spring-boot/docs/howto.md

# How Do I Do That With Spring Boot?

Here is a starting point for a potentially large collection of micro
HOWTO guides. If you want to add a placeholder for a question without
an answer, put it at the top (at header level 2) and we can fill in
the gaps later.

### General Advice and Other Sources of Information

There is a really useful `AutoConfigurationReport` available in any
Spring Boot `ApplicationContext`. You will see it automatically if a
context fails to start, and also if you enable DEBUG logging for
Spring Boot. If you use the Actuator there is also an endpoint
`/autoconfig` that renders the report in JSON. Use that to debug the
application and see what features have been added (and which not) by
Spring Boot at runtime.

Many more questions can be answered by looking at the source code and
Javadocs. Some rules of thumb:

* Look for classes called `*AutoConfiguration` and read their sources,
  in particular the `@Conditional*` annotations to find out what
  features they enable and when. In those clases...

* Look for classes that are `@ConfigurationProperties`
  (e.g. [`ServerProperties`](https://github.com/spring-projects/spring-boot/blob/master/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ServerProperties.java?source=c))
  and read from there the available external configuration
  options. The `@ConfigurationProperties` has a `name` attribute which
  acts as a prefix to external properties, thus `ServerProperties` has
  `name="server"` and its configuration properties are `server.port`,
  `server.address` etc.

* Look for use of `RelaxedEnvironment` to pull configuration values
  explicitly out of the `Environment`. It often is used with a prefix.

* Look for `@Value` annotations that bind directly to the
  `Environment`. This is less flexible than the `RelaxedEnvironment`
  approach, but does allow some relaxed binding, specifically for OS
  environment variables (so `CAPITALS_AND_UNDERSCORES` are synonyms
  for `period.separated`).

* Look for `@ConditionalOnExpression` annotations that switch features
  on and off in response to SpEL expressions, normally evaluated with
  placeholders resolved from the `Environment`.


## Write a JSON REST Service

Any Spring `@RestController` in a Spring Boot application should
render JSON response by default as long as Jackson2 is on the
classpath. For example:

```java
@RestController
public class MyController {

    @RequestMapping("/thing")
    public MyThing thing() {
        return new MyThing();
    }

}
```

As long as `MyThing` can be serialized by Jackson2 (e.g. a normal POJO
or Groovy object) then `http://localhost:8080/thing` will serve a JSON
representation of it by default. Sometimes in a browser you might see
XML responses (but by default only if `MyThing` was a JAXB object)
because browsers tend to send accept headers that prefer XML.

## Customize the Jackson ObjectMapper

Spring MVC (client and server side) uses `HttpMessageConverters` to
negotiate content conversion in an HTTP exchange. If Jackson is on the
classpath you already get a default converter with a vanilla
`ObjectMapper`. Spring Boot has some features to make it easier to
customize this behaviour.

The smallest change that might work is to just add beans of type
`Module` to your context. They will be registered with the default
`ObjectMapper` and then injected into the default message
converter. To replace the default `ObjectMapper` completely, define a
`@Bean` of that type and mark it as `@Primary`.

In addition, if your context contains any beans of type `ObjectMapper`
then all of the `Module` beans will be registered with all of the
mappers. So there is a global mechanism for contributing custom
modules when you add new features to your application.

Finally, if you provide any `@Beans` of type
`MappingJackson2HttpMessageConverter` then they will replace the
default value in the MVC configuration. Also, a convenience bean is
provided of type `HttpMessageConverters` (always available if you use
the default MVC configuration) which has some useful methods to access
the default and user-enhanced message converters.

See also the [section on `HttpMessageConverters`](#message.converters)
and the `WebMvcAutoConfiguration` source code for more details.

