Core Features

The “Liveness” state of an application tells whether its internal state allows it to work correctly, or recover by itself if it is currently failing. A broken “Liveness” state means that the application is in a state that it cannot recover from, and the infrastructure should restart the application.

The internal state of Spring Boot applications is mostly represented by the Spring ApplicationContext. If the application context has started successfully, Spring Boot assumes that the application is in a valid state. An application is considered live as soon as the context has been refreshed, see Spring Boot application lifecycle and related Application Events.

The “Readiness” state of an application tells whether the application is ready to handle traffic. A failing “Readiness” state tells the platform that it should not route traffic to the application for now. This typically happens during startup, while CommandLineRunner and ApplicationRunner components are being processed, or at any time if the application decides that it is too busy for additional traffic.

An application is considered ready as soon as application and command-line runners have been called, see Spring Boot application lifecycle and related Application Events.

Application components can retrieve the current availability state at any time, by injecting the ApplicationAvailability interface and calling methods on it. More often, applications will want to listen to state updates or update the state of the application.

For example, we can export the "Readiness" state of the application to a file so that a Kubernetes "exec Probe" can look at this file:

We can also update the state of the application, when the application breaks and cannot recover:

Spring Boot provides Kubernetes HTTP probes for "Liveness" and "Readiness" with Actuator Health Endpoints. You can get more guidance about deploying Spring Boot applications on Kubernetes in the dedicated section.

By default, when a specified config data location does not exist, Spring Boot will throw a ConfigDataLocationNotFoundException and your application will not start.

If you want to specify a location, but you do not mind if it does not always exist, you can use the optional: prefix. You can use this prefix with the spring.config.location and spring.config.additional-location properties, as well as with spring.config.import declarations.

For example, a spring.config.import value of optional:file:./myconfig.properties allows your application to start, even if the myconfig.properties file is missing.

If you want to ignore all ConfigDataLocationNotFoundExceptions and always continue to start your application, you can use the spring.config.on-not-found property. Set the value to ignore using SpringApplication.setDefaultProperties(…​) or with a system/environment variable.

If a config file location includes the * character for the last path segment, it is considered a wildcard location. Wildcards are expanded when the config is loaded so that immediate subdirectories are also checked. Wildcard locations are particularly useful in an environment such as Kubernetes when there are multiple sources of config properties.

For example, if you have some Redis configuration and some MySQL configuration, you might want to keep those two pieces of configuration separate, while requiring that both those are present in an application.properties file. This might result in two separate application.properties files mounted at different locations such as /config/redis/application.properties and /config/mysql/application.properties. In such a case, having a wildcard location of config/*/, will result in both files being processed.

By default, Spring Boot includes config/*/ in the default search locations. It means that all subdirectories of the /config directory outside of your jar will be searched.

You can use wildcard locations yourself with the spring.config.location and spring.config.additional-location properties.

As well as application property files, Spring Boot will also attempt to load profile-specific files using the naming convention application-{profile}. For example, if your application activates a profile named prod and uses YAML files, then both application.yml and application-prod.yml will be considered.

Profile-specific properties are loaded from the same locations as standard application.properties, with profile-specific files always overriding the non-specific ones. If several profiles are specified, a last-wins strategy applies. For example, if profiles prod,live are specified by the spring.profiles.active property, values in application-prod.properties can be overridden by those in application-live.properties.

The Environment has a set of default profiles (by default, [default]) that are used if no active profiles are set. In other words, if no profiles are explicitly activated, then properties from application-default are considered.

Application properties may import further config data from other locations using the spring.config.import property. Imports are processed as they are discovered, and are treated as additional documents inserted immediately below the one that declares the import.

For example, you might have the following in your classpath application.properties file:

This will trigger the import of a dev.properties file in current directory (if such a file exists). Values from the imported dev.properties will take precedence over the file that triggered the import. In the above example, the dev.properties could redefine spring.application.name to a different value.

An import will only be imported once no matter how many times it is declared. The order an import is defined inside a single document within the properties/yaml file does not matter. For instance, the two examples below produce the same result:

In both of the above examples, the values from the my.properties file will take precedence over the file that triggered its import.

Several locations can be specified under a single spring.config.import key. Locations will be processed in the order that they are defined, with later imports taking precedence.

Some cloud platforms cannot add a file extension to volume mounted files. To import these extensionless files, you need to give Spring Boot a hint so that it knows how to load them. You can do this by putting an extension hint in square brackets.

For example, suppose you have a /etc/config/myconfig file that you wish to import as yaml. You can import it from your application.properties using the following:

When running applications on a cloud platform (such as Kubernetes) you often need to read config values that the platform supplies. It is not uncommon to use environment variables for such purposes, but this can have drawbacks, especially if the value is supposed to be kept secret.

As an alternative to environment variables, many cloud platforms now allow you to map configuration into mounted data volumes. For example, Kubernetes can volume mount both ConfigMaps and Secrets.

