commit	790460a27878ab916df7c4fed8a895c2c899fded	[log] [tgz]
author	Jake Wharton <jw@squareup.com>	Thu May 11 21:05:11 2017 -0400
committer	Jake Wharton <jw@squareup.com>	Thu May 11 21:05:11 2017 -0400
tree	2001060d3c5f45de98de9ec37f15da57d8bafc55
parent	1075139e9b3724c32cc8fcd930c67de51ba8136b [diff]

tree: 2001060d3c5f45de98de9ec37f15da57d8bafc55

README.md

KotlinPoet

KotlinPoet is a Kotlin and Java API for generating .kt source files.

Source file generation can be useful when doing things such as annotation processing or interacting with metadata files (e.g., database schemas, protocol formats). By generating code, you eliminate the need to write boilerplate while also keeping a single source of truth for the metadata.

Example

Here's a (boring) HelloWorld class:

package com.example.helloworld

fun main(vararg args: String) {
  println("Hello, KotlinPoet!")
}

And this is the (exciting) code to generate it with KotlinPoet:

FunSpec main = FunSpec.builder("main")
    .addModifiers(Modifier.PUBLIC, Modifier.STATIC)
    .returns(void.class)
    .addParameter(String[].class, "args")
    .addStatement("%T.out.println(%S)", System.class, "Hello, KotlinPoet!")
    .build();

TypeSpec helloWorld = TypeSpec.classBuilder("HelloWorld")
    .addModifiers(Modifier.PUBLIC, Modifier.FINAL)
    .addFun(main)
    .build();

JavaFile javaFile = JavaFile.builder("com.example.helloworld", helloWorld)
    .build();

javaFile.writeTo(System.out);

To declare the main function, we've created a FunSpec “main” configured with modifiers, return type, parameters and code statements. We add the main function to a HelloWorld class, and then add that to a HelloWorld.java file.

In this case we write the file to System.out, but we could also get it as a string (JavaFile.toString()) or write it to the file system (KotlinPoet.writeTo()).

The KDoc catalogs the complete KotlinPoet API, which we explore below.

Code & Control Flow

Most of KotlinPoet‘s API uses plain old immutable Java objects. There’s also builders, function chaining and varargs to make the API friendly. KotlinPoet offers models for classes & interfaces (TypeSpec), fields (FieldSpec), functions & constructors (FunSpec), parameters (ParameterSpec) and annotations (AnnotationSpec).

But the body of functions and constructors is not modeled. There's no expression class, no statement class or syntax tree nodes. Instead, KotlinPoet uses strings for code blocks:

FunSpec main = FunSpec.builder("main")
    .addCode(""
        + "int total = 0;\n"
        + "for (int i = 0; i < 10; i++) {\n"
        + "  total += i;\n"
        + "}\n")
    .build();

Which generates this:

void main() {
  int total = 0;
  for (int i = 0; i < 10; i++) {
    total += i;
  }
}

The manual semicolons, line wrapping, and indentation are tedious and so KotlinPoet offers APIs to make it easier. There's addStatement() which takes care of semicolons and newline, and beginControlFlow() + endControlFlow() which are used together for braces, newlines, and indentation:

FunSpec main = FunSpec.builder("main")
    .addStatement("int total = 0")
    .beginControlFlow("for (int i = 0; i < 10; i++)")
    .addStatement("total += i")
    .endControlFlow()
    .build();

This example is lame because the generated code is constant! Suppose instead of just adding 0 to 10, we want to make the operation and range configurable. Here's a function that generates a function:

private FunSpec computeRange(String name, int from, int to, String op) {
  return FunSpec.builder(name)
      .returns(int.class)
      .addStatement("int result = 0")
      .beginControlFlow("for (int i = " + from + "; i < " + to + "; i++)")
      .addStatement("result = result " + op + " i")
      .endControlFlow()
      .addStatement("return result")
      .build();
}

And here's what we get when we call computeRange("multiply10to20", 10, 20, "*"):

int multiply10to20() {
  int result = 0;
  for (int i = 10; i < 20; i++) {
    result = result * i;
  }
  return result;
}

Functions generating functions! And since KotlinPoet generates source instead of bytecode, you can read through it to make sure it's right.