<span id="message.converters"/>
## Customize the @ResponseBody Rendering

Spring uses `HttpMessageConverters` to render `@ResponseBody` (or
responses from `@RestControllers`). You can contribute additional
converters by simply adding beans of that type in a Spring Boot
context. If a bean you add is of a type that would have been included
by default anyway (like `MappingJackson2HttpMessageConverter` for JSON
conversions) then it will replace the default value. A convenience
bean is provided of type `HttpMessageConverters` (always available if you
use the default MVC configuration) which has some useful methods to
access the default and user-enhanced message converters (useful, for
example if you want to manually inject them into a custom
`RestTemplate`).

As in normal MVC usage, any `WebMvcConfigurerAdapter` beans that you
provide can also contribute converters by overriding the
`configureMessageConverters` method, but unlike with normal MVC, you
can supply only additional converters that you need (because Spring
Boot uses the same mechanism to contribute its defaults). Finally, if
you opt out of the Spring Boot default MVC configuration by providing
your own `@EnableWebMvc` configuration, then you can take control
completely and do everything manually using `getMessageConverters`
from `WebMvcConfigurationSupport`.

See the `WebMvcAutoConfiguration` source code for more details.

## Add a Servlet, Filter or ServletContextListener to an Application

`Servlet`, `Filter`, `ServletContextListener` and the other listeners
supported by the Servlet spec can be added to your application as
`@Bean` definitions. Be very careful that they don't cause eager
initialization of too many other beans because they have to be
installed in th container very early in the application lifecycle
(e.g. it's not a good idea to have them depend on your `DataSource` or
JPA configuration). You can work around restrictions like that by
initializing them lazily when first used instead of on initialization.

In the case of `Filters` and `Servlets` you can also add mappings and
init parameters by adding a `FilterRegistrationBean` or
`ServletRegistrationBean` instead of or as well as the underlying
component.

## Configure Tomcat

Generally you can follow the advice [here](#discover.options) about
`@ConfigurationProperties` (`ServerProperties` is the main one here),
but also look at `EmbeddedServletContainerCustomizer` and various
Tomcat specific `*Customizers` that you can add in one of those. The
Tomcat APIs are quite rich so once you have access to the
`TomcatEmbeddedServletContainerFactory` you can modify it in a number
of ways. Or the nuclear option is to add your own
`TomcatEmbeddedServletContainerFactory`.

## Configure Jetty

Generally you can follow the advice [here](#discover.options) about
`@ConfigurationProperties` (`ServerProperties` is the main one here),
but also look at `EmbeddedServletContainerCustomizer`. The Jetty APIs
are quite rich so once you have access to the
`JettyEmbeddedServletContainerFactory` you can modify it in a number
of ways. Or the nuclear option is to add your own
`JettyEmbeddedServletContainerFactory`.

## Terminate SSL in Tomcat

Add a `EmbeddedServletContainerCustomizer` and in that add a
`TomcatConnectorCustomizer` that sets up the connector to be secure:

```java
@Bean
public EmbeddedServletContainerCustomizer containerCustomizer(){
    return new EmbeddedServletContainerCustomizer() {
        @Override
        public void customize(ConfigurableEmbeddedServletContainerFactory factory) {
            if(factory instanceof TomcatEmbeddedServletContainerFactory){
                TomcatEmbeddedServletContainerFactory containerFactory = (TomcatEmbeddedServletContainerFactory) factory;
                containerFactory.addConnectorCustomizers(new TomcatConnectorCustomizer() {

                    @Override
                    public void customize(Connector connector) {

                        connector.setPort(serverPort);
                        connector.setSecure(true);
                        connector.setScheme("https");
                        connector.setAttribute("keyAlias", "tomcat");
                        connector.setAttribute("keystorePass", "password");
                        try {
                            connector.setAttribute("keystoreFile", ResourceUtils.getFile("src/ssl/tomcat.keystore").getAbsolutePath());
                        } catch (FileNotFoundException e) {
                            throw new IllegalStateException("Cannot load keystore", e);
                        }
                        connector.setAttribute("clientAuth", "false");
                        connector.setAttribute("sslProtocol", "TLS");
                        connector.setAttribute("SSLEnabled", true);

                });
            }
        }
    };
}
```

## Reload Static Content (E.g. Thymeleaf Templates) Without Restarting the Container

If you are using Thymeleaf, then set
`spring.thymeleaf.cache=false`. See `ThymeleafAutoConfiguration` for
other template customization options.