There are two common volume mount patterns that can be used:

For the first case, you can import the YAML or Properties file directly using spring.config.import as described above. For the second case, you need to use the configtree: prefix so that Spring Boot knows it needs to expose all the files as properties.

As an example, let’s imagine that Kubernetes has mounted the following volume:

The contents of the username file would be a config value, and the contents of password would be a secret.

To import these properties, you can add the following to your application.properties or application.yaml file:

You can then access or inject myapp.username and myapp.password properties from the Environment in the usual way.

If you have multiple config trees to import from the same parent folder you can use a wildcard shortcut. Any configtree: location that ends with /*/ will import all immediate children as config trees.

For example, given the following volume:

You can use configtree:/etc/config/*/ as the import location:

This will add db.username, db.password, mq.username and mq.password properties.

Configuration trees can also be used for Docker secrets. When a Docker swarm service is granted access to a secret, the secret gets mounted into the container. For example, if a secret named db.password is mounted at location /run/secrets/, you can make db.password available to the Spring environment using the following:

The values in application.properties and application.yml are filtered through the existing Environment when they are used, so you can refer back to previously defined values (for example, from System properties or environment variables). The standard ${name} property-placeholder syntax can be used anywhere within a value. Property placeholders can also specify a default value using a : to separate the default value from the property name, for example ${name:default}.

The use of placeholders with and without defaults is shown in the following example:

Assuming that the username property has not been set elsewhere, app.description will have the value MyApp is a Spring Boot application written by Unknown.

Spring Boot allows you to split a single physical file into multiple logical documents which are each added independently. Documents are processed in order, from top to bottom. Later documents can override the properties defined in earlier ones.

For application.yml files, the standard YAML multi-document syntax is used. Three consecutive hyphens represent the end of one document, and the start of the next.

For example, the following file has two logical documents:

For application.properties files a special #--- comment is used to mark the document splits:

It is sometimes useful to only activate a given set of properties when certain conditions are met. For example, you might have properties that are only relevant when a specific profile is active.

You can conditionally activate a properties document using spring.config.activate.*.

The following activation properties are available:

For example, the following specifies that the second document is only active when running on Kubernetes, and only when either the “prod” or “staging” profiles are active:

YAML documents need to be converted from their hierarchical format to a flat structure that can be used with the Spring Environment. For example, consider the following YAML document:

In order to access these properties from the Environment, they would be flattened as follows:

Likewise, YAML lists also need to be flattened. They are represented as property keys with [index] dereferencers. For example, consider the following YAML:

The preceding example would be transformed into these properties:

Spring Framework provides two convenient classes that can be used to load YAML documents. The YamlPropertiesFactoryBean loads YAML as Properties and the YamlMapFactoryBean loads YAML as a Map.

You can also use the YamlPropertySourceLoader class if you want to load YAML as a Spring PropertySource.

It is possible to bind a bean declaring standard JavaBean properties as shown in the following example:

The preceding POJO defines the following properties:

The example in the previous section can be rewritten in an immutable fashion as shown in the following example:

In this setup, the @ConstructorBinding annotation is used to indicate that constructor binding should be used. This means that the binder will expect to find a constructor with the parameters that you wish to have bound. If you are using Java 16 or later, constructor binding can be used with records. In this case, unless your record has multiple constructors, there is no need to use @ConstructorBinding.

Nested members of a @ConstructorBinding class (such as Security in the example above) will also be bound through their constructor.

Default values can be specified using @DefaultValue on a constructor parameter or, when using Java 16 or later, a record component. The conversion service will be applied to coerce the String value to the target type of a missing property.

Referring to the previous example, if no properties are bound to Security, the MyProperties instance will contain a null value for security. If you wish you return a non-null instance of Security even when no properties are bound to it, you can use an empty @DefaultValue annotation to do so:

Spring Boot provides infrastructure to bind @ConfigurationProperties types and register them as beans. You can either enable configuration properties on a class-by-class basis or enable configuration property scanning that works in a similar manner to component scanning.

Sometimes, classes annotated with @ConfigurationProperties might not be suitable for scanning, for example, if you’re developing your own auto-configuration or you want to enable them conditionally. In these cases, specify the list of types to process using the @EnableConfigurationProperties annotation. This can be done on any @Configuration class, as shown in the following example:

To use configuration property scanning, add the @ConfigurationPropertiesScan annotation to your application. Typically, it is added to the main application class that is annotated with @SpringBootApplication but it can be added to any @Configuration class. By default, scanning will occur from the package of the class that declares the annotation. If you want to define specific packages to scan, you can do so as shown in the following example:

We recommend that @ConfigurationProperties only deal with the environment and, in particular, does not inject other beans from the context. For corner cases, setter injection can be used or any of the *Aware interfaces provided by the framework (such as EnvironmentAware if you need access to the Environment). If you still want to inject other beans using the constructor, the configuration properties bean must be annotated with @Component and use JavaBean-based property binding.

