[[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 how to run your applications. 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, make your development process a little easier.

If you are 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
<<using-boot-dependency-management,_dependency management_>> and 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 are not particularly well supported.



[[using-boot-dependency-management]]
=== Dependency Management
Each release of Spring Boot provides a curated list of dependencies that it supports. In
practice, you do not need to provide a version for any of these dependencies in your
build configuration, as Spring Boot manages that for you. When you upgrade Spring
Boot itself, these dependencies are upgraded as well in a consistent way.

NOTE: You can still specify a version and override Spring Boot's recommendations if you
need to do so.

The curated list contains all the spring modules that you can use with Spring Boot as
well as a refined list of third party libraries. The list is available as a standard
<<using-boot-maven-without-a-parent,Bills of Materials (`spring-boot-dependencies`)>>
that can be used with both <<using-boot-maven-parent-pom,Maven>> and
<<using-boot-gradle,Gradle>>.

WARNING: Each release of Spring Boot is associated with a base version of the Spring
Framework. We **highly** recommend that you not specify its version.



[[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 1.8 as the default compiler level.
* UTF-8 source encoding.
* A <<using-boot-dependency-management,Dependency Management section>>, inherited from
the spring-boot-dependencies pom, that manages the versions of common dependencies. This
dependency management lets you omit <version> tags for those dependencies when used in
your own pom.
* Sensible
https://maven.apache.org/plugins/maven-resources-plugin/examples/filter.html[resource
filtering].
* Sensible plugin configuration (http://www.mojohaus.org/exec-maven-plugin/[exec plugin],
https://github.com/ktoso/maven-git-commit-id-plugin[Git commit ID], and
https://maven.apache.org/plugins/maven-shade-plugin/[shade]).
* Sensible resource filtering for `application.properties` and `application.yml`
including profile-specific files (for example, `application-dev.properties` and
`application-dev.yml`)

Note that, since the `application.properties` and `application.yml` files accept Spring
style placeholders (`${...}`), the Maven filtering is changed to use `@..@` placeholders.
(You can override that by setting a Maven property called `resource.delimiter`.)



[[using-boot-maven-parent-pom]]
==== Inheriting the Starter Parent
To configure your project to inherit from the `spring-boot-starter-parent`, set the
`parent` as follows:

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

With that setup, you can also override individual dependencies by overriding a property
in your own project. For instance, to upgrade to another Spring Data release train, you
would add the following to your `pom.xml`:

[source,xml,indent=0,subs="verbatim,quotes,attributes"]
----
	<properties>
		<spring-data-releasetrain.version>Fowler-SR2</spring-data-releasetrain.version>
	</properties>
----

TIP: Check the
{github-code}/spring-boot-project/spring-boot-dependencies/pom.xml[`spring-boot-dependencies` pom]
for a list of supported properties.



[[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 prefer to explicitly
declare all your Maven configuration.

If you do not 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, as follows:

[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>
----

The preceding sample setup does not let you override individual dependencies by using a
property, as explained above. To achieve the same result, you need to add an entry in the
`dependencyManagement` of your project **before** the `spring-boot-dependencies` entry.
For instance, to upgrade to another Spring Data release train, you could add the
following element to your `pom.xml`:

[source,xml,indent=0,subs="verbatim,quotes,attributes"]
----
	<dependencyManagement>
		<dependencies>
			<!-- Override Spring Data release train provided by Spring Boot -->
			<dependency>
				<groupId>org.springframework.data</groupId>
				<artifactId>spring-data-releasetrain</artifactId>
				<version>Fowler-SR2</version>
				<type>pom</type>
				<scope>import</scope>
			</dependency>
			<dependency>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-dependencies</artifactId>
				<version>{spring-boot-version}</version>
				<type>pom</type>
				<scope>import</scope>
			</dependency>
		</dependencies>
	</dependencyManagement>
----

NOTE: In the preceding example, we specify a _BOM_, but any dependency type can be
overridden in the same way.



[[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, as shown in the following example:

[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 need to add only the plugin.
There is no need to configure it unless you want to change the settings defined in the
parent.



