Use static factory methods and proxyBeanMethods = false

It is better to be explicit about beans of type `Module` and make
sure they don't interfere with Spring trying to enhance
`@Configuration` classes. The tests are now clear of the pesky
warning logs, and there is a new section in the README.

README.md

@@ -97,7 +97,7 @@ bindings. Example:
 public static class TestConfig {
 
     @Bean
-    public MyModule myModule() {
+    public static MyModule myModule() {
         return new MyModule();
     }
 
@@ -112,6 +112,35 @@ public static class TestConfig {
 The `Service` was defined in the Guice module `MyModule`, and then it
 was be bound to the autowired `spam()` method when Spring started.
 
+## Configuration Class Enhancements
+
+Note that the `Module` bean definition in the example above is 
+declared in a `static` method. This is intentional and can be used to
+avoid accidentally preventing Spring from being able to enhance the
+parent `@Configuration` class. The default behaviour for Spring is to
+create a proxy for `@Configuration` classes so the `@Bean` methods
+can call each other and Spring will preserve the singleton nature of
+the bean factory - one bean of each type per bean id.
+
+If you see logs like this when the context starts:
+
+```
+Mar 21, 2022 8:30:35 AM org.springframework.context.annotation.ConfigurationClassPostProcessor enhanceConfigurationClasses
+INFO: Cannot enhance @Configuration bean definition 'TestConfig' since its singleton instance has been created too early. The typical cause is a non-static @Bean method with a BeanDefinitionRegistryPostProcessor return type: Consider declaring such methods as 'static'.
+```
+
+that is a sign that you might want to use static methods to define
+any beans of type `Module`. It is logged at INFO because Spring 
+doesn't know if it was intentional or not. Most likely it was
+unintentional, and it is better to avoid nasty surprises later 
+if we can.
+
+Another way to avoid the warning is to declare the parent class as 
+`@Configuration(proxyBeanMethods = false)` so that Spring knows that
+you don't even want to enhance the class. It's not a bad idea to use
+that flag wherever you can because it saves some time on start up
+if the proxy doesn't need to be created.
+
 ## Using Guice as an API for accessing a Spring ApplicationContext
 
 In this case the main feature is an `Injector` implementation that

src/main/java/org/springframework/guice/annotation/ModuleRegistryConfiguration.java

@@ -76,7 +76,7 @@ import org.springframework.guice.module.SpringModule;
  * @author Howard Yuan
  *
  */
-@Configuration
+@Configuration(proxyBeanMethods = false)
 @Order(Ordered.HIGHEST_PRECEDENCE)
 class ModuleRegistryConfiguration implements BeanDefinitionRegistryPostProcessor, ApplicationContextAware {
 

src/test/java/org/springframework/guice/BeanPostProcessorTests.java

@@ -97,7 +97,7 @@ public class BeanPostProcessorTests {
 	static class BeanPostProcessorTestConfig {
 
 		@Bean
-		PostProcessorRegistrar postProcessorRegistrar() {
+		static PostProcessorRegistrar postProcessorRegistrar() {
 			return new PostProcessorRegistrar();
 		}
 
@@ -112,7 +112,7 @@ public class BeanPostProcessorTests {
 		}
 
 		@Bean
-		Module someGuiceModule() {
+		static Module someGuiceModule() {
 			return new AbstractModule() {
 
 				@Override

src/test/java/org/springframework/guice/BindingAnnotationTests.java

@@ -222,7 +222,7 @@ public class BindingAnnotationTests {
 		}
 
 		@Bean
-		AbstractModule module() {
+		static AbstractModule module() {
 			return new AbstractModule() {
 				@Override
 				protected void configure() {

src/test/java/org/springframework/guice/BindingDeduplicationTests.java

@@ -83,7 +83,7 @@ public class BindingDeduplicationTests {
 		}
 
 		@Bean
-		Module module() {
+		static Module module() {
 			return new AbstractModule() {
 				@Override
 				protected void configure() {

src/test/java/org/springframework/guice/DuplicateNamesDifferentTypesTests.java

@@ -93,7 +93,7 @@ public class DuplicateNamesDifferentTypesTests {
 	static class DuplicateNamesDifferentTypesTestsConfig {
 
 		@Bean
-		Module module() {
+		static Module module() {
 			return new AbstractModule() {
 				@Override
 				protected void configure() {

src/test/java/org/springframework/guice/ElementVisitorTests.java

@@ -105,7 +105,7 @@ public class ElementVisitorTests {
 		}
 
 		@Bean
-		Module module() {
+		static Module module() {
 			return new AbstractModule() {
 				@Override
 				protected void configure() {

src/test/java/org/springframework/guice/LazyInitializationTests.java

@@ -71,7 +71,7 @@ public class LazyInitializationTests {
 	static class GuiceConfig {
 
