8028368: There is no description whether or not java.util.ResourceBundle is thread-safe

Reviewed-by: okutsu

src/share/classes/java/util/ListResourceBundle.java

@@ -105,6 +105,12 @@ import sun.util.ResourceBundleEnumeration;
  * }
  * </pre>
  * </blockquote>
+ *
+ * <p>
+ * The implementation of a {@code ListResourceBundle} subclass must be thread-safe
+ * if it's simultaneously used by multiple threads. The default implementations
+ * of the methods in this class are thread-safe.
+ *
  * @see ResourceBundle
  * @see PropertyResourceBundle
  * @since JDK1.1

src/share/classes/java/util/PropertyResourceBundle.java

@@ -100,6 +100,11 @@ import sun.util.ResourceBundleEnumeration;
  * </blockquote>
  *
  * <p>
+ * The implementation of a {@code PropertyResourceBundle} subclass must be
+ * thread-safe if it's simultaneously used by multiple threads. The default
+ * implementations of the non-abstract methods in this class are thread-safe.
+ *
+ * <p>
  * <strong>Note:</strong> PropertyResourceBundle can be constructed either
  * from an InputStream or a Reader, which represents a property file.
  * Constructing a PropertyResourceBundle instance from an InputStream requires

src/share/classes/java/util/ResourceBundle.java

@@ -184,6 +184,13 @@ import sun.util.locale.LocaleObjectCache;
  * subclass.  Your subclasses must override two methods: <code>handleGetObject</code>
  * and <code>getKeys()</code>.
  *
+ * <p>
+ * The implementation of a {@code ResourceBundle} subclass must be thread-safe
+ * if it's simultaneously used by multiple threads. The default implementations
+ * of the non-abstract methods in this class, and the methods in the direct
+ * known concrete subclasses {@code ListResourceBundle} and
+ * {@code PropertyResourceBundle} are thread-safe.
+ *
  * <h3>ResourceBundle.Control</h3>
  *
  * The {@link ResourceBundle.Control} class provides information necessary