%L for Literals

The string-concatenation in calls to beginControlFlow() and addStatement is distracting. Too many operators. To address this, KotlinPoet offers a syntax inspired-by but incompatible-with String.format(). It accepts %L to emit a literal value in the output. This works just like Formatter's %s:

private FunSpec computeRange(String name, int from, int to, String op) {
  return FunSpec.builder(name)
      .returns(int.class)
      .addStatement("int result = 0")
      .beginControlFlow("for (int i = %L; i < %L; i++)", from, to)
      .addStatement("result = result %L i", op)
      .endControlFlow()
      .addStatement("return result")
      .build();
}

Literals are emitted directly to the output code with no escaping. Arguments for literals may be strings, primitives, and a few KotlinPoet types described below.

%S for Strings

When emitting code that includes string literals, we can use %S to emit a string, complete with wrapping quotation marks and escaping. Here's a program that emits 3 functions, each of which returns its own name:

public static void main(String[] args) throws Exception {
  TypeSpec helloWorld = TypeSpec.classBuilder("HelloWorld")
      .addModifiers(Modifier.PUBLIC, Modifier.FINAL)
      .addFun(whatsMyName("slimShady"))
      .addFun(whatsMyName("eminem"))
      .addFun(whatsMyName("marshallMathers"))
      .build();

  JavaFile javaFile = JavaFile.builder("com.example.helloworld", helloWorld)
      .build();

  javaFile.writeTo(System.out);
}

private static FunSpec whatsMyName(String name) {
  return FunSpec.builder(name)
      .returns(String.class)
      .addStatement("return %S", name)
      .build();
}

In this case, using %S gives us quotation marks:

public final class HelloWorld {
  String slimShady() {
    return "slimShady";
  }

  String eminem() {
    return "eminem";
  }

  String marshallMathers() {
    return "marshallMathers";
  }
}

%T for Types

We Java programmers love our types: they make our code easier to understand. And KotlinPoet is on board. It has rich built-in support for types, including automatic generation of import statements. Just use %T to reference types:

FunSpec today = FunSpec.builder("today")
    .returns(Date.class)
    .addStatement("return new %T()", Date.class)
    .build();

TypeSpec helloWorld = TypeSpec.classBuilder("HelloWorld")
    .addModifiers(Modifier.PUBLIC, Modifier.FINAL)
    .addFun(today)
    .build();

JavaFile javaFile = JavaFile.builder("com.example.helloworld", helloWorld)
    .build();

javaFile.writeTo(System.out);

That generates the following .java file, complete with the necessary import:

package com.example.helloworld;

import java.util.Date;

public final class HelloWorld {
  Date today() {
    return new Date();
  }
}

We passed Date.class to reference a class that just-so-happens to be available when we‘re generating code. This doesn’t need to be the case. Here‘s a similar example, but this one references a class that doesn’t exist (yet):

ClassName hoverboard = ClassName.get("com.mattel", "Hoverboard");

FunSpec today = FunSpec.builder("tomorrow")
    .returns(hoverboard)
    .addStatement("return new %T()", hoverboard)
    .build();

And that not-yet-existent class is imported as well:

package com.example.helloworld;

import com.mattel.Hoverboard;

public final class HelloWorld {
  Hoverboard tomorrow() {
    return new Hoverboard();
  }
}

The ClassName type is very important, and you‘ll need it frequently when you’re using KotlinPoet. It can identify any declared class. Declared types are just the beginning of Java's rich type system: we also have arrays, parameterized types, wildcard types, and type variables. KotlinPoet has classes for building each of these:

ClassName hoverboard = ClassName.get("com.mattel", "Hoverboard");
ClassName list = ClassName.get("java.util", "List");
ClassName arrayList = ClassName.get("java.util", "ArrayList");
TypeName listOfHoverboards = ParameterizedTypeName.get(list, hoverboard);

