Ignore @ImportAutoConfiguration exclude errors

Update `ImportAutoConfigurationImportSelector` to ignore excludes for
classes that aren't loaded. Since the import classes for tests tend to
be much more limited, the exception isn't really helpful.

Closes gh-6809

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/EnableAutoConfigurationImportSelector.java

@@ -161,20 +161,31 @@ public class EnableAutoConfigurationImportSelector
 
 	private void checkExcludedClasses(List<String> configurations,
 			Set<String> exclusions) {
-		StringBuilder message = new StringBuilder();
+		List<String> invalidExcludes = new ArrayList<String>();
 		for (String exclusion : exclusions) {
 			if (ClassUtils.isPresent(exclusion, getClass().getClassLoader())
 					&& !configurations.contains(exclusion)) {
-				message.append("\t- ").append(exclusion).append(String.format("%n"));
+				invalidExcludes.add(exclusion);
 			}
 		}
-		if (!message.toString().isEmpty()) {
-			throw new IllegalStateException(String.format(
-					"The following classes could not be excluded because they are"
-							+ " not auto-configuration classes:%n%s",
-					message.toString()));
+		if (!invalidExcludes.isEmpty()) {
+			handleInvalidExcludes(invalidExcludes);
 		}
+	}
 
+	/**
+	 * Handle any invalid excludes that have been specified.
+	 * @param invalidExcludes the list of invalid excludes (will always have at least on
+	 * element)
+	 */
+	protected void handleInvalidExcludes(List<String> invalidExcludes) {
+		StringBuilder message = new StringBuilder();
+		for (String exclude : invalidExcludes) {
+			message.append("\t- ").append(exclude).append(String.format("%n"));
+		}
+		throw new IllegalStateException(String
+				.format("The following classes could not be excluded because they are"
+						+ " not auto-configuration classes:%n%s", message));
 	}
 
 	/**

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/ImportAutoConfigurationImportSelector.java

@@ -146,4 +146,9 @@ class ImportAutoConfigurationImportSelector
 		return super.getOrder() - 1;
 	}
 
+	@Override
+	protected void handleInvalidExcludes(List<String> invalidExcludes) {
+		// Ignore for test
+	}
+
 }

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/ImportAutoConfigurationImportSelectorTests.java

@@ -116,6 +116,15 @@ public class ImportAutoConfigurationImportSelectorTests {
 		assertThat(imports).containsOnly(FreeMarkerAutoConfiguration.class.getName());
 	}
 
+	@Test
+	public void exclusionsWithoutImport() throws Exception {
+		AnnotationMetadata annotationMetadata = new SimpleMetadataReaderFactory()
+				.getMetadataReader(ExclusionWithoutImport.class.getName())
+				.getAnnotationMetadata();
+		String[] imports = this.importSelector.selectImports(annotationMetadata);
+		assertThat(imports).containsOnly(FreeMarkerAutoConfiguration.class.getName());
+	}
+
 	@Test
 	public void exclusionsAliasesAreApplied() throws Exception {
 		AnnotationMetadata annotationMetadata = new SimpleMetadataReaderFactory()
@@ -149,6 +158,12 @@ public class ImportAutoConfigurationImportSelectorTests {
 
 	}
 
+	@ImportOne
+	@ImportAutoConfiguration(exclude = ThymeleafAutoConfiguration.class)
+	static class ExclusionWithoutImport {
+
+	}
+
 	@SelfAnnotating
 	static class ImportWithSelfAnnotatingAnnotation {