Vert.x API Generation

Render documentation

> mvn clean package -Pdocs
> open target/docs/vertx-codegen/java/index.html

Helper projects

• Codegen CLI: a codegen CLI to help code generating files.
• Codegen starter: a codegen Starter that can be forked to create a new Vert.x code generator

API generator

A code generator is a class extending io.vertx.codegen.Generator loaded by a custom io.vertx.codegen.GeneratorLoader declared as a META-INF/services/io.vertx.codegen.GeneratorLoader JVM service.

There can be as many generators as you like.

Generated output

A generator can create 3 different kinds of output: Java classes, resources and anything else

Generated Java classes

A generator declaring a filename that matches a Java FQN followed by .java suffix will have its content generated as a Java class. This class will be automatically compiled by the same compiler (that's a Java compiler feature).

The generated files are handled by the Java compiler (-s option), usually build tools configures the compiler to store them in a specific build location, for instance Maven by default uses the target/generated-sources/annotations directory.

The following generators use it:

• Data object converters
• Service proxy and service handler
• RxJava-ified classes API
• Groovy extension methods API

Generated resources

A generator declaring a filename prefixed by resources/ will have its content generated as a compiler resource. This resource will be stored in the generated sources directory and the generated classes directory.

The generated files are handled by the Java compiler (-s option), usually build tools configures the compiler to store them in a specific build location, for instance Maven by default uses the target/generated-sources/annotations directory.

The following generators use it:

• JavaScript generator
• Ruby generator

Other generated files

Anything else will be stored in the file system using the filename, when the filename is relative (it usually is) the target path will be resolved agains the codegen.output directory.

The following generators use it:

• Ceylon generator
• Scala generator
• Kotlin extension methods

When the codegen.output is not specified, the generated files are discarded.

Relocation

Sometimes you want to have a generator to output its files in another directory, you can do that with the codegen.output.generator-name compiler option:

<codegen.output.data_object_converters>generated</codegen.output.data_object_converters>

The generator will store its content in the codegen.output/generated directory instead as a Java class.

Processor configuration

You can configure the CodeGenProcessor as any Java annotation processor, here is how to do with Maven:

<pluginManagement>
  <plugins>
    <!-- Configure the execution of the compiler to execute the codegen processor -->
    <plugin>
      <artifactId>maven-compiler-plugin</artifactId>
      <version>3.8.1</version>
      <configuration>
        <source>1.8</source>
        <target>1.8</target>
        <encoding>${project.build.sourceEncoding}</encoding>
        <!-- Important: there are issues with apt and incremental compilation in the maven-compiler-plugin -->
        <useIncrementalCompilation>false</useIncrementalCompilation>
      </configuration>
      <executions>
        <execution>
          <id>default-compile</id>
          <configuration>
            <annotationProcessors>
              <annotationProcessor>io.vertx.codegen.CodeGenProcessor</annotationProcessor>
            </annotationProcessors>
             <!-- It is new option since v3.5 to instruct compiler detect annotation processors classpath -->
            <annotationProcessorPaths>
              <path>
                <groupId>io.vertx</groupId>
                <artifactId>vertx-codegen</artifactId>
                <version>${vertx.version}</version>
              </path>
              <!-- ... more path such as vertx-service-proxy/vertx-rx-java2 depends on what you want to generate ... -->
            </annotationProcessorPaths>
            <compilerArgs>
              <arg>-Acodegen.output=${project.basedir}/src/main</arg>
            </compilerArgs>
          </configuration>
        </execution>
      </executions>
    </plugin>
  </plugins>
</pluginManagement>

And here is a configuration example for Gradle (since Gradle 5.0):

Gradle Groovy

dependencies {
    compileOnly("io.vertx:vertx-codegen:4.0.2")
    // more vertx-service-proxy/vertx-rx-java2
    // compileOnly("io.vertx:vertx-rx-java2:4.0.2")
    annotationProcessor("io.vertx:vertx-codegen:4.0.2")
}

task annotationProcessing(type: JavaCompile, group: 'other') { // codegen
  source = sourceSets.main.java
  classpath = configurations.compile
  destinationDir = project.file('${project.buildDir}/generated/main/java')
  options.annotationProcessorPath = configurations.compileClasspath
  options.compilerArgs = [
    "-proc:only",
    "-processor", "io.vertx.codegen.CodeGenProcessor",
    "-Acodegen.output=${project.projectDir}/src/main"
  ]
}

compileJava {
  dependsOn annotationProcessing
}

sourceSets {
  main {
    java {
      srcDirs += '${project.buildDir}/generated/main/java'
    }
  }
}