This style of configuration works particularly well with the SpringApplication external YAML configuration, as shown in the following example:

To work with @ConfigurationProperties beans, you can inject them in the same way as any other bean, as shown in the following example:

As well as using @ConfigurationProperties to annotate a class, you can also use it on public @Bean methods. Doing so can be particularly useful when you want to bind properties to third-party components that are outside of your control.

To configure a bean from the Environment properties, add @ConfigurationProperties to its bean registration, as shown in the following example:

Any JavaBean property defined with the another prefix is mapped onto that AnotherComponent bean in manner similar to the preceding SomeProperties example.

Spring Boot uses some relaxed rules for binding Environment properties to @ConfigurationProperties beans, so there does not need to be an exact match between the Environment property name and the bean property name. Common examples where this is useful include dash-separated environment properties (for example, context-path binds to contextPath), and capitalized environment properties (for example, PORT binds to port).

As an example, consider the following @ConfigurationProperties class:

With the preceding code, the following properties names can all be used:

When lists are configured in more than one place, overriding works by replacing the entire list.

For example, assume a MyPojo object with name and description attributes that are null by default. The following example exposes a list of MyPojo objects from MyProperties:

Consider the following configuration:

If the dev profile is not active, MyProperties.list contains one MyPojo entry, as previously defined. If the dev profile is enabled, however, the list still contains only one entry (with a name of my another name and a description of null). This configuration does not add a second MyPojo instance to the list, and it does not merge the items.

When a List is specified in multiple profiles, the one with the highest priority (and only that one) is used. Consider the following example:

In the preceding example, if the dev profile is active, MyProperties.list contains one MyPojo entry (with a name of my another name and a description of null). For YAML, both comma-separated lists and YAML lists can be used for completely overriding the contents of the list.

For Map properties, you can bind with property values drawn from multiple sources. However, for the same property in multiple sources, the one with the highest priority is used. The following example exposes a Map<String, MyPojo> from MyProperties:

Consider the following configuration:

If the dev profile is not active, MyProperties.map contains one entry with key key1 (with a name of my name 1 and a description of my description 1). If the dev profile is enabled, however, map contains two entries with keys key1 (with a name of dev name 1 and a description of my description 1) and key2 (with a name of dev name 2 and a description of dev description 2).

Spring Boot attempts to coerce the external application properties to the right type when it binds to the @ConfigurationProperties beans. If you need custom type conversion, you can provide a ConversionService bean (with a bean named conversionService) or custom property editors (through a CustomEditorConfigurer bean) or custom Converters (with bean definitions annotated as @ConfigurationPropertiesBinding).

Spring Boot attempts to validate @ConfigurationProperties classes whenever they are annotated with Spring’s @Validated annotation. You can use JSR-303 javax.validation constraint annotations directly on your configuration class. To do so, ensure that a compliant JSR-303 implementation is on your classpath and then add constraint annotations to your fields, as shown in the following example:

To ensure that validation is always triggered for nested properties, even when no properties are found, the associated field must be annotated with @Valid. The following example builds on the preceding MyProperties example:

You can also add a custom Spring Validator by creating a bean definition called configurationPropertiesValidator. The @Bean method should be declared static. The configuration properties validator is created very early in the application’s lifecycle, and declaring the @Bean method as static lets the bean be created without having to instantiate the @Configuration class. Doing so avoids any problems that may be caused by early instantiation.

The @Value annotation is a core container feature, and it does not provide the same features as type-safe configuration properties. The following table summarizes the features that are supported by @ConfigurationProperties and @Value:

If you define a set of configuration keys for your own components, we recommend you group them in a POJO annotated with @ConfigurationProperties. Doing so will provide you with structured, type-safe object that you can inject into your own beans.

SpEL expressions from application property files are not processed at time of parsing these files and populating the environment. However, it is possible to write a SpEL expression in @Value. If the value of a property from an application property file is a SpEL expression, it will be evaluated when consumed through @Value.

If your terminal supports ANSI, color output is used to aid readability. You can set spring.output.ansi.enabled to a supported value to override the auto-detection.

