To learn more about the capabilities of the Dependency Management Plugin, please refer to
its {dependency-management-plugin-documentation}[documentation].



[[build-tool-plugins-gradle-packaging]]
=== Packaging executable jar and war files
Once the `spring-boot` plugin has been applied to your project it will automatically
attempt to rewrite archives to make them executable using the
<<build-tool-plugins-gradle-repackage-configuration,`bootRepackage` task>>. You
should configure your project to build a jar or war (as appropriate) in the usual way.

The main class that you want to launch can either be specified using a configuration
option, or by adding a `Main-Class` attribute to the manifest. If you don't specify a
main class the plugin will search for a class with a
`public static void main(String[] args)` method.

TIP: Check <<build-tool-plugins-gradle-repackage-configuration>> for a full list of
configuration options.

To build and run a project artifact, you can type the following:

[indent=0]
----
	$ gradle build
	$ java -jar build/libs/mymodule-0.0.1-SNAPSHOT.jar
----

To build a war file that is both executable and deployable into an external container,
you need to mark the embedded container dependencies as belonging to the war plugin's
`providedRuntime` configuration, e.g.:

[source,groovy,indent=0,subs="verbatim,attributes"]
----
	...
	apply plugin: 'war'

	war {
		baseName = 'myapp'
		version =  '0.5.0'
	}

	repositories {
		jcenter()
		maven { url "https://repo.spring.io/libs-snapshot" }
	}

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

TIP: See the "`<<howto-create-a-deployable-war-file>>`" section for more details on
how to create a deployable war file.



[[build-tool-plugins-gradle-running-applications]]
=== Running a project in-place
To run a project in place without building a jar first you can use the "`bootRun`" task:

[indent=0]
----
	$ gradle bootRun
----

If <<using-spring-boot.adoc#using-boot-devtools,`devtools`>> has been added to your project
it will automatically monitor your application for changes. Alternatively, you can also
run the application so that your static classpath resources (i.e. in `src/main/resources`
by default) are reloadable in the live application, which can be helpful at development
time.

[source,groovy,indent=0,subs="verbatim,attributes"]
----
	bootRun {
		addResources = true
	}
----

Making static classpath resources reloadable means that `bootRun` does not use the output
of the `processResources` task, i.e., when invoked using `bootRun`, your application will
use the resources in their unprocessed form.



[[build-tool-plugins-gradle-global-configuration]]
=== Spring Boot plugin configuration
The gradle plugin automatically extends your build script DSL with a `springBoot` element
for global configuration of the Boot plugin. Set the appropriate properties as you would
with any other Gradle extension (see below for a list of configuration options):

[source,groovy,indent=0,subs="verbatim,attributes"]
----
	springBoot {
		backupSource = false
	}
----



[[build-tool-plugins-gradle-repackage-configuration]]
=== Repackage configuration
The plugin adds a `bootRepackage` task which you can also configure directly, e.g.:

[source,groovy,indent=0,subs="verbatim,attributes"]
----
	bootRepackage {
		mainClass = 'demo.Application'
	}
----

The following configuration options are available:

[cols="2,4"]
|===
|Name |Description

|`enabled`
|Boolean flag to switch the repackager off (sometimes useful if you
want the other Boot features but not this one)

|`mainClass`
|The main class that should be run. If not specified, and you have applied the application
 plugin, the `mainClassName` project property will be used. If the application plugin has
 not been applied or no `mainClassName` has been specified, the archive will be searched
 for a suitable class. "Suitable" means a unique class with a well-formed `main()` method
 (if more than one is found the build will fail). If you have applied the application
 plugin, the main class can also be specified via its "run" task (`main` property) and/or
 its "startScripts" task (`mainClassName` property) as an alternative to using the
 "springBoot" configuration.

|`classifier`
|A file name segment (before the extension) to add to the archive, so that the original is
 preserved in its original location. Defaults to `null` in which case the archive is
 repackaged in place. The default is convenient for many purposes, but if you want to use
 the original jar as a dependency in another project you must use a classifier to define
 the executable archive.

