Code generation on the Java VM 2019-12-05

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Code generation on the Java VM Sean Sullivan 2019-12-05

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Code generation Java Kotlin Scala

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Why code generation?

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less boilerplate more consistency Why code generation?

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class names parameter names method names exception names Consistency error handling logging JSON serialization network I/O

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Code generation is not a new idea

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2001: Apache Axis

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wsdl2java August 2001

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2003: Apache XmlBeans

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September 2003 SchemaCodeGenerator

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The most common use case for source code generation…

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generating client libraries for REST API’s

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bytecode source code Two kinds of code generation

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Code generation Bytecode generation

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Bytecode generators produce Java class deﬁnitions

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ByteBuddy ObjectWeb ASM cglib Bytecode libraries Javassist Commons BCEL AspectJ

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Spring Framework

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cglib spring-core

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cglib spring-beans

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Micronaut framework

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“Unlike Spring which relies exclusively on runtime reﬂection and proxies, Micronaut uses compile time data to implement dependency injection” micronaut https://docs.micronaut.io/latest/guide/ioc.html

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“This goal is achieved through the use of Java’s annotation processors" micronaut https://docs.micronaut.io/latest/guide/index.html

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micronaut https://github.com/micronaut-projects/micronaut-core Annotation processors

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micronaut ObjectWeb ASM https://github.com/micronaut-projects/micronaut-core

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Mockito

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mockito 2.x ByteBuddy https://github.com/mockito/mockito

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“Mockito switched from CGLIB to the great library ByteBuddy to make mocks” https://github.com/mockito/mockito/wiki/What%27s-new-in-Mockito-2 mockito 2.x

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Datadog Java agent

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Datadog Java agent ByteBuddy https://github.com/DataDog/dd-trace-java

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How can I implement a custom Java agent?

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Java agent example https://github.com/sullis/java-lang-instrument-example/

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Java agent example “premain” method https://github.com/sullis/java-lang-instrument-example/

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Java agent example https://github.com/sullis/java-lang-instrument-example/ ByteBuddy AgentBuilder

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Code generation Source code generation

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java.lang.StringBuilder java.io.StringWriter Source generation

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javapoet CodeModel jdeparser2 javaparser Source generation

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kotlinpoet kotlinx-ast kastree Source generation

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scalameta treehugger.scala Source generation

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JavaPoet

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JavaPoet https://github.com/square/javapoet

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package com.example.helloworld; public final class HelloWorld { public static void main(String[] args) { System.out.println("Hello, JavaPoet!"); } } JavaPoet

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import com.squareup.javapoet.*; MethodSpec main = MethodSpec.methodBuilder("main") .addModifiers(Modifier.PUBLIC, Modifier.STATIC) .returns(void.class) .addParameter(String[].class, "args") .addStatement("$T.out.println($S)", System.class, "Hello, JavaPoet!") .build(); TypeSpec helloWorld = TypeSpec.classBuilder("HelloWorld") .addModifiers(Modifier.PUBLIC, Modifier.FINAL) .addMethod(main) .build(); JavaFile javaFile = JavaFile.builder("com.example.helloworld", helloWorld) .build(); javaFile.writeTo(System.out); JavaPoet method builder class builder ﬁle builder

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AWS SDK for Java v2

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AWS SDK for Java v2 “To provide SDK support for the many services that AWS owns, the AWS SDKs make extensive use of code generation” https://aws.amazon.com/blogs/developer/aws-sdk-for-java-2-x-released/

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AWS SDK for Java v2

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JavaPoet AWS SDK for Java v2

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Dagger

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“Dagger 2 is the ﬁrst to implement the full stack with generated code” Dagger 2.x “Dependency injection frameworks have existed for years with a whole variety of APIs for conﬁguring and injecting” https://dagger.dev/users-guide.html

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Dagger 2.x https://dagger.dev/users-guide.html

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JavaPoet Dagger 2.x

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REST API code generators apibuilder project guardrail project openapi-generator

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ApiBuilder project

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https://www.apibuilder.io/ ApiBuilder

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https://github.com/apicollective/apibuilder-generator/ ApiBuilder

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ApiBuilder demo

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Guardrail project

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https://guardrail.dev/ Guardrail

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https://github.com/twilio/guardrail Guardrail

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https://github.com/twilio/guardrail Guardrail: Scala ScalaMeta

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https://github.com/twilio/guardrail Guardrail: Java JavaParser

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Guardrail example

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guardrail-maven-plugin com.twilio guardrail-maven-plugin_2.12 0.54.5 …

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guardrail-maven-plugin java client dropwizard ${project.basedir}/src/main/openapi/petstore.json com.example.clients.petstore Java client

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guardrail-maven-plugin

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openapi-generator project

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openapi-generator

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openapi-generator

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openapi-generator

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Final thoughts code generation is a powerful technique use a code generator to generate client libraries contribute to open source projects