Color coding is configured by using the %clr conversion word. In its simplest form, the converter colors the output according to the log level, as shown in the following example:

The following table describes the mapping of log levels to colors:

Alternatively, you can specify the color or style that should be used by providing it as an option to the conversion. For example, to make the text yellow, use the following setting:

The following colors and styles are supported:

The <springProfile> tag lets you optionally include or exclude sections of configuration based on the active Spring profiles. Profile sections are supported anywhere within the <configuration> element. Use the name attribute to specify which profile accepts the configuration. The <springProfile> tag can contain a profile name (for example staging) or a profile expression. A profile expression allows for more complicated profile logic to be expressed, for example production & (eu-central | eu-west). Check the reference guide for more details. The following listing shows three sample profiles:

The <springProperty> tag lets you expose properties from the Spring Environment for use within Logback. Doing so can be useful if you want to access values from your application.properties file in your Logback configuration. The tag works in a similar way to Logback’s standard <property> tag. However, rather than specifying a direct value, you specify the source of the property (from the Environment). If you need to store the property somewhere other than in local scope, you can use the scope attribute. If you need a fallback value (in case the property is not set in the Environment), you can use the defaultValue attribute. The following example shows how to expose properties for use within Logback:

If Spring MVC is available, a regular MVC-based application context is configured. If you have only Spring WebFlux, we will detect that and configure a WebFlux-based application context instead.

If both are present, Spring MVC takes precedence. If you want to test a reactive web application in this scenario, you must set the spring.main.web-application-type property:

If you are familiar with the Spring Test Framework, you may be used to using @ContextConfiguration(classes=…​) in order to specify which Spring @Configuration to load. Alternatively, you might have often used nested @Configuration classes within your test.

When testing Spring Boot applications, this is often not required. Spring Boot’s @*Test annotations search for your primary configuration automatically whenever you do not explicitly define one.

The search algorithm works up from the package that contains the test until it finds a class annotated with @SpringBootApplication or @SpringBootConfiguration. As long as you structured your code in a sensible way, your main configuration is usually found.

If you want to customize the primary configuration, you can use a nested @TestConfiguration class. Unlike a nested @Configuration class, which would be used instead of your application’s primary configuration, a nested @TestConfiguration class is used in addition to your application’s primary configuration.

If your application uses component scanning (for example, if you use @SpringBootApplication or @ComponentScan), you may find top-level configuration classes that you created only for specific tests accidentally get picked up everywhere.

As we have seen earlier, @TestConfiguration can be used on an inner class of a test to customize the primary configuration. When placed on a top-level class, @TestConfiguration indicates that classes in src/test/java should not be picked up by scanning. You can then import that class explicitly where it is required, as shown in the following example:

If your application expects arguments, you can have @SpringBootTest inject them using the args attribute.

By default, @SpringBootTest does not start the server but instead sets up a mock environment for testing web endpoints.

With Spring MVC, we can query our web endpoints using MockMvc or WebTestClient, as shown in the following example:

With Spring WebFlux endpoints, you can use WebTestClient as shown in the following example:

If you need to start a full running server, we recommend that you use random ports. If you use @SpringBootTest(webEnvironment=WebEnvironment.RANDOM_PORT), an available port is picked at random each time your test runs.

The @LocalServerPort annotation can be used to inject the actual port used into your test. For convenience, tests that need to make REST calls to the started server can additionally @Autowire a WebTestClient, which resolves relative links to the running server and comes with a dedicated API for verifying responses, as shown in the following example:

This setup requires spring-webflux on the classpath. If you can not or will not add webflux, Spring Boot also provides a TestRestTemplate facility:

To customize the WebTestClient bean, configure a WebTestClientBuilderCustomizer bean. Any such beans are called with the WebTestClient.Builder that is used to create the WebTestClient.

As the test context framework caches context, JMX is disabled by default to prevent identical components to register on the same domain. If such test needs access to an MBeanServer, consider marking it dirty as well:

Regardless of your classpath, meter registries, except the in-memory backed, are not auto-configured when using @SpringBootTest.

If you need to export metrics to a different backend as part of an integration test, annotate it with @AutoConfigureMetrics.

When running tests, it is sometimes necessary to mock certain components within your application context. For example, you may have a facade over some remote service that is unavailable during development. Mocking can also be useful when you want to simulate failures that might be hard to trigger in a real environment.

Spring Boot includes a @MockBean annotation that can be used to define a Mockito mock for a bean inside your ApplicationContext. You can use the annotation to add new beans or replace a single existing bean definition. The annotation can be used directly on test classes, on fields within your test, or on @Configuration classes and fields. When used on a field, the instance of the created mock is also injected. Mock beans are automatically reset after each test method.

The following example replaces an existing RemoteService bean with a mock implementation:

Additionally, you can use @SpyBean to wrap any existing bean with a Mockito spy. See the Javadoc for full details.

Spring Boot’s auto-configuration system works well for applications but can sometimes be a little too much for tests. It often helps to load only the parts of the configuration that are required to test a “slice” of your application. For example, you might want to test that Spring MVC controllers are mapping URLs correctly, and you do not want to involve database calls in those tests, or you might want to test JPA entities, and you are not interested in the web layer when those tests run.

The spring-boot-test-autoconfigure module includes a number of annotations that can be used to automatically configure such “slices”. Each of them works in a similar way, providing a @…​Test annotation that loads the ApplicationContext and one or more @AutoConfigure…​ annotations that can be used to customize auto-configuration settings.

