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编译期注解之JavaPoet

栏目:综合技术时间:2017-01-23 20:21:27

注解系列

注解基础

APT

JavaPoet

0x00 概述

上1篇限于篇幅只介绍了APT,这篇来继续介绍javapoet,是square公司的开源库。正如其名,java诗人,通过注解来生成java源文件,通常要使用javapoet这个库与Filer配合使用。主要和注解配适用来干掉那些重复的模板代码(如butterknife
和databinding所做的事情),固然你也能够使用这个技术让你的代码更加的炫酷。

0x01 简单使用

使用之前要先引入这个库

compile 'com.squareup:javapoet:1.7.0'

javapoet是用来生成代码的,需要借助

经常使用类

使用javapoet前需要了解4个经常使用类

  • MethodSpec 代表1个构造函数或方法声明。
  • TypeSpec 代表1个类,接口,或枚举声明。
  • FieldSpec 代表1个成员变量,1个字段声明。
  • JavaFile包括1个顶级类的Java文件。

国际惯例先自动生成1个helloWorld类
定义1个编译期注解

@Retention(RetentionPolicy.CLASS)
@Target(ElementType.TYPE)
public @interface clazz_hello {
    String value();
}

然后看下helloworld的注解处理器

@AutoService(Processor.class)
public class HelloWorldProcess extends AbstractProcessor {

    private Filer filer;

    @Override
    public synchronized void init(ProcessingEnvironment processingEnv) {
        super.init(processingEnv);
        filer = processingEnv.getFiler();
    }

    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        for (TypeElement element : annotations) {
            if (element.getQualifiedName().toString().equals(clazz_hello.class.getCanonicalName())) {
                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();

                try {
                    JavaFile javaFile = JavaFile.builder("com.xsf", helloWorld)
                            .addFileComment(" This codes are generated automatically. Do not modify!")
                            .build();
                    javaFile.writeTo(filer);
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
        return true;
    }

    @Override
    public Set<String> getSupportedAnnotationTypes() {
        Set<String> annotations = new LinkedHashSet<>();
        annotations.add(clazz_hello.class.getCanonicalName());
        return annotations;
    }

    @Override
    public SourceVersion getSupportedSourceVersion() {
        return SourceVersion.latestSupported();
    }
}

这样就会在app-build-source-apt-debug-com.xsf文件夹下生成这个文件

0x02 使用进阶

方法&控制流:

  • 添加方法 addcodeaddstatement
    对与无需类引入的极简代码可以直接使用addCode
MethodSpec main = MethodSpec.methodBuilder("main")
    .addCode(""
        + "int total = 0;\n"
        + "for (int i = 0; i < 10; i++) {\n"
        + "  total += i;\n"
        + "}\n")
    .build();

生成的是

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

要是需要import的方法,如上面的.addStatement("$T.out.println($S)", System.class, "Hello, JavaPoet!") 就需要使用.addStatement来声明

  • 更优雅的流控制

beginControlFlow 流开启
addStatement 处理语句
endControlFlow()流结束

如上面的用流改写就是

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

占位符

javapoet里面提供了占位符来帮助我们更好地生成代码

  • $L 字面常量(Literals)
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();
}

这个就是1个for循环,op负责加减乘除等符号

  • $S 字符串常量(String)
  • $T 类型(Types)

最大的特点是自动导入包,

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);

生成的代码以下,而且会自动导包

package com.example.helloworld;

import java.util.Date;

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

如果我们想要导入自己写的类怎样办?上面的例子是传入系统的class,这里也提供1种方式,通过ClassName.get(”类的路径”,”类名“),结合$T可以生成

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();

然后生成

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;
  }
}

在导入包这里,javapoet 一样支持import static,通过addStaticImport

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

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();
  • $N 命名(Names),通常指我们自己生成的方法名或变量名等等

比如这样的代码块

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');
}

我们可以传递hexDigit()来代替。

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();

构建类的元素

  • Methods

方法的修饰,如Modifiers.ABSTRACT

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

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

这将会生成以下代码

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

固然Methods需要和MethodSpec.Builder配置来增加方法参数、异常、javadoc、注解等。

  • 构造器

这个其实也是个函数方法而已,因此可使用MethodSpec来生成构造器方法。比如:

MethodSpec flux = MethodSpec.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)
    .addMethod(flux)
    .build();

将会生成

public class HelloWorld {
  private final String greeting;

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

之前我们是通过addstatement直接设置参数,其实参数也有自己的1个专用类ParameterSpec,我们可使用ParameterSpec.builder()来生成参数,然后MethodSpec的addParameter去使用,这样更加优雅。

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();

生成的代码

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

可使用FieldSpec去声明字段,然后加到Method中处理

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();

然后生成代码

public class HelloWorld {
  private final String android;

  private final String robot;
}

通常Builder可以更加详细的创建字段的内容,比如javadoc、annotations或初始化字段参数等,如:

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

对应生成的代码

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

接口方法必须是PUBLIC ABSTRACT并且接口字段必须是PUBLIC STATIC FINAL ,使用TypeSpec.interfaceBuilder

以下

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();

生成的代码以下

public interface HelloWorld {
  String ONLY_THING_THAT_IS_CONSTANT = "change";

  void beep();
}
  • 枚举类型

使用TypeSpec.enumBuilder来创建,使用addEnumConstant来添加

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

生成的代码

public enum Roshambo {
  ROCK,

  SCISSORS,

  PAPER
}

更复杂的类型也能够支持,如重写、注解等

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 $S", "avalanche!")
            .build())
        .build())
    .addEnumConstant("SCISSORS", TypeSpec.anonymousClassBuilder("$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();

生成代码

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;
  }
}
  • 匿名内部类

需要使用Type.anonymousInnerClass(""),通常可使用$L占位符来指代

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();

生成代码

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

定义匿名内部类的1个特别辣手的问题是参数的构造。在上面的代码中我们传递了不带参数的空字符串。TypeSpec.anonymousClassBuilder(“”)。

  • 注解

注解使用起来比较简单

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

生成代码

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

通过AnnotationSpec.builder() 可以对注解设置属性:

MethodSpec logRecord = MethodSpec.methodBuilder("recordEvent")
    .addModifiers(Modifier.PUBLIC, Modifier.ABSTRACT)
    .addAnnotation(AnnotationSpec.builder(Headers.class)
        .addMember("accept", "$S", "application/json; charset=utf⑻")
        .addMember("userAgent", "$S", "Square Cash")
        .build())
    .addParameter(LogRecord.class, "logRecord")
    .returns(LogReceipt.class)
    .build();

代码生成以下

@Headers(
    accept = "application/json; charset=utf⑻",
    userAgent = "Square Cash"
)
LogReceipt recordEvent(LogRecord logRecord);

注解一样可以注解其他注解,通过$L援用如

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", "$S", "application/json; charset=utf⑻")
            .build())
        .addMember("value", "$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();

生成代码

@HeaderList({
    @Header(name = "Accept", value = "application/json; charset=utf⑻"),
    @Header(name = "User-Agent", value = "Square Cash")
})
LogReceipt recordEvent(LogRecord logRecord);

0x03 后续

在javapoet之前有javawriter,但javapoet有着更强大的代码模型,并且对类的理解更加到位,因此推荐使用javapoet

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