[[using-boot-gradle]]
=== Gradle
To learn about using Spring Boot with Gradle, please refer to the documentation for
Spring Boot's Gradle plugin:

* Reference ({spring-boot-gradle-plugin}/reference/html[HTML] and
{spring-boot-gradle-plugin}/reference/pdf/spring-boot-gradle-plugin-reference.pdf[PDF])
* {spring-boot-gradle-plugin}/api[API]

[[using-boot-ant]]
===  Ant
It is possible to build a Spring Boot project using Apache Ant+Ivy. The
`spring-boot-antlib` "`AntLib`" module is also available to help Ant create executable
jars.

To declare dependencies, a typical `ivy.xml` file looks something like the following
example:

[source,xml,indent=0]
----
	<ivy-module version="2.0">
		<info organisation="org.springframework.boot" module="spring-boot-sample-ant" />
		<configurations>
			<conf name="compile" description="everything needed to compile this module" />
			<conf name="runtime" extends="compile" description="everything needed to run this module" />
		</configurations>
		<dependencies>
			<dependency org="org.springframework.boot" name="spring-boot-starter"
				rev="${spring-boot.version}" conf="compile" />
		</dependencies>
	</ivy-module>
----

A typical `build.xml` looks like the following example:

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

		<property name="spring-boot.version" value="{spring-boot-version}" />

		<target name="resolve" description="--> retrieve dependencies with ivy">
			<ivy:retrieve pattern="lib/[conf]/[artifact]-[type]-[revision].[ext]" />
		</target>

		<target name="classpaths" depends="resolve">
			<path id="compile.classpath">
				<fileset dir="lib/compile" includes="*.jar" />
			</path>
		</target>

		<target name="init" depends="classpaths">
			<mkdir dir="build/classes" />
		</target>

		<target name="compile" depends="init" description="compile">
			<javac srcdir="src/main/java" destdir="build/classes" classpathref="compile.classpath" />
		</target>

		<target name="build" depends="compile">
			<spring-boot:exejar destfile="build/myapp.jar" classes="build/classes">
				<spring-boot:lib>
					<fileset dir="lib/runtime" />
				</spring-boot:lib>
			</spring-boot:exejar>
		</target>
	</project>
----

TIP: If you do not want to use the `spring-boot-antlib` module, see the
_<<howto.adoc#howto-build-an-executable-archive-with-ant>>_ "`How-to`" .



[[using-boot-starter]]
=== Starters
Starters 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 technologies 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, include the `spring-boot-starter-data-jpa` dependency in your project.

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 **official** 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 lets you
search dependencies by name. For example, with the appropriate Eclipse or STS plugin
installed, you can press `ctrl-space` in the POM editor and type
"`spring-boot-starter`" for a complete list.

As explained in the "`<<spring-boot-features#boot-features-custom-starter,Creating Your
Own Starter>>`" section, third party starters should not start with `spring-boot`, as it
is reserved for official Spring Boot artifacts. Rather, a third-party starter typically
starts with the name of the project. For example, a third-party starter project called
`thirdpartyproject` would typically be named `thirdpartyproject-spring-boot-starter`.
****

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

.Spring Boot application starters
include::../../../target/generated-resources/application-starters.adoc[]

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 starters
include::../../../target/generated-resources/production-starters.adoc[]

Finally, Spring Boot also includes the following starters that can be used if you want to
exclude or swap specific technical facets:

.Spring Boot technical starters
include::../../../target/generated-resources/technical-starters.adoc[]

TIP: For a list of additional community contributed starters, see the
{github-master-code}/spring-boot-project/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 does not 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
the `@ComponentScan`, `@EntityScan`, or `@SpringBootApplication` annotations, since every
class from every jar is 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 <<using-boot-using-springbootapplication-annotation,
`@SpringBootApplication` 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 `@SpringBootApplication` annotated class
is used to search for `@Entity` items. Using a root package also allows component
scan to apply only on your project.

TIP: If you don't want to use `@SpringBootApplication`, the `@EnableAutoConfiguration`
and `@ComponentScan` annotations that it imports defines that behaviour so you can also
use that instead.

The following listing shows a typical layout:

[indent=0]
----
	com
	 +- example
	     +- myapplication
	         +- Application.java
	         |
	         +- customer
	         |   +- Customer.java
	         |   +- CustomerController.java
	         |   +- CustomerService.java
	         |   +- CustomerRepository.java
	         |
	         +- order
	             +- Order.java
	             +- OrderController.java
	             +- OrderService.java
	             +- OrderRepository.java
----

The `Application.java` file would declare the `main` method, along with the basic
`@SpringBootApplication`, as follows:

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

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

	@SpringBootApplication
	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 use
`SpringApplication` with XML sources, we generally recommend that your primary source be
a single `@Configuration` class. Usually the class that defines the `main` method is a
good candidate as the primary `@Configuration`.

