Improve documentation of custom implementations for parameterized tests

This commit documents that custom implementations of APIs must be
either top-level or static nested classes.

This commit also ensures code listings for example implementations are
separated if appropriate.

documentation/src/docs/asciidoc/user-guide/writing-tests.adoc

@@ -983,13 +983,20 @@ primitive type.
 [[writing-tests-parameterized-tests-sources-ArgumentsSource]]
 ===== @ArgumentsSource
 
-`@ArgumentsSource` can be used to specify a custom, reusable `ArgumentsProvider`.
+`@ArgumentsSource` can be used to specify a custom, reusable `ArgumentsProvider`. Note
+that an implementation of `ArgumentsProvider` must be declared as either a top-level
+class or as a `static` nested class.
 
 [source,java,indent=0]
 ----
 include::{testDir}/example/ParameterizedTestDemo.java[tags=ArgumentsSource_example]
 ----
 
+[source,java,indent=0]
+----
+include::{testDir}/example/ParameterizedTestDemo.java[tags=ArgumentsProvider_example]
+----
+
 
 [[writing-tests-parameterized-tests-argument-conversion]]
 ==== Argument Conversion
@@ -1072,7 +1079,8 @@ method_ or a _factory constructor_ as defined below.
   accepts a single `String` argument and returns an instance of the target type. The name
   of the method can be arbitrary and need not follow any particular convention.
 - __factory constructor__: a non-private constructor in the target type that accepts a
-  single `String` argument.
+  single `String` argument. Note that the target type must be declared as either a
+  top-level class or as a `static` nested class.
 
 NOTE: If multiple _factory methods_ are discovered, they will be ignored. If a _factory
 method_ and a _factory constructor_ are discovered, the factory method will be used
@@ -1087,18 +1095,29 @@ as the title of the book.
 include::{testDir}/example/ParameterizedTestDemo.java[tags=implicit_fallback_conversion_example]
 ----
 
+[source,java,indent=0]
+----
+include::{testDir}/example/ParameterizedTestDemo.java[tags=implicit_fallback_conversion_example_Book]
+----
+
 [[writing-tests-parameterized-tests-argument-conversion-explicit]]
 ===== Explicit Conversion
 
 Instead of relying on implicit argument conversion you may explicitly specify an
 `ArgumentConverter` to use for a certain parameter using the `@ConvertWith` annotation
-like in the following example.
+like in the following example. Note that an implementation of `ArgumentConverter` must be
+declared as either a top-level class or as a `static` nested class.
 
 [source,java,indent=0]
 ----
 include::{testDir}/example/ParameterizedTestDemo.java[tags=explicit_conversion_example]
 ----
 
+[source,java,indent=0]
+----
+include::{testDir}/example/ParameterizedTestDemo.java[tags=explicit_conversion_example_ToStringArgumentConverter]
+----
+
 Explicit argument converters are meant to be implemented by test and extension authors.
 Thus, `junit-jupiter-params` only provides a single explicit argument converter that may
 also serve as a reference implementation: `JavaTimeArgumentConverter`. It is used via the
@@ -1139,13 +1158,20 @@ _aggregators_.
 To use a custom aggregator simply implement the `{ArgumentsAggregator}` interface and
 register it via the `@AggregateWith` annotation on a compatible parameter in the
 `@ParameterizedTest` method. The result of the aggregation will then be provided as an
-argument for the corresponding parameter when the parameterized test is invoked.
+argument for the corresponding parameter when the parameterized test is invoked. Note
+that an implementation of `ArgumentsAggregator` must be declared as either a top-level
+class or as a `static` nested class.
 