To test that object JSON serialization and deserialization is working as expected, you can use the @JsonTest annotation. @JsonTest auto-configures the available supported JSON mapper, which can be one of the following libraries:

If you need to configure elements of the auto-configuration, you can use the @AutoConfigureJsonTesters annotation.

Spring Boot includes AssertJ-based helpers that work with the JSONAssert and JsonPath libraries to check that JSON appears as expected. The JacksonTester, GsonTester, JsonbTester, and BasicJsonTester classes can be used for Jackson, Gson, Jsonb, and Strings respectively. Any helper fields on the test class can be @Autowired when using @JsonTest. The following example shows a test class for Jackson:

If you use Spring Boot’s AssertJ-based helpers to assert on a number value at a given JSON path, you might not be able to use isEqualTo depending on the type. Instead, you can use AssertJ’s satisfies to assert that the value matches the given condition. For instance, the following example asserts that the actual number is a float value close to 0.15 within an offset of 0.01.

To test whether Spring MVC controllers are working as expected, use the @WebMvcTest annotation. @WebMvcTest auto-configures the Spring MVC infrastructure and limits scanned beans to @Controller, @ControllerAdvice, @JsonComponent, Converter, GenericConverter, Filter, HandlerInterceptor, WebMvcConfigurer, WebMvcRegistrations, and HandlerMethodArgumentResolver. Regular @Component and @ConfigurationProperties beans are not scanned when the @WebMvcTest annotation is used. @EnableConfigurationProperties can be used to include @ConfigurationProperties beans.

Often, @WebMvcTest is limited to a single controller and is used in combination with @MockBean to provide mock implementations for required collaborators.

@WebMvcTest also auto-configures MockMvc. Mock MVC offers a powerful way to quickly test MVC controllers without needing to start a full HTTP server.

If you use HtmlUnit and Selenium, auto-configuration also provides an HtmlUnit WebClient bean and/or a Selenium WebDriver bean. The following example uses HtmlUnit:

If you have Spring Security on the classpath, @WebMvcTest will also scan WebSecurityConfigurer beans. Instead of disabling security completely for such tests, you can use Spring Security’s test support. More details on how to use Spring Security’s MockMvc support can be found in this howto.html how-to section.

To test that Spring WebFlux controllers are working as expected, you can use the @WebFluxTest annotation. @WebFluxTest auto-configures the Spring WebFlux infrastructure and limits scanned beans to @Controller, @ControllerAdvice, @JsonComponent, Converter, GenericConverter, WebFilter, and WebFluxConfigurer. Regular @Component and @ConfigurationProperties beans are not scanned when the @WebFluxTest annotation is used. @EnableConfigurationProperties can be used to include @ConfigurationProperties beans.

Often, @WebFluxTest is limited to a single controller and used in combination with the @MockBean annotation to provide mock implementations for required collaborators.

@WebFluxTest also auto-configures WebTestClient, which offers a powerful way to quickly test WebFlux controllers without needing to start a full HTTP server.

You can use @DataCassandraTest to test Cassandra applications. By default, it configures a CassandraTemplate, scans for @Table classes, and configures Spring Data Cassandra repositories. Regular @Component and @ConfigurationProperties beans are not scanned when the @DataCassandraTest annotation is used. @EnableConfigurationProperties can be used to include @ConfigurationProperties beans. (For more about using Cassandra with Spring Boot, see "data.html", earlier in this chapter.)

The following example shows a typical setup for using Cassandra tests in Spring Boot:

You can use the @DataJpaTest annotation to test JPA applications. By default, it scans for @Entity classes and configures Spring Data JPA repositories. If an embedded database is available on the classpath, it configures one as well. SQL queries are logged by default by setting the spring.jpa.show-sql property to true. This can be disabled using the showSql() attribute of the annotation.

Regular @Component and @ConfigurationProperties beans are not scanned when the @DataJpaTest annotation is used. @EnableConfigurationProperties can be used to include @ConfigurationProperties beans.

By default, data JPA tests are transactional and roll back at the end of each test. See the relevant section in the Spring Framework Reference Documentation for more details. If that is not what you want, you can disable transaction management for a test or for the whole class as follows:

Data JPA tests may also inject a TestEntityManager bean, which provides an alternative to the standard JPA EntityManager that is specifically designed for tests.

A JdbcTemplate is also available if you need that. The following example shows the @DataJpaTest annotation in use:

In-memory embedded databases generally work well for tests, since they are fast and do not require any installation. If, however, you prefer to run tests against a real database you can use the @AutoConfigureTestDatabase annotation, as shown in the following example:

@JdbcTest is similar to @DataJpaTest but is for tests that only require a DataSource and do not use Spring Data JDBC. By default, it configures an in-memory embedded database and a JdbcTemplate. Regular @Component and @ConfigurationProperties beans are not scanned when the @JdbcTest annotation is used. @EnableConfigurationProperties can be used to include @ConfigurationProperties beans.

