From 1871c9626de328758b74e14e0f993ce121c4fe92 Mon Sep 17 00:00:00 2001 From: David Harrigan Date: Tue, 12 Sep 2017 14:36:52 +0100 Subject: [PATCH] Kotlin documentation readability improvements Issue: SPR-15659 --- src/docs/asciidoc/kotlin.adoc | 220 ++++++++++++++++++---------------- 1 file changed, 117 insertions(+), 103 deletions(-) diff --git a/src/docs/asciidoc/kotlin.adoc b/src/docs/asciidoc/kotlin.adoc index 6a79d310b1..a91808a990 100644 --- a/src/docs/asciidoc/kotlin.adoc +++ b/src/docs/asciidoc/kotlin.adoc @@ -8,12 +8,12 @@ == Introduction https://kotlinlang.org[Kotlin] is a statically-typed language targeting the JVM (and other platforms) -which allows to write concise and elegant code while providing a very good -https://kotlinlang.org/docs/reference/java-interop.html[interoperability] with libraries -written in Java. +which allows writing concise and elegant code while providing a very good +https://kotlinlang.org/docs/reference/java-interop.html[interoperability] with +existing libraries written in Java. Spring Framework 5 introduces first-class support for Kotlin and allows developers to write -Spring + Kotlin applications almost like if Spring Framework was a native Kotlin framework. +Spring + Kotlin applications almost as if the Spring Framework was a native Kotlin framework. == Requirements == @@ -22,47 +22,47 @@ https://bintray.com/bintray/jcenter/org.jetbrains.kotlin%3Akotlin-stdlib[`kotlin (or one of its https://bintray.com/bintray/jcenter/org.jetbrains.kotlin%3Akotlin-stdlib-jre7[`kotlin-stdlib-jre7`] / https://bintray.com/bintray/jcenter/org.jetbrains.kotlin%3Akotlin-stdlib-jre8[`kotlin-stdlib-jre8`] variants) and https://bintray.com/bintray/jcenter/org.jetbrains.kotlin%3Akotlin-reflect[`kotlin-reflect`] -to be present on the classpath. They are provided by default if you bootstrap a Kotlin project on +to be present on the classpath. They are provided by default if one bootstraps a Kotlin project on https://start.spring.io/#!language=kotlin[start.spring.io]. == Extensions Thanks to its great https://kotlinlang.org/docs/reference/java-interop.html[Java interoperability] and to https://kotlinlang.org/docs/reference/extensions.html[Kotlin extensions], Spring -Framework Kotlin API is leveraging the regular Java's one, completed by a few Kotlin specific API -available out of the box in Spring Framework artifacts. +Framework Kotlin APIs leverage regular Java APIs and are additionally enhanced by a few Kotlin specific APIs +available out of the box within Spring Framework 5 artifacts. {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/[Spring Framework KDoc API] lists -and documents all the Kotlin extensions and DSL available. +and documents all the Kotlin extensions and DSLs available. [NOTE] ==== -Keep in mind that Kotlin extensions need to be imported to be used. That means for example that -`GenericApplicationContext.registerBean` Kotlin extension will be available only if -`import org.springframework.context.support.registerBean` is present in your imports. -That said, like with static imports, your IDE should automatically suggest them in most cases. +Keep in mind that Kotlin extensions need to be imported to be used. This means +for example that the `GenericApplicationContext.registerBean` Kotlin extension +will only be available if `import org.springframework.context.support.registerBean` is imported. +That said, similar to static imports, an IDE should automatically suggest the import in most cases. ==== For example, https://kotlinlang.org/docs/reference/inline-functions.html#reified-type-parameters[Kotlin reified type parameters] provide a workaround for JVM https://docs.oracle.com/javase/tutorial/java/generics/erasure.html[generics type erasure], and Spring Framework provides some extensions to take advantage of this feature. -That allows to provide a better Kotlin API `RestTemplate`, the new `WebClient` from Spring -WebFlux and for various other API. +This allows for a better Kotlin API `RestTemplate`, the new `WebClient` from Spring +WebFlux and for various other APIs. [NOTE] ==== -Other libraries like Reactor or Spring Data also provide Kotlin extensions for their API -in order to allow a better Kotlin development experience. +Other libraries like Reactor and Spring Data also provide Kotlin extensions +for their APIs, thus giving a better Kotlin development experience overall. ==== -To retrieve a list of `Foo` objects in Java you have to write: +To retrieve a list of `Foo` objects in Java, one would normally write: [source,java] ---- Flux