 		@Bean
-		GuiceModule guiceModule() {
+		static GuiceModule guiceModule() {
 			return new GuiceModule();
 		}
 

src/test/java/org/springframework/guice/MapWiringTests.java

@@ -49,7 +49,7 @@ public class MapWiringTests {
 	static class ModulesConfig {
 
 		@Bean
-		TestConfig testConfig() {
+		static TestConfig testConfig() {
 			return new TestConfig();
 		}
 

src/test/java/org/springframework/guice/ModuleFilteringTests.java

@@ -93,7 +93,7 @@ public class ModuleFilteringTests {
 		}
 
 		@Bean
-		Module module() {
+		static Module module() {
 			return new AbstractModule() {
 
 				@Override

src/test/java/org/springframework/guice/PrivateModuleTests.java

@@ -121,7 +121,7 @@ public class PrivateModuleTests {
 		}
 
 		@Bean
-		Module module() {
+		static Module module() {
 			return new AbstractModule() {
 				@Override
 				protected void configure() {

src/test/java/org/springframework/guice/PrototypeScopedBeanTests.java

@@ -49,7 +49,7 @@ public class PrototypeScopedBeanTests {
 	static class ModulesConfig {
 
 		@Bean
-		Module guiceModule() {
+		static Module guiceModule() {
 			return new AbstractModule() {
 				@Override
 				protected void configure() {

src/test/java/org/springframework/guice/ProvidesSupplierWiringTests.java

@@ -71,7 +71,7 @@ public class ProvidesSupplierWiringTests {
 	static class ModulesConfig {
 
 		@Bean
-		TestConfig testConfig() {
+		static TestConfig testConfig() {
 			return new TestConfig();
 		}
 

src/test/java/org/springframework/guice/ScopingTests.java

@@ -91,7 +91,7 @@ public class ScopingTests {
 	static class ScopingTestsConfig {
 
 		@Bean
-		Module module() {
+		static Module module() {
 			return new AbstractModule() {
 				@Override
 				protected void configure() {

src/test/java/org/springframework/guice/SpringAutowiredCollectionTests.java

@@ -61,7 +61,7 @@ public class SpringAutowiredCollectionTests {
 		}
 
 		@Bean
-		GuiceModule guiceServiceModule() {
+		static GuiceModule guiceServiceModule() {
 			return new GuiceModule();
 		}
 

src/test/java/org/springframework/guice/SuperClassTests.java

@@ -361,7 +361,7 @@ public class SuperClassTests {
 	static class DisableJITConfig {
 
 		@Bean
-		AbstractModule disableJITModule() {
+		static AbstractModule disableJITModule() {
 			return new AbstractModule() {
 				@Override
 				protected void configure() {

src/test/java/org/springframework/guice/annotation/EnableGuiceModulesTests.java

@@ -119,7 +119,7 @@ public class EnableGuiceModulesTests {
 
 	}
 
-	@Configuration
+	@Configuration(proxyBeanMethods = false)
 	@EnableGuiceModules
 	protected static class ModuleConfig extends AbstractModule {
 
@@ -135,12 +135,12 @@ public class EnableGuiceModulesTests {
 
 	}
 
-	@Configuration
+	@Configuration(proxyBeanMethods = false)
 	@EnableGuiceModules
 	protected static class ModuleBeanConfig {
 
 		@Bean
-		public MyModule module() {
+		public static MyModule module() {
 			return new MyModule();
 		}
 
@@ -204,7 +204,7 @@ public class EnableGuiceModulesTests {
 		}
 
 		@Bean
-		public MyGuiceModule bazModule() {
+		public static MyGuiceModule bazModule() {
 			return new MyGuiceModule();
 		}
 

src/test/java/org/springframework/guice/annotation/ModuleBeanWiringTests.java

@@ -56,7 +56,7 @@ public class ModuleBeanWiringTests extends AbstractCompleteWiringTests {
 	}
 
 	@EnableGuiceModules
-	@Configuration
+	@Configuration(proxyBeanMethods = false)
 	public static class TestConfig extends AbstractModule {
 
 		@Autowired

src/test/java/org/springframework/guice/annotation/ModuleNamedBeanWiringTests.java

@@ -56,7 +56,7 @@ public class ModuleNamedBeanWiringTests extends AbstractCompleteWiringTests {
 	}
 
 	@EnableGuiceModules
-	@Configuration
+	@Configuration(proxyBeanMethods = false)
 	public static class TestConfig extends AbstractModule {
 
 		@Autowired

src/test/java/org/springframework/guice/module/DevelepmentStageInjectorTest.java

@@ -65,7 +65,7 @@ public class DevelepmentStageInjectorTest {
 	static class ModulesConfig {
 
 		@Bean
-		TestGuiceModule testGuiceModule() {
+		static TestGuiceModule testGuiceModule() {
 			return new TestGuiceModule();
 		}
 

src/test/java/org/springframework/guice/module/SpringModuleWrappedTests.java

@@ -79,7 +79,7 @@ public class SpringModuleWrappedTests {
 	protected static class ModuleProviderConfig {
 
 		@Bean
-		public ProviderModule module() {
+		public static ProviderModule module() {
 			return new ProviderModule();
 		}