
[[using-boot]]
= Using Spring Boot

[partintro]
--
This section goes into more detail about how you should use Spring Boot. It covers topics
such as build systems, auto-configuration and run/deployment options. We also cover some
Spring Boot best practices. Although there is nothing particularly special about
Spring Boot (it is just another library that you can consume), there are a few
recommendations that, when followed, will make your development process just a
little easier.

If you're just starting out with Spring Boot, you should probably read the
_<<getting-started.adoc#getting-started, Getting Started>>_ guide before diving into
this section.
--



[[using-boot-build-systems]]
== Build systems
It is strongly recommended that you choose a build system that supports _dependency
management_, and one that can consume artifacts published to the "`Maven Central`"
repository. We would recommend that you choose Maven or Gradle. It is possible to get
Spring Boot to work with other build systems (Ant for example), but they will not be
particularly well supported.



[[using-boot-maven]]
=== Maven
Maven users can inherit from the `spring-boot-starter-parent` project to obtain sensible
defaults. The parent project provides the following features:

* Java 1.6 as the default compiler level.
* UTF-8 source encoding.
* A Dependency Management section, allowing you to omit `<version>` tags for common
  dependencies, inherited from the `spring-boot-dependencies` POM.
* Sensible https://maven.apache.org/plugins/maven-resources-plugin/examples/filter.html[resource filtering].
* Sensible plugin configuration (http://mojo.codehaus.org/exec-maven-plugin/[exec plugin],
  http://maven.apache.org/surefire/maven-surefire-plugin/[surefire],
  https://github.com/ktoso/maven-git-commit-id-plugin[Git commit ID],
  http://maven.apache.org/plugins/maven-shade-plugin/[shade]).
* Sensible resource filtering for `application.properties` and `application.yml`

On the last point: since the default config files files accept
Spring style placeholders (`${...}`) the Maven filtering is changed to
use `@..@` placeholders (you can override that with a Maven property
`resource.delimiter`).



[[using-boot-maven-parent-pom]]
==== Inheriting the starter parent
To configure your project to inherit from the `spring-boot-starter-parent` simply set
the `parent`:

[source,xml,indent=0,subs="verbatim,quotes,attributes"]
----
	<!-- Inherit defaults from Spring Boot -->
	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>{spring-boot-version}</version>
	</parent>
----

NOTE: You should only need to specify the Spring Boot version number on this dependency.
If you import additional starters, you can safely omit the version number.



[[using-boot-maven-without-a-parent]]
==== Using Spring Boot without the parent POM
Not everyone likes inheriting from the `spring-boot-starter-parent` POM. You may have your
own corporate standard parent that you need to use, or you may just prefer to explicitly
declare all your Maven configuration.

If you don't want to use the `spring-boot-starter-parent`, you can still keep the benefit
of the dependency management (but not the plugin management) by using a `scope=import`
dependency:

[source,xml,indent=0,subs="verbatim,quotes,attributes"]
----
	<dependencyManagement>
 		<dependencies>
			<dependency>
				<!-- Import dependency management from Spring Boot -->
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-dependencies</artifactId>
				<version>{spring-boot-version}</version>
				<type>pom</type>
		        <scope>import</scope>
			</dependency>
		</dependencies>
	</dependencyManagement>
----



[[using-boot-maven-java-version]]
==== Changing the Java version
The `spring-boot-starter-parent` chooses fairly conservative Java compatibility. If you
want to follow our recommendation and use a later Java version you can add a
`java.version` property:

[source,xml,indent=0,subs="verbatim,quotes,attributes"]
----
	<properties>
		<java.version>1.8</java.version>
	</properties>
----



[[using-boot-maven-plugin]]
==== Using the Spring Boot Maven plugin
Spring Boot includes a <<build-tool-plugins.adoc#build-tool-plugins-maven-plugin, Maven plugin>>
that can package the project as an executable jar. Add the plugin to your `<plugins>`
section if you want to use it:

[source,xml,indent=0,subs="verbatim,quotes,attributes"]
----
	<build>
		<plugins>
			<plugin>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-maven-plugin</artifactId>
			</plugin>
		</plugins>
	</build>
----

NOTE: If you use the Spring Boot starter parent pom, you only need to add the plugin,
there is no need for to configure it unless you want to change the settings defined in
the parent.



[[using-boot-gradle]]
=== Gradle
Gradle users can directly import "`starter POMs`" in their `dependencies` section. Unlike
Maven, there is no "`super parent`" to import to share some configuration.