|`withJarTask`
|The name or value of the `Jar` task (defaults to all tasks of type `Jar`) which is used
 to locate the archive to repackage.

|`customConfiguration`
|The name of the custom configuration which is used to populate the nested lib directory
 (without specifying this you get all compile and runtime dependencies).

|`executable`
|Boolean flag to indicate if jar files are fully executable on Unix like operating
 systems. Defaults to `false`.

|`embeddedLaunchScript`
|The embedded launch script to prepend to the front of the jar if it is fully executable.
 If not specified the 'Spring Boot' default script will be used.

|`embeddedLaunchScriptProperties`
|Additional properties that to be expanded in the launch script. The default script
 supports a `mode` property which can contain the values `auto`, `service` or `run`.

|`excludeDevtools`
|Boolean flag to indicate if the devtools jar should be excluded from the repackaged
archives. Defaults to `true`.
|===



[[build-tool-plugins-gradle-repackage-custom-configuration]]
=== Repackage with custom Gradle configuration
Sometimes it may be more appropriate to not package default dependencies resolved from
`compile`, `runtime` and `provided` scopes. If the created executable jar file
is intended to be run as it is, you need to have all dependencies nested inside it;
however, if the plan is to explode a jar file and run the main class manually, you may already
have some of the libraries available via `CLASSPATH`. This is a situation where
you can repackage your jar with a different set of dependencies.

Using a custom
configuration will automatically disable dependency resolving from
`compile`, `runtime` and `provided` scopes. Custom configuration can be either
defined globally (inside the `springBoot` section) or per task.

[source,groovy,indent=0,subs="verbatim,attributes"]
----
	task clientJar(type: Jar) {
		appendix = 'client'
		from sourceSets.main.output
		exclude('**/*Something*')
	}

	task clientBoot(type: BootRepackage, dependsOn: clientJar) {
		withJarTask = clientJar
		customConfiguration = "mycustomconfiguration"
	}
----

In above example, we created a new `clientJar` Jar task to package a customized
file set from your compiled sources. Then we created a new `clientBoot`
BootRepackage task and instructed it to work with only `clientJar` task and
`mycustomconfiguration`.

[source,groovy,indent=0,subs="verbatim,attributes"]
----
	configurations {
		mycustomconfiguration.exclude group: 'log4j'
	}

	dependencies {
		mycustomconfiguration configurations.runtime
	}
----

The configuration that we are referring to in `BootRepackage` is a normal
{gradle-dsl}/org.gradle.api.artifacts.Configuration.html[Gradle
configuration]. In the above example we created a new configuration named
`mycustomconfiguration` instructing it to derive from a `runtime` and exclude the `log4j`
group. If the `clientBoot` task is executed, the repackaged boot jar will have all
dependencies from `runtime` but no `log4j` jars.



[[build-tool-plugins-gradle-configuration-options]]
==== Configuration options
The following configuration options are available:

[cols="2,4"]
|===
|Name |Description

|`mainClass`
|The main class that should be run by the executable archive.

|`providedConfiguration`
|The name of the provided configuration (defaults to `providedRuntime`).

|`backupSource`
|If the original source archive should be backed-up before being repackaged (defaults
 to `true`).

|`customConfiguration`
|The name of the custom configuration.

|`layout`
|The type of archive, corresponding to how the dependencies are laid out inside
 (defaults to a guess based on the archive type). See
 <<build-tool-plugins-gradle-configuration-layouts,available layouts for more details>>.

|`layoutFactory`
|A layout factory that can be used if a custom layout is required. Alternative layouts
can be provided by 3rd parties. Layout factories are only used when `layout` is not
specified.

|`requiresUnpack`
|A list of dependencies (in the form "`groupId:artifactId`" that must be unpacked from
 fat jars in order to run. Items are still packaged into the fat jar, but they will be
 automatically unpacked when it runs.
|===