TIP: Many Spring configuration examples have been published on the Internet that use XML
configuration. If possible, always try to use the equivalent Java-based configuration.
Searching for `+Enable*+` annotations can be a good starting point.



[[using-boot-importing-configuration]]
=== Importing Additional Configuration Classes
You need not 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 pick up 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 `@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 Spring Boot auto-configures 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 `@SpringBootApplication` or `@EnableAutoConfiguration`
annotation. We generally recommend that you add one or the other to your primary
`@Configuration` class only.



[[using-boot-replacing-auto-configuration]]
=== Gradually Replacing Auto-configuration
Auto-configuration is non-invasive. 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 backs away.

If you need to find out what auto-configuration is currently being applied, and why,
start your application with the `--debug` switch. Doing so enables debug logs for a
selection of core loggers and logs a conditions report to the console.



[[using-boot-disabling-specific-auto-configuration]]
=== Disabling Specific Auto-configuration Classes
If you find that specific auto-configuration classes that you do not want are being
applied, you can use the exclude attribute of `@EnableAutoConfiguration` to disable them,
as shown in the following example:

[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 {
	}
----

If the class is not on the classpath, you can use the `excludeName` attribute of the
annotation and specify the fully qualified name instead. Finally, you can also control
the list of auto-configuration classes to exclude by using the
`spring.autoconfigure.exclude` property.

TIP: You can define exclusions both at the annotation level and by using the property.

[[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) and using `@Autowired` (to do 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.) are
automatically registered as Spring Beans.

The following example shows a `@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;
		}

		// ...

	}
----

If a bean has one constructor, you can omit the `@Autowired`, as shown in the following
example:

[source,java,indent=0]
----
	@Service
	public class DatabaseAccountService implements AccountService {

		private final RiskAssessor riskAssessor;

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

		// ...

	}
----

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



[[using-boot-using-springbootapplication-annotation]]
== Using the @SpringBootApplication Annotation
Many Spring Boot developers like their apps to use auto-configuration, component scan and
be able to define extra configuration on their "application class". A single
`@SpringBootApplication` annotation can be used to enable those three features, that is:

* `@EnableAutoConfiguration`: enable <<using-boot-auto-configuration,Spring Boot's
auto-configuration mechanism>>
* `@ComponentScan`: enable `@Component` scan on the package where the application is
located (see <<using-boot-structuring-your-code,the best practices>>)
* `@Configuration`: allow to register extra beans in the context or import additional
configuration classes

The `@SpringBootApplication` annotation is equivalent to using `@Configuration`,
`@EnableAutoConfiguration`, and `@ComponentScan` with their default attributes, as shown
in the following example:


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

	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);
		}

	}
----

NOTE: `@SpringBootApplication` also provides aliases to customize the attributes of
`@EnableAutoConfiguration` and `@ComponentScan`.

[NOTE]
====
None of these features are mandatory and you may chose to replace this single annotation
by any of the features that it enables. For instance, you may not want to use component
scan in your application:

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

	import org.springframework.boot.SpringApplication;
	import org.springframework.context.annotation.ComponentScan
	import org.springframework.context.annotation.Configuration;
	import org.springframework.context.annotation.Import;

	@Configuration
	@EnableAutoConfiguration
	@Import({ MyConfig.class, MyAnotherConfig.class })
	public class Application {

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

	}
----

In this example, `Application` is just like any other Spring Boot application except that
`@Component`-annotated classes are not detected automatically and the user-defined beans
are imported explicitly (see `@Import`).
====



[[using-boot-running-your-application]]
== Running Your Application
One of the biggest advantages of packaging your application as a 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 do not 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, you first need to import your project. Import steps 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 cannot directly import your project into your IDE, you may be able to generate IDE
metadata by using a build plugin. Maven includes plugins for
https://maven.apache.org/plugins/maven-eclipse-plugin/[Eclipse] and
https://maven.apache.org/plugins/maven-idea-plugin/[IDEA]. Gradle offers plugins for
{gradle-user-guide}/userguide.html[various IDEs].