By default, JDBC tests are transactional and roll back at the end of each test. See the relevant section in the Spring Framework Reference Documentation for more details. If that is not what you want, you can disable transaction management for a test or for the whole class, as follows:

If you prefer your test to run against a real database, you can use the @AutoConfigureTestDatabase annotation in the same way as for DataJpaTest. (See "Auto-configured Data JPA Tests".)

@DataJdbcTest is similar to @JdbcTest but is for tests that use Spring Data JDBC repositories. By default, it configures an in-memory embedded database, a JdbcTemplate, and Spring Data JDBC repositories. Regular @Component and @ConfigurationProperties beans are not scanned when the @DataJdbcTest annotation is used. @EnableConfigurationProperties can be used to include @ConfigurationProperties beans.

By default, Data JDBC tests are transactional and roll back at the end of each test. See the relevant section in the Spring Framework Reference Documentation for more details. If that is not what you want, you can disable transaction management for a test or for the whole test class as shown in the JDBC example.

If you prefer your test to run against a real database, you can use the @AutoConfigureTestDatabase annotation in the same way as for DataJpaTest. (See "Auto-configured Data JPA Tests".)

You can use @JooqTest in a similar fashion as @JdbcTest but for jOOQ-related tests. As jOOQ relies heavily on a Java-based schema that corresponds with the database schema, the existing DataSource is used. If you want to replace it with an in-memory database, you can use @AutoConfigureTestDatabase to override those settings. (For more about using jOOQ with Spring Boot, see "data.html", earlier in this chapter.) Regular @Component and @ConfigurationProperties beans are not scanned when the @JooqTest annotation is used. @EnableConfigurationProperties can be used to include @ConfigurationProperties beans.

@JooqTest configures a DSLContext. The following example shows the @JooqTest annotation in use:

JOOQ tests are transactional and roll back at the end of each test by default. If that is not what you want, you can disable transaction management for a test or for the whole test class as shown in the JDBC example.

You can use @DataMongoTest to test MongoDB applications. By default, it configures an in-memory embedded MongoDB (if available), configures a MongoTemplate, scans for @Document classes, and configures Spring Data MongoDB repositories. Regular @Component and @ConfigurationProperties beans are not scanned when the @DataMongoTest annotation is used. @EnableConfigurationProperties can be used to include @ConfigurationProperties beans. (For more about using MongoDB with Spring Boot, see "data.html", earlier in this chapter.)

The following class shows the @DataMongoTest annotation in use:

In-memory embedded MongoDB generally works well for tests, since it is fast and does not require any developer installation. If, however, you prefer to run tests against a real MongoDB server, you should exclude the embedded MongoDB auto-configuration, as shown in the following example:

You can use @DataNeo4jTest to test Neo4j applications. By default, it scans for @Node classes, and configures Spring Data Neo4j repositories. Regular @Component and @ConfigurationProperties beans are not scanned when the @DataNeo4jTest annotation is used. @EnableConfigurationProperties can be used to include @ConfigurationProperties beans. (For more about using Neo4J with Spring Boot, see "data.html", earlier in this chapter.)

The following example shows a typical setup for using Neo4J tests in Spring Boot:

By default, Data Neo4j tests are transactional and roll back at the end of each test. See the relevant section in the Spring Framework Reference Documentation for more details. If that is not what you want, you can disable transaction management for a test or for the whole class, as follows:

You can use @DataRedisTest to test Redis applications. By default, it scans for @RedisHash classes and configures Spring Data Redis repositories. Regular @Component and @ConfigurationProperties beans are not scanned when the @DataRedisTest annotation is used. @EnableConfigurationProperties can be used to include @ConfigurationProperties beans. (For more about using Redis with Spring Boot, see "data.html", earlier in this chapter.)

The following example shows the @DataRedisTest annotation in use:

You can use @DataLdapTest to test LDAP applications. By default, it configures an in-memory embedded LDAP (if available), configures an LdapTemplate, scans for @Entry classes, and configures Spring Data LDAP repositories. Regular @Component and @ConfigurationProperties beans are not scanned when the @DataLdapTest annotation is used. @EnableConfigurationProperties can be used to include @ConfigurationProperties beans. (For more about using LDAP with Spring Boot, see "data.html", earlier in this chapter.)

The following example shows the @DataLdapTest annotation in use:

In-memory embedded LDAP generally works well for tests, since it is fast and does not require any developer installation. If, however, you prefer to run tests against a real LDAP server, you should exclude the embedded LDAP auto-configuration, as shown in the following example:

You can use the @RestClientTest annotation to test REST clients. By default, it auto-configures Jackson, GSON, and Jsonb support, configures a RestTemplateBuilder, and adds support for MockRestServiceServer. Regular @Component and @ConfigurationProperties beans are not scanned when the @RestClientTest annotation is used. @EnableConfigurationProperties can be used to include @ConfigurationProperties beans.

