Document default message handling in MessageSource.getMessage variants

spring-context/src/main/java/org/springframework/context/MessageSource.java

 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2019 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.
@@ -26,10 +26,10 @@ import org.springframework.lang.Nullable;
  *
  * <p>Spring provides two out-of-the-box implementations for production:
  * <ul>
- * <li>{@link org.springframework.context.support.ResourceBundleMessageSource},
- * built on top of the standard {@link java.util.ResourceBundle}
- * <li>{@link org.springframework.context.support.ReloadableResourceBundleMessageSource},
- * being able to reload message definitions without restarting the VM
+ * <li>{@link org.springframework.context.support.ResourceBundleMessageSource}: built
+ * on top of the standard {@link java.util.ResourceBundle}, sharing its limitations.
+ * <li>{@link org.springframework.context.support.ReloadableResourceBundleMessageSource}:
+ * highly configurable, in particular with respect to reloading message definitions.
  * </ul>
  *
  * @author Rod Johnson
@@ -41,16 +41,17 @@ public interface MessageSource {
 
 	/**
 	 * Try to resolve the message. Return default message if no message was found.
-	 * @param code the code to lookup up, such as 'calculator.noRateSet'. Users of
-	 * this class are encouraged to base message names on the relevant fully
-	 * qualified class name, thus avoiding conflict and ensuring maximum clarity.
+	 * @param code the code to lookup up, e.g. 'calculator.noRateSet'.
+	 * MessageSource users are encouraged to base message names on qualified class
+	 * or package names, avoiding potential conflicts and ensuring maximum clarity.
 	 * @param args an array of arguments that will be filled in for params within
 	 * the message (params look like "{0}", "{1,date}", "{2,time}" within a message),
-	 * or {@code null} if none.
+	 * or {@code null} if none
 	 * @param defaultMessage a default message to return if the lookup fails
 	 * @param locale the locale in which to do the lookup
-	 * @return the resolved message if the lookup was successful;
-	 * otherwise the default message passed as a parameter
+	 * @return the resolved message if the lookup was successful, otherwise
+	 * the default message passed as a parameter (which may be {@code null})
+	 * @see #getMessage(MessageSourceResolvable, Locale)
 	 * @see java.text.MessageFormat
 	 */
 	@Nullable
@@ -58,13 +59,16 @@ public interface MessageSource {
 
 	/**
 	 * Try to resolve the message. Treat as an error if the message can't be found.
-	 * @param code the code to lookup up, such as 'calculator.noRateSet'
+	 * @param code the code to lookup up, e.g. 'calculator.noRateSet'.
+	 * MessageSource users are encouraged to base message names on qualified class
+	 * or package names, avoiding potential conflicts and ensuring maximum clarity.
 	 * @param args an array of arguments that will be filled in for params within
 	 * the message (params look like "{0}", "{1,date}", "{2,time}" within a message),
-	 * or {@code null} if none.
+	 * or {@code null} if none
 	 * @param locale the locale in which to do the lookup
-	 * @return the resolved message
-	 * @throws NoSuchMessageException if the message wasn't found
+	 * @return the resolved message (never {@code null})
+	 * @throws NoSuchMessageException if no corresponding message was found
+	 * @see #getMessage(MessageSourceResolvable, Locale)
 	 * @see java.text.MessageFormat
 	 */
 	String getMessage(String code, @Nullable Object[] args, Locale locale) throws NoSuchMessageException;
@@ -76,9 +80,15 @@ public interface MessageSource {
 	 * since at the time of calling this method we aren't able to determine if the
 	 * {@code defaultMessage} property of the resolvable is {@code null} or not.
 	 * @param resolvable the value object storing attributes required to resolve a message
+	 * (may include a default message)
 	 * @param locale the locale in which to do the lookup
-	 * @return the resolved message
-	 * @throws NoSuchMessageException if the message wasn't found
+	 * @return the resolved message (never {@code null} since even a
+	 * {@code MessageSourceResolvable}-provided default message needs to be non-null)
+	 * @throws NoSuchMessageException if no corresponding message was found
+	 * (and no default message was provided by the {@code MessageSourceResolvable})
+	 * @see MessageSourceResolvable#getCodes()
+	 * @see MessageSourceResolvable#getArguments()
+	 * @see MessageSourceResolvable#getDefaultMessage()
 	 * @see java.text.MessageFormat
 	 */
 	String getMessage(MessageSourceResolvable resolvable, Locale locale) throws NoSuchMessageException;