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

	repositories { jcenter() }
	dependencies {
		compile("org.springframework.boot:spring-boot-starter-web:{spring-boot-version}")
	}
----

The <<build-tool-plugins.adoc#build-tool-plugins-gradle-plugin, `spring-boot-gradle-plugin`>>
is also available and provides tasks to create executable jars and run projects from
source. It also adds a `ResolutionStrategy` that enables you to
<<build-tool-plugins-gradle-dependencies-without-versions, omit the version number
for "`blessed`" dependencies>>:

[source,groovy,indent=0,subs="attributes"]
----
	buildscript {
		repositories { jcenter() }
		dependencies {
			classpath("org.springframework.boot:spring-boot-gradle-plugin:{spring-boot-version}")
		}
	}

	apply plugin: 'java'
	apply plugin: 'spring-boot'

	repositories { jcenter() }
	dependencies {
		compile("org.springframework.boot:spring-boot-starter-web")
		testCompile("org.springframework.boot:spring-boot-starter-test")
	}
----



[[using-boot-ant]]
===  Ant
It is possible to build a Spring Boot project using Apache Ant, however, no special
support or plugins are provided. Ant scripts can use the Ivy dependency system to import
starter POMs.

See the _<<howto.adoc#howto-build-an-executable-archive-with-ant>>_ "`How-to`" for more
complete instructions.



[[using-boot-starter-poms]]
=== Starter POMs
Starter POMs are a set of convenient dependency descriptors that you can include in your
application. You get a one-stop-shop for all the Spring and related technology that you
need, without having to hunt through sample code and copy paste loads of dependency
descriptors. For example, if you want to get started using Spring and JPA for database
access, just include the `spring-boot-starter-data-jpa` dependency in your project, and
you are good to go.

The starters contain a lot of the dependencies that you need to get a project up and
running quickly and with a consistent, supported set of managed transitive dependencies.

.What's in a name
****
All starters follow a similar naming pattern; `+spring-boot-starter-*+`, where `+*+` is
a particular type of application. This naming structure is intended to help when you need
to find a starter. The Maven integration in many IDEs allow you to search dependencies by
name. For example, with the appropriate Eclipse or STS plugin installed, you can simply
hit `ctrl-space` in the POM editor and type ''spring-boot-starter'' for a complete list.
****

The following application starters are provided by Spring Boot under the
`org.springframework.boot` group:

.Spring Boot application starters
|===
| Name | Description

|`spring-boot-starter`
|The core Spring Boot starter, including auto-configuration support, logging and YAML.

|`spring-boot-starter-actuator`
|Production ready features to help you monitor and manage your application.

|`spring-boot-starter-amqp`
|Support for the "`Advanced Message Queuing Protocol`" via `spring-rabbit`.

|`spring-boot-starter-aop`
|Support for aspect-oriented programming including `spring-aop` and AspectJ.

|`spring-boot-starter-batch`
|Support for "`Spring Batch`" including HSQLDB database.

|`spring-boot-starter-cloud-connectors`
|Support for "`Spring Cloud Connectors`" which simplifies connecting to services in cloud
platforms like Cloud Foundry and Heroku.

|`spring-boot-starter-data-elasticsearch`
|Support for the Elasticsearch search and analytics engine including
`spring-data-elasticsearch`.

|`spring-boot-starter-data-gemfire`
|Support for the GemFire distributed data store including `spring-data-gemfire`.

|`spring-boot-starter-data-jpa`
|Support for the "`Java Persistence API`" including `spring-data-jpa`, `spring-orm`
and Hibernate.

|`spring-boot-starter-data-mongodb`
|Support for the MongoDB NoSQL Database, including `spring-data-mongodb`.

|`spring-boot-starter-data-rest`
|Support for exposing Spring Data repositories over REST via `spring-data-rest-webmvc`.

|`spring-boot-starter-data-solr`
|Support for the Apache Solr search platform, including `spring-data-solr`.

|`spring-boot-starter-freemarker`
|Support for the FreeMarker templating engine

|`spring-boot-starter-groovy-templates`
|Support for the Groovy templating engine

|`spring-boot-starter-hornetq`
|Support for "`Java Message Service API`" via HornetQ.

|`spring-boot-starter-integration`
|Support for common `spring-integration` modules.

|`spring-boot-starter-jdbc`
|Support for JDBC databases.

|`spring-boot-starter-jersey`
|Support for the Jersey RESTful Web Services framework.

|`spring-boot-starter-jta-atomikos`
|Support for JTA distributed transactions via Atomikos.

|`spring-boot-starter-jta-bitronix`
|Support for JTA distributed transactions via Bitronix.