users = client.get().retrieve().bodyToFlux(User.class) ---- -While in Kotlin with Spring Framework extensions, you are able to write: +Whilst with Kotlin and Spring Framework extensions, one is able to write: [source,kotlin] ---- @@ -71,19 +71,19 @@ val users = client.get().retrieve().bodyToFlux() val users : Flux = client.get().retrieve().bodyToFlux() ---- -Like in Java, `users` in Kotlin is strongly typed, but Kotlin clever type inference allows -shorter syntax. +As in Java, `users` in Kotlin is strongly typed, but Kotlin's clever type inference allows +for a shorter syntax. == Null-safety One of Kotlin's key features is https://kotlinlang.org/docs/reference/null-safety.html[null-safety] -which allows to deal with `null` values at compile time rather than bumping into the famous -`NullPointerException` at runtime. This makes your applications safer through clean nullability -declarations, expressing "value or no value" semantics without paying the cost of wrapper like `Optional`. +which cleanly deals with `null` values at compile time rather than bumping into the famous +`NullPointerException` at runtime. This makes applications safer through nullability +declarations and expressing "value or no value" semantics without paying the cost of wrappers like `Optional`. (Kotlin allows using functional constructs with nullable values; check out this http://www.baeldung.com/kotlin-null-safety[comprehensive guide to Kotlin null-safety].) -Although Java does not allow to express null-safety in its type-system, Spring Framework now +Although Java does not allow one to express null-safety in its type-system, Spring Framework now provides https://jira.spring.io/browse/SPR-15540[null-safety of the whole Spring Framework API] via tooling-friendly annotations: @@ -91,16 +91,16 @@ via tooling-friendly annotations: * `@Nullable` annotations where specific parameters or return values can be `null`. Both annotations are meta-annotated with https://jcp.org/en/jsr/detail?id=305[JSR 305] -meta-annotations (a dormant JSR but supported by tools like IDEA, Findbugs, etc.) +annotations (a dormant JSR but supported by tools like IDEA, Findbugs, etc.) to provide useful warnings to Java developers. -On the Kotlin side - as of the https://blog.jetbrains.com/kotlin/2017/08/kotlin-1-1-4-is-out/[Kotlin 1.1.4 release] - +On the Kotlin side - as of https://blog.jetbrains.com/kotlin/2017/08/kotlin-1-1-4-is-out/[Kotlin 1.1.4 release] - these annotations https://github.com/Kotlin/KEEP/blob/jsr-305/proposals/jsr-305-custom-nullability-qualifiers.md[are recognized by Kotlin] -in order to provide null-safety for the whole Spring Framework API. That means you should -never have `NullPointerException` in your code when using Spring Framework and Kotlin because +in order to provide null-safety for the whole Spring Framework API. That means +one should never experience a `NullPointerException` when using Spring Framework and Kotlin because the compiler will not allow it. -For now, you need to use a `-Xjsr305-annotations=enable` flag (specified via the +For now, one needs to use a `-Xjsr305-annotations=enable` flag (specified via the `freeCompilerArgs` property with Maven or Gradle Kotlin plugins), but that should become the default behavior in an upcoming release of Kotlin. @@ -109,7 +109,7 @@ until Kotlin 1.1.5 is released (it will fix https://youtrack.jetbrains.com/issue [NOTE] ==== -Other libraries like Reactor or Spring Data leverage these annotations to provide +Other libraries like Reactor and Spring Data leverage these annotations to provide null-safe APIs for Kotlin developers. ==== @@ -120,13 +120,13 @@ via primary constructors, immutable classes data binding and function optional p with default values. Kotlin parameter names are recognized via a dedicated `KotlinReflectionParameterNameDiscoverer` -which allows to find interface method parameter names without requiring Java 8 `-parameters` -compiler flag. +which allows finding interface method parameter names without requiring the Java 8 `-parameters` +compiler flag enabled during compliation. https://github.com/FasterXML/jackson-module-kotlin[Jackson Kotlin module] which is required -for serializing / deserializing JSON data is automatically registered when present in the -classpath, and a warning message will be logged if Jackson + Kotlin are detected without -Jackson Kotlin module. +for serializing / deserializing JSON data is automatically registered when +found in the classpath and a warning message will be logged if Jackson and Kotlin are +detected without the Jackson Kotlin module present. [NOTE] ==== @@ -136,12 +136,13 @@ As of Spring Boot 2.0, Jackson Kotlin module is automatically provided via the J == Annotations Spring Framework also takes advantage of https://kotlinlang.org/docs/reference/null-safety.html[Kotlin null-safety] -to determine if an HTTP parameter is required without having to define explicitly the `required` attribute. -That means `@RequestParam name: String?