Gradle Kotlin

dependencies {
    compileOnly("io.vertx:vertx-codegen:4.0.2")
    // more vertx-service-proxy/vertx-rx-java2
    // compileOnly("io.vertx:vertx-rx-java2:4.0.2")
    annotationProcessor("io.vertx:vertx-codegen:4.0.2")
}

tasks.register<JavaCompile>("annotationProcessing") {
    group = "other"
    source = sourceSets.getByName(SourceSet.MAIN_SOURCE_SET_NAME).java
    destinationDir = project.file("${project.buildDir}/generated/main/java")
    classpath = configurations.compileClasspath.get()
    options.annotationProcessorPath = configurations.compileClasspath.get()
    options.compilerArgs = listOf(
        "-proc:only",
        "-processor", "io.vertx.codegen.CodeGenProcessor",
        "-Acodegen.output=${project.projectDir}/src/main"
    )
}

tasks.compileJava {
    dependsOn(tasks.named("annotationProcessing"))
}

sourceSets {
    main {
        java {
            srcDirs(project.file("${project.buildDir}/generated/main/java"))
        }
    }
}

Besides you can use the processor classified dependency that declares the annotation processor as a META-INF/services/javax.annotation.processing.Processor, if you do so, code generation happens automatically:

<dependency>
  <groupid>io.vertx</groupId>
  <artifactId>vertx-codegen</artifactId>
  <classifier>processor</classifier>
</dependency>

You still need to configure the codegen.output for generating files non resources/classes as the processors requires this option to know where to place them.

The processor is configured by a few options

• codegen.output : where the non Java classes / non resources are stored
• codegen.output.<generator-name> : relocate the output of to another directory
• codegen.generators : a comma separated list of generators, each expression is a regex, allow to filter undesired generators

API constraints

In order for code generation to work effectively, certain constraints are put on the Java interfaces.

The constraints are

• The API must be described as a set of Java interfaces, classes are not permitted
• Default methods are not permitted
• Nested interfaces are not permitted
• All interfaces to have generation performed on them must be annotated with the io.vertx.codegen.annotations.VertxGen annotation
• Fluent methods (methods which return a reference to this) must be annotated with the io.vertx.codegen.annotations.Fluent annotation
• Data object classes (classes which provide data (e.g. configuration) to methods) must be annotated with the io.vertx.codegen.annotations.DataObject annotation
• Data object classes must provide a constructor which takes a single io.vertx.core.json.JsonObject or java.lang.String parameter.
• Methods where the return value must be cached in the API shim must be annotated with the io.vertx.codegen.annotations.CacheReturn annotation
• Only certain types are allowed as parameter or return value types for any API methods (defined below).
• Custom enums should be annotated with @VertxGen, although this is not mandatory to allow the usage of existing Java enums
• JsonMapper implementations must expose an instance as a public static final [JsonDesdeType] INSTANCE field

Permitted types

We define the following set Basic of basic types:

• any primitive type
• any boxed primitive type
• java.lang.String

We define Json as the set of types io.vertx.core.json.JsonObject and io.vertx.core.json.JsonArray

We define DataObject:

• The set of user defined API types which are defined in its own class and annotated with @DataObject
• The set of types that have an associated mapper declared with the @Mapper annotation

We define TypeVar as the set of of types variables where the variable is either declared by its generic method or its generic type

We define Api as the set of user defined API types which are defined in its own interface and annotated with @VertxGen

We define JavaType as the set of any Java type that does not belong to Basic, Json, DataObject, TypeVar and Api, e.g java.net.Socket. Methods are not allowed to declare such type by default and can be annotated with @GenIgnore(GenIgnore.PERMITTED_TYPE) to allow them. Such method limit the translation of the method to other languages, so it should be used with care. It is useful to allow method previously annotated with @GenIgnore to be available in code generator like RxJava that can handle Java types.

We define Parameterized as the set of user defined API types which are defined in its own interface and annotated with @VertxGen where the type parameters belong to:

• the type java.lang.Void
• the set Basic
• the set Json
• the set DataObject
• any enum type
• the set Api
• the set TypeVar

We define ContainerValueType as the set of any Java type that belongs to:

• the set Basic
• the set Json
• any enum type
• the set Api
• the set DataObject
• the set JavaType
• java.lang.Object

The following set Return of types are permitted as return types from any API method:

• void
• the set Basic
• the set Json
• the set DataObject
• any enum type
• java.lang.Throwable
• the set TypeVar
• java.lang.Object
• the set Api
• the set Parameterized
• the set JavaType
• type java.util.List<C>, java.util.Set<C> or java.util.Map<String, C> where C belongs to ContainerValueType

The following set Param of types are permitted as parameters to any API method:

• the set Basic
• the set Json
• the set DataObject
• any enum type
• the type java.lang.Throwable
• the set TypeVar
• java.lang.Object
• the set Api
• the set JavaType
• the set Parameterized
• the type java.lang.Class<T> where <T> is among
  • the set Basic
  • the set Json
  • the set Api
  • the set JavaType
• type java.util.List<C>, java.util.Set<C> or java.util.Map<String, C> where C belongs to ContainerValueType

In addition any `Api method can have as parameter:

• io.vertx.java.core.Handler<io.vertx.java.core.AsyncResult<HA>> where HA contains
  • the set Return where void is interpreted as java.lang.Void minus java.lang.Throwable
• io.vertx.java.core.Handler<H> where H contains
  • the set Return where void is interpreted as java.lang.Void
• java.util.Function<T, R> where T contains Return and R contains Param

Notes:

• Why no support for data object in Map param values ?

