Make AbsTstCtxBootstrapper.resolveContextLoader protected

This commit increases the extensibility of
AbstractTestContextBootstrapper by making the resolveContextLoader()
and resolveExplicitContextLoaderClass() methods protected instead of
private.

Furthermore, resolveContextLoader() now throws an IllegalStateException
if getDefaultContextLoaderClass() returns null.

Issue: SPR-12682

spring-test/src/main/java/org/springframework/test/context/support/AbstractTestContextBootstrapper.java

 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2015 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.
@@ -390,8 +390,10 @@ public abstract class AbstractTestContextBootstrapper implements TestContextBoot
 	 * (i.e., as if we were traversing up the class hierarchy)
 	 * @return the resolved {@code ContextLoader} for the supplied {@code testClass}
 	 * (never {@code null})
+	 * @throws IllegalStateException if {@link #getDefaultContextLoaderClass(Class)}
+	 * returns {@code null}
 	 */
-	private ContextLoader resolveContextLoader(Class<?> testClass,
+	protected ContextLoader resolveContextLoader(Class<?> testClass,
 			List<ContextConfigurationAttributes> configAttributesList) {
 
 		Assert.notNull(testClass, "Class must not be null");
@@ -400,6 +402,9 @@ public abstract class AbstractTestContextBootstrapper implements TestContextBoot
 		Class<? extends ContextLoader> contextLoaderClass = resolveExplicitContextLoaderClass(configAttributesList);
 		if (contextLoaderClass == null) {
 			contextLoaderClass = getDefaultContextLoaderClass(testClass);
+			if (contextLoaderClass == null) {
+				throw new IllegalStateException("getDefaultContextLoaderClass() must not return null");
+			}
 		}
 		if (logger.isTraceEnabled()) {
 			logger.trace(String.format("Using ContextLoader class [%s] for test class [%s]",
@@ -428,7 +433,7 @@ public abstract class AbstractTestContextBootstrapper implements TestContextBoot
 	 * @throws IllegalArgumentException if supplied configuration attributes are
 	 * {@code null} or <em>empty</em>
 	 */
-	private Class<? extends ContextLoader> resolveExplicitContextLoaderClass(
+	protected Class<? extends ContextLoader> resolveExplicitContextLoaderClass(
 			List<ContextConfigurationAttributes> configAttributesList) {
 
 		Assert.notEmpty(configAttributesList, "ContextConfigurationAttributes list must not be empty");
@@ -451,12 +456,14 @@ public abstract class AbstractTestContextBootstrapper implements TestContextBoot
 	}
 
 	/**
-	 * Determine the default {@link ContextLoader} class to use for the supplied
-	 * test class.
+	 * Determine the default {@link ContextLoader} {@linkplain Class class}
+	 * to use for the supplied test class.
 	 * <p>The class returned by this method will only be used if a {@code ContextLoader}
 	 * class has not been explicitly declared via {@link ContextConfiguration#loader}.
 	 * @param testClass the test class for which to retrieve the default
 	 * {@code ContextLoader} class
+	 * @return the default {@code ContextLoader} class for the supplied test class
+	 * (never {@code null})
 	 */
 	protected abstract Class<? extends ContextLoader> getDefaultContextLoaderClass(Class<?> testClass);