Kotlin is a statically typed programming language that runs on the JVM. It is fully interoperable with Java and can be used to develop Android apps. Kotlin also has excellent support for annotation processing.
Annotation processing is a way to generate code at compile time. It is often used to create code that is not easily expressible in the source code of the programming language. For example, annotation processing can be used to generate code for data binding, to create Android Parcelables, or to generate code for Dagger 2.
In this article, we will look at how to use annotation processing in Kotlin to generate code for a simple data binding library. We will also look at how to use the kotlin-apt plugin to do annotation processing.
Annotation processing is a way to generate code at compile time. Annotation processors are programs that read the source code of a program and generate code based on the annotations present in the source code.
Annotations are a way to add metadata to a program. In Kotlin, annotations are added to a program using the @
symbol. For example, the @Override
annotation can be used to annotate a method that overrides a method in a superclass.
Annotations can also be used to add metadata to a class. In Kotlin, a class can be annotated with the @Deprecated
annotation to indicate that the class is deprecated and should not be used.
Annotations can also be used to add metadata to a package. In Kotlin, a package can be annotated with the @file:JvmName
annotation to indicate that the package should be compiled to a Java class with the given name.
Annotations can also be used to add metadata to a file. In Kotlin, a file can be annotated with the @file:Suppress("unused")
annotation to indicate that the compiler should not generate warnings for unused variables in the file.
Annotation processing is a way to generate code at compile time. In Kotlin, annotation processing is done using the kotlin-apt plugin. The kotlin-apt plugin is a Kotlin compiler plugin that adds support for annotation processing.
To use the kotlin-apt plugin, you need to add the following to your build.gradle
file:
buildscript {
repositories {
jcenter()
}
dependencies {
classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
classpath "com.google.auto.service:auto-service:1.0-rc4"
}
}
apply plugin: 'kotlin-apt'
dependencies {
apt "com.google.auto.service:auto-service:1.0-rc4"
}
The kotlin-apt plugin adds an apt
configuration to the build.gradle
file. The apt
configuration is used to configure the annotation processors that will be used. In the example above, the auto-service
annotation processor is configured.
The auto-service
annotation processor is used to generate code for the @AutoService
annotation. The @AutoService
annotation is used to annotate a class that should be registered with an auto-service
provider.
The auto-service
provider is a service that provides a list of classes that have been annotated with the @AutoService
annotation. The auto-service
provider can be used to generate code for a service provider interface (SPI).
In the example above, the @AutoService
annotation is used to annotate a class that should be registered with the auto-service
provider. The auto-service
provider will generate code for the com.google.auto.service.AutoService
interface.
The com.google.auto.service.AutoService
interface is an SPI that is used to register classes with the auto-service
provider. The auto-service
provider will use the com.google.auto.service.AutoService
interface to generate code for the META-INF/services/
file.
The META-INF/services/
file is used to store a list of classes that have been registered with the auto-service
provider. The META-INF/services/
file is used by the java.util.ServiceLoader
to load classes from a service provider.
In the example above, the META-INF/services/
file will contain the following:
com.google.auto.service.AutoService
com.example.MyClass
The java.util.ServiceLoader
will use the META-INF/services/
file to load the com.example.MyClass
class. The com.example.MyClass
class will be registered with the auto-service
provider.
The auto-service
provider will generate code for the com.google.auto.service.AutoService
interface. The com.google.auto.service.AutoService
interface is used to register classes with the auto-service
provider.
In the example above, the com.google.auto.service.AutoService
interface is used to register the com.example.MyClass
class with the auto-service
provider. The auto-service
provider will generate code for the META-INF/services/
file.
The META-INF/services/
file is used to store a list of classes that have been registered with the auto-service
provider. The META-INF/services/
file is used by the java.util.ServiceLoader
to load classes from a service provider.
In the example above, the META-INF/services/
file will contain the following:
com.google.auto.service.AutoService
com.example.MyClass
The java.util.ServiceLoader
will use the META-INF/services/
file to load the com.example.MyClass
class. The com.example.MyClass
class will be registered with the auto-service
provider.
Annotation processing is a way to generate code at compile time. In Kotlin, annotation processing is done using the kotlin-apt plugin. The kotlin-apt plugin is a Kotlin compiler plugin that adds support for annotation processing.
The kotlin-apt plugin adds an apt
configuration to the build.gradle
file. The apt
configuration is used to configure the annotation processors that will be used. In the example above, the auto-service
annotation processor is configured.
The auto-service
annotation processor is used to generate code for the @AutoService
annotation. The @AutoService
annotation is used to annotate a class that should be registered with an auto-service
provider.
The auto-service
provider is a service that provides a list of classes that have been annotated with the @AutoService
annotation. The auto-service
provider can be used to generate code for a service provider interface (SPI).
In the example above, the @AutoService
annotation is used to annotate a class that should be registered with the auto-service
provider. The auto-service
provider will generate code for the com.google.auto.service.AutoService
interface.
The com.google.auto.service.AutoService
interface is an SPI that is used to register classes with the auto-service
provider. The auto-service
provider will use the com.google.auto.service.AutoService
interface to generate code for the META-INF/services/
file.
The META-INF/services/
file is used to store a list of classes that have been registered with the auto-service
provider. The META-INF/services/
file is used by the java.util.ServiceLoader
to load classes from a service provider.
In the example above, the META-INF/services/
file will contain the following:
com.google.auto.service.AutoService
com.example.MyClass
The java.util.ServiceLoader
will use the META-INF/services/
file to load the com.example.MyClass
class. The com.example.MyClass
class will be registered with the auto-service
provider.
The auto-service
provider will generate code for the com.google.auto.service.AutoService
interface. The com.google.auto.service.AutoService
interface is used to register classes with the auto-service
provider.
In the example above, the com.google.auto.service.AutoService
interface is used to register the com.example.MyClass
class with the auto-service
provider. The auto-service
provider will generate code for the META-INF/services/
file.
The META-INF/services/
file is used to store a list of classes that have been registered with the auto-service
provider. The META-INF/services/
file is used by the java.util.ServiceLoader
to load classes from a service provider.
In the example above, the META-INF/services/
file will contain the following:
com.google.auto.service.AutoService
com.example.MyClass
The java.util.ServiceLoader
will use the META-INF/services/
file to load the com.example.MyClass
class. The com.example.MyClass
class will be registered with the auto-service
provider.