TIP: If you accidentally run a web application twice, you see a "`Port already in use`"
error. STS users can use the `Relaunch` button rather than the `Run` button 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`, as shown in the following example:

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

It is also possible to run a packaged application with remote debugging support enabled.
Doing so lets you attach a debugger to your packaged application, as shown in the
following example:

[indent=0,subs="attributes"]
----
	$ java -Xdebug -Xrunjdwp:server=y,transport=dt_socket,address=8000,suspend=n \
	       -jar target/myapplication-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 that can be used to quickly compile
and run your application. Applications run in an exploded form, as they do in your IDE.
The following example shows a typical Maven command to run a Spring Boot application:

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

You might also want to use the `MAVEN_OPTS` operating system environment variable, as
shown in the following example:

[indent=0,subs="attributes"]
----
	$ export MAVEN_OPTS=-Xmx1024m
----



[[using-boot-running-with-the-gradle-plugin]]
=== Using the Gradle Plugin
The Spring Boot Gradle plugin also includes a `bootRun` task that can be used to run your
application in an exploded form. The `bootRun` task is added whenever you apply the
`org.springframework.boot` and `java` plugins and is shown in the following example:

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

You might also want to use the `JAVA_OPTS` operating system environment variable, as
shown in the following example:

[indent=0,subs="attributes"]
----
	$ export JAVA_OPTS=-Xmx1024m
----



[[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,
https://zeroturnaround.com/software/jrebel/[JRebel] can be used.

The
`spring-boot-devtools` module also includes support for quick application restarts.
See the <<using-boot-devtools>> section later in this chapter and the
<<howto.adoc#howto-hotswapping, Hot swapping "`How-to`">> for details.



[[using-boot-devtools]]
== Developer Tools
Spring Boot includes an additional set of tools that can make the application
development experience a little more pleasant. The `spring-boot-devtools` module can be
included in any project to provide additional development-time features. To include
devtools support, add the module dependency to your build, as shown in the following
listings for Maven and Gradle:

.Maven
[source,xml,indent=0,subs="verbatim,quotes,attributes"]
----
	<dependencies>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-devtools</artifactId>
			<optional>true</optional>
		</dependency>
	</dependencies>
----

.Gradle
[source,groovy,indent=0,subs="attributes"]
----
	dependencies {
		compile("org.springframework.boot:spring-boot-devtools")
	}
----

NOTE: Developer tools are automatically disabled when running a fully packaged
application. If your application is launched from `java -jar` or if it is started from a
special classloader, then it is considered a "`production application`". Flagging the
dependency as optional in Maven or using `compileOnly` in Gradle is a best practice that
prevents devtools from being transitively applied to other modules that use your project.

TIP: Repackaged archives do not contain devtools by default. If you want to use a
<<using-boot-devtools-remote,certain remote devtools feature>>, you need to disable the
`excludeDevtools` build property to include it. The property is supported with both the
Maven and Gradle plugins.



[[using-boot-devtools-property-defaults]]
=== Property Defaults
Several of the libraries supported by Spring Boot use caches to improve performance. For
example, <<spring-boot-features#boot-features-spring-mvc-template-engines,template
engines>> cache compiled templates to avoid repeatedly parsing template files. Also,
Spring MVC can add HTTP caching headers to responses when serving static resources.

While caching is very beneficial in production, it can be counter-productive during
development, preventing you from seeing the changes you just made in your application.
For this reason, spring-boot-devtools disables the caching options by default.

Cache options are usually configured by settings in your `application.properties` file.
For example, Thymeleaf offers the `spring.thymeleaf.cache` property. Rather than needing
to set these properties manually, the `spring-boot-devtools` module automatically applies
sensible development-time configuration.

TIP: For a complete list of the properties that are applied by the devtools, see
{sc-spring-boot-devtools}/env/DevToolsPropertyDefaultsPostProcessor.{sc-ext}[DevToolsPropertyDefaultsPostProcessor].



[[using-boot-devtools-restart]]
=== Automatic Restart
Applications that use `spring-boot-devtools` automatically restart whenever files on the
classpath change. This can be a useful feature when working in an IDE, as it gives a very
fast feedback loop for code changes. By default, any entry on the classpath that points
to a folder is monitored for changes. Note that certain resources, such as static assets
and view templates, <<using-boot-devtools-restart-exclude, do not need to restart the
application>>.