[[build-tool-plugins-gradle-configuration-layouts]]
==== Available layouts

The `layout` attribute configures the format of the archive and whether the bootstrap
loader should be included or not. The following layouts are available:

[cols="2,4,1"]
|===
|Name |Description |Executable

|`JAR`
|Regular executable
 <<appendix-executable-jar-format.adoc#executable-jar-jar-file-structure,JAR layout>>.
|Yes

|`WAR`
|Executable
 <<appendix-executable-jar-format.adoc#executable-jar-war-file-structure,WAR layout>>.
 `provided` dependencies are placed in `WEB-INF/lib-provided` to avoid any clash when
 the `war` is deployed in a servlet container.
|Yes

|`ZIP` (alias to `DIR`)
|Similar to `JAR` layout, using
 <<appendix-executable-jar-format.adoc#executable-jar-property-launcher-features,`PropertiesLauncher`>>.
| Yes

|`MODULE`
|Bundle dependencies (excluding those with `provided` scope) and project resources.
|No

|`NONE`
|Bundle all dependencies and project resources.
|No
|===



[[build-tool-plugins-gradle-configuration-custom-repackager]]
==== Using a custom layout
If you have custom requirements for how to arrange the dependencies and loader classes
inside the repackaged jar, you can use a custom layout. Any library which defines one
or more `LayoutFactory` implementations can be added to the build script dependencies
and then the layout factory becomes available in the `springBoot` configuration.
For example:

[source,groovy,indent=0,subs="verbatim,attributes"]
----
buildscript {
	repositories {
		mavenCentral()
	}
	dependencies {
		classpath("org.springframework.boot:spring-boot-gradle-plugin:{spring-boot-version}")
		classpath("com.example:custom-layout:1.0.0")
	}
}

springBoot {
	layoutFactory = new com.example.CustomLayoutFactory()
}
----

NOTE: If there is only one custom `LayoutFactory` on the build classpath and it is
listed in `META-INF/spring.factories` then it is unnecessary to explicitly set it in the
`springBoot` configuration. Layout factories are only used when no explicit `layout` is
specified.



[[build-tool-plugins-understanding-the-gradle-plugin]]
=== Understanding how the Gradle plugin works
When `spring-boot` is applied to your Gradle project a default task named `bootRepackage`
is created automatically. The `bootRepackage` task depends on Gradle `assemble` task, and
when executed, it tries to find all jar artifacts whose qualifier is empty (i.e. tests and
sources jars are automatically skipped).

Due to the fact that `bootRepackage` finds 'all' created jar artifacts, the order of
Gradle task execution is important. Most projects only create a single jar file, so
usually this is not an issue; however, if you are planning to create a more complex
project setup, with custom `Jar` and `BootRepackage` tasks, there are few tweaks to
consider.

If you are 'just' creating custom jar files from your project you can simply disable
default `jar` and `bootRepackage` tasks:

[source,groovy,indent=0,subs="verbatim,attributes"]
----
	jar.enabled = false
	bootRepackage.enabled = false
----

Another option is to instruct the default `bootRepackage` task to only work with a
default `jar` task.

[source,groovy,indent=0,subs="verbatim,attributes"]
----
	bootRepackage.withJarTask = jar
----

If you have a default project setup where the main jar file is created and repackaged,
'and' you still want to create additional custom jars, you can combine your custom
repackage tasks together and use `dependsOn` so that the `bootJars` task will run after
the default `bootRepackage` task is executed:

[source,groovy,indent=0,subs="verbatim,attributes"]
----
	task bootJars
	bootJars.dependsOn = [clientBoot1,clientBoot2,clientBoot3]
	build.dependsOn(bootJars)
----

All the above tweaks are usually used to avoid situations where an already created boot
jar is repackaged again. Repackaging an existing boot jar will not break anything, but
you may find that it includes unnecessary dependencies.