## Reload Java Classes Without Restarting the Container

Modern IDEs (Eclipse, IDEA etc.) all support hot swapping of bytecode,
so if you make a change that doesn't affect class or method signatures
it should reload cleanly with no side effects.

[Spring Loaded](https://github.com/spring-projects/spring-loaded) goes
a little further in that it can reload class definitions with changes
in the method signatures. With some customization it can force an
`ApplicationContext` to refresh itself (but there is no general
mechanism to ensure that would be safe for a running application
anyway, so it would only ever be a development time trick probably).

<span id="build.hierarchy"/>
## Build an ApplicationContext Hierarchy (Adding a Parent or Root Context)

The
[`SpringApplicationBuilder`](https://github.com/spring-projects/spring-boot/blob/master/spring-boot/src/main/java/org/springframework/boot/builder/SpringApplicationBuilder.java)
has methods specifically designed for the purpose of building a
hierarchy, e.g.

```java
SpringApplicationBuilder application = new SpringApplicationBuilder();
application.sources(Parent.class).child(Application.class).run(args);
```

There are some restrictions, e.g. the parent aplication context is
*not* a `WebApplicationContext`.  Both parent and child are executed
with the same `Environment` constructed in the usual way to include
command line arguments.  Any `ServletContextAware` components all have
to go in the child context, otherwise there is no way for Spring Boot
to create the `ServletContext` in time.

## Convert an Existing Application to Spring Boot

For a non-web application it should be easy (throw away the code that
creates your `ApplicationContext` and replace it with calls to
`SpringApplication` or `SpringApplicationBuilder`). Spring MVC web
applications are generally amenable to first creating a deployable WAR
application, and then migrating it later to an executable WAR and/or
JAR.  Useful reading is in the
[Getting Started Guide on Converting a JAR to a WAR](http://spring.io/guides/gs/convert-jar-to-war/).

Create a deployable WAR by extending `SpringBootServletInitializer`
(e.g. in a class called `Application`), and add the Spring Boot
`@EnableAutoConfiguration` annotation. Example:

```
@Configuration
@EnableAutoConfiguration
@ComponentScan
public class Application extends SpringBootServletInitializer {

	@Override
	protected SpringApplicationBuilder configure(SpringApplicationBuilder application) {
		return application.sources(Application.class);
	}

}
```

Remember that whatever you put in the "sources" is just a Spring
`ApplicationContext` and normally anything that already works should
work here. There might be some beans you can remove later and let
Spring Boot provide its own defaults for them, but it should be
possible to get something working first.

Static resources can be moved to `/public` (or `/static` or
`/resources` or `/META-INFO/resources`) in the classpath root. Same
for `messages.properties` (Spring Boot detects this automatically in
the root of the classpath).

Vanilla usage of Spring `DispatcherServlet` and Spring Security should
require no further changes. If you have other features in your
application, using other servlets or filters, for instance then you
may need to add some configuration to your `Application` context,
replacing those elements from the `web.xml` as follows:

* A `@Bean` of type `Servlet` or `ServletRegistrationBean` installs
  that bean in the container as if it was a `<servlet/>` and
  `<servlet-mapping/>` in `web.xml`

* A `@Bean` of type `Filter` or `FilterRegistrationBean` behaves
  similarly (like a `<filter/>` and `<filter-mapping/>`.

* An `ApplicationContext` in an XML file can be added to an `@Import`
  in your `Application`. Or simple cases where annotation
  configuration is heavily used already can be recreated in a few
  lines as `@Bean` definitions.

