Improve diagnostics with @ServiceConnection on non-static field

Closes gh-34795

spring-boot-project/spring-boot-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/service/connection/ServiceConnectionContextCustomizerFactory.java

@@ -17,6 +17,7 @@
 package org.springframework.boot.test.autoconfigure.service.connection;
 
 import java.lang.reflect.Field;
+import java.lang.reflect.Modifier;
 import java.util.ArrayList;
 import java.util.List;
 
@@ -62,6 +63,9 @@ class ServiceConnectionContextCustomizerFactory implements ContextCustomizerFact
 
 	private ContainerConnectionSource<?, ?, ?> createSource(Field field,
 			MergedAnnotation<ServiceConnection> annotation) {
+		if (!Modifier.isStatic(field.getModifiers())) {
+			throw new IllegalStateException("@ServiceConnection field '%s' must be static".formatted(field.getName()));
+		}
 		Class<? extends ConnectionDetails> connectionDetailsType = getConnectionDetailsType(annotation);
 		Object fieldValue = getFieldValue(field);
 		Assert.isInstanceOf(GenericContainer.class, fieldValue,

spring-boot-project/spring-boot-test-autoconfigure/src/test/java/org/springframework/boot/test/autoconfigure/service/connection/ServiceConnectionContextCustomizerFactoryTests.java

@@ -24,6 +24,7 @@ import org.springframework.boot.autoconfigure.service.connection.ConnectionDetai
 import org.springframework.boot.test.autoconfigure.service.connection.ServiceConnectionContextCustomizerFactoryTests.ServiceConnections.NestedClass;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
 
 /**
  * Tests for {@link ServiceConnectionContextCustomizerFactory}.
@@ -55,6 +56,13 @@ public class ServiceConnectionContextCustomizerFactoryTests {
 		assertThat(customizer.getSources()).hasSize(3);
 	}
 
+	@Test
+	void whenClassHasNonStaticServiceConnectionThenCreateShouldFailWithHelpfulIllegalStateException() {
+		assertThatIllegalStateException()
+			.isThrownBy(() -> this.factory.createContextCustomizer(NonStaticServiceConnection.class, null))
+			.withMessage("@ServiceConnection field 'service' must be static");
+	}
+
 	static class NoServiceConnections {
 
 	}
@@ -77,6 +85,13 @@ public class ServiceConnectionContextCustomizerFactoryTests {
 
 	}
 
+	static class NonStaticServiceConnection {
+
+		@ServiceConnection(TestConnectionDetails.class)
+		private GenericContainer<?> service = new GenericContainer<>("example");
+
+	}
+
 	static class TestConnectionDetails implements ConnectionDetails {
 
 	}