|`spring-boot-starter-mail`
|Support for `javax.mail`.

|`spring-boot-starter-mobile`
|Support for `spring-mobile`

|`spring-boot-starter-redis`
|Support for the REDIS key-value data store, including `spring-redis`.

|`spring-boot-starter-security`
|Support for `spring-security`.

|`spring-boot-starter-social-facebook`
|Support for `spring-social-facebook`.

|`spring-boot-starter-social-linkedin`
|Support for `spring-social-linkedin`.

|`spring-boot-starter-social-twitter`
|Support for `spring-social-twitter`.

|`spring-boot-starter-test`
|Support for common test dependencies, including JUnit, Hamcrest and Mockito along with
 the `spring-test` module.

|`spring-boot-starter-thymeleaf`
|Support for the Thymeleaf templating engine, including integration with Spring.

|`spring-boot-starter-velocity`
|Support for the Velocity templating engine

|`spring-boot-starter-web`
|Support for full-stack web development, including Tomcat and `spring-webmvc`.

|`spring-boot-starter-websocket`
|Support for WebSocket development.

|`spring-boot-starter-ws`
|Support for Spring Web Services
|===

In addition to the application starters, the following starters can be used to
add _<<production-ready-features.adoc#production-ready, production ready>>_ features.

.Spring Boot production ready starters
|===
| Name | Description

|`spring-boot-starter-actuator`
|Adds production ready features such as metrics and monitoring.

|`spring-boot-starter-remote-shell`
|Adds remote `ssh` shell support.
|===

Finally, Spring Boot includes some starters that can be used if you want to exclude or
swap specific technical facets.

.Spring Boot technical starters
|===
| Name | Description

|`spring-boot-starter-jetty`
|Imports the Jetty HTTP engine (to be used as an alternative to Tomcat)

|`spring-boot-starter-log4j`
|Support the Log4J logging framework

|`spring-boot-starter-logging`
|Import Spring Boot's default logging framework (Logback).

|`spring-boot-starter-tomcat`
|Import Spring Boot's default HTTP engine (Tomcat).

|`spring-boot-starter-undertow`
|Imports the Undertow HTTP engine (to be used as an alternative to Tomcat)
|===

TIP: For a list of additional community contributed starter POMs, see the
{github-master-code}/spring-boot-starters/README.adoc[README file] in the
`spring-boot-starters` module on GitHub.



[[using-boot-structuring-your-code]]
== Structuring your code
Spring Boot does not require any specific code layout to work, however, there are some
best practices that help.



[[using-boot-using-the-default-package]]
=== Using the "`default`" package
When a class doesn't include a `package` declaration it is considered to be in the
"`default package`". The use of the "`default package`" is generally discouraged, and
should be avoided. It can cause particular problems for Spring Boot applications that
use `@ComponentScan`, `@EntityScan` or `@SpringBootApplication` annotations, since every
class from every jar, will be read.

TIP: We recommend that you follow Java's recommended package naming conventions
and use a reversed domain name (for example, `com.example.project`).



[[using-boot-locating-the-main-class]]
=== Locating the main application class
We generally recommend that you locate your main application class in a root package
above other classes. The `@EnableAutoConfiguration` annotation is often placed on your
main class, and it implicitly defines a base "`search package`" for certain items. For
example, if you are writing a JPA application, the package of the
`@EnableAutoConfiguration` annotated class will be used to search for `@Entity` items.

Using a root package also allows the `@ComponentScan` annotation to be used without
needing to specify a `basePackage` attribute. You can also use the
`@SpringBootApplication` annotation if your main class is in the root package.

Here is a typical layout:

[indent=0]
----
	com
	 +- example
	     +- myproject
	         +- Application.java
	         |
	         +- domain
	         |   +- Customer.java
	         |   +- CustomerRepository.java
	         |
	         +- service
	         |   +- CustomerService.java
	         |
	         +- web
	             +- CustomerController.java
----

The `Application.java` file would declare the `main` method, along with the basic
`@Configuration`.

[source,java,indent=0]
----
	package com.example.myproject;

	import org.springframework.boot.SpringApplication;
	import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
	import org.springframework.context.annotation.ComponentScan;
	import org.springframework.context.annotation.Configuration;

	@Configuration
	@EnableAutoConfiguration
	@ComponentScan
	public class Application {

		public static void main(String[] args) {
			SpringApplication.run(Application.class, args);
		}

	}
----



[[using-boot-configuration-classes]]
== Configuration classes
Spring Boot favors Java-based configuration. Although it is possible to call
`SpringApplication.run()` with an XML source, we generally recommend that your primary
source is a `@Configuration` class. Usually the class that defines the `main` method
is also a good candidate as the primary `@Configuration`.

