[BAEL-9374] A Guide to @ClassTemplate in JUnit 5

 - implement class template that runs date-formatting tests under multiple locales

Author: martin-blazevic

testing-modules/junit-5-advanced-3/src/main/java/com/baeldung/classtemplate/extended/DateFormatter.java

@@ -0,0 +1,16 @@
+package com.baeldung.classtemplate.extended;
+
+import java.time.LocalDate;
+import java.time.format.DateTimeFormatter;
+import java.time.format.FormatStyle;
+import java.util.Locale;
+
+class DateFormatter {
+
+    public String format(LocalDate date) {
+        DateTimeFormatter formatter = DateTimeFormatter.ofLocalizedDate(FormatStyle.LONG)
+            .withLocale(Locale.getDefault());
+
+        return date.format(formatter);
+    }
+}

testing-modules/junit-5-advanced-3/src/test/java/com/baeldung/classtemplate/extended/DateFormatterLocaleUnitTest.java

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+package com.baeldung.classtemplate.extended;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.time.LocalDate;
+import java.time.format.DateTimeFormatter;
+import java.time.format.FormatStyle;
+import java.util.Locale;
+
+import org.junit.jupiter.api.ClassTemplate;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+@ClassTemplate
+@ExtendWith(DateLocaleClassTemplateProvider.class)
+class DateFormatterLocaleUnitTest {
+
+    private static final Logger LOG = LoggerFactory.getLogger(DateFormatterLocaleUnitTest.class);
+
+    private final DateFormatter formatter = new DateFormatter();
+
+    @Test
+    void givenDefaultLocale_whenFormattingDate_thenMatchesLocalizedOutput() {
+        LocalDate date = LocalDate.of(2025, 9, 30);
+
+        DateTimeFormatter expectedFormatter = DateTimeFormatter.ofLocalizedDate(FormatStyle.LONG)
+            .withLocale(Locale.getDefault());
+
+        String expected = date.format(expectedFormatter);
+        String formatted = formatter.format(date);
+
+        LOG.info("Locale: {}, Expected: {}, Formatted: {}", Locale.getDefault(), expected, formatted);
+
+        assertEquals(expected, formatted);
+    }
+
+}

testing-modules/junit-5-advanced-3/src/test/java/com/baeldung/classtemplate/extended/DateLocaleClassTemplateProvider.java

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+package com.baeldung.classtemplate.extended;
+
+import java.util.List;
+import java.util.Locale;
+import java.util.stream.Stream;
+
+import org.junit.jupiter.api.extension.ClassTemplateInvocationContext;
+import org.junit.jupiter.api.extension.ClassTemplateInvocationContextProvider;
+import org.junit.jupiter.api.extension.Extension;
+import org.junit.jupiter.api.extension.ExtensionContext;
+
+class DateLocaleClassTemplateProvider implements ClassTemplateInvocationContextProvider {
+
+    @Override
+    public boolean supportsClassTemplate(ExtensionContext context) {
+        return true;
+    }
+
+    @Override
+    public Stream<ClassTemplateInvocationContext> provideClassTemplateInvocationContexts(ExtensionContext context) {
+        return Stream.of(invocationContext(Locale.US), invocationContext(Locale.GERMANY), invocationContext(Locale.ITALY), invocationContext(Locale.JAPAN));
+    }
+
+    private ClassTemplateInvocationContext invocationContext(Locale locale) {
+        return new ClassTemplateInvocationContext() {
+
+            @Override
+            public String getDisplayName(int invocationIndex) {
+                return "Locale: " + locale.getDisplayName();
+            }
+
+            @Override
+            public List<Extension> getAdditionalExtensions() {
+                return List.of(new LocaleExtension(locale));
+            }
+        };
+    }
+}
+

testing-modules/junit-5-advanced-3/src/test/java/com/baeldung/classtemplate/extended/LocaleExtension.java

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+package com.baeldung.classtemplate.extended;
+
+import org.junit.jupiter.api.extension.BeforeEachCallback;
+import org.junit.jupiter.api.extension.AfterEachCallback;
+import org.junit.jupiter.api.extension.ExtensionContext;
+
+import java.util.Locale;
+
+class LocaleExtension implements BeforeEachCallback, AfterEachCallback {
+
+    private final Locale locale;
+    private Locale previous;
+
+    public LocaleExtension(Locale locale) {
+        this.locale = locale;
+    }
+
+    @Override
+    public void beforeEach(ExtensionContext context) {
+        previous = Locale.getDefault();
+        Locale.setDefault(locale);
+    }
+
+    @Override
+    public void afterEach(ExtensionContext context) {
+        Locale.setDefault(previous);
+    }
+}
+