Improve error reporting when non mandatory options are misconfigured

spring-shell-core/src/main/java/org/springframework/shell/core/command/annotation/support/CommandFactoryBean.java

@@ -53,6 +53,7 @@
  * @author Janne Valkealahti
  * @author Piotr Olaszewski
  * @author Mahmoud Ben Hassine
+ * @author Jay Choi
  */
 public class CommandFactoryBean implements ApplicationContextAware, FactoryBean<Command> {
 
@@ -127,6 +128,11 @@ private List<CommandOption> getCommandOptions() {
 							"Either shortName or longName (or both) must be provided for option on parameter '"
 									+ parameter.getName() + "'");
 				}
+				if (!required && defaultValue.isEmpty()) {
+					String optionName = longName.isEmpty() ? "-" + shortName : "--" + longName;
+					throw new IllegalArgumentException("Optional option '" + optionName + "' on parameter '"
+							+ parameter.getName() + "' must have a default value.");
+				}
 				CommandOption commandOption = CommandOption.with()
 					.shortName(shortName)
 					.longName(longName)

spring-shell-core/src/test/java/org/springframework/shell/core/command/annotation/support/CommandFactoryBeanTests.java

@@ -0,0 +1,103 @@
+/*
+ * Copyright 2026-present 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.shell.core.command.annotation.support;
+
+import java.lang.reflect.Method;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.context.ApplicationContext;
+import org.springframework.shell.core.command.annotation.Command;
+import org.springframework.shell.core.command.annotation.Option;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+/**
+ * Tests for {@link CommandFactoryBean}
+ *
+ * @author Jay Choi
+ */
+class CommandFactoryBeanTests {
+
+	private ApplicationContext context;
+
+	@BeforeEach
+	void setUp() {
+		context = mock(ApplicationContext.class);
+		TestCommands testCommands = new TestCommands();
+		when(context.getBean(TestCommands.class)).thenReturn(testCommands);
+	}
+
+	@Test
+	void optionalOptionWithoutDefaultValueShouldFail() throws Exception {
+		// given
+		Method method = TestCommands.class.getMethod("optionalWithoutDefault", String.class);
+		CommandFactoryBean factoryBean = createFactoryBean(method);
+
+		// when & then
+		IllegalArgumentException exception = assertThrows(IllegalArgumentException.class, factoryBean::getObject);
+		assertTrue(exception.getMessage().contains("must have a default value"));
+	}
+
+	@Test
+	void optionalOptionWithDefaultValueShouldSucceed() throws Exception {
+		// given
+		Method method = TestCommands.class.getMethod("optionalWithDefault", String.class);
+		CommandFactoryBean factoryBean = createFactoryBean(method);
+
+		// when & then
+		assertDoesNotThrow(factoryBean::getObject);
+	}
+
+	@Test
+	void requiredOptionWithoutDefaultValueShouldSucceed() throws Exception {
+		// given
+		Method method = TestCommands.class.getMethod("requiredWithoutDefault", String.class);
+		CommandFactoryBean factoryBean = createFactoryBean(method);
+
+		// when & then
+		assertDoesNotThrow(factoryBean::getObject);
+	}
+
+	private CommandFactoryBean createFactoryBean(Method method) {
+		CommandFactoryBean factoryBean = new CommandFactoryBean(method);
+		factoryBean.setApplicationContext(context);
+		return factoryBean;
+	}
+
+	// Test command class
+	static class TestCommands {
+
+		@Command(name = "test1")
+		public void optionalWithoutDefault(@Option(longName = "name") String name) {
+		}
+
+		@Command(name = "test2")
+		public void optionalWithDefault(@Option(longName = "name", defaultValue = "World") String name) {
+		}
+
+		@Command(name = "test3")
+		public void requiredWithoutDefault(@Option(longName = "name", required = true) String name) {
+		}
+
+	}
+
+}