FunSpec beyond = FunSpec.builder("beyond")
    .returns(listOfHoverboards)
    .addStatement("%T result = new %T<>()", listOfHoverboards, arrayList)
    .addStatement("result.add(new %T())", hoverboard)
    .addStatement("result.add(new %T())", hoverboard)
    .addStatement("result.add(new %T())", hoverboard)
    .addStatement("return result")
    .build();

KotlinPoet will decompose each type and import its components where possible.

package com.example.helloworld;

import com.mattel.Hoverboard;
import java.util.ArrayList;
import java.util.List;

public final class HelloWorld {
  List<Hoverboard> beyond() {
    List<Hoverboard> result = new ArrayList<>();
    result.add(new Hoverboard());
    result.add(new Hoverboard());
    result.add(new Hoverboard());
    return result;
  }
}

Import static

KotlinPoet supports import static. It does it via explicitly collecting type member names. Let's enhance the previous example with some static sugar:

...
ClassName namedBoards = ClassName.get("com.mattel", "Hoverboard", "Boards");

FunSpec beyond = FunSpec.builder("beyond")
    .returns(listOfHoverboards)
    .addStatement("%T result = new %T<>()", listOfHoverboards, arrayList)
    .addStatement("result.add(%T.createNimbus(2000))", hoverboard)
    .addStatement("result.add(%T.createNimbus(\"2001\"))", hoverboard)
    .addStatement("result.add(%T.createNimbus(%T.THUNDERBOLT))", hoverboard, namedBoards)
    .addStatement("%T.sort(result)", Collections.class)
    .addStatement("return result.isEmpty() ? %T.emptyList() : result", Collections.class)
    .build();

TypeSpec hello = TypeSpec.classBuilder("HelloWorld")
    .addFun(beyond)
    .build();

JavaFile.builder("com.example.helloworld", hello)
    .addStaticImport(hoverboard, "createNimbus")
    .addStaticImport(namedBoards, "*")
    .addStaticImport(Collections.class, "*")
    .build();

KotlinPoet will first add your import static block to the file as configured, match and mangle all calls accordingly and also import all other types as needed.

package com.example.helloworld;

import static com.mattel.Hoverboard.Boards.*;
import static com.mattel.Hoverboard.createNimbus;
import static java.util.Collections.*;

import com.mattel.Hoverboard;
import java.util.ArrayList;
import java.util.List;

class HelloWorld {
  List<Hoverboard> beyond() {
    List<Hoverboard> result = new ArrayList<>();
    result.add(createNimbus(2000));
    result.add(createNimbus("2001"));
    result.add(createNimbus(THUNDERBOLT));
    sort(result);
    return result.isEmpty() ? emptyList() : result;
  }
}

%N for Names

Generated code is often self-referential. Use %N to refer to another generated declaration by its name. Here's a function that calls another:

public String byteToHex(int b) {
  char[] result = new char[2];
  result[0] = hexDigit((b >>> 4) & 0xf);
  result[1] = hexDigit(b & 0xf);
  return new String(result);
}

public char hexDigit(int i) {
  return (char) (i < 10 ? i + '0' : i - 10 + 'a');
}

When generating the code above, we pass the hexDigit() function as an argument to the byteToHex() functino using %N:

FunSpec hexDigit = FunSpec.builder("hexDigit")
    .addParameter(int.class, "i")
    .returns(char.class)
    .addStatement("return (char) (i < 10 ? i + '0' : i - 10 + 'a')")
    .build();

FunSpec byteToHex = FunSpec.builder("byteToHex")
    .addParameter(int.class, "b")
    .returns(String.class)
    .addStatement("char[] result = new char[2]")
    .addStatement("result[0] = %N((b >>> 4) & 0xf)", hexDigit)
    .addStatement("result[1] = %N(b & 0xf)", hexDigit)
    .addStatement("return new String(result)")
    .build();

Code block format strings

Code blocks may specify the values for their placeholders in a few ways. Only one style may be used for each operation on a code block.

