Slimming down a Spring Boot app for testing

One of my favorite things about Spring Boot is the ability to launch an application in embedded mode and do some pseudo-integration-testing (that is, it's integration testing because I'm able to call the embedded app over the loopback network, as if the test were running on a different machine).   Of course you can launch your 'real' application that lives in your 'main' source folder, and you can enable / disable parts of the application with Spring profiles.

But what if you want to create a specialized Spring Boot app, just for testing?   Well, you can!

For example, in src/test/org/example/test/app/TestServer.kt we can make an app class (btw, this is Kotlin, just so you know):

package org.example.test.app

...imports blah...

@SpringBootApplication()
@Import(SomeConfig::class)
class TestGraphQLClientApp {
    /**
     * Defines the main resolvers: Query and Mutation.
     */
    @Bean
    fun resolvers(query: GraphQLQueryResolver) = listOf(query)

}

• This is a GraphQL server, and I'm going to test a simple GraphQL client with it.   There are more components behind the scenes.
• I'm putting it in the app sub-package to avoid loading any component in the main app or anywhere else unintentionally.    Remember that a Spring Boot app class implies a 'component scan' of the package it lives in, and all sub-packages!
• SomeConfig is imported, and this brings in whatever components from the main code or elsewhere.
• Specialized test components can be defined in packages or sub-packages.

We can make a test like this:

package org.example.test

... imports blah ...

@RunWith(SpringJUnit4ClassRunner::class)
@SpringBootTest(classes = [TestGraphQLClientApp::class, HttpClientConfiguration::class],
        webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
class GraphQLClientTest {
    companion object : KLogging()

    @LocalServerPort
    private val port: Int = 0

    @Autowired
    private lateinit var factory: RestTemplateFactory

    // These have to be 'by lazy' because Spring will inject the fields they rely on after init.

    private val template by lazy { factory.createRestTemplate() }

    private val baseUrl by lazy { "http://localhost:$port/graphql" }

    private val client by lazy { GraphQLClient(baseUrl, template) }

    @Test
    fun basicClientTest() {
        client.query("query { foo }").also { value ->
            logger.info { prettyPrint(value) }
            assertEquals("foo", assertHasField(value, "data", "foo").asText())
        }
        client.query("query { getThing(id: \"12345-ABC\") { one two } }").also {
            logger.info { prettyPrint(it) }
        }
    }
}

• Note that the default 'properties' will be loaded from application.properties or application.yml.  If we want to override this, we should probably make a profile and use it from the test.

So what's the problem?   The problem is that, in this particular context I have JPA and a few other Spring Boot 'starter' dependencies.   So, when the test class starts the Spring Boot app, it launches:

1. A JDBC Data Source
2. Liquibase (my preferred data migration tool)
3. JPA and Hibernate

Those are all great tools, and it's super convenient to have all these 'auto starters' but they are not needed in this particular test.   How to turn them off and "slim" down my application?   There are two approaches:

1. Create a profile, and disable some things in that profile - This works for the autostart modules that support it, but not all of them do.
2. Use 'exclude' in SpringBootApplication to disable the autostart modules.

Using exclude is easy, and since it prevents Spring from loading the autostart modules in the first place, it can reduce startup time.   So for a simple web app, we can disable all the database modules:

@SpringBootApplication(exclude = [
    LiquibaseAutoConfiguration::class,
    DataSourceAutoConfiguration::class,
    DataSourceTransactionManagerAutoConfiguration::class,
    HibernateJpaAutoConfiguration::class])

That's all we need to do!   Now the test app starts up in about 9 seconds.

See also:

• Using the @SpringBootApplication Annotation
• SpringBootApplication annotation
• https://stackoverflow.com/a/49276219/266167