Once the WAR is working we make it executable by adding a `main`
method to our `Application`, e.g.

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

Applications can fall into more than one category:

* Servlet 3.0 applications with no `web.xml`
* Applications with a `web.xml`
* Applications with a context hierarchy and
* Those without a context hierarchy

All of these should be amenable to translation, but each might require
slightly different tricks.

Servlet 3.0 applications might translate pretty easily if they already
use the Spring Servlet 3.0 initializer support classes. Normally all
the code from an existing `WebApplicationInitializer` can be moved
into a `SpringBootServletInitializer`. If your existing application
has more than one `ApplicationContext` (e.g. if it uses
`AbstractDispatcherServletInitializer`) then you might be able to
squish all your context sources into a single `SpringApplication`. The
main complication you might encounter is if that doesn't work and you
need to maintain the context hierarchy. See the
[entry on building a hierarchy](#build.hierarchy) for examples. An
existing parent context that contains web-specific features will
usually need to be broken up so that all the `ServletContextAware`
components are in the child context.

Applications that are not already Spring applications might be
convertible to a Spring Boot application, and the guidance above might
help, but your mileage may vary.

## Serve Static Content

Spring Boot by default will serve static content from a folder called
`/static` (or `/public` or or `/resources` or `/META-INF/resources`)
in the classpath or from the root of the `ServeltContext`.  It uses
the `ResourceHttpRequestHandler` from Spring MVC so you can modify
that behaviour by adding your own `WebMvcConfigurerAdapter` and
overriding the `addResourceHandlers` method.

By default in a standalone web application the default servlet from
the container is also enabled, and acts as a fallback, serving content
from the root of the `ServletContext` if Spring decides not to handle
it. Most of the time this will not happen unless you modify the
deafult MVC configuration because Spring will always be able to handle
requests through the `DispatcherServlet`.

In addition to the 'standard' static resource locations above, a
special case is made for
[Webjars content](http://www.webjars.org/). Any resources with a path
in `/webjars/**` will be served from jar files if they are packaged in
the Webjars format.

For more detail look at the
[`WebMvcAutoConfiguration`](https://github.com/spring-projects/spring-boot/blob/master/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/WebMvcAutoConfiguration.java?source=c)
source code.

## Switch off the Spring DispatcherServlet

Spring Boot wants to serve all content from the root of your
application "/" down. If you would rather map your own servlet to that
URL you can do it, but of course you may lose some of the other Boot
MVC features. To add your own servlet and map it to the root resource
just declare a `@Bean` of type `Servlet` and give it the special bean
name "dispatcherServlet". (You can also create a bean of a different
type with that name if you want to switch it off and not replace it.)

## Switch off the Default MVC Configuration

The easiest way to take complete control over MVC configuration is to
provide your own `@Configuration` with the `@EnableWebMvc`
annotation. This will leave all MVC configuration in your hands.

## Change the HTTP Port

In a standalone application the main HTTP port defaults to 8080, but
can be set with `server.port` (e.g. in `application.properties` or as
a System property). Thanks to relaxed binding of `Environment` values
you can also use `SERVER_PORT` (e.g. as an OS environment variable).

To scan for a free port (using OS natives to prevent clashes) use
`server.port=0`. To switch off the HTTP endpoints completely, but
still create a `WebApplicationContext`, use `server.port=-1` (this is
sometimes useful for testing).

For more detail look at the
[`ServerProperties`](https://github.com/spring-projects/spring-boot/blob/master/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ServerProperties.java?source=c)
source code.

## Change the HTTP Port or Address of the Actuator Endpoints

In a standalone application the Actuator HTTP port defaults to the
same as the main HTTP port. To make the application listen on a
different port set the external property `management.port`. To listen
on a completely different network address (e.g. if you have an
internal network for management and an external one for user
applications) you can also set `management.address` to a valid IP
address that the server is able to bind to.

