Document CommandNotFoundMessageProvider

- Relates #778

spring-shell-docs/src/main/asciidoc/using-shell-customization-commandnotfound.adoc

@@ -0,0 +1,44 @@
+[[using-shell-customization-commandnotfound]]
+=== Command Not Found
+ifndef::snippets[:snippets: ../../test/java/org/springframework/shell/docs]
+
+On default a missing command is handled via `CommandNotFoundResultHandler`
+and outputs a simple message:
+
+====
+[source, text]
+----
+shell:>missing
+No command found for 'missing'
+----
+====
+
+Internally `CommandNotFoundResultHandler` is using `CommandNotFoundMessageProvider`
+which is a simple function taking a `ProviderContext` and returning a text
+message. Below is an example what a custom message provider might look like.
+
+====
+[source, java, indent=0]
+----
+include::{snippets}/CommandNotFoundSnippets.java[tag=custom-provider]
+----
+====
+
+It's possible to change this implementation by defining it as a bean.
+
+====
+[source, java, indent=0]
+----
+include::{snippets}/CommandNotFoundSnippets.java[tag=provider-bean-1]
+----
+====
+
+`CommandNotFoundResultHandler` is a functional interface so it can
+be writter as a lambda.
+
+====
+[source, java, indent=0]
+----
+include::{snippets}/CommandNotFoundSnippets.java[tag=provider-bean-2]
+----
+====

spring-shell-docs/src/main/asciidoc/using-shell-customization.adoc

@@ -6,3 +6,5 @@ This section describes how you can customize the shell.
 include::using-shell-customization-styling.adoc[]
 
 include::using-shell-customization-logging.adoc[]
+
+include::using-shell-customization-commandnotfound.adoc[]

spring-shell-docs/src/test/java/org/springframework/shell/docs/CommandNotFoundSnippets.java

@@ -0,0 +1,63 @@
+/*
+ * Copyright 2023 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.docs;
+
+import java.util.List;
+import java.util.Map;
+
+import org.springframework.context.annotation.Bean;
+import org.springframework.shell.command.CommandRegistration;
+import org.springframework.shell.result.CommandNotFoundMessageProvider;
+
+@SuppressWarnings("unused")
+class CommandNotFoundSnippets {
+
+	// tag::custom-provider[]
+	class CustomProvider implements CommandNotFoundMessageProvider {
+
+		@Override
+		public String apply(ProviderContext context) {
+			// parsed commands without options
+			List<String> commands = context.commands();
+			// actual error, usually CommandNotFound exception
+			Throwable error = context.error();
+			// access to registrations at this time
+			Map<String, CommandRegistration> registrations = context.registrations();
+			// raw text input from a user
+			String text = context.text();
+			return "My custom message";
+		}
+	}
+	// end::custom-provider[]
+
+	class Dump1 {
+
+		// tag::provider-bean-1[]
+		@Bean
+		CommandNotFoundMessageProvider provider1() {
+			return new CustomProvider();
+		}
+		// end::provider-bean-1[]
+
+		// tag::provider-bean-2[]
+		@Bean
+		CommandNotFoundMessageProvider provider2() {
+			return ctx -> "My custom message";
+		}
+		// end::provider-bean-2[]
+	}
+
+}