Instance Methods

You can declare methods in your interfaces, e.g.

interface MyInterface {

    void doSomething(String foo);

}

Default method works as well

interface MyInterface {

    default String doSomething(String foo) {
      return foo != null ? new StringBuilder(foo).reverse().toString() : null;
    }

}

Static methods

You can declare static methods in your interfaces, e.g.

interface MyInterface {

    static MyInterface newInterface(String foo) {
      return new ....
    }

}

Fields

You can declare fields in your interfaces, e.g.

interface MyInterface {

    int SOME_CONSTANT = 4;

}

Super interfaces

Interfaces can extend other interfaces which also have the @VertxGen annotation.

Concrete/abstract interfaces

Interfaces annotated with @VertxGen can either be concrete or abstract, such information is important for languages not supporting multiple class inheritance like Groovy:

• interfaces annotated with @VertxGen(concrete = false) are meant to be extended by concrete interfaces and can inherit from abstract interfaces only.
• interfaces annotated with @VertxGen or @VertxGen(concrete = true) are implemented directly by Vertx and can inherit at most one other concrete interface and any abstract interface

Ignoring methods

If you do not wish a method to be used for generation you can annotate it with the @GenIgnore annotation.

Modules

Generated types must belong to a module: a java package annotated with @ModuleGen that defines a module. Such file is created in a file package-info.java.

A module must define:

• a name used when generating languages that don't follow Java package naming, like JavaScript or Ruby.
• a groupPackage to define the package of the group used for generating the generated package names (for Groovy, RxJava or Ceylon generation):

@ModuleGen(name = "acme", groupPackage="com.acme")
package com.acme.myservice;

The group package must be a prefix of the annotated module, it defines the naming of the generate packages o for the modules that belongs to the same group, in this case:

• com.acme.groovy... for Groovy API
• com.acme.rxjava... for RxJava API

For this particular com.acme.myservice module we have:

• com.acme.groovy.myservice for Groovy API
• com.acme.rxjava.myservice for RxJava API

Vert.x Apis uses the io.vertx group package and vertx-XYZ name, this naming is exclusively reserved to Vert.x Apis.

NOTE: using Maven coordinates for name and group package is encouraged: the name corresponding to the Maven artifactId and the group package corresponding to the groupId.

Data objects

A Data object is a type that can be converted back and forth to a Json type.

You can declare data objects by:

• Defining an annotated @Mapper method for it
• Or annotating the type itself with @DataObject

Json mappers

A json mapper for type T is a method that maps any object of type Type, where J can be:

• JsonArray or JsonObject
• a concrete type extending Number such as Long or Double
• String
• Boolean

Json mapped types can be used anywhere a json types used are.

A json mapper turns any Java type into a data object type.

You can declare them as public static methods:

@Mapper
public static String serialize(ZonedDateTime date) {
  return date.toString();
}

@Mapper
public static ZonedDateTime deserialize(String s) {
  return ZonedDateTime.parse(s);
}

@DataObject annotated types

A @DataObject annotated type is a Java class with the only purpose to be a container for data.

• Codegen recognizes the type as deserializable when the annotated type has a io.vertx.core.json.JsonObject constructor
• The mapper recognizes the type as serializable when the annotated type has a io.vertx.core.json.JsonObject toJson() method

Data object conversion recognize the following types as member of any @DataObject:

• the set Basic
• these specific types
  • io.vertx.core.Buffer
  • java.time.Instant
• the set Json
• any data object class annotated with @DataObject
• type java.util.List<C> where C contains
  • the specific io.vertx.core.Buffer type
  • the set Basic
  • the set Json
  • any @DataObject
  • the Object type : the List<Object> acts like a JsonArray
• type java.util.Map<String, C> where C contains
  • the specific io.vertx.core.Buffer type
  • the set Basic
  • the set Json
  • any @DataObject
  • the Object type : the Map<String, Object> acts like a JsonMap

This is also used for data object cheatsheet generation.

Enums

Enum types can be freely used in an API, custom enum types should be annotated with @VertxGen to allow processing of the enum. This is not mandatory to allow the reuse the existing Java enums.

Enums can be processed for providing more idiomatic APIs in some languages.