javapoet 学习笔记

javapoet是一个开源项目，用于动态生成java源码。使用这个插件，你可以new出一个method，然后设置它的入参返回值，方法体等，最后输出这个方法的java源码。
原本考虑使用CodeModel这个插件，奈何这个插件过于古老且早已不在维护，以及更重要的一点，使用codemodel创建class实在过于繁琐，想想以后的工作量就让我窒息。

 

[infobox title="基础语法"][/infobox]

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);

 

如上是一个简单的Demo，构建了一个类和一个方法，其实这代码是构建AST的过程，这些使用到的类都包含了emit方法，含义和toString一致，用于输出当前元素的java源码，通过递归调用生成整个源码，使用起来和JSqlParse差不多，不停的new一个个的元素，然后配置组装各种元素。只不过jSqlParse还支持代码解析到AST实现动态修改。
而javapoet没办法把java源码转换成对象，也就没办法修改现成的代码。

控制流 最终生成{}代码块
beginControlFlow
nextControlFlow
endControlFlow

迭代循环体 $L 最终的操作和string.format类似
beginControlFlow("for (int i = $L; i < $L; i++)", from, to)

$S 表示字符串变量
addStatement("return $S", name)

$T 表示类型 会生成对应的import
addStatement("return new $T()", Date.class)

ClassName 构建一个编译环境不存在的类
ClassName hoverboard = ClassName.get("com.mattel", "Hoverboard");

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

静态导入
addStaticImport(namedBoards, "*")

$N 调用类中已定义的方法
.addStatement("result[1] = $N(b & 0xf)", hexDigit)

CodeBlock 代码块格式化

// zfc占位符 增强版 $indexL index > 0
CodeBlock.builder().add("I ate $2L $1L", "tacos", 3).build().toString()



// 直接读取属性

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

定义抽象方法和抽象类

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

构造函数

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();

方法参数

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();

变创建量

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

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

定义接口 方法和属性的修饰符固定不能变 但输出的时候 又会忽略

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();

定于一个枚举类

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

一个比较复杂的枚举

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();

匿名内部类

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();

添加注解

.addAnnotation(Override.class)
.addAnnotation(AnnotationSpec.builder(Headers.class)
        .addMember("accept", "$S", "application/json; charset=utf-8")
        .addMember("userAgent", "$S", "Square Cash")
        .build())

注解组合

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();

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