BAEL-4209: Difference between e.getMessage() and e.getLocalizedMessage() (#10222)

* - initial commit of sample code

* BAEL-4209:  Moving the localized exception module into core-java-exceptions-3.

* BAEL-4209: Removed the old files for localizing exception messages.

core-java-modules/core-java-exceptions-3/src/main/java/com/baeldung/exceptions/localization/LocalizedException.java

@@ -0,0 +1,47 @@
+package com.baeldung.exceptions.localization;
+
+import java.util.Locale;
+
+public class LocalizedException extends Exception {
+    
+    private static final long serialVersionUID = 1L;
+
+    private final String messageKey;
+    private final Locale locale;
+
+    public LocalizedException(String messageKey) {
+        this(messageKey, Locale.getDefault());
+    }
+
+    public LocalizedException(String messageKey, Locale locale) {
+        this.messageKey = messageKey;
+        this.locale = locale;
+    }
+
+    /**
+     * @return a localized message based on the messageKey provided at instantiation.
+     */
+    public String getMessage() {
+
+        /*
+         * This is a deliberate role reversal of the default implementation of getLocalizedMessage.
+         * some logging frameworks like Log4J 1 & 2 and Logback will use getMessage instead of
+         * getLocalizedMessage when logging Throwables. If we want to use these frameworks in client
+         * applications to log localized messages, then we'll need to override getMessage in a
+         * similar fashion to return the appropriate content.  Or, you can call getLocalizedMessage
+         * on your own to create the log content.
+         */
+        return getLocalizedMessage();
+    }
+
+    /**
+     * @return a localized message based on the messageKey provided at instantiation.
+     */
+    public String getLocalizedMessage() {
+
+        /*
+         * java.util.logging uses getLocalizedMessage when logging Throwables.
+         */
+        return Messages.getMessageForLocale(messageKey, locale);
+    }
+}

core-java-modules/core-java-exceptions-3/src/main/java/com/baeldung/exceptions/localization/Messages.java

@@ -0,0 +1,27 @@
+package com.baeldung.exceptions.localization;
+
+import java.util.Locale;
+import java.util.ResourceBundle;
+
+public class Messages {
+
+    /**
+     * Retrieves the value for the messageKey from the locale-specific messages.properties, or from
+     * the base messages.properties for unsupported locales.
+     *
+     * @param messageKey The key for the message in the messages.properties ResourceBundle.
+     * @param locale The locale to search the message key.
+     * @return The value defined for the messageKey in the provided locale.
+     */
+    public static String getMessageForLocale(String messageKey, Locale locale) {
+
+        /*
+         * For more complex implementations, you will want a var-args parameter for MessageFormat
+         * substitutions. Then we can read the value from the bundle and pass the value with the
+         * substitutions to MessageFormat to create the final message value.
+         */
+        return ResourceBundle.getBundle("messages", locale)
+            .getString(messageKey);
+    }
+
+}

core-java-modules/core-java-exceptions-3/src/main/resources/messages.properties

@@ -0,0 +1 @@
+message.exception = I am an exception.

core-java-modules/core-java-exceptions-3/src/main/resources/messages_fr.properties

@@ -0,0 +1 @@
+message.exception = Je suis une exception.

core-java-modules/core-java-exceptions-3/src/test/java/com/baeldung/exceptions/localization/LocalizedExceptionUnitTest.java

@@ -0,0 +1,61 @@
+package com.baeldung.exceptions.localization;
+
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Test;
+
+import java.util.Locale;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class LocalizedExceptionUnitTest {
+
+    private Locale originalDefaultLocale;
+
+    @Before
+    public void saveOriginalDefaultLocale() {
+        originalDefaultLocale = Locale.getDefault();
+    }
+
+    @After
+    public void restoreOriginalDefaultLocale() {
+        Locale.setDefault(originalDefaultLocale);
+    }
+
+    @Test
+    public void givenUsEnglishDefaultLocale_whenLocalizingMessage_thenMessageComesFromDefaultMessages() {
+        Locale.setDefault(Locale.US);
+
+        LocalizedException localizedException = new LocalizedException("message.exception");
+        String usEnglishLocalizedExceptionMessage = localizedException.getLocalizedMessage();
+
+        assertThat(usEnglishLocalizedExceptionMessage).isEqualTo("I am an exception.");
+    }
+
+    @Test
+    public void givenFranceFrenchDefaultLocale_whenLocalizingMessage_thenMessageComesFromFrenchTranslationMessages() {
+        Locale.setDefault(Locale.FRANCE);
+
+        LocalizedException localizedException = new LocalizedException("message.exception");
+        String franceFrenchLocalizedExceptionMessage = localizedException.getLocalizedMessage();
+
+        assertThat(franceFrenchLocalizedExceptionMessage).isEqualTo("Je suis une exception.");
+    }
+
+    @Test
+    public void givenUsEnglishProvidedLocale_whenLocalizingMessage_thenMessageComesFromDefaultMessage() {
+        LocalizedException localizedException = new LocalizedException("message.exception", Locale.US);
+        String usEnglishLocalizedExceptionMessage = localizedException.getLocalizedMessage();
+
+        assertThat(usEnglishLocalizedExceptionMessage).isEqualTo("I am an exception.");
+    }
+
+    @Test
+    public void givenFranceFrenchProvidedLocale_whenLocalizingMessage_thenMessageComesFromFrenchTranslationMessages() {
+        LocalizedException localizedException = new LocalizedException("message.exception", Locale.FRANCE);
+        String franceFrenchLocalizedExceptionMessage = localizedException.getLocalizedMessage();
+
+        assertThat(franceFrenchLocalizedExceptionMessage).isEqualTo("Je suis une exception.");
+    }
+
+}

core-java-modules/core-java-exceptions-3/src/test/java/com/baeldung/exceptions/localization/MessagesUnitTest.java

@@ -0,0 +1,25 @@
+package com.baeldung.exceptions.localization;
+
+import org.junit.Test;
+
+import java.util.Locale;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class MessagesUnitTest {
+
+    @Test
+    public void givenUsEnglishLocale_whenRetrievingMessage_thenEnglishTranslationIsReturned() {
+        String translatedMessage = Messages.getMessageForLocale("message.exception", Locale.US);
+
+        assertThat(translatedMessage).isEqualTo("I am an exception.");
+    }
+
+    @Test
+    public void givenFranceFrenchLocale_whenRetrievingMessage_thenFrenchTranslationIsReturned() {
+        String translatedMessage = Messages.getMessageForLocale("message.exception", Locale.FRANCE);
+
+        assertThat(translatedMessage).isEqualTo("Je suis une exception.");
+    }
+
+}