Encapsulate intelligent class loader access

According to the javadoc, Thread#getContextClassLoader() can return
null. In that case, we want to fall back to the system class loader.

Author: ctrueden

src/main/java/org/scijava/Context.java

@@ -440,6 +440,18 @@ public static List<Class<? extends Service>> serviceClassList(
 			Arrays.asList(serviceClasses) : Arrays.asList(Service.class);
 	}
 
+	/**
+	 * Gets the class loader to use. This will be the current thread's context
+	 * class loader if non-null; otherwise it will be the system class loader.
+	 * 
+	 * @see Thread#getContextClassLoader()
+	 * @see ClassLoader#getSystemClassLoader()
+	 */
+	public static ClassLoader getClassLoader() {
+		final ClassLoader contextCL = Thread.currentThread().getContextClassLoader();
+		return contextCL != null ? contextCL : ClassLoader.getSystemClassLoader();
+	}
+
 	// -- Helper methods --
 
 	private List<Field> getParameterFields(final Object o) {
@@ -530,8 +542,7 @@ private String createMissingServiceMessage(
 		final Class<? extends Service> serviceType)
 	{
 		final String nl = System.getProperty("line.separator");
-		final ClassLoader classLoader = //
-			Thread.currentThread().getContextClassLoader();
+		final ClassLoader classLoader = getClassLoader();
 		final StringBuilder msg = new StringBuilder(
 			"Required service is missing: " + serviceType.getName() + nl);
 		msg.append("Context: " + this + nl);
@@ -569,5 +580,4 @@ private static List<Class<? extends Service>> services(final boolean empty) {
 	private static boolean strict() {
 		return !"false".equals(System.getProperty(STRICT_PROPERTY));
 	}
-
 }

src/main/java/org/scijava/annotations/AnnotationCombiner.java

@@ -43,6 +43,7 @@
 import java.util.HashSet;
 import java.util.Set;
 
+import org.scijava.Context;
 import org.scijava.util.Combiner;
 import org.scijava.util.FileUtils;
 
@@ -65,7 +66,7 @@ public void combine(File outputDirectory) throws Exception {
 		}
 		final Set<String> annotationFiles = getAnnotationFiles();
 
-		final ClassLoader loader = Thread.currentThread().getContextClassLoader();
+		final ClassLoader loader = Context.getClassLoader();
 
 		log("");
 		log("Writing annotations to " + outputDirectory.getAbsolutePath());
@@ -92,7 +93,7 @@ public Set<String> getAnnotationFiles() throws IOException {
 
 		for (final String prefix : new String[] { PREFIX, LEGACY_PREFIX }) {
 			final Enumeration<URL> directories =
-				Thread.currentThread().getContextClassLoader().getResources(prefix);
+				Context.getClassLoader().getResources(prefix);
 			while (directories.hasMoreElements()) {
 				final URL url = directories.nextElement();
 				for (final URL annotationIndexURL : FileUtils.listContents(url)) {

src/main/java/org/scijava/annotations/EclipseHelper.java

@@ -43,6 +43,7 @@
 import java.util.jar.JarFile;
 import java.util.jar.Manifest;
 
+import org.scijava.Context;
 import org.scijava.util.FileUtils;
 
 /**
@@ -297,7 +298,7 @@ else if (file.isDirectory()) {
 	 */
 	public static void main(final String... args) {
 		System.setProperty(FORCE_ANNOTATION_INDEX_PROPERTY, "true");
-		updateAnnotationIndex(Thread.currentThread().getContextClassLoader());
+		updateAnnotationIndex(Context.getClassLoader());
 	}
 
 }

src/main/java/org/scijava/plugin/DefaultPluginFinder.java

@@ -38,6 +38,7 @@
 import java.util.regex.Pattern;
 import java.util.regex.PatternSyntaxException;
 
+import org.scijava.Context;
 import org.scijava.annotations.Index;
 import org.scijava.annotations.IndexItem;
 
@@ -113,8 +114,8 @@ private PluginInfo<SciJavaPlugin> createInfo(
 	}
 
 	private ClassLoader getClassLoader() {
-		if (customClassLoader != null) return customClassLoader;
-		return Thread.currentThread().getContextClassLoader();
+		return customClassLoader != null ? //
+			customClassLoader : Context.getClassLoader();
 	}
 
 	// -- Helper classes --

src/main/java/org/scijava/util/FileUtils.java

@@ -58,6 +58,8 @@
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
+import org.scijava.Context;
+
 /**
  * Useful methods for working with file paths.
  * 
@@ -664,7 +666,7 @@ public static Map<String, URL> findResources(final String regex,
 		final String pathPrefix, final File baseDirectory)
 	{
 		// scan URL resource paths first
-		final ClassLoader loader = Thread.currentThread().getContextClassLoader();
+		final ClassLoader loader = Context.getClassLoader();
 		final ArrayList<URL> urls = new ArrayList<>();
 		try {
 			urls.addAll(Collections.list(loader.getResources(pathPrefix + "/")));

src/main/java/org/scijava/util/POM.java

@@ -43,6 +43,7 @@
 
 import javax.xml.parsers.ParserConfigurationException;
 
+import org.scijava.Context;
 import org.scijava.Versioned;
 import org.xml.sax.SAXException;
 
@@ -280,8 +281,7 @@ public static POM getPOM(final Class<?> c, final String groupId,
 	public static List<POM> getAllPOMs() {
 		// find all META-INF/maven/ folders on the classpath
 		final String pomPrefix = "META-INF/maven/";
-		final ClassLoader classLoader =
-			Thread.currentThread().getContextClassLoader();
+		final ClassLoader classLoader = Context.getClassLoader();
 		final Enumeration<URL> resources;
 		try {
 			resources = classLoader.getResources(pomPrefix);

src/main/java/org/scijava/util/ServiceCombiner.java

@@ -45,6 +45,8 @@
 
 import javax.xml.ws.Service;
 
+import org.scijava.Context;
+
 /**
  * Combines {@link Service} information from all JAR files on the classpath.
  * 
@@ -61,8 +63,7 @@ public void combine(final File outputDirectory) throws IOException {
 		final Map<String, StringBuilder> files =
 			new HashMap<>();
 		final Enumeration<URL> directories =
-			Thread.currentThread().getContextClassLoader().getResources(
-				SERVICES_PREFIX);
+			Context.getClassLoader().getResources(SERVICES_PREFIX);
 
 		// Iterate over all the service files
 		while (directories.hasMoreElements()) {

src/main/java/org/scijava/util/Types.java

@@ -69,6 +69,8 @@
 import java.util.Objects;
 import java.util.Set;
 
+import org.scijava.Context;
+
 /**
  * Utility class for working with generic types, fields and methods.
  * <p>
@@ -221,8 +223,8 @@ public static Class<?> load(final String name, final ClassLoader classLoader,
 
 		// load the class!
 		try {
-			final ClassLoader cl = classLoader == null ? //
-				Thread.currentThread().getContextClassLoader() : classLoader;
+			final ClassLoader cl = //
+				classLoader != null ? classLoader : Context.getClassLoader();
 			return cl.loadClass(className);
 		}
 		catch (final Throwable t) {