Add support for lists with nested structure

Previously, a nested structure of a Maven Plugin configuration can be
only configured for a single parameter. This commit adds a dedicated
method to add a new entry with a nested structure every time it is
called. This effectively allows to configure nested "list" structure
such as listing dependency exclusions.

Closes gh-1165

initializr-generator/src/main/java/io/spring/initializr/generator/buildsystem/maven/MavenPlugin.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.
@@ -284,12 +284,29 @@ public class MavenPlugin {
 		}
 
 		/**
-		 * Configure the parameter with the specified {@code name}.
+		 * Add the specified parameter with a nested structure.
+		 * @param name the name of the parameter
+		 * @param consumer a consumer to further configure the parameter
+		 * @return this for method chaining
+		 * @see #configure(String, Consumer) name
+		 */
+		public ConfigurationBuilder add(String name, Consumer<ConfigurationBuilder> consumer) {
+			ConfigurationBuilder nestedConfiguration = new ConfigurationBuilder();
+			consumer.accept(nestedConfiguration);
+			this.settings.add(new Setting(name, nestedConfiguration));
+			return this;
+		}
+
+		/**
+		 * Configure the parameter with the specified {@code name}. If no parameter with
+		 * that name exists, it is created. If the parameter already exists, the consumer
+		 * can be used to further tune the nested structure.
 		 * @param name the name of the parameter
 		 * @param consumer a consumer to further configure the parameter
 		 * @return this for method chaining
 		 * @throws IllegalArgumentException if a parameter with the same name is
 		 * registered with a single value
+		 * @see #add(String, Consumer)
 		 */
 		public ConfigurationBuilder configure(String name, Consumer<ConfigurationBuilder> consumer) {
 			Object value = this.settings.stream().filter((candidate) -> candidate.getName().equals(name)).findFirst()

initializr-generator/src/test/java/io/spring/initializr/generator/buildsystem/maven/MavenPluginTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.
@@ -54,6 +54,35 @@ class MavenPluginTests {
 				"false");
 	}
 
+	@SuppressWarnings("unchecked")
+	@Test
+	void configurationParameterWithNestedValuesCanBeAdded() {
+		MavenPlugin plugin = plugin("com.example", "test-plugin")
+				.configuration((configuration) -> configuration.configure("items", (items) -> {
+					items.add("item", (firstItem) -> firstItem.add("name", "one"));
+					items.add("item", (secondItem) -> secondItem.add("name", "two"));
+				})).build();
+		assertThat(plugin.getConfiguration().getSettings()).hasSize(1);
+		Setting setting = plugin.getConfiguration().getSettings().get(0);
+		assertThat(setting.getName()).isEqualTo("items");
+		assertThat(setting.getValue()).isInstanceOf(List.class);
+		List<Setting> values = (List<Setting>) setting.getValue();
+		assertThat(values.stream().map(Setting::getName)).containsExactly("item", "item");
+		assertThat(values.stream().map(Setting::getValue)).anySatisfy((value) -> {
+			assertThat(value).isInstanceOf(List.class);
+			List<Setting> itemValues = (List<Setting>) value;
+			assertThat(itemValues.stream().map(Setting::getName)).containsExactly("name");
+			assertThat(itemValues.stream().map(Setting::getValue)).containsExactly("one");
+		});
+		assertThat(values.stream().map(Setting::getValue)).anySatisfy((value) -> {
+			assertThat(value).isInstanceOf(List.class);
+			List<Setting> itemValues = (List<Setting>) value;
+			assertThat(itemValues.stream().map(Setting::getName)).containsExactly("name");
+			assertThat(itemValues.stream().map(Setting::getValue)).containsExactly("two");
+		});
+		assertThat(values).hasSize(2);
+	}
+
 	@Test
 	@SuppressWarnings("unchecked")
 	void configurationParameterWithNestedValuesCanBeCustomized() {