LoadTimeWeavingConfiguration should not rely on private field injection

Issue: SPR-12776
(cherry picked from commit 1259671)

spring-context/src/main/java/org/springframework/context/annotation/LoadTimeWeavingConfiguration.java

@@ -26,9 +26,6 @@ import org.springframework.context.weaving.DefaultContextLoadTimeWeaver;
 import org.springframework.core.annotation.AnnotationAttributes;
 import org.springframework.core.type.AnnotationMetadata;
 import org.springframework.instrument.classloading.LoadTimeWeaver;
-import org.springframework.util.Assert;
-
-import static org.springframework.context.weaving.AspectJWeavingEnabler.*;
 
 /**
  * {@code @Configuration} class that registers a {@link LoadTimeWeaver} bean.
@@ -47,7 +44,6 @@ public class LoadTimeWeavingConfiguration implements ImportAware, BeanClassLoade
 
 	private AnnotationAttributes enableLTW;
 
-	@Autowired(required = false)
 	private LoadTimeWeavingConfigurer ltwConfigurer;
 
 	private ClassLoader beanClassLoader;
@@ -56,9 +52,16 @@ public class LoadTimeWeavingConfiguration implements ImportAware, BeanClassLoade
 	@Override
 	public void setImportMetadata(AnnotationMetadata importMetadata) {
 		this.enableLTW = AnnotationConfigUtils.attributesFor(importMetadata, EnableLoadTimeWeaving.class);
-		Assert.notNull(this.enableLTW,
+		if (this.enableLTW == null) {
+			throw new IllegalArgumentException(
 					"@EnableLoadTimeWeaving is not present on importing class " + importMetadata.getClassName());
 		}
+	}
+
+	@Autowired(required = false)
+	public void setLoadTimeWeavingConfigurer(LoadTimeWeavingConfigurer ltwConfigurer) {
+		this.ltwConfigurer = ltwConfigurer;
+	}
 
 	@Override
 	public void setBeanClassLoader(ClassLoader beanClassLoader) {
@@ -87,7 +90,7 @@ public class LoadTimeWeavingConfiguration implements ImportAware, BeanClassLoade
 				// AJ weaving is disabled -> do nothing
 				break;
 			case AUTODETECT:
-				if (this.beanClassLoader.getResource(ASPECTJ_AOP_XML_RESOURCE) == null) {
+				if (this.beanClassLoader.getResource(AspectJWeavingEnabler.ASPECTJ_AOP_XML_RESOURCE) == null) {
 					// No aop.xml present on the classpath -> treat as 'disabled'
 					break;
 				}