Automatically append query string parameters during a call to Spring's RestTemplate

RestTemplateExtensions.kt

import org.springframework.http.ResponseEntity
import org.springframework.web.client.RestTemplate

/**
 * This method behaves like 'getForEntity', but automatically extends the uriTemplate
 * with the queryString part, adding only query parameters for which the value is not null.
 * Map keys of the 'queryParams' will be used as keys in the queryString.
 *
 * See unit tests for excellent usage examples.
 *
 * We're using this instead of the much more comfortable UriComponentsBuilder, because
 * the http.client.requests metric requires the RestTemplate to be used with a templateUri
 * and separate uriVariables to not overflow the metrics pipeline with all the variable
 * values.
 */
inline fun <reified T : Any> RestTemplate.getForEntityWithQuery(
    pathTemplate: String,
    pathParams: Map<String, Any>,
    queryParams: Map<String, Any?>
): ResponseEntity<T> {
    // use SortedMap to avoid URIs varying with the hashing implementation
    val nonNullQueryParams = queryParams.filterNotNullValues().toSortedMap()
    val queryStringTemplate =
        if (nonNullQueryParams.isEmpty()) ""
        else "?" + nonNullQueryParams.map { it.key }.joinToString("&") { "$it={$it}" }

    return getForEntity(pathTemplate + queryStringTemplate, T::class.java, pathParams + nonNullQueryParams)
}

// Explicitly .map on the Map so it autocasts the ? away.
fun <K, V> Map<K, V?>.filterNotNullValues(): Map<K, V> =
    mapNotNull { (key, nullableValue) ->
        nullableValue?.let { nonNullValue -> key to nonNullValue }
    }.toMap()

RestUtilitiesKtTest.kt

package de.friday.services

import io.mockk.mockk
import io.mockk.verify
import org.assertj.core.api.Assertions.assertThat
import org.junit.Test
import org.springframework.web.client.RestTemplate

class RestUtilitiesKtTest {
    val restTemplate = mockk<RestTemplate>(relaxed = true)

    @Test
    fun optionalQueryParams_allNull() {
        restTemplate.getForEntityWithQuery<String>(
            "api/policies/{no}/",
            mapOf("no" to "123"),
            mapOf("name" to null as Any?, "age" to null))

        verify {
            restTemplate.getForEntity("api/policies/{no}/", String::class.java, mapOf("no" to "123"))
        }
    }

    @Test
    fun optionalQueryParams_oneNotNull() {
        restTemplate.getForEntityWithQuery<String>(
            "api/policies/{no}/",
            mapOf("no" to "123"),
            mapOf("name" to null as Any?, "age" to 23))

        verify {
            restTemplate.getForEntity("api/policies/{no}/?age={age}", String::class.java,
                mapOf("age" to 23, "no" to "123"))
        }
    }

    @Test
    fun optionalQueryParams_twoNotNull_noPathParams() {
        restTemplate.getForEntityWithQuery<String>(
            "api/something/",
            mapOf(),
            mapOf("name" to "Jack", "age" to 23))

        verify {
            restTemplate.getForEntity("api/something/?age={age}&name={name}", String::class.java,
                mapOf("age" to 23, "name" to "Jack"))
        }
    }
}