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javapoet

javapoet 是用于生成 .java 文件的Java库

Posted by Shon on 2018-07-24

JavaPoet

javapoet 是用于生成 .java 文件的Java库

program

Example

Feature

program

  • ControlFlow
  • PlaceHolder
    • $L
    • $S
    • $T
    • $N
  • CodeBlock Format
    • relative
    • positional
    • for map
  • Import
    • $T
    • classname (static method、field)
  • Class
    • Class
    • Interface
    • Enums
    • Anonymous Inner Classes
  • Method
    • Field
    • Method
    • Constructor
  • other
    • Annonation
    • JavaDoc

Example

需生成的文件

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package com.example.helloworld;

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

javapoet生成代码

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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);

Feature

code & control flow

javapoet 为类、接口、变量、方法（包含构造方法）、参数以及注解提供了友好的链式Api,而对于方法的内容却是使用字符串来生成代码，方法中的分号，换行，缩紧很繁琐。
因此，提供了

  • addStatement() 控制分号和换行
  • beginControlFlow() + endControlFlow() 一同使用控制括号、换行和缩进.

例如：

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

我们可以直接写成字符串的形式(不优雅)

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

而写成这样是不是就好多了呢

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

$L

和java的占位符%s一样，javapoet提供了$L

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private MethodSpec computeRange(String name, int from, int to, String op) {
  return MethodSpec.methodBuilder(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();
}

$S

用于生成string，生成的String是带引号的，如：

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

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

  javaFile.writeTo(System.out);
}

private static MethodSpec whatsMyName(String name) {
  return MethodSpec.methodBuilder(name)
      .returns(String.class)
      .addStatement("return $S", name)
      .build();
}

生成的代码：

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

  String eminem() {
    return "eminem";
  }

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

$T

javapoet对于类型有着丰富的内置支持，包括自动导包，可以直接使用$T引用类型，如：

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MethodSpec today = MethodSpec.methodBuilder("today")
    .returns(Date.class)
    .addStatement("return new $T()", Date.class)
    .build();

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

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

javaFile.writeTo(System.out);

生成代码：

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package com.example.helloworld;

import java.util.Date;

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

上面的例子，恰巧我们引用的是一个存在的类，但这个不是必须的，我们可以使用 className。className可以定义任何申明的类型，包括数组，参数化类型，通配符类型以及类型变量。
简单例子：

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

MethodSpec today = MethodSpec.methodBuilder("tomorrow")
    .returns(hoverboard)
    .addStatement("return new $T()", hoverboard)
    .build();

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package com.example.helloworld;

import com.mattel.Hoverboard;

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

再看个复杂点的：

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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);

MethodSpec beyond = MethodSpec.methodBuilder("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();

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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

javapoet可以通过收集类成员名称支持静态变量的导入，语法糖如下：

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

MethodSpec beyond = MethodSpec.methodBuilder("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")
    .addMethod(beyond)
    .build();

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

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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

生成的代码通常是自引用的,javapoet可以通过使用$N引用其他生成代码的名字来引用其他代码：

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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');
}

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

MethodSpec byteToHex = MethodSpec.methodBuilder("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

代码块有几种不同的方式指定占位符的值，注意的是： 每个代码块的每个操作只能是一种方式：

  • Relative Arguments(普通方式)

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

  • Positional Argument（在占位符前面加一个index,从1开始）

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

  • Named Arguments(使用$argumentName:X语法，参数名规则：a-z, A-Z, 0-9, and _；首字母小写)

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

Methods

方法没什么好说，贴个栗子；不过需要注意的是，在javapoet中使用的 javax.lang.model.element.Modifier,是无法在Android运行的，只能用于代码生成。

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MethodSpec flux = MethodSpec.methodBuilder("flux")
    .addModifiers(Modifier.ABSTRACT, Modifier.PROTECTED)
    .build();

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

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

Constructors

构造方法也可以看作方法，唯一不同的是，在生成的文件中，Javapoet会把它放在其他方法前面。

Parameters

  • ParameterSpec.builder()
  • MethodSpec.methodBuilder(“xx”).addParameter()
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ParameterSpec android = ParameterSpec.builder(String.class, "android")
    .addModifiers(Modifier.FINAL)
    .build();

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

Fields

变量很简单，可以使用 initializer()初始化变量。

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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();

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public class HelloWorld {
  private final String android;

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

Interfaces

  • 接口方法要加上 PUBLIC ABSTRACT
  • 接口变量要加上 PUBLIC STATIC FINAL
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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())
    .addMethod(MethodSpec.methodBuilder("beep")
        .addModifiers(Modifier.PUBLIC, Modifier.ABSTRACT)
        .build())
    .build();
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public interface HelloWorld {
  String ONLY_THING_THAT_IS_CONSTANT = "change";

  void beep();
}

Enums

使用 enumBuilder 创建枚举类型，addEnumConstant() 添加枚举值

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

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public enum Roshambo {
  ROCK,

  SCISSORS,

  PAPER
}

看个复杂的栗子,支持重写方法，构造方法

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

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

  SCISSORS("peace"),

  PAPER("flat");

  private final String handsign;

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

Anonymous Inner Classes

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TypeSpec comparator = TypeSpec.anonymousClassBuilder("")
    .addSuperinterface(ParameterizedTypeName.get(Comparator.class, String.class))
    .addMethod(MethodSpec.methodBuilder("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")
    .addMethod(MethodSpec.methodBuilder("sortByLength")
        .addParameter(ParameterizedTypeName.get(List.class, String.class), "strings")
        .addStatement("$T.sort($N, $L)", Collections.class, "strings", comparator)
        .build())
    .build();

Annotations

简单注解

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MethodSpec toString = MethodSpec.methodBuilder("toString")
    .addAnnotation(Override.class)
    .returns(String.class)
    .addModifiers(Modifier.PUBLIC)
    .addStatement("return $S", "Hoverboard")
    .build();

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

使用 AnnotationSpec.builder() 为注解添加属性：

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MethodSpec logRecord = MethodSpec.methodBuilder("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();

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

注解值也可以是注解本身，使用$L内嵌注解：

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MethodSpec logRecord = MethodSpec.methodBuilder("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()

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

Javadoc

变量、方法、类可以使用注释文档：

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MethodSpec dismiss = MethodSpec.methodBuilder("dismiss")
    .addJavadoc("Hides {@code message} from the caller's history. Other\n"
        + "participants in the conversation will continue to see the\n"
        + "message in their own history unless they also delete it.\n")
    .addJavadoc("\n")
    .addJavadoc("<p>Use {@link #delete($T)} to delete the entire\n"
        + "conversation for all participants.\n", Conversation.class)
    .addModifiers(Modifier.PUBLIC, Modifier.ABSTRACT)
    .addParameter(Message.class, "message")
    .build();

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/**
  * Hides {@code 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.
  *
  * <p>Use {@link #delete(Conversation)} to delete the entire
  * conversation for all participants.
  */
 void dismiss(Message message);

至此，语法糖介绍完毕！

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