Add centralized service loader utility

This allows library consumers to control the classloader in applications
with non-trivial classloading requirements.

Author: nicobrevin

cdm/core/src/main/java/ucar/nc2/util/NcServiceLoader.java

@@ -0,0 +1,50 @@
+package ucar.nc2.util;
+
+import java.util.ServiceLoader;
+
+/**
+ * Centralized access to the jdk's built in {@link ServiceLoader} utility to support using the NetCDF library in
+ * applications with non-trivial class loading requirements.
+ */
+public class NcServiceLoader {
+
+  private static ClassLoader classLoader;
+
+  public static <S> ServiceLoader<S> load(Class<S> service) {
+    ClassLoader useClassLoader;
+
+    if (classLoader == null) {
+      useClassLoader = Thread.currentThread().getContextClassLoader();
+    } else {
+      useClassLoader = classLoader;
+    }
+
+    return ServiceLoader.load(service, useClassLoader);
+  }
+
+  /**
+   * Services will be loaded using whatever happens to be the current thread's context class loader at the time
+   * services are loaded. This is the default behaviour and will work for applications that use the NetCDF library
+   * without any special class-loading requirements.
+   */
+  public static void useContextClassLoader() {
+    classLoader = null;
+  }
+
+  /**
+   * Services will be loaded using the same classloader that loaded this class, assumed to be
+   * the same one that will load the rest of the netcdf library. This option is suitable for applications that
+   * use only the built-in functionality, i.e they do not extend the library via the {@link ServiceLoader} mechanism
+   */
+  public static void usePackageClassLoader() {
+    classLoader = NcServiceLoader.class.getClassLoader();
+  }
+
+  /**
+   * Services will be loaded using a specific custom class loader. This class loader should 'descend' from the one that
+   * loaded the NetCDF library code, otherwise any services loaded are likely to throw {@link ClassCastException}s
+   */
+  public static void useCustomClassLoader(ClassLoader useClassLoader) {
+    classLoader = useClassLoader;
+  }
+}