Validate excluded autoconfiguration classes

Closes gh-7407
See gh-6865

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

@@ -85,6 +85,7 @@ public class EnableAutoConfigurationImportSelector
 					attributes);
 			configurations = removeDuplicates(configurations);
 			Set<String> exclusions = getExclusions(metadata, attributes);
+			checkExcludedClasses(configurations, exclusions);
 			configurations.removeAll(exclusions);
 			configurations = sort(configurations);
 			recordWithConditionEvaluationReport(configurations, exclusions);
@@ -157,6 +158,23 @@ public class EnableAutoConfigurationImportSelector
 		return EnableAutoConfiguration.class;
 	}
 
+	private void checkExcludedClasses(List<String> configurations,
+			Set<String> exclusions) {
+		StringBuilder message = new StringBuilder();
+		for (String exclusion : exclusions) {
+			if (ClassUtils.isPresent(exclusion, getClass().getClassLoader())
+					&& !configurations.contains(exclusion)) {
+				message.append("\t- ").append(exclusion).append("\n");
+			}
+		}
+		if (!message.toString().isEmpty()) {
+			throw new IllegalStateException(
+					"The following classes could not be excluded because they are not auto-configuration classes:\n"
+							+ message.toString());
+		}
+
+	}
+
 	/**
 	 * Return any exclusions that limit the candidate configurations.
 	 * @param metadata the source metadata

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

@@ -19,7 +19,9 @@ package org.springframework.boot.autoconfigure;
 import java.util.List;
 
 import org.junit.Before;
+import org.junit.Rule;
 import org.junit.Test;
+import org.junit.rules.ExpectedException;
 import org.mockito.Mock;
 import org.mockito.MockitoAnnotations;
 
@@ -29,6 +31,7 @@ import org.springframework.boot.autoconfigure.condition.ConditionEvaluationRepor
 import org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration;
 import org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration;
 import org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration;
+import org.springframework.context.annotation.Configuration;
 import org.springframework.core.annotation.AnnotationAttributes;
 import org.springframework.core.io.DefaultResourceLoader;
 import org.springframework.core.io.support.SpringFactoriesLoader;
@@ -60,6 +63,9 @@ public class EnableAutoConfigurationImportSelectorTests {
 	@Mock
 	private AnnotationAttributes annotationAttributes;
 
+	@Rule
+	public ExpectedException expected = ExpectedException.none();
+
 	@Before
 	public void setup() {
 		MockitoAnnotations.initMocks(this);
@@ -166,6 +172,47 @@ public class EnableAutoConfigurationImportSelectorTests {
 		assertThat(imports).isEmpty();
 	}
 
+	@Test
+	public void nonAutoConfigurationClassExclusionsShouldThrowException() throws Exception {
+		this.expected.expect(IllegalStateException.class);
+		configureExclusions(new String[] { TestConfiguration.class.getName() },
+				new String[0], new String[0]);
+		this.importSelector.selectImports(this.annotationMetadata);
+	}
+
+	@Test
+	public void nonAutoConfigurationClassNameExclusionsWhenPresentOnClassPathShouldThrowException()
+			throws Exception {
+		this.expected.expect(IllegalStateException.class);
+		configureExclusions(new String[0],
+				new String[] { "org.springframework.boot.autoconfigure.EnableAutoConfigurationImportSelectorTests.TestConfiguration" },
+				new String[0]);
+		this.importSelector.selectImports(this.annotationMetadata);
+	}
+
+	@Test
+	public void  nonAutoConfigurationPropertyExclusionsWhenPresentOnClassPathShouldThrowException()
+			throws Exception {
+		this.expected.expect(IllegalStateException.class);
+		configureExclusions(new String[0],
+				new String[0],
+				new String[] { "org.springframework.boot.autoconfigure.EnableAutoConfigurationImportSelectorTests.TestConfiguration" });
+		this.importSelector.selectImports(this.annotationMetadata);
+	}
+
+	@Test
+	public void nameAndPropertyExclusionsWhenNotPresentOnClasspathShouldNotThrowException()
+			throws Exception {
+		configureExclusions(new String[0],
+				new String[] { "org.springframework.boot.autoconfigure.DoesNotExist1" },
+				new String[] { "org.springframework.boot.autoconfigure.DoesNotExist2" });
+		this.importSelector.selectImports(this.annotationMetadata);
+		assertThat(ConditionEvaluationReport.get(this.beanFactory).getExclusions())
+				.contains("org.springframework.boot.autoconfigure.DoesNotExist1");
+		assertThat(ConditionEvaluationReport.get(this.beanFactory).getExclusions())
+				.contains("org.springframework.boot.autoconfigure.DoesNotExist2");
+	}
+
 	private void configureExclusions(String[] classExclusion, String[] nameExclusion,
 			String[] propertyExclusion) {
 		String annotationName = EnableAutoConfiguration.class.getName();
@@ -187,4 +234,9 @@ public class EnableAutoConfigurationImportSelectorTests {
 				getClass().getClassLoader());
 	}
 
+	@Configuration
+	private class TestConfiguration {
+
+	}
+
 }