 [source,java,indent=0]
 ----
 include::{testDir}/example/ParameterizedTestDemo.java[tags=ArgumentsAggregator_example]
 ----
 
+[source,java,indent=0]
+----
+include::{testDir}/example/ParameterizedTestDemo.java[tags=ArgumentsAggregator_example_PersonAggregator]
+----
+
 If you find yourself repeatedly declaring `@AggregateWith(MyTypeAggregator.class)` for
 multiple parameterized test methods across your codebase, you may wish to create a custom
 _composed annotation_ such as `@CsvToMyType` that is meta-annotated with
@@ -1157,6 +1183,11 @@ action with a custom `@CsvToPerson` annotation.
 include::{testDir}/example/ParameterizedTestDemo.java[tags=ArgumentsAggregator_with_custom_annotation_example]
 ----
 
+[source,java,indent=0]
+----
+include::{testDir}/example/ParameterizedTestDemo.java[tags=ArgumentsAggregator_with_custom_annotation_example_CsvToPerson]
+----
+
 
 [[writing-tests-parameterized-tests-display-names]]
 ==== Customizing Display Names

documentation/src/test/java/example/ParameterizedTestDemo.java

@@ -201,15 +201,15 @@ void testWithArgumentsSource(String argument) {
 
 	// end::ArgumentsSource_example[]
 	static
-	// tag::ArgumentsSource_example[]
+	// tag::ArgumentsProvider_example[]
 	public class MyArgumentsProvider implements ArgumentsProvider {
 
 		@Override
 		public Stream<? extends Arguments> provideArguments(ExtensionContext context) {
 			return Stream.of("foo", "bar").map(Arguments::of);
 		}
 	}
-	// end::ArgumentsSource_example[]
+	// end::ArgumentsProvider_example[]
 
 	// tag::ParameterResolver_example[]
 	@BeforeEach
@@ -246,7 +246,7 @@ void testWithImplicitFallbackArgumentConversion(Book book) {
 
 	// end::implicit_fallback_conversion_example[]
 	static
-	// tag::implicit_fallback_conversion_example[]
+	// tag::implicit_fallback_conversion_example_Book[]
 	public class Book {
 
 		private final String title;
@@ -263,7 +263,7 @@ public String getTitle() {
 			return this.title;
 		}
 	}
-	// end::implicit_fallback_conversion_example[]
+	// end::implicit_fallback_conversion_example_Book[]
 
 	// @formatter:off
 	// tag::explicit_conversion_example[]
@@ -277,7 +277,7 @@ void testWithExplicitArgumentConversion(
 
 	// end::explicit_conversion_example[]
 	static
-	// tag::explicit_conversion_example[]
+	// tag::explicit_conversion_example_ToStringArgumentConverter[]
 	public class ToStringArgumentConverter extends SimpleArgumentConverter {
 
 		@Override
@@ -286,7 +286,7 @@ protected Object convert(Object source, Class<?> targetType) {
 			return String.valueOf(source);
 		}
 	}
-	// end::explicit_conversion_example[]
+	// end::explicit_conversion_example_ToStringArgumentConverter[]
 
 	// tag::explicit_java_time_converter[]
 	@ParameterizedTest
@@ -337,7 +337,7 @@ void testWithArgumentsAggregator(@AggregateWith(PersonAggregator.class) Person p
 
     // end::ArgumentsAggregator_example[]
     static
-    // tag::ArgumentsAggregator_example[]
+    // tag::ArgumentsAggregator_example_PersonAggregator[]
     public class PersonAggregator implements ArgumentsAggregator {
         @Override
         public Person aggregateArguments(ArgumentsAccessor arguments, ParameterContext context) {
@@ -347,7 +347,7 @@ public Person aggregateArguments(ArgumentsAccessor arguments, ParameterContext c
                               arguments.get(3, LocalDate.class));
         }
     }
-    // end::ArgumentsAggregator_example[]
+    // end::ArgumentsAggregator_example_PersonAggregator[]
 	// @formatter:on
 
 	// @formatter:off
@@ -360,13 +360,15 @@ public Person aggregateArguments(ArgumentsAccessor arguments, ParameterContext c
     void testWithCustomAggregatorAnnotation(@CsvToPerson Person person) {
         // perform assertions against person
     }
+    // end::ArgumentsAggregator_with_custom_annotation_example[]
 
+    // tag::ArgumentsAggregator_with_custom_annotation_example_CsvToPerson[]
     @Retention(RetentionPolicy.RUNTIME)
     @Target(ElementType.PARAMETER)
     @AggregateWith(PersonAggregator.class)
     public @interface CsvToPerson {
     }
-    // end::ArgumentsAggregator_with_custom_annotation_example[]
+    // end::ArgumentsAggregator_with_custom_annotation_example_CsvToPerson[]
 	// @formatter:on
 
 	// tag::custom_display_names[]