TIP: Many Spring configuration examples have been published on the Internet that use XML
configuration. Always try to use the equivalent Java-base configuration if possible.
Searching for `+enable*+` annotations can be a good starting point.



[[using-boot-importing-configuration]]
=== Importing additional configuration classes
You don't need to put all your `@Configuration` into a single class. The `@Import`
annotation can be used to import additional configuration classes. Alternatively, you
can use `@ComponentScan` to automatically pickup all Spring components, including
`@Configuration` classes.



[[using-boot-importing-xml-configuration]]
=== Importing XML configuration
If you absolutely must use XML based configuration, we recommend that you still start
with a `@Configuration` class. You can then use an additional `@ImportResource`
annotation to load XML configuration files.



[[using-boot-auto-configuration]]
== Auto-configuration
Spring Boot auto-configuration attempts to automatically configure your Spring
application based on the jar dependencies that you have added. For example, If
`HSQLDB` is on your classpath, and you have not manually configured any database
connection beans, then we will auto-configure an in-memory database.

You need to opt-in to auto-configuration by adding the `@EnableAutoConfiguration` or
`@SpringBootApplication` annotations to one of your `@Configuration` classes.

TIP: You should only ever add one `@EnableAutoConfiguration` annotation. We generally
recommend that you add it to your primary `@Configuration` class.



[[using-boot-replacing-auto-configuration]]
=== Gradually replacing auto-configuration
Auto-configuration is noninvasive,  at any point you can start to define your own
configuration to replace specific parts of the auto-configuration. For example, if
you add your own `DataSource` bean, the default embedded database support will back away.

If you need to find out what auto-configuration is currently being applied, and why,
starting your application with the `--debug` switch. This will log an auto-configuration
report to the console.



[[using-boot-disabling-specific-auto-configutation]]
=== Disabling specific auto-configuration
If you find that specific auto-configure classes are being applied that you don't want,
you can use the exclude attribute of `@EnableAutoConfiguration` to disable them.

[source,java,indent=0]
----
	import org.springframework.boot.autoconfigure.*;
	import org.springframework.boot.autoconfigure.jdbc.*;
	import org.springframework.context.annotation.*;

	@Configuration
	@EnableAutoConfiguration(exclude={DataSourceAutoConfiguration.class})
	public class MyConfiguration {
	}
----



[[using-boot-spring-beans-and-dependency-injection]]
== Spring Beans and dependency injection
You are free to use any of the standard Spring Framework techniques to define your beans
and their injected dependencies. For simplicity, we often find that using `@ComponentScan`
to find your beans, in combination with `@Autowired` constructor injection works well.

If you structure your code as suggested above (locating your application class in a root
package), you can add `@ComponentScan` without any arguments. All of your application
components (`@Component`, `@Service`, `@Repository`, `@Controller` etc.) will be
automatically registered as Spring Beans.

Here is an example `@Service` Bean that uses constructor injection to obtain a
required `RiskAssessor` bean.

[source,java,indent=0]
----
	package com.example.service;

	import org.springframework.beans.factory.annotation.Autowired;
	import org.springframework.stereotype.Service;

	@Service
	public class DatabaseAccountService implements AccountService {

		private final RiskAssessor riskAssessor;

		@Autowired
		public DatabaseAccountService(RiskAssessor riskAssessor) {
			this.riskAssessor = riskAssessor;
		}

		// ...

	}
----

TIP: Notice how using constructor injection allows the `riskAssessor` field to be marked
as `final`, indicating that it cannot be subsequently changed.



[[using-boot-using-springbootapplication-annotation]]
== Using the @SpringBootApplication annotation
Many Spring Boot developers always have their main class annotated with `@Configuration`,
`@EnableAutoConfiguration` and `@ComponentScan`. Since these annotations are so frequently
used together (especially if you follow the <<using-boot-structuring-your-code, best practices>>
above), Spring Boot provides a convenient `@SpringBootApplication` alternative.

The `@SpringBootApplication` annotation is equivalent to using `@Configuration`,
`@EnableAutoConfiguration` and `@ComponentScan` with their default attributes:


[source,java,indent=0]
----
	package com.example.myproject;

	import org.springframework.boot.SpringApplication;
	import org.springframework.boot.autoconfigure.SpringBootApplication;

	@SpringBootApplication // same as @Configuration @EnableAutoConfiguration @ComponentScan
	public class Application {

		public static void main(String[] args) {
			SpringApplication.run(Application.class, args);
		}

	}
----



[[using-boot-running-your-application]]
== Running your application
One of the biggest advantages of packaging your application as jar and using an embedded
HTTP server is that you can run your application as you would any other. Debugging Spring
Boot applications is also easy; you don't need any special IDE plugins or extensions.

