Remove reflection for SimpleRequestExpectationManager creation

Closes gh-32867

spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/web/client/MockServerRestTemplateCustomizer.java

@@ -19,6 +19,7 @@ package org.springframework.boot.test.web.client;
 import java.util.Collections;
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.function.Supplier;
 
 import org.springframework.beans.BeanUtils;
 import org.springframework.boot.web.client.RestTemplateBuilder;
@@ -47,27 +48,33 @@ import org.springframework.web.client.RestTemplate;
  * the related server.
  *
  * @author Phillip Webb
+ * @author Moritz Halbritter
  * @since 1.4.0
  * @see #getServer()
  * @see #getServer(RestTemplate)
  */
 public class MockServerRestTemplateCustomizer implements RestTemplateCustomizer {
 
-	private Map<RestTemplate, RequestExpectationManager> expectationManagers = new ConcurrentHashMap<>();
+	private final Map<RestTemplate, RequestExpectationManager> expectationManagers = new ConcurrentHashMap<>();
 
-	private Map<RestTemplate, MockRestServiceServer> servers = new ConcurrentHashMap<>();
+	private final Map<RestTemplate, MockRestServiceServer> servers = new ConcurrentHashMap<>();
 
-	private final Class<? extends RequestExpectationManager> expectationManager;
+	private final Supplier<? extends RequestExpectationManager> expectationManagerSupplier;
 
 	private boolean detectRootUri = true;
 
 	public MockServerRestTemplateCustomizer() {
-		this.expectationManager = SimpleRequestExpectationManager.class;
+		this(SimpleRequestExpectationManager::new);
 	}
 
 	public MockServerRestTemplateCustomizer(Class<? extends RequestExpectationManager> expectationManager) {
+		this(() -> BeanUtils.instantiateClass(expectationManager));
 		Assert.notNull(expectationManager, "ExpectationManager must not be null");
-		this.expectationManager = expectationManager;
+	}
+
+	public MockServerRestTemplateCustomizer(Supplier<? extends RequestExpectationManager> expectationManagerSupplier) {
+		Assert.notNull(expectationManagerSupplier, "ExpectationManagerSupplier must not be null");
+		this.expectationManagerSupplier = expectationManagerSupplier;
 	}
 
 	/**
@@ -91,7 +98,7 @@ public class MockServerRestTemplateCustomizer implements RestTemplateCustomizer
 	}
 
 	protected RequestExpectationManager createExpectationManager() {
-		return BeanUtils.instantiateClass(this.expectationManager);
+		return this.expectationManagerSupplier.get();
 	}
 
 	public MockRestServiceServer getServer() {

spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/web/client/MockServerRestTemplateCustomizerTests.java

@@ -16,6 +16,8 @@
 
 package org.springframework.boot.test.web.client;
 
+import java.util.function.Supplier;
+
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
@@ -35,6 +37,7 @@ import static org.springframework.test.web.client.response.MockRestResponseCreat
  * Tests for {@link MockServerRestTemplateCustomizer}.
  *
  * @author Phillip Webb
+ * @author Moritz Halbritter
  */
 class MockServerRestTemplateCustomizerTests {
 
@@ -55,10 +58,19 @@ class MockServerRestTemplateCustomizerTests {
 
 	@Test
 	void createWhenExpectationManagerClassIsNullShouldThrowException() {
-		assertThatIllegalArgumentException().isThrownBy(() -> new MockServerRestTemplateCustomizer(null))
+		Class<? extends RequestExpectationManager> expectationManager = null;
+		assertThatIllegalArgumentException().isThrownBy(() -> new MockServerRestTemplateCustomizer(expectationManager))
 				.withMessageContaining("ExpectationManager must not be null");
 	}
 
+	@Test
+	void createWhenExpectationManagerSupplierIsNullShouldThrowException() {
+		Supplier<? extends RequestExpectationManager> expectationManagerSupplier = null;
+		assertThatIllegalArgumentException()
+				.isThrownBy(() -> new MockServerRestTemplateCustomizer(expectationManagerSupplier))
+				.withMessageContaining("ExpectationManagerSupplier must not be null");
+	}
+
 	@Test
 	void createShouldUseExpectationManagerClass() {
 		MockServerRestTemplateCustomizer customizer = new MockServerRestTemplateCustomizer(
@@ -68,6 +80,15 @@ class MockServerRestTemplateCustomizerTests {
 				.isInstanceOf(UnorderedRequestExpectationManager.class);
 	}
 
+	@Test
+	void createShouldUseSupplier() {
+		MockServerRestTemplateCustomizer customizer = new MockServerRestTemplateCustomizer(
+				UnorderedRequestExpectationManager::new);
+		customizer.customize(new RestTemplate());
+		assertThat(customizer.getServer()).extracting("expectationManager")
+				.isInstanceOf(UnorderedRequestExpectationManager.class);
+	}
+
 	@Test
 	void detectRootUriShouldDefaultToTrue() {
 		MockServerRestTemplateCustomizer customizer = new MockServerRestTemplateCustomizer(