.Triggering a restart
****
As DevTools monitors classpath resources, the only way to trigger a restart is to update
the classpath. The way in which you cause the classpath to be updated depends on the IDE
that you are using. In Eclipse, saving a modified file causes the classpath to be updated
and triggers a restart. In IntelliJ IDEA, building the project
(`Build +->+ Build Project`) has the same effect.
****

[NOTE]
====
As long as forking is enabled, you can also start your application by using the supported
build plugins (Maven and Gradle), since DevTools needs an isolated application
classloader to operate properly. By default, Gradle and Maven do that when they detect
DevTools on the classpath.

====

TIP: Automatic restart works very well when used with LiveReload.
<<using-boot-devtools-livereload,See the LiveReload section>> for details. If you use
JRebel, automatic restarts are disabled in favor of dynamic class reloading. Other
devtools features (such as LiveReload and property overrides) can still be used.

NOTE: DevTools relies on the application context's shutdown hook to close it during a
restart. It does not work correctly if you have disabled the shutdown hook
(`SpringApplication.setRegisterShutdownHook(false)`).

NOTE: When deciding if an entry on the classpath should trigger a restart when it
changes, DevTools automatically ignores projects named `spring-boot`,
`spring-boot-devtools`, `spring-boot-autoconfigure`, `spring-boot-actuator`, and
`spring-boot-starter`.

NOTE: DevTools needs to customize the `ResourceLoader` used by the `ApplicationContext`.
If your application provides one already, it is going to be wrapped. Direct override of
the `getResource` method on the `ApplicationContext` is not supported.

[[using-spring-boot-restart-vs-reload]]
.Restart vs Reload
****
The restart technology provided by Spring Boot works by using two classloaders. Classes
that do not change (for example, those from third-party jars) are loaded into a _base_
classloader. Classes that you are actively developing are loaded into a _restart_
classloader. When the application is restarted, the _restart_ classloader is thrown away
and a new one is created. This approach means that application restarts are typically
much faster than "`cold starts`", since the _base_ classloader is already available and
populated.

If you find that restarts are not quick enough for your applications or you encounter
classloading issues, you could consider reloading technologies such as
https://zeroturnaround.com/software/jrebel/[JRebel] from ZeroTurnaround. These work by
rewriting classes as they are loaded to make them more amenable to reloading.
****

[[using-boot-devtools-restart-logging-condition-delta]]
==== Logging changes in condition evaluation
By default, each time your application restarts, a report showing the condition evaluation
delta is logged. The report shows the changes to your application's auto-configuration as
you make changes such as adding or removing beans and setting configuration properties.

To disable the logging of the report, set the following property:

[indent=0]
----
	spring.devtools.restart.log-condition-evaluation-delta=false
----


[[using-boot-devtools-restart-exclude]]
==== Excluding Resources
Certain resources do not necessarily need to trigger a restart when they are changed. For
example, Thymeleaf templates can be edited in-place. By default, changing resources
in `/META-INF/maven`, `/META-INF/resources`, `/resources`, `/static`, `/public`, or
`/templates` does not trigger a restart but does trigger a
<<using-boot-devtools-livereload, live reload>>. If you want to customize these
exclusions, you can use the `spring.devtools.restart.exclude` property. For example, to
exclude only `/static` and `/public` you would set the following property:

[indent=0]
----
	spring.devtools.restart.exclude=static/**,public/**
----

TIP: If you want to keep those defaults and _add_ additional exclusions, use the
`spring.devtools.restart.additional-exclude` property instead.


[[using-boot-devtools-restart-additional-paths]]
==== Watching Additional Paths
You may want your application to be restarted or reloaded when you make changes to files
that are not on the classpath. To do so, use the
`spring.devtools.restart.additional-paths` property to configure additional paths to
watch for changes. You can use the `spring.devtools.restart.exclude` property
<<using-boot-devtools-restart-exclude, described earlier>> to control whether changes
beneath the additional paths trigger a full restart or a
<<using-boot-devtools-livereload, live reload>>.



[[using-boot-devtools-restart-disable]]
==== Disabling Restart
If you do not want to use the restart feature, you can disable it by using the
`spring.devtools.restart.enabled` property. In most cases, you can set this property in
your `application.properties` (doing so still initializes the restart classloader, but it
does not watch for file changes).