[[build-tool-plugins-gradle-publishing-artifacts-to-a-maven-repository]]
=== Publishing artifacts to a Maven repository using Gradle
If you are <<build-tool-plugins-gradle-dependencies-without-versions, declaring
dependencies without versions>> and you want to publish artifacts to a Maven repository
you will need to configure the Maven publication with details of Spring Boot's
dependency management. This can be achieved by configuring it to publish poms that
inherit from `spring-boot-starter-parent` or that import dependency management from
`spring-boot-dependencies`. The exact details of this configuration depend on how you're
using Gradle and how you're trying to publish the artifacts.



[[build-tool-plugins-gradle-publishing-artifacts-to-a-maven-repository-inherit]]
==== Configuring Gradle to produce a pom that inherits dependency management
The following is an example of configuring Gradle to generate a pom that inherits
from `spring-boot-starter-parent`. Please refer to the
{gradle-user-guide}/userguide.html[Gradle User Guide] for further information.

[source,groovy,indent=0,subs="verbatim,attributes"]
----
	uploadArchives {
		repositories {
			mavenDeployer {
				pom {
					project {
						parent {
							groupId "org.springframework.boot"
							artifactId "spring-boot-starter-parent"
							version "{spring-boot-version}"
						}
					}
				}
			}
		}
	}
----



[[build-tool-plugins-gradle-publishing-artifacts-to-a-maven-repository-import]]
==== Configuring Gradle to produce a pom that imports dependency management
The following is an example of configuring Gradle to generate a pom that imports
the dependency management provided by `spring-boot-dependencies`. Please refer to the
https://www.gradle.org/docs/current/userguide/userguide.html[Gradle User Guide] for
further information.

[source,groovy,indent=0,subs="verbatim,attributes"]
----
	uploadArchives {
		repositories {
			mavenDeployer {
				pom {
					project {
						dependencyManagement {
							dependencies {
								dependency {
									groupId "org.springframework.boot"
									artifactId "spring-boot-dependencies"
									version "{spring-boot-version}"
									type "pom"
									scope "import"
								}
							}
						}
					}
				}
			}
		}
	}
----



[[build-tool-plugins-antlib]]
== Spring Boot AntLib module
The Spring Boot AntLib module provides basic Spring Boot support for Apache Ant. You can
use the module to create executable jars. To use the module you need to declare an
additional `spring-boot` namespace in your `build.xml`:

[source,xml,indent=0]
----
	<project xmlns:ivy="antlib:org.apache.ivy.ant"
		xmlns:spring-boot="antlib:org.springframework.boot.ant"
		name="myapp" default="build">
		...
	</project>
----

You'll need to remember to start Ant using the `-lib` option, for example:

[indent=0,subs="verbatim,quotes,attributes"]
----
	$ ant -lib <folder containing spring-boot-antlib-{spring-boot-version}.jar>
----

TIP: The "`Using Spring Boot`" section includes a more complete example of
<<using-spring-boot.adoc#using-boot-ant, using Apache Ant with `spring-boot-antlib`>>


=== Spring Boot Ant tasks
Once the `spring-boot-antlib` namespace has been declared, the following additional
tasks are available.



==== spring-boot:exejar
The `exejar` task can be used to creates a Spring Boot executable jar. The following
attributes are supported by the task:

[cols="1,2,2"]
|====
|Attribute |Description |Required

|`destfile`
|The destination jar file to create
|Yes

|`classes`
|The root directory of Java class files
|Yes

|`start-class`
|The main application class to run
|No _(default is first class found declaring a `main` method)_
|====

The following nested elements can be used with the task:

[cols="1,4"]
|====
|Element |Description

|`resources`
|One or more {ant-manual}/Types/resources.html#collection[Resource Collections]
describing a set of {ant-manual}/Types/resources.html[Resources] that should be added to
the content of the created +jar+ file.

|`lib`
|One or more {ant-manual}/Types/resources.html#collection[Resource Collections]
that should be added to the set of jar libraries that make up the runtime dependency
classpath of the application.
|====