Relative Arguments

Pass an argument value for each placeholder in the format string to CodeBlock.add(). In each example, we generate code to say “I ate 3 tacos”

CodeBlock.builder().add("I ate %L %L", 3, "tacos")

Positional Arguments

Place an integer index (1-based) before the placeholder in the format string to specify which argument to use.

CodeBlock.builder().add("I ate %2L %1L", "tacos", 3)

Named Arguments

Use the syntax %argumentName:X where X is the format character and call CodeBlock.addNamed() with a map containing all argument keys in the format string. Argument names use characters in a-z, A-Z, 0-9, and _, and must start with a lowercase character.

Map<String, Object> map = new LinkedHashMap<>();
map.put("food", "tacos");
map.put("count", 3);
CodeBlock.builder().addNamed("I ate %count:L %food:L", map)

Functions

All of the above functions have a code body. Use Modifiers.ABSTRACT to get a function without any body. This is only legal if the enclosing class is either abstract or an interface.

FunSpec flux = FunSpec.builder("flux")
    .addModifiers(Modifier.ABSTRACT, Modifier.PROTECTED)
    .build();

TypeSpec helloWorld = TypeSpec.classBuilder("HelloWorld")
    .addModifiers(Modifier.PUBLIC, Modifier.ABSTRACT)
    .addFun(flux)
    .build();

Which generates this:

public abstract class HelloWorld {
  protected abstract void flux();
}

The other modifiers work where permitted. Note that when specifying modifiers, KotlinPoet uses javax.lang.model.element.Modifier, a class that is not available on Android. This limitation applies to code-generating-code only; the output code runs everywhere: JVMs, Android, and GWT.

Functions also have parameters, exceptions, varargs, KDoc, annotations, type variables, and a return type. All of these are configured with FunSpec.Builder.

Constructors

FunSpec is a slight misnomer; it can also be used for constructors:

FunSpec flux = FunSpec.constructorBuilder()
    .addModifiers(Modifier.PUBLIC)
    .addParameter(String.class, "greeting")
    .addStatement("this.%N = %N", "greeting", "greeting")
    .build();

TypeSpec helloWorld = TypeSpec.classBuilder("HelloWorld")
    .addModifiers(Modifier.PUBLIC)
    .addField(String.class, "greeting", Modifier.PRIVATE, Modifier.FINAL)
    .addFun(flux)
    .build();

Which generates this:

public class HelloWorld {
  private final String greeting;

  public HelloWorld(String greeting) {
    this.greeting = greeting;
  }
}

For the most part, constructors work just like functions. When emitting code, KotlinPoet will place constructors before functions in the output file.

Parameters

Declare parameters on functions and constructors with either ParameterSpec.builder() or FunSpec's convenient addParameter() API:

ParameterSpec android = ParameterSpec.builder(String.class, "android")
    .addModifiers(Modifier.FINAL)
    .build();

FunSpec welcomeOverlords = FunSpec.builder("welcomeOverlords")
    .addParameter(android)
    .addParameter(String.class, "robot", Modifier.FINAL)
    .build();

Though the code above to generate android and robot parameters is different, the output is the same:

void welcomeOverlords(final String android, final String robot) {
}

The extended Builder form is necessary when the parameter has annotations (such as @Nullable).

Fields

Like parameters, fields can be created either with builders or by using convenient helper functions:

FieldSpec android = FieldSpec.builder(String.class, "android")
    .addModifiers(Modifier.PRIVATE, Modifier.FINAL)
    .build();

TypeSpec helloWorld = TypeSpec.classBuilder("HelloWorld")
    .addModifiers(Modifier.PUBLIC)
    .addField(android)
    .addField(String.class, "robot", Modifier.PRIVATE, Modifier.FINAL)
    .build();

Which generates:

public class HelloWorld {
  private final String android;

  private final String robot;
}

The extended Builder form is necessary when a field has KDoc, annotations, or a field initializer. Field initializers use the same String.format()-like syntax as the code blocks above:

FieldSpec android = FieldSpec.builder(String.class, "android")
    .addModifiers(Modifier.PRIVATE, Modifier.FINAL)
    .initializer("%S + %L", "Lollipop v.", 5.0d)
    .build();