If you need to _completely_ disable restart support (for example, because it does not work
with a specific library), you need to set the `spring.devtools.restart.enabled` `System`
property to `false` before calling `SpringApplication.run(...)`, as shown in the
following example:

[source,java,indent=0]
----
	public static void main(String[] args) {
		System.setProperty("spring.devtools.restart.enabled", "false");
		SpringApplication.run(MyApp.class, args);
	}
----



[[using-boot-devtools-restart-triggerfile]]
==== Using a Trigger File
If you work with an IDE that continuously compiles changed files, you might prefer to
trigger restarts only at specific times. To do so, you can use a "`trigger file`", which
is a special file that must be modified when you want to actually trigger a restart
check. Changing the file only triggers the check and the restart only occurs if
Devtools has detected it has to do something. The trigger file can be updated manually or
with an IDE plugin.

To use a trigger file, set the `spring.devtools.restart.trigger-file` property to the
path of your trigger file.

TIP: You might want to set `spring.devtools.restart.trigger-file` as a
<<using-boot-devtools-globalsettings,global setting>>, so that all your projects behave
in the same way.



[[using-boot-devtools-customizing-classload]]
==== Customizing the Restart Classloader
As described earlier in the <<using-spring-boot-restart-vs-reload>> section, restart
functionality is implemented by using two classloaders. For most applications, this
approach works well. However, it can sometimes cause classloading issues.

By default, any open project in your IDE is loaded with the "`restart`" classloader, and
any regular `.jar` file is loaded with the "`base`" classloader. If you work on a
multi-module project, and not every module is imported into your IDE, you may need to
customize things. To do so, you can create a `META-INF/spring-devtools.properties` file.

The `spring-devtools.properties` file can contain properties prefixed with
`restart.exclude` and `restart.include`. The `include` elements are items that should be
pulled up into the "`restart`" classloader, and the `exclude` elements are items that
should be pushed down into the "`base`" classloader. The value of the property is a regex
pattern that is applied to the classpath, as shown in the following example:

[source,properties,indent=0]
----
	restart.exclude.companycommonlibs=/mycorp-common-[\\w-]+\.jar
	restart.include.projectcommon=/mycorp-myproj-[\\w-]+\.jar
----

NOTE: All property keys must be unique. As long as a property starts with
`restart.include.` or `restart.exclude.` it is considered.

TIP: All `META-INF/spring-devtools.properties` from the classpath are loaded. You can
package files inside your project, or in the libraries that the project consumes.



[[using-boot-devtools-known-restart-limitations]]
==== Known Limitations
Restart functionality does not work well with objects that are deserialized by using a
standard `ObjectInputStream`. If you need to deserialize data, you may need to use
Spring's `ConfigurableObjectInputStream` in combination with
`Thread.currentThread().getContextClassLoader()`.

Unfortunately, several third-party libraries deserialize without considering the context
classloader. If you find such a problem, you need to request a fix with the original
authors.



[[using-boot-devtools-livereload]]
=== LiveReload
The `spring-boot-devtools` module includes an embedded LiveReload server that can be used
to trigger a browser refresh when a resource is changed. LiveReload browser extensions
are freely available for Chrome, Firefox and Safari from
https://livereload.com/extensions/[livereload.com].

If you do not want to start the LiveReload server when your application runs, you can set
the `spring.devtools.livereload.enabled` property to `false`.

NOTE: You can only run one LiveReload server at a time. Before starting your application,
ensure that no other LiveReload servers are running. If you start multiple applications
from your IDE, only the first has LiveReload support.



[[using-boot-devtools-globalsettings]]
=== Global Settings
You can configure global devtools settings by adding a file named
`.spring-boot-devtools.properties` to your `$HOME` folder (note that the filename starts
with "`.`"). Any properties added to this file apply to _all_ Spring Boot applications on
your machine that use devtools. For example, to configure restart to always use a
<<using-boot-devtools-restart-triggerfile, trigger file>>, you would add the following
property:

.~/.spring-boot-devtools.properties
[source,properties,indent=0]
----
	spring.devtools.reload.trigger-file=.reloadtrigger
----



[[using-boot-devtools-remote]]
=== Remote Applications
The Spring Boot developer tools are not limited to local development. You can also
use several features when running applications remotely. Remote support is opt-in. To
enable it, you need to make sure that `devtools` is included in the repackaged archive,
as shown in the following listing:

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

Then you need to set a `spring.devtools.remote.secret` property, as shown in the
following example:

[source,properties,indent=0]
----
	spring.devtools.remote.secret=mysecret
----

WARNING: Enabling `spring-boot-devtools` on a remote application is a security risk. You
should never enable support on a production deployment.