For more detail look at the
[`ManagementServerProperties`](https://github.com/spring-projects/spring-boot/blob/master/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/properties/ManagementServerProperties.java?source=c)
source code.

## Customize "Whitelabel" Error Page

The Actuator installs a "whitelabel" error page that you will see in
browser client if you encounter a server error (machine clients
consuming JSON and other media types should see a sensible response
with the right error code). To switch it off you can set
`error.whitelabel.enabled=false`, but normally in addition or
alternatively to that you will want to add your own error page
replacing the whitelabel one. If you are using Thymeleaf you can do
this by adding an "error.html" template. In general what you need is a
`View` that resolves with a name of "error", and/or a `@Controller`
that handles the "/error" path. Unless you replaced some of the
default configuration you should find a `BeanNameViewResolver` in your
`ApplicationContext` so a `@Bean` with id "error" would be a simple
way of doing that.  Look at `ErrorMvcAutoConfiguration` for more
options.

## Use 'Short' Command Line Arguments

Some people like to use (for example) `--port=9000` instead of
`--server.port=9000` to set configuration properties on the command
line. You can easily enable this by using placeholders in
`application.properties`, e.g.

```properties
server.port: ${port:8080}
```

> Note that in this specific case the port binding will work in a PaaS
> environment like Heroku and Cloud Foundry, since in those two
> platforms the `PORT` environment variable is set automatically and
> Spring can bind to capitalized synonyms for `Environment`
> properties.

## Configure Logback for Logging

Spring Boot has no mandatory logging dependence, except for the
`commons-logging` API, of which there are many implementations to
choose from. To use [Logback](http://logback.qos.ch) you need to
include it, and some bindings for `commons-logging` on the classpath.
The simplest way to do that is through the starter poms which all
depend on `spring-boot-start-logging`.  For a web application you only
need the web starter since it depends transitively on the logging
starter. E.g. in Maven:

```xml
<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-web</artifactId>
</dependency>
```

Spring Boot has a `LoggingSystem` abstraction that attempts to select
a system depending on the contents of the classpath. If Logback is
available it is the first choice. So if you put a `logback.xml` in the
root of your classpath it will be picked up from there. Spring Boot
provides a default base configuration that you can include if you just
want to set levels, e.g.

```xml
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
	<include resource="org/springframework/boot/logging/logback/base.xml"/>
	<logger name="org.springframework.web" level="DEBUG"/>
</configuration>
```

If you look at the default `logback.xml` in the spring-boot JAR you
will see that it uses some useful System properties which the
`LoggingSystem` takes care of creating for you. These are:

* `${PID}` the current process ID
* `${LOG_FILE}` if `logging.file` was set in Boot's external configuration
* `${LOG_PATH` if `logging.path` was set (representing a directory for
  log files to live in)

Spring Boot also provides some nice ANSI colour terminal output on a
console (but not in a log file) using a custom Logback converter. See
the default `base.xml` configuration for details.

If Groovy is on the classpath you should be able to configure Logback
with `logback.groovy` as well (it will be given preference if
present).

## Configure Log4j for Logging

Spring Boot supports [Log4j](http://logging.apache.org/log4j/1.x/) for
logging configuration, but it has to be on the classpath. If you are
using the starter poms for assembling dependencies that means you have
to exclude logback and then include log4j back. If you aren't using
the starter poms then you need to provide `commons-logging` (at least)
in addition to Log4j.

The simplest path to using Log4j is probably through the starter poms,
even though it requires some jiggling with excludes, e.g. in Maven:

```xml
<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-web</artifactId>
	<exclusions>
		<exclusion>
			<groupId>${project.groupId}</groupId>
			<artifactId>spring-boot-starter-logging</artifactId>
		</exclusion>
	</exclusions>
</dependency>
<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-log4j</artifactId>
</dependency>
```

Note the use of the log4j starter to gather together the dependencies
for common logging requirements (e.g. including having Tomcat use
`java.util.logging` but configure the output using Log4j). See the
[Actuator Log4j Sample]() for more detail and to see it in action.