Which generates:

private final String android = "Lollipop v." + 5.0;

Interfaces

KotlinPoet has no trouble with interfaces. Note that interface functions must always be PUBLIC ABSTRACT and interface fields must always be PUBLIC STATIC FINAL. These modifiers are necessary when defining the interface:

TypeSpec helloWorld = TypeSpec.interfaceBuilder("HelloWorld")
    .addModifiers(Modifier.PUBLIC)
    .addField(FieldSpec.builder(String.class, "ONLY_THING_THAT_IS_CONSTANT")
        .addModifiers(Modifier.PUBLIC, Modifier.STATIC, Modifier.FINAL)
        .initializer("%S", "change")
        .build())
    .addFun(FunSpec.builder("beep")
        .addModifiers(Modifier.PUBLIC, Modifier.ABSTRACT)
        .build())
    .build();

But these modifiers are omitted when the code is generated. These are the defaults so we don't need to include them for javac's benefit!

public interface HelloWorld {
  String ONLY_THING_THAT_IS_CONSTANT = "change";

  void beep();
}

Enums

Use enumBuilder to create the enum type, and addEnumConstant() for each value:

TypeSpec helloWorld = TypeSpec.enumBuilder("Roshambo")
    .addModifiers(Modifier.PUBLIC)
    .addEnumConstant("ROCK")
    .addEnumConstant("SCISSORS")
    .addEnumConstant("PAPER")
    .build();

To generate this:

public enum Roshambo {
  ROCK,

  SCISSORS,

  PAPER
}

Fancy enums are supported, where the enum values override functions or call a superclass constructor. Here's a comprehensive example:

TypeSpec helloWorld = TypeSpec.enumBuilder("Roshambo")
    .addModifiers(Modifier.PUBLIC)
    .addEnumConstant("ROCK", TypeSpec.anonymousClassBuilder("%S", "fist")
        .addFun(FunSpec.builder("toString")
            .addAnnotation(Override.class)
            .addModifiers(Modifier.PUBLIC)
            .addStatement("return %S", "avalanche!")
            .build())
        .build())
    .addEnumConstant("SCISSORS", TypeSpec.anonymousClassBuilder("%S", "peace")
        .build())
    .addEnumConstant("PAPER", TypeSpec.anonymousClassBuilder("%S", "flat")
        .build())
    .addField(String.class, "handsign", Modifier.PRIVATE, Modifier.FINAL)
    .addFun(FunSpec.constructorBuilder()
        .addParameter(String.class, "handsign")
        .addStatement("this.%N = %N", "handsign", "handsign")
        .build())
    .build();

Which generates this:

public enum Roshambo {
  ROCK("fist") {
    @Override
    public void toString() {
      return "avalanche!";
    }
  },

  SCISSORS("peace"),

  PAPER("flat");

  private final String handsign;

  Roshambo(String handsign) {
    this.handsign = handsign;
  }
}

Anonymous Inner Classes

In the enum code, we used Types.anonymousInnerClass(). Anonymous inner classes can also be used in code blocks. They are values that can be referenced with %L:

TypeSpec comparator = TypeSpec.anonymousClassBuilder("")
    .addSuperinterface(ParameterizedTypeName.get(Comparator.class, String.class))
    .addFun(FunSpec.builder("compare")
        .addAnnotation(Override.class)
        .addModifiers(Modifier.PUBLIC)
        .addParameter(String.class, "a")
        .addParameter(String.class, "b")
        .returns(int.class)
        .addStatement("return %N.length() - %N.length()", "a", "b")
        .build())
    .build();

TypeSpec helloWorld = TypeSpec.classBuilder("HelloWorld")
    .addFun(FunSpec.builder("sortByLength")
        .addParameter(ParameterizedTypeName.get(List.class, String.class), "strings")
        .addStatement("%T.sort(%N, %L)", Collections.class, "strings", comparator)
        .build())
    .build();

This generates a function that contains a class that contains a function:

void sortByLength(List<String> strings) {
  Collections.sort(strings, new Comparator<String>() {
    @Override
    public int compare(String a, String b) {
      return a.length() - b.length();
    }
  });
}

One particularly tricky part of defining anonymous inner classes is the arguments to the superclass constructor. In the above code we‘re passing the empty string for no arguments: TypeSpec.anonymousClassBuilder(""). To pass different parameters use KotlinPoet’s code block syntax with commas to separate arguments.