The specific beans that you want to test should be specified by using the value or components attribute of @RestClientTest, as shown in the following example:

You can use the @AutoConfigureRestDocs annotation to use Spring REST Docs in your tests with Mock MVC, REST Assured, or WebTestClient. It removes the need for the JUnit extension in Spring REST Docs.

@AutoConfigureRestDocs can be used to override the default output directory (target/generated-snippets if you are using Maven or build/generated-snippets if you are using Gradle). It can also be used to configure the host, scheme, and port that appears in any documented URIs.

Each slice provides one or more @AutoConfigure…​ annotations that namely defines the auto-configurations that should be included as part of a slice. Additional auto-configurations can be added on a test-by-test basis by creating a custom @AutoConfigure…​ annotation or by adding @ImportAutoConfiguration to the test as shown in the following example:

Alternatively, additional auto-configurations can be added for any use of a slice annotation by registering them in META-INF/spring.factories as shown in the following example:

If you structure your code in a sensible way, your @SpringBootApplication class is used by default as the configuration of your tests.

It then becomes important not to litter the application’s main class with configuration settings that are specific to a particular area of its functionality.

Assume that you are using Spring Batch and you rely on the auto-configuration for it. You could define your @SpringBootApplication as follows:

Because this class is the source configuration for the test, any slice test actually tries to start Spring Batch, which is definitely not what you want to do. A recommended approach is to move that area-specific configuration to a separate @Configuration class at the same level as your application, as shown in the following example:

Test slices exclude @Configuration classes from scanning. For example, for a @WebMvcTest, the following configuration will not include the given WebMvcConfigurer bean in the application context loaded by the test slice:

The configuration below will, however, cause the custom WebMvcConfigurer to be loaded by the test slice.

Another source of confusion is classpath scanning. Assume that, while you structured your code in a sensible way, you need to scan an additional package. Your application may resemble the following code:

Doing so effectively overrides the default component scan directive with the side effect of scanning those two packages regardless of the slice that you chose. For instance, a @DataJpaTest seems to suddenly scan components and user configurations of your application. Again, moving the custom directive to a separate class is a good way to fix this issue.

Spock 2.x can be used to test a Spring Boot application. To do so, add a dependency on Spock’s spock-spring module to your application’s build. spock-spring integrates Spring’s test framework into Spock. See the documentation for Spock’s Spring module for further details.

ConfigDataApplicationContextInitializer is an ApplicationContextInitializer that you can apply to your tests to load Spring Boot application.properties files. You can use it when you do not need the full set of features provided by @SpringBootTest, as shown in the following example:

TestPropertyValues lets you quickly add properties to a ConfigurableEnvironment or ConfigurableApplicationContext. You can call it with key=value strings, as follows:

OutputCapture is a JUnit Extension that you can use to capture System.out and System.err output. To use add @ExtendWith(OutputCaptureExtension.class) and inject CapturedOutput as an argument to your test class constructor or test method as follows:

TestRestTemplate is a convenience alternative to Spring’s RestTemplate that is useful in integration tests. You can get a vanilla template or one that sends Basic HTTP authentication (with a username and password). In either case, the template is fault tolerant. This means that it behaves in a test-friendly way by not throwing exceptions on 4xx and 5xx errors. Instead, such errors can be detected through the returned ResponseEntity and its status code.

It is recommended, but not mandatory, to use the Apache HTTP Client (version 4.3.2 or better). If you have that on your classpath, the TestRestTemplate responds by configuring the client appropriately. If you do use Apache’s HTTP client, some additional test-friendly features are enabled:

TestRestTemplate can be instantiated directly in your integration tests, as shown in the following example:

Alternatively, if you use the @SpringBootTest annotation with WebEnvironment.RANDOM_PORT or WebEnvironment.DEFINED_PORT, you can inject a fully configured TestRestTemplate and start using it. If necessary, additional customizations can be applied through the RestTemplateBuilder bean. Any URLs that do not specify a host and port automatically connect to the embedded server, as shown in the following example:

The @ConditionalOnClass and @ConditionalOnMissingClass annotations let @Configuration classes be included based on the presence or absence of specific classes. Due to the fact that annotation metadata is parsed by using ASM, you can use the value attribute to refer to the real class, even though that class might not actually appear on the running application classpath. You can also use the name attribute if you prefer to specify the class name by using a String value.

This mechanism does not apply the same way to @Bean methods where typically the return type is the target of the condition: before the condition on the method applies, the JVM will have loaded the class and potentially processed method references which will fail if the class is not present.

To handle this scenario, a separate @Configuration class can be used to isolate the condition, as shown in the following example:

The @ConditionalOnBean and @ConditionalOnMissingBean annotations let a bean be included based on the presence or absence of specific beans. You can use the value attribute to specify beans by type or name to specify beans by name. The search attribute lets you limit the ApplicationContext hierarchy that should be considered when searching for beans.