` with be treated as not required and `@RequestParam name: String` as required. -This is also supported on Spring Messaging `@Header` annotation. +to determine if a HTTP parameter is required without having to explicitly +define the `required` attribute. That means `@RequestParam name: String?` will be treated +as not required and conversely `@RequestParam name: String` as being required. +This feature is also supported on the Spring Messaging `@Header` annotation. In a similar fashion, Spring bean injection with `@Autowired` or `@Inject` uses this information -to know if a bean is required or not. `@Autowired lateinit var foo: Foo` implies that a bean +to determine if a bean is required or not. `@Autowired lateinit var foo: Foo` implies that a bean of type `Foo` must be registered in the application context while `@Autowired lateinit var foo: Foo?` won’t raise an error if such bean does not exist. @@ -150,20 +151,21 @@ won’t raise an error if such bean does not exist. Spring Framework 5 introduces a new way to register beans in a functional way using lambdas as an alternative to XML or JavaConfig (`@Configuration` and `@Bean`). In a nutshell, it makes it possible to register beans with a lambda that acts as a `FactoryBean`. -It is very efficient and does not require any reflection or CGLIB proxies. +This mechanism is very efficient as it does not require any reflection or CGLIB proxies. -In Java you will for example write: +In Java, one may for example write: [source,java] ---- GenericApplicationContext context = new GenericApplicationContext(); context.registerBean(Foo.class); context.registerBean(Bar.class, () -> new - Bar(context.getBean(Foo.class)) + Bar(context.getBean(Foo.class)) ); ---- -While in Kotlin, reified type parameters and `GenericApplicationContext` Kotlin extensions allow to simply write: +Whilst in Kotlin with reified type parameters and `GenericApplicationContext` +Kotlin extensions one can instead simply write: [source,kotlin] ---- @@ -175,8 +177,9 @@ val context = GenericApplicationContext().apply { In order to allow a more declarative approach and cleaner syntax, Spring Framework provides a {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/org.springframework.context.support/-bean-definition-dsl/[Kotlin bean definition DSL] -It declares an `ApplicationContextInitializer` via a clean declarative API which allows -you to deal with profiles and `Environment` for customizing how your beans are registered. +It declares an `ApplicationContextInitializer` via a clean declarative API +which enables one to deal with profiles and `Environment` for customizing +how beans are registered. [source,kotlin] ---- @@ -216,7 +219,7 @@ In this example, `Routes(ref(), ref())` is the equivalent of `Routes(ref()` is a shortcut for `applicationContext.getBean(UserHandler::class.java)`. -This `beans()` function can then be used to register beans on your application context. +This `beans()` function can then be used to register beans on the application context. [source,kotlin] ---- @@ -228,8 +231,8 @@ val context = GenericApplicationContext().apply { [NOTE] ==== -This DSL is programmatic, thus also allows custom registration logic of beans via `if` expression, -`for` loop or any other Kotlin constructs. +This DSL is programmatic, thus it allows custom registration logic of beans +via an `if` expression, a `for` loop or any other Kotlin constructs. ==== See https://github.com/sdeleuze/spring-kotlin-functional/blob/3d12ab102c28f4761bd6a0736e2f585713eb2243/src/main/kotlin/functional/Beans.kt[spring-kotlin-functional beans declaration] @@ -239,9 +242,9 @@ for a concrete example. ==== Spring Boot is based on Java Config and https://github.com/spring-projects/spring-boot/issues/8115[does not provide specific support for functional bean definition yet], -but you can experimentally use functional bean definitions via its `ApplicationContextInitializer` support, +but one can experimentally use functional bean definitions via Spring Boot's `ApplicationContextInitializer` support, see