## Test a Spring Boot Application

A Spring Boot application is just a Spring `ApplicationContext` so
nothing very special has to be done to test it beyond what you would
normally do with a vanilla Spring context. One thing to watch out for
though is that the external properties, logging and other features of
Spring Boot are only installed in the context by default if you use
`SpringApplication` to create it. Spring Boot has a special Spring
`@ContextConfiguration` annotation, so you can use this for example
(from the JPA Sample):

```java
@RunWith(SpringJUnit4ClassRunner.class)
@SpringApplicationConfiguration(classes = SampleDataJpaApplication.class)
public class CityRepositoryIntegrationTests {

	@Autowired
	CityRepository repository;

...
```

To use the `@SpringApplicationConfiguration` you need the test jar on
your classpath (recommended Maven co-ordinates
"org.springframework.boot:spring-boot-starter-test"). The context
loader guesses whether you want to test a web application or not
(e.g. with `MockMVC`) by looking for the `@WebAppConfiguration`
annotation.  (`MockMVC` and `@WebAppConfiguration` are from the Spring
Test support library).

<span id="main.properties"/>
## Externalize the Configuration of SpringApplication

A `SpringApplication` has bean properties (mainly setters) so you can
use its Java API as you create the application to modify its
behaviour. Or you can externalize the configuration using properties
in `spring.main.*`. E.g. in `application.properties` you might have

```properties
spring.main.web_environment: false
spring.main.show_banner: false
```

and then the Spring Boot banner will not be printed on startup, and
the application will not be a web application.

## Create a Non-Web Application

Not all Spring applications have to be web applications (or web
services). If you want to execute some code in a `main` method, but
also bootstrap a Spring application to set up the infrastructure to
use, then it's easy with the `SpringApplication` features of Spring
Boot. A `SpringApplication` changes its `ApplicationContext` class
depending on whether it thinks it needs a web application or not. The
first thing you can do to help it is to just leave the web
depdendencies off the classpath. If you can't do that (e.g. you are
running 2 applications from the same code base) then you can
explicitly call `SpringApplication.setWebEnvironment(false)`, or set
the `applicationContextClass` property (through the Java API or with
[external properties](#main.properties)). Application code that you
want to run as your business logic can be implemented as a
`CommandLineRunner` and dropped into the context as a `@Bean`
definition.

## Create a Deployable WAR File

Use the `SpringBootServletInitializer` base class, which is picked up
by Spring's Servlet 3.0 support on deployment. Add an extension of
that to your project and build a WAR file as normal. For more detail,
see the ["Converting a JAR Project to a WAR" guide][gs-war] on the
spring.io website.

The WAR file can also be executable if you use the Spring Boot build
tools. In that case the embedded container classes (to launch Tomcat
for instance) have to be added to the WAR in a `lib-provided`
directory. The tools will take care of that as long as the
dependencies are marked as "provided" in Maven or Gradle. Here's a
Maven example
[in the Boot Samples](https://github.com/spring-projects/spring-boot/blob/master/spring-boot-samples/spring-boot-sample-traditional/pom.xml).

[gs-war]: http://spring.io/guides/gs/convert-jar-to-war

## Create a Deployable WAR File for older Servlet Containers

Older Servlet containers don't have support for the
`ServletContextInitializer` bootstrap process used in Servlet 3.0. You
can still use Spring and Spring Boot in these containers but you are
going to need to add a `web.xml` to your application and configure it
to load an `ApplicationContext` via a `DispatcherServlet`.

TODO: add some detail.

## Configure a DataSource

Spring Boot will create a `DataSource` for you if you have
`spring-jdbc` and some other things on the classpath. Here's the
algorithm for choosing a specific implementation.