Annotations

Simple annotations are easy:

FunSpec toString = FunSpec.builder("toString")
    .addAnnotation(Override.class)
    .returns(String.class)
    .addModifiers(Modifier.PUBLIC)
    .addStatement("return %S", "Hoverboard")
    .build();

Which generates this function with an @Override annotation:

  @Override
  public String toString() {
    return "Hoverboard";
  }

Use AnnotationSpec.builder() to set properties on annotations:

FunSpec logRecord = FunSpec.builder("recordEvent")
    .addModifiers(Modifier.PUBLIC, Modifier.ABSTRACT)
    .addAnnotation(AnnotationSpec.builder(Headers.class)
        .addMember("accept", "%S", "application/json; charset=utf-8")
        .addMember("userAgent", "%S", "Square Cash")
        .build())
    .addParameter(LogRecord.class, "logRecord")
    .returns(LogReceipt.class)
    .build();

Which generates this annotation with accept and userAgent properties:

@Headers(
    accept = "application/json; charset=utf-8",
    userAgent = "Square Cash"
)
LogReceipt recordEvent(LogRecord logRecord);

When you get fancy, annotation values can be annotations themselves. Use %L for embedded annotations:

FunSpec logRecord = FunSpec.builder("recordEvent")
    .addModifiers(Modifier.PUBLIC, Modifier.ABSTRACT)
    .addAnnotation(AnnotationSpec.builder(HeaderList.class)
        .addMember("value", "%L", AnnotationSpec.builder(Header.class)
            .addMember("name", "%S", "Accept")
            .addMember("value", "%S", "application/json; charset=utf-8")
            .build())
        .addMember("value", "%L", AnnotationSpec.builder(Header.class)
            .addMember("name", "%S", "User-Agent")
            .addMember("value", "%S", "Square Cash")
            .build())
        .build())
    .addParameter(LogRecord.class, "logRecord")
    .returns(LogReceipt.class)
    .build();

Which generates this:

@HeaderList({
    @Header(name = "Accept", value = "application/json; charset=utf-8"),
    @Header(name = "User-Agent", value = "Square Cash")
})
LogReceipt recordEvent(LogRecord logRecord);

Note that you can call addMember() multiple times with the same property name to populate a list of values for that property.

KDoc

Fields, functions, and types can be documented with KDoc:

FunSpec dismiss = FunSpec.builder("dismiss")
    .addKdoc("Hides `message` from the caller's history. Other participants in the\n"
        + "conversation will continue to see the message in their own history\n"
        + " unless they also delete it.\n")
    .addKdoc("\n")
    .addKdoc("Use [Message.delete(%T)] to delete the entire conversation\n"
        + "for all participants.\n", Conversation.class)
    .addModifiers(Modifier.PUBLIC, Modifier.ABSTRACT)
    .addParameter(Message.class, "message")
    .build();

Which generates this:

  /**
   * Hides `message` from the caller's history. Other participants in the
   * conversation will continue to see the message in their own history
   * unless they also delete it.
   *
   * Use [Message.delete(Conversation)] to delete the entire conversation
   * for all participants.
   */
  void dismiss(Message message);

Use %T when referencing types in Javadoc to get automatic imports.

Download

Download the latest .jar or depend via Maven:

<dependency>
  <groupId>com.squareup</groupId>
  <artifactId>kotlinpoet</artifactId>
  <version>0.1.0</version>
</dependency>

or Gradle:

compile 'com.squareup:kotlinpoet:0.1.0'

Snapshots of the development version are available in Sonatype's snapshots repository.

License

Copyright 2017 Square, Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.