Remote devtools support is provided in two parts: a server-side endpoint that accepts
connections and a client application that you run in your IDE. The server component is
automatically enabled when the `spring.devtools.remote.secret` property is set. The
client component must be launched manually.



==== Running the Remote Client Application
The remote client application is designed to be run from within your IDE. You need to run
`org.springframework.boot.devtools.RemoteSpringApplication` with the same classpath as
the remote project that you connect to. The application's single required argument is the
remote URL to which it connects.

For example, if you are using Eclipse or STS and you have a project named `my-app` that
you have deployed to Cloud Foundry, you would do the following:

* Select `Run Configurations...` from the `Run` menu.
* Create a new `Java Application` "`launch configuration`".
* Browse for the `my-app` project.
* Use `org.springframework.boot.devtools.RemoteSpringApplication` as the main class.
* Add `+++https://myapp.cfapps.io+++` to the `Program arguments` (or whatever your remote
URL is).

A running remote client might resemble the following listing:

[indent=0,subs="attributes"]
----
	  .   ____          _                                              __ _ _
	 /\\ / ___'_ __ _ _(_)_ __  __ _          ___               _      \ \ \ \
	( ( )\___ | '_ | '_| | '_ \/ _` |        | _ \___ _ __  ___| |_ ___ \ \ \ \
	 \\/  ___)| |_)| | | | | || (_| []::::::[]   / -_) '  \/ _ \  _/ -_) ) ) ) )
	  '  |____| .__|_| |_|_| |_\__, |        |_|_\___|_|_|_\___/\__\___|/ / / /
	 =========|_|==============|___/===================================/_/_/_/
	 :: Spring Boot Remote :: {spring-boot-version}

	2015-06-10 18:25:06.632  INFO 14938 --- [           main] o.s.b.devtools.RemoteSpringApplication   : Starting RemoteSpringApplication on pwmbp with PID 14938 (/Users/pwebb/projects/spring-boot/code/spring-boot-devtools/target/classes started by pwebb in /Users/pwebb/projects/spring-boot/code/spring-boot-samples/spring-boot-sample-devtools)
	2015-06-10 18:25:06.671  INFO 14938 --- [           main] s.c.a.AnnotationConfigApplicationContext : Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@2a17b7b6: startup date [Wed Jun 10 18:25:06 PDT 2015]; root of context hierarchy
	2015-06-10 18:25:07.043  WARN 14938 --- [           main] o.s.b.d.r.c.RemoteClientConfiguration    : The connection to http://localhost:8080 is insecure. You should use a URL starting with 'https://'.
	2015-06-10 18:25:07.074  INFO 14938 --- [           main] o.s.b.d.a.OptionalLiveReloadServer       : LiveReload server is running on port 35729
	2015-06-10 18:25:07.130  INFO 14938 --- [           main] o.s.b.devtools.RemoteSpringApplication   : Started RemoteSpringApplication in 0.74 seconds (JVM running for 1.105)
----

NOTE: Because the remote client is using the same classpath as the real application it
can directly read application properties. This is how the `spring.devtools.remote.secret`
property is read and passed to the server for authentication.

TIP: It is always advisable to use `https://` as the connection protocol, so that traffic
is encrypted and passwords cannot be intercepted.

TIP: If you need to use a proxy to access the remote application, configure the
`spring.devtools.remote.proxy.host` and `spring.devtools.remote.proxy.port` properties.



[[using-boot-devtools-remote-update]]
==== Remote Update
The remote client monitors your application classpath for changes in the same way as the
<<using-boot-devtools-restart,local restart>>. Any updated resource is pushed to the
remote application and (_if required_) triggers a restart. This can be helpful if you
iterate on a feature that uses a cloud service that you do not have locally. Generally,
remote updates and restarts are much quicker than a full rebuild and deploy cycle.

NOTE: Files are only monitored when the remote client is running. If you change a file
before starting the remote client, it is not pushed to the remote server.



[[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 understand how you can use Spring Boot and 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.