* We prefer the Tomcat pooling `DataSource` for its performance and
  concurrency, so if that is available we always choose it.
* If commons-dbcp is available we will use that, but we don't
  recommend it in production.
* If neither of those is available but an embedded database is then we
  create one of those for you (preference order is h2, then Apache
  Derby, then hsqldb).

The pooling `DataSource` option is controlled by external
configuration properties in `spring.datasource.*` for example:

```properties
spring.datasource.url: jdbc:mysql://localhost/test
spring.datasource.username: root
spring.datasource.password:
spring.datasource.driverClassName: com.mysql.jdbc.Driver
```

The `@ConfigurationProperties` for `spring.datasource` are defined in
`AbstractDataSourceConfiguration` (so see there for more options).

For a pooling `DataSource` to be created we need to be able to verify
that a valid `Driver` class is available, so we check for that before
doing anything. I.e. if you set
`spring.datasource.driverClassName=com.mysql.jdbc.Driver` then that
class has to be loadable.

To override the default settings just define a `@Bean` of your own of
type `DataSource`. See
[`DataSourceAutoConfiguration`]((https://github.com/spring-projects/spring-boot/blob/master/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jdbc/DataSourceAutoConfiguration.java))
for more details.

## Use Spring Data Repositories

Spring Data can create implementations for you of `@Repository`
interfaces of various flavours. Spring Boot will handle all of that
for you as long as those `@Repositories` are included in a
`@ComponentScan`.

For many applications all you will need is to put the right Spring
Data dependencies on your classpath (there is a
"spring-boot-starter-data-jpa" for JPA and for Mongodb you only nee
dto add "spring-datamongodb"), create some repository interfaces to
handle your `@Entity` objects, and then add a `@ComponentScan` that
covers those packages. Examples are in the
[JPA sample](https://github.com/spring-projects/spring-boot/tree/master/spring-boot-samples/spring-boot-sample-data-jpa)
or the
[Mongodb sample](https://github.com/spring-projects/spring-boot/tree/master/spring-boot-samples/spring-boot-sample-data-mongodb).

## Configure JPA Properties

Spring JPA already provides some vendor-independent configuration
options (e.g. for SQL logging) and Spring Boot exposes those, and a
few more for hibernate as external configuration properties. The most
common options to set are

```properties
spring.jpa.hibernate.ddl-auto: create-drop
spring.jpa.hibernate.naming_strategy: org.hibernate.cfg.ImprovedNamingStrategy
spring.jpa.database: H2
spring.jpa.show-sql: true
```

(Because of relaxed data binding hyphens or underscores should work
equally well as property keys.)  The `ddl-auto` setting is a special
case in that it has different defaults depending on whether you are
using an embedded database ("create-drop") or not ("none"). In
addition all properties in `spring.jpa.properties.*` are passed
through as normal JPA properties (with the prefix stripped) when the
local `EntityManagerFactory` is created.

See
[`HibernateJpaAutoConfiguration`](https://github.com/spring-projects/spring-boot/blob/master/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/orm/jpa/HibernateJpaAutoConfiguration.java)
and
[`JpaBaseConfiguration`](https://github.com/spring-projects/spring-boot/blob/master/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/orm/jpa/JpaBaseConfiguration.java)
for more details.

<span id="discover.options"/>
## Discover Built-in Options for External Properties

Spring Boot binds external properties from `application.properties`
(or `.yml`) (and other places) into an application at runtime.  There
is not (and technically cannot be) an exhaustive list of all supported
properties in a single location because contributions can come from
additional JAR files on your classpath.  There is a sample
[`application.yml`](https://github.com/spring-projects/spring-boot/blob/master/docs/application.yml)
with a non-exhaustive and possibly inaccurate list of properties
supported by Spring Boot vanilla with autoconfiguration. The
definitive list comes from searching the source code for
`@ConfigurationProperties` and `@Value` annotations, as well as the
occasional use of `RelaxedEnvironment`
(c.f. [here](https://github.com/spring-projects/spring-boot/blob/master/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/orm/jpa/HibernateJpaAutoConfiguration.java?source=c#L65)).