https://stackoverflow.com/questions/45935931/how-to-use-functional-bean-definition-kotlin-dsl-with-spring-boot-and-spring-w/46033685#46033685[this Stack Overflow answer] -for more details and up to date informations. +for more details and up-to-date information. ==== == Web @@ -250,7 +253,8 @@ for more details and up to date informations. Spring Framework now comes with a {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/org.springframework.web.reactive.function.server/-router-function-dsl/[Kotlin routing DSL] -that allows you to leverage the <> with clean and idiomatic Kotlin code: +that allows one to leverage the <> for writing clean and idiomatic Kotlin code: [source,kotlin] ---- @@ -274,9 +278,9 @@ router { [NOTE] ==== -This DSL is programmatic, thus also allows custom registration logic of beans via `if` expression, -`for` loop or any other Kotlin constructs. That can be useful when routes need to be registered -depending on dynamic data (from a database for example). +This DSL is programmatic, thus it allows custom registration logic of beans +via an `if` expression, a `for` loop or any other Kotlin constructs. That can be useful when routes need to be registered +depending on dynamic data (for example, from a database). ==== See https://github.com/mixitconf/mixit/tree/bad6b92bce6193f9b3f696af9d416c276501dbf1/src/main/kotlin/mixit/web/routes[MiXiT project routes] @@ -286,17 +290,19 @@ for a concrete example. As of version 4.3, Spring Framework provides a http://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/web/servlet/view/script/ScriptTemplateView.html[ScriptTemplateView] -to render templates using script engines that supports https://www.jcp.org/en/jsr/detail?id=223[JSR-223] -and Spring Framework 5 go even further by extending this feature to WebFlux and supporting +to render templates using script engines that supports +https://www.jcp.org/en/jsr/detail?id=223[JSR-223]. +Spring Framework 5 goes even further by extending this feature to WebFlux and supporting https://jira.spring.io/browse/SPR-15064[i18n and nested templates]. -Kotlin provides such support and allows to render Kotlin based templates, see +Kotlin provides similar support and allows the rendering of Kotlin based templates, see https://github.com/spring-projects/spring-framework/commit/badde3a479a53e1dd0777dd1bd5b55cb1021cf9e[this commit] for details. This enables some interesting use cases like writing type-safe templates using -https://github.com/Kotlin/kotlinx.html[kotlinx.html] DSL or simply Kotlin multiline `String` with interpolation. +https://github.com/Kotlin/kotlinx.html[kotlinx.html] DSL or simply using Kotlin multiline `String` with interpolation. -This can allow you to write this kind of templates with full autocompletion and refactoring support in your IDE: +This can allow one to write Kotlin templates with full autocompletion and +refactoring support in a supported IDE: [source,kotlin] ---- @@ -317,33 +323,35 @@ project for more details. == Spring projects in Kotlin -This section provides a focus on some specific hints and recommendations worth to know when -developing Spring projects in Kotlin. +This section provides a focus on some specific hints and recommendations worth +knowing when developing Spring projects in Kotlin. === Final by default By default, https://discuss.kotlinlang.org/t/classes-final-by-default/166[all classes in Kotlin are `final`]. The `open` modifier on a class is the opposite of Java's `final`: it allows others to -inherit from this class. Same for member functions that need to be marked as `open` to be overridden. +inherit from this class. This also applies to member functions, in that they need to be marked as `open` to +be overridden. -While Kotlin JVM-friendly design is generally frictionless with Spring, this specific point -can prevent your application to start if not taken in account because Spring beans proxified -with CGLIB - like `@Configuration` classes - need to be inherited at runtime for technical -reasons. +Whilst Kotlin's JVM-friendly design is generally frictionless with Spring, +this specific Kotlin feature can prevent the application from starting, if this fact is not taken in +consideration. This is because Spring beans are normally proxified with CGLIB +- such as `@Configuration` classes - which need to be inherited at runtime for technical reasons. -Before Kotlin 1.0.6, you needed to add an `open` keyword on each class and member -functions of Spring beans proxified with CGLIB like `@Configuration` classes. +Before Kotlin 1.0.6, one needed to add an `open` keyword on each class and member +functions of Spring beans proxified with CGLIB such as `@Configuration` classes. Fortunately, Kotlin 1.0.6+ now provides a https://kotlinlang.org/docs/reference/compiler-plugins.html#kotlin-spring-compiler-plugin[`kotlin-spring`] -plugin that open classes and their member functions by default for classes annotated or meta-annotated with one of the following annotation: +plugin that automatically opens classes and their member functions for types +annotated or meta-annotated with one of the following annotations: * `@Component` * `@Async` * `@Transactional` * `@Cacheable` -Meta-annotations support means that classes annotated with`@Configuration`, `@Controller`, +Meta-annotations support means that types annotated with `@Configuration`, `@Controller`, `@RestController`, `@Service` or `@Repository` are automatically opened since these annotations are meta-annotated with `@Component`. @@ -351,7 +359,7 @@ http://start.spring.io/#!language=kotlin[start.spring.io] enables it by default. === Injecting dependencies -Try to favor constructor injection with `val` read-only (and non-nullable when possible) +Our recommendation is to try and favor constructor injection with `val` read-only (and non-nullable when possible) https://kotlinlang.org/docs/reference/properties.html[properties]. [source,kotlin] @@ -365,12 +373,13 @@ class YourBean( [NOTE] ==== -As of Spring Framework 4.3, classes with a single constructor get its parameters +As of Spring Framework 4.3, classes with a single constructor have its parameters automatically autowired, that's why there is no need for `@Autowired constructor` -in the example above. +in the example shown above. ==== -If you really need to use field injection, use `lateinit var`: +If one really needs to use field injection, use the `lateinit var` construct, +i.e., [source,kotlin] ---- @@ -387,13 +396,14 @@ class YourBean { === Injecting configuration properties -In Java, you can inject configuration properties using annotations like `@Value("${property}")`, +In Java, one can inject configuration properties using annotations like `@Value("${property}")`, however in Kotlin `$` is a reserved character that is used for https://kotlinlang.org/docs/reference/idioms.html#string-interpolation[string interpolation]. -In order to use `@Value` in Kotlin, you have to escape the `$` character by writing `@Value("\${property}")`. +Therefore, if one wishes to use the `@Value` annotation in Kotlin, the `$` +character will need to be escaped by writing `@Value("\${property}")`. -As an alternative, you can also customize the properties placeholder prefix by declaring -the following beans in your configuration: +As an alternative, it is possible to customize the properties placeholder prefix by declaring +the following configuration beans: [source,kotlin] ---- @@ -403,8 +413,9 @@ fun propertyConfigurer() = PropertySourcesPlaceholderConfigurer().apply { } ---- -If you have any existing code (like Spring Boot actuators or `@LocalServerPort`) that is -using the `${...}` syntax, you should declare the following beans in your configuration: +Existing code (like Spring Boot actuators or `@LocalServerPort`) that +uses the `${...}` syntax, can be customised with configuration beans, like +this: [source,kotlin] ---- @@ -420,10 +431,11 @@ fun defaultPropertyConfigurer() = PropertySourcesPlaceholderConfigurer() [NOTE] ==== -If you are using Spring Boot, you would probably be interested in using +If Spring Boot is being used, then https://docs.spring.io/spring-boot/docs/current/reference/html/boot-features-external-config.html#boot-features-external-config-typesafe-configuration-properties[`@ConfigurationProperties`] -instead of `@Value`, but currently you have to use it with nullable `var` (which is far from ideal) -properties since immutable classes initialized by constructor are not supported yet. +instead of `@Value` annotations can be used, but currently this only works with nullable `var` +properties (which is far from ideal) since immutable classes initialized by +constructors are not yet supported. See these issues about https://github.com/spring-projects/spring-boot/issues/8762[`@ConfigurationProperties` binding for immutable POJOs] and https://github.com/spring-projects/spring-boot/issues/1254[`@ConfigurationProperties` binding on interfaces] for more details. @@ -432,12 +444,12 @@ for more details. === Annotation array attributes Kotlin annotations are mostly similar to Java ones, but array attributes - which are -extensively used in Spring - behaves differently. As