Support fully-qualified factory method names in @⁠TestBean

Prior to this commit, @⁠TestBean factory methods had to be defined in
the test class, one of its superclasses, or in an implemented
interface. However, users may wish to define common factory methods in
external classes that can be shared easily across multiple test classes
simply by referencing an external method via a fully-qualified method
name.

To address that, this commit introduces support for referencing a
@⁠TestBean factory method via its fully-qualified method name following
the syntax <fully-qualified class name>#<method name>.

Closes gh-33125

spring-test/src/test/java/org/springframework/test/context/bean/override/convention/TestBeanForExternalFactoryMethodIntegrationTests.java

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2002-2024 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.test.context.bean.override.convention;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.test.context.junit.jupiter.SpringJUnitConfig;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Integration tests for {@link TestBean} that use bean factory methods defined
+ * in external classes.
+ *
+ * @author Sam Brannen
+ * @since 6.2
+ */
+@SpringJUnitConfig
+class TestBeanForExternalFactoryMethodIntegrationTests {
+
+	@TestBean(methodName = "org.springframework.test.context.bean.override.example.TestBeanFactory#createTestMessage")
+	String message;
+
+
+	@Test
+	void test() {
+		assertThat(message).isEqualTo("test");
+	}
+
+
+	@Configuration
+	static class Config {
+
+		@Bean
+		String message() {
+			return "prod";
+		}
+	}
+
+}

spring-test/src/test/java/org/springframework/test/context/bean/override/convention/TestBeanOverrideProcessorTests.java

@@ -24,6 +24,7 @@ import org.junit.jupiter.api.Test;
 import org.springframework.core.annotation.AnnotationUtils;
 import org.springframework.lang.NonNull;
 import org.springframework.test.context.bean.override.example.ExampleService;
+import org.springframework.test.context.bean.override.example.TestBeanFactory;
 import org.springframework.util.ReflectionUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -91,6 +92,64 @@ class TestBeanOverrideProcessorTests {
 				.withMessage("At least one candidate method name is required");
 	}
 
+	@Test
+	void findTestBeanFactoryMethodByFullyQualifiedName() {
+		Class<?> clazz = getClass();
+		Class<?> returnType = String.class;
+		String methodName = TestBeanFactory.class.getName() + "#createTestMessage";
+
+		Method method = this.processor.findTestBeanFactoryMethod(clazz, returnType, methodName);
+
+		assertThat(method).isEqualTo(ReflectionUtils.findMethod(TestBeanFactory.class, "createTestMessage"));
+	}
+
+	@Test
+	void findTestBeanFactoryMethodByFullyQualifiedNameWithNonexistentMethod() {
+		Class<?> clazz = getClass();
+		Class<?> returnType = String.class;
+		String factoryClassName = TestBeanFactory.class.getName();
+		String methodName = factoryClassName + "#bogus";
+
+		assertThatIllegalStateException()
+				.isThrownBy(() -> this.processor.findTestBeanFactoryMethod(clazz, returnType, methodName))
+				.withMessage("No static method found named %s in %s with return type %s",
+						"bogus", factoryClassName, returnType.getName());
+	}
+
+	@Test
+	void findTestBeanFactoryMethodByFullyQualifiedNameWithNonexistentClass() {
+		Class<?> clazz = getClass();
+		Class<?> returnType = String.class;
+		String methodName = "org.example.Bogus#createTestBean";
+
+		assertThatIllegalStateException()
+				.isThrownBy(() -> this.processor.findTestBeanFactoryMethod(clazz, returnType, methodName))
+				.withMessage("Failed to load class for fully-qualified method name: %s", methodName)
+				.withCauseInstanceOf(ClassNotFoundException.class);
+	}
+
+	@Test
+	void findTestBeanFactoryMethodByFullyQualifiedNameWithMissingMethodName() {
+		Class<?> clazz = getClass();
+		Class<?> returnType = String.class;
+		String methodName = TestBeanFactory.class.getName() + "#";
+
+		assertThatIllegalArgumentException()
+				.isThrownBy(() -> this.processor.findTestBeanFactoryMethod(clazz, returnType, methodName))
+				.withMessage("No method name present in fully-qualified method name: %s", methodName);
+	}
+
+	@Test
+	void findTestBeanFactoryMethodByFullyQualifiedNameWithMissingClassName() {
+		Class<?> clazz = getClass();
+		Class<?> returnType = String.class;
+		String methodName = "#createTestBean";
+
+		assertThatIllegalArgumentException()
+				.isThrownBy(() -> this.processor.findTestBeanFactoryMethod(clazz, returnType, methodName))
+				.withMessage("No class name present in fully-qualified method name: %s", methodName);
+	}
+
 	@Test
 	void createMetaDataForUnknownExplicitMethod() throws Exception {
 		Class<?> clazz = ExplicitMethodNameTestCase.class;
@@ -176,7 +235,6 @@ class TestBeanOverrideProcessorTests {
 
 	static class BaseTestCase {
 
-		@TestBean(methodName = "factory")
 		public String field;
 
 		static String factory() {