## Set the Active Spring Profiles

The Spring `Environment` has an API for this, but normally you would
set a System profile (`spring.profiles.active`) or an OS environment
variable (`SPRING_PROFILES_ACTIVE`). E.g. launch your application with
a `-D...` argument (remember to put it before the main class or jar
archive):

```
java -jar -Dspring.profiles.active=production demo-0.0.1-SNAPSHOT.jar
```

In Spring Boot you can also set the active profile in
`application.properties`, e.g.

```properties
spring.profiles.active: production
```

A value set this is replaced by the System property or environment
variable setting, but not by the `SpringApplicationBuilder.profiles()`
method. Thus the latter Java API can be used to augment the profiles
without changing the defaults.

## Change the Location of External Properties of an Application

By default properties from different sources are added to the Spring
`Environment` in a defined order, and the precedence for resolution is
1) commandline, 2) filesystem (current working directory)
`application.properties`, 3) classpath `application.properties`.

A nice way to augment and modify this is to add `@PropertySource`
annotations to your application sources. Classes passed to the
`SpringApplication` static convenience methods, and those added using
`setSources()` are inspected to see if they have `@PropertySources`
and if they do those properties are added to the `Environment` early
enough to be used in all phases of the `ApplicationContext`
lifecycle. Properties added in this way have precendence over any
added using the default locations, but have lower priority than system
properties, environment variables or the command line.

You can also provide System properties (or environment variables) to
change the default behaviour:

* `config.name` (`CONFIG_NAME`), defaults to `application` as the root
  of the file name
* `config.location` (`CONFIG_LOCATION`) is a comma-separated list of
  files to load. A separate `Environment` property source is set up
  for each document found, so the priority order is most significant
  first. Defaults to
  `file:./application.properties,classpath:application.properties`. If
  YAML is used then those files are also added to the list by default.

See `ConfigFileApplicationContextInitializer` for more detail.

## Use YAML for External Properties

YAML is a superset of JSON and as such is a very convenient syntax for
storing external properties in a hierarchical format. E.g.

```yaml
spring:
  application:
    name: cruncher
  datasource:
    driverClassName: com.mysql.jdbc.Driver
    url: jdbc:mysql://localhost/test
server:
  port: 9000
```

Create a file called `application.yml` and stick it in the root of
your classpath, and also add `snake-yaml` to your classpath (Maven
co-ordinates `org.yaml:snake-yaml`). A YAML file is parsed to a Java
`Map<String,Object>` (like a JSON object), and Spring Boot flattens
the maps so that it is 1-level deep and has period-separated keys, a
lot like people are used to with `Properties` files in Java.

The example YAML above corresponds to an `application.properties` file

```properties
spring.application.name: cruncher
spring.datasource.driverClassName: com.mysql.jdbc.Driver
spring.datasource.url: jdbc:mysql://localhost/test
server.port: 9000
```

## Change Configuration Depending on the Environment

A YAML file is actually a sequence of documents separated by `---`
lines, and each document is parsed separately to a flattened map.

If a YAML document contains a `spring.profiles` key, then the
profiles value (comma-separated list of profiles) is fed into the
Spring `Environment.acceptsProfiles()` and if any of those profiles is
active that document is included in the final merge (otherwise not).

Example:

```yaml
server:
  port: 9000

---

spring:
  profiles: development
server:
  port: 9001

---

spring:
  profiles: production
server:
  port: 0
```

In this example the default port is 9000, but if the Spring profile
"development" is active then the port is 9001, and if "production" is
active then it is 0.

The YAML documents are merged in the order they are encountered (so
later values override earlier ones).

To do the same thing with properties files you can use
`application-${profile}.properties` to specify profile-specific
values.