explained in https://kotlinlang.org/docs/reference/annotations.html[Kotlin documentation] -unlike other attributes, `value` attribute name can be omitted and when it is an array +extensively used in Spring - behave differently. As explained in https://kotlinlang.org/docs/reference/annotations.html[Kotlin documentation] +unlike other attributes, the `value` attribute name can be omitted and when it is an array attribute it is specified as a `vararg` parameter. -To understand what that means more concretely, let's take `@RequestMapping`, which is one -of the most used Spring annotation as an example. This Java annotation is declared as following: +To understand what that means, let's take `@RequestMapping`, which is one +of the most widely used Spring annotations as an example. This Java annotation is declared as: [source,java] ---- @@ -455,36 +467,38 @@ public @interface RequestMapping { } ---- -The typical use case for `@RequestMapping` is to map an handler method to a specific path -+ method. In Java, it is possible to specify single value for annotation array attribute, -they will be automatically converted to arrays. That's why you can write +The typical use case for `@RequestMapping` is to map a handler method to a specific path +and method. In Java, it is possible to specify a single value for the +annotation array attribute and it will be automatically converted to an array. + +That's why one can write `@RequestMapping(value = "/foo", method = RequestMethod.GET)` or `@RequestMapping(path = "/foo", method = RequestMethod.GET)`. -In Kotlin, you will have to write `@RequestMapping("/foo", method = arrayOf(RequestMethod.GET))`. +However, in Kotlin, one will have to write `@RequestMapping("/foo", method = arrayOf(RequestMethod.GET))`. The variant using `path` is not recommended as it need to be written `@RequestMapping(path = arrayOf("/foo"), method = arrayOf(RequestMethod.GET))`. -A workaround for this specific `method` attribute (the most common one) is to use shortcut -annotation like `@GetMapping`, `@PostMapping`, etc. +A workaround for this specific `method` attribute (the most common one) is to +use a shortcut annotation such as `@GetMapping` or `@PostMapping`, etc. [NOTE] ==== -Remininder: if you don't specify `@RequestMapping` `method` attribute, all HTTP methods will be matched, -not just `GET` ones. +Remininder: if the `@RequestMapping` `method` attribute is not specified, all HTTP methods will be matched, +not only the `GET` methods. ==== -Improving syntax and consistency of Kotlin annotation array attributes is discussed in +Improving the syntax and consistency of Kotlin annotation array attributes is discussed in https://youtrack.jetbrains.com/issue/KT-11235[this Kotlin language design issue]. === Testing -Kotlin allows to specify meaningful test function names betweeen backticks, -and as of JUnit 5 Kotlin test classes can use `@TestInstance(TestInstance.Lifecycle.PER_CLASS)` -to enable a single instantiation of test classes which allows to use `@BeforeAll` and `@AfterAll` +Kotlin allows one to specify meaningful test function names between backticks, +and as of JUnit 5 Kotlin test classes can use the `@TestInstance(TestInstance.Lifecycle.PER_CLASS)` +annotation to enable a single instantiation of test classes which allows the use of `@BeforeAll` and `@AfterAll` annotations on non-static methods, which is a good fit for Kotlin. -It is also now possible to change the default behavior to `PER_CLASS` thanks to a +It is now possible to change the default behavior to `PER_CLASS` thanks to a `junit-platform.properties` file with a `junit.jupiter.testinstance.lifecycle.default = per_class` property. @@ -528,13 +542,13 @@ Boot 2 project on https://start.spring.io/#!language=kotlin[start.spring.io]. It is also possible to create a standalone WebFlux project as described in https://spring.io/blog/2017/08/01/spring-framework-5-kotlin-apis-the-functional-way[this blog post]. -=== Choosing your web flavor +=== Choosing the web flavor Spring Framework now comes with 2 different web stacks: <> and <>. -Spring WebFlux is recommended if you want to create applications that will deal with latency, -long-lived connections, streaming scenarios or simply if you want to use the web functional +Spring WebFlux is recommended if one wants to create applications that will deal with latency, +long-lived connections, streaming scenarios or simply if one wants to use the web functional Kotlin DSL. For other use cases, Spring MVC and its annotation-based programming model is a perfectly