==== Examples
.Specify +start-class+
[source,xml,indent=0]
----
	<spring-boot:exejar destfile="target/my-application.jar"
			classes="target/classes" start-class="com.foo.MyApplication">
		<resources>
			<fileset dir="src/main/resources" />
		</resources>
		<lib>
			<fileset dir="lib" />
		</lib>
	</spring-boot:exejar>
----

.Detect +start-class+
[source,xml,indent=0]
----
	<exejar destfile="target/my-application.jar" classes="target/classes">
		<lib>
			<fileset dir="lib" />
		</lib>
	</exejar>
----



=== spring-boot:findmainclass
The `findmainclass` task is used internally by `exejar` to locate a class declaring a
`main`. You can also use this task directly in your build if needed. The following
attributes are supported

[cols="1,2,2"]
|====
|Attribute |Description |Required

|`classesroot`
|The root directory of Java class files
|Yes _(unless `mainclass` is specified)_

|`mainclass`
|Can be used to short-circuit the `main` class search
|No

|`property`
|The Ant property that should be set with the result
|No _(result will be logged if unspecified)_
|====



==== Examples
.Find and log
[source,xml,indent=0]
----
	<findmainclass classesroot="target/classes" />
----

.Find and set
[source,xml,indent=0]
----
	<findmainclass classesroot="target/classes" property="main-class" />
----

.Override and set
[source,xml,indent=0]
----
	<findmainclass mainclass="com.foo.MainClass" property="main-class" />
----



[[build-tool-plugins-other-build-systems]]
== Supporting other build systems
If you want to use a build tool other than Maven, Gradle or Ant, you will likely need to
develop your own plugin. Executable jars need to follow a specific format and certain
entries need to be written in an uncompressed form (see the
_<<appendix-executable-jar-format.adoc#executable-jar, executable jar format>>_ section
in the appendix for details).

The Spring Boot Maven and Gradle plugins both make use of `spring-boot-loader-tools` to
actually generate jars. You are also free to use this library directly yourself if you
need to.



[[build-tool-plugins-repackaging-archives]]
=== Repackaging archives
To repackage an existing archive so that it becomes a self-contained executable archive
use `org.springframework.boot.loader.tools.Repackager`. The `Repackager` class takes a
single constructor argument that refers to an existing jar or war archive. Use one of the
two available `repackage()` methods to either replace the original file or write to a new
destination. Various settings can also be configured on the repackager before it is
run.



[[build-tool-plugins-nested-libraries]]
=== Nested libraries
When repackaging an archive you can include references to dependency files using the
`org.springframework.boot.loader.tools.Libraries` interface. We don't provide any
concrete implementations of `Libraries` here as they are usually build system specific.

If your archive already includes libraries you can use `Libraries.NONE`.



[[build-tool-plugins-find-a-main-class]]
=== Finding a main class
If you don't use `Repackager.setMainClass()` to specify a main class, the repackager will
use https://asm.ow2.org/[ASM] to read class files and attempt to find a suitable class
with a `public static void main(String[] args)` method. An exception is thrown if more
than one candidate is found.



[[build-tool-plugins-repackage-implementation]]
=== Example repackage implementation
Here is a typical example repackage:

[source,java,indent=0]
----
	Repackager repackager = new Repackager(sourceJarFile);
	repackager.setBackupSource(false);
	repackager.repackage(new Libraries() {
				@Override
				public void doWithLibraries(LibraryCallback callback) throws IOException {
					// Build system specific implementation, callback for each dependency
					// callback.library(new Library(nestedFile, LibraryScope.COMPILE));
				}
			});
----



[[build-tool-plugins-whats-next]]
== What to read next
If you're interested in how the build tool plugins work you can
look at the {github-code}/spring-boot-tools[`spring-boot-tools`] module on GitHub. More
technical details of the <<appendix-executable-jar-format.adoc#executable-jar, executable
jar format>> are covered in the appendix.

If you have specific build-related questions you can check out the
"`<<howto.adoc#howto, how-to>>`" guides.