NOTE: This section only covers jar based packaging, If you choose to package your
application as a war file you should refer to your server and IDE documentation.



[[using-boot-running-from-an-ide]]
=== Running from an IDE
You can run a Spring Boot application from your IDE as a simple Java application, however,
first you will need to import your project. Import steps will vary depending on your IDE
and build system. Most IDEs can import Maven projects directly, for example Eclipse users
can select `Import...` -> `Existing Maven Projects` from the `File` menu.

If you can't directly import your project into your IDE, you may be able to generate IDE
metadata using a build plugin. Maven includes plugins for
http://maven.apache.org/plugins/maven-eclipse-plugin/[Eclipse] and
http://maven.apache.org/plugins/maven-idea-plugin/[IDEA]; Gradle offers plugins
for http://www.gradle.org/docs/current/userguide/ide_support.html[various IDEs].

TIP: If you accidentally run a web application twice you will see a "`Port already in
use`" error. STS users can use the `Relaunch` button rather than `Run` to ensure that
any existing instance is closed.



[[using-boot-running-as-a-packaged-application]]
=== Running as a packaged application
If you use the Spring Boot Maven or Gradle plugins to create an executable jar you can
run your application using `java -jar`. For example:

[indent=0,subs="attributes"]
----
	$ java -jar target/myproject-0.0.1-SNAPSHOT.jar
----

It is also possible to run a packaged application with remote debugging support enabled.
This allows you to attach a debugger to your packaged application:

[indent=0,subs="attributes"]
----
	$ java -Xdebug -Xrunjdwp:server=y,transport=dt_socket,address=8000,suspend=n \
	       -jar target/myproject-0.0.1-SNAPSHOT.jar
----



[[using-boot-running-with-the-maven-plugin]]
=== Using the Maven plugin
The Spring Boot Maven plugin includes a `run` goal which can be used to quickly compile
and run your application. Applications run in an exploded form, and you can edit
resources for instant "`hot`" reload.

[indent=0,subs="attributes"]
----
	$ mvn spring-boot:run
----

You might also want to use the useful operating system environment variable:

[indent=0,subs="attributes"]
----
	$ export MAVEN_OPTS=-Xmx1024m -XX:MaxPermSize=128M -Djava.security.egd=file:/dev/./urandom
----

(The "`egd`" setting is to speed up Tomcat startup by giving it a faster source of
entropy for session keys.)



[[using-boot-running-with-the-gradle-plugin]]
=== Using the Gradle plugin
The Spring Boot Gradle plugin also includes a `run` goal which can be used to run
your application in an exploded form. The `bootRun` task is added whenever you import
the `spring-boot-plugin`

[indent=0,subs="attributes"]
----
	$ gradle bootRun
----

You might also want to use this useful operating system environment variable:

[indent=0,subs="attributes"]
----
	$ export JAVA_OPTS=-Xmx1024m -XX:MaxPermSize=128M -Djava.security.egd=file:/dev/./urandom
----



[[using-boot-hot-swapping]]
=== Hot swapping
Since Spring Boot applications are just plain Java applications, JVM hot-swapping should
work out of the box. JVM hot swapping is somewhat limited with the bytecode that it can
replace, for a more complete solution the
https://github.com/spring-projects/spring-loaded[Spring Loaded] project, or
http://zeroturnaround.com/software/jrebel/[JRebel] can be used.

See the <<howto.adoc#howto-hotswapping, Hot swapping "`How-to`">> section for details.



[[using-boot-packaging-for-production]]
== Packaging your application for production
Executable jars can be used for production deployment. As they are self-contained, they
are also ideally suited for cloud-based deployment.

For additional "`production ready`" features, such as health, auditing and metric REST
or JMX end-points; consider adding `spring-boot-actuator`. See
_<<production-ready-features.adoc#production-ready>>_ for details.



[[using-boot-whats-next]]
== What to read next
You should now have good understanding of how you can use Spring Boot along with some best
practices that you should follow. You can now go on to learn about specific
_<<spring-boot-features#boot-features, Spring Boot features>>_ in depth, or you
could skip ahead and read about the
"`<<production-ready-features#production-ready, production ready>>`" aspects of Spring
Boot.