When placed on a @Bean method, the target type defaults to the return type of the method, as shown in the following example:

In the preceding example, the someService bean is going to be created if no bean of type SomeService is already contained in the ApplicationContext.

The @ConditionalOnProperty annotation lets configuration be included based on a Spring Environment property. Use the prefix and name attributes to specify the property that should be checked. By default, any property that exists and is not equal to false is matched. You can also create more advanced checks by using the havingValue and matchIfMissing attributes.

The @ConditionalOnResource annotation lets configuration be included only when a specific resource is present. Resources can be specified by using the usual Spring conventions, as shown in the following example: file:/home/user/test.dat.

The @ConditionalOnWebApplication and @ConditionalOnNotWebApplication annotations let configuration be included depending on whether the application is a “web application”. A servlet-based web application is any application that uses a Spring WebApplicationContext, defines a session scope, or has a ConfigurableWebEnvironment. A reactive web application is any application that uses a ReactiveWebApplicationContext, or has a ConfigurableReactiveWebEnvironment.

The @ConditionalOnWarDeployment annotation lets configuration be included depending on whether the application is a traditional WAR application that is deployed to a container. This condition will not match for applications that are run with an embedded server.

The @ConditionalOnExpression annotation lets configuration be included based on the result of a SpEL expression.

If you need to test an auto-configuration that only operates in a servlet or reactive web application context, use the WebApplicationContextRunner or ReactiveWebApplicationContextRunner respectively.

It is also possible to test what happens when a particular class and/or package is not present at runtime. Spring Boot ships with a FilteredClassLoader that can easily be used by the runner. In the following example, we assert that if MyService is not present, the auto-configuration is properly disabled:

You should make sure to provide a proper namespace for your starter. Do not start your module names with spring-boot, even if you use a different Maven groupId. We may offer official support for the thing you auto-configure in the future.

As a rule of thumb, you should name a combined module after the starter. For example, assume that you are creating a starter for "acme" and that you name the auto-configure module acme-spring-boot and the starter acme-spring-boot-starter. If you only have one module that combines the two, name it acme-spring-boot-starter.

If your starter provides configuration keys, use a unique namespace for them. In particular, do not include your keys in the namespaces that Spring Boot uses (such as server, management, spring, and so on). If you use the same namespace, we may modify these namespaces in the future in ways that break your modules. As a rule of thumb, prefix all your keys with a namespace that you own (for example acme).

Make sure that configuration keys are documented by adding field javadoc for each property, as shown in the following example:

Here are some rules we follow internally to make sure descriptions are consistent:

Make sure to trigger meta-data generation so that IDE assistance is available for your keys as well. You may want to review the generated metadata (META-INF/spring-configuration-metadata.json) to make sure your keys are properly documented. Using your own starter in a compatible IDE is also a good idea to validate that quality of the metadata.

The autoconfigure module contains everything that is necessary to get started with the library. It may also contain configuration key definitions (such as @ConfigurationProperties) and any callback interface that can be used to further customize how the components are initialized.

Spring Boot uses an annotation processor to collect the conditions on auto-configurations in a metadata file (META-INF/spring-autoconfigure-metadata.properties). If that file is present, it is used to eagerly filter auto-configurations that do not match, which will improve startup time. It is recommended to add the following dependency in a module that contains auto-configurations:

If you have defined auto-configurations directly in your application, make sure to configure the spring-boot-maven-plugin to prevent the repackage goal from adding the dependency into the fat jar:

With Gradle 4.5 and earlier, the dependency should be declared in the compileOnly configuration, as shown in the following example:

With Gradle 4.6 and later, the dependency should be declared in the annotationProcessor configuration, as shown in the following example:

The starter is really an empty jar. Its only purpose is to provide the necessary dependencies to work with the library. You can think of it as an opinionated view of what is required to get started.

Do not make assumptions about the project in which your starter is added. If the library you are auto-configuring typically requires other starters, mention them as well. Providing a proper set of default dependencies may be hard if the number of optional dependencies is high, as you should avoid including dependencies that are unnecessary for a typical usage of the library. In other words, you should not include optional dependencies.

Spring Boot provides an idiomatic way to run an application with runApplication<MyApplication>(*args) as shown in the following example:

This is a drop-in replacement for SpringApplication.run(MyApplication::class.java, *args). It also allows customization of the application as shown in the following example:

Kotlin extensions provide the ability to extend existing classes with additional functionality. The Spring Boot Kotlin API makes use of these extensions to add new Kotlin specific conveniences to existing APIs.

TestRestTemplate extensions, similar to those provided by Spring Framework for RestOperations in Spring Framework, are provided. Among other things, the extensions make it possible to take advantage of Kotlin reified type parameters.