** 在之前的“Java中的屠龍之術：如何修改語法樹”中，我們詳細介紹瞭如何使用Javac原始碼提供的工具類來修改語法樹。**

而在此基礎上，有一款開源工具javapoet可以更加快捷地生成位元組碼，實現原理其實也就是對JavaAPT的封裝，然而Javapoet有一個侷限性，就是隻能生成新的.class檔案，卻無法修改原有的類，這也是它的一大侷限性所在。接下來就讓我們看看它的使用方法把。

## 0x00 概述

註解系列

*   [註解基礎](https://link.juejin.cn?target=https%3A%2F%2Fmp.weixin.qq.com%2Fs%3F__biz%3DMzU5OTgzNzM0NA%3D%3D%26mid%3D100000012%26idx%3D1%26sn%3Dea419145d678c6a39c3981cb8512b219%26scene%3D19%26token%3D1145503795%26lang%3Dzh_CN%23wechat_redirect "註解基礎")
*   [JavaPoet](https://link.juejin.cn?target=https%3A%2F%2Fmp.weixin.qq.com%2Fs%3F__biz%3DMzU5OTgzNzM0NA%3D%3D%26mid%3D100000024%26idx%3D1%26sn%3D118f2761faee835af670d52922a69228%26scene%3D19%26token%3D1145503795%26lang%3Dzh_CN%23wechat_redirect "JavaPoet")
*   [編譯期註解處理之APT](https://link.juejin.cn?target=https%3A%2F%2Fmp.weixin.qq.com%2Fs%3F__biz%3DMzU5OTgzNzM0NA%3D%3D%26mid%3D100000019%26idx%3D1%26sn%3D035fc80b9c8128ea10a11b5759f70a21%26scene%3D19%26token%3D1145503795%26lang%3Dzh_CN%23wechat_redirect "編譯期註解處理之APT")

上一篇限於篇幅只介紹了APT，這篇來繼續介紹[javapoet](https://link.juejin.cn?target=https%3A%2F%2Fgithub.com%2Fsquare%2Fjavapoet "javapoet")，是square公司的開源庫。正如其名，java詩人，透過註解來生成java原始檔，通常要使用javapoet這個庫與Filer配合使用。主要和註解配合用來幹掉那些重複的模板程式碼(如butterknife 和databinding所做的事情)，當然你也可以使用這個技術讓你的程式碼更加的炫酷。

## 0x01 簡單使用

使用之前要先引入這個庫

```
compile 'com.squareup:javapoet:1.7.0'
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

javapoet是用來生成程式碼的，需要藉助

> 常用類

使用javapoet前需要了解8個常用類

| 類名 | 作用 |
| --- | --- |
| MethodSpec | 代表一個建構函式或方法宣告 |
| TypeSpec | 代表一個類，介面，或者列舉宣告 |
| FieldSpec | 代表一個成員變數，一個欄位宣告 |
| JavaFile | 包含一個頂級類的Java檔案 |
| ParameterSpec | 用來建立引數 |
| AnnotationSpec | 用來建立註解 |
| ClassName | 用來包裝一個類 |
| TypeName | 型別，如在新增返回值型別是使用 TypeName.VOID |

除此之外 JavaPoet提供了一套自定義的字串格式化規則，常用的有

| 格式化規則 | 表示含義 |
| --- | --- |
| $L | 字面量 |
| $S | 字串 |
| $T | 類、介面 |
| $N | 變數 |

## 0x02 使用進階

下面由淺入深，循序漸進的說明用法

> 方法&控制流：

*   新增方法 `addcode` 和 `addstatement` 對與無需類引入的極簡程式碼可以直接使用`addCode`

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

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

生成的是

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

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

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`來宣告

*   更優雅的流控制

`beginControlFlow` 流開啟 `addStatement` 處理語句 `endControlFlow()`流結束

如上面的用流改寫就是

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

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

> 佔位符

javapoet裡面提供了佔位符來幫助我們更好地生成程式碼

*   $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();
}
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```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

這個就是一個for迴圈，op負責加減乘除等符號

*   $S 字串常量（String）
*   $T 型別(Types)

`最大的特點是自動匯入包`

```
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);

複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

生成的程式碼如下，而且會自動導包

```
package com.example.helloworld;

import java.util.Date;

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

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

*   $N 命名(Names),通常指我們自己生成的方法名或者變數名等等

比如這樣的程式碼塊

```
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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```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

我們可以傳遞`hexDigit()`來代替。

```
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();
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

> 獲取對應類

有兩種方式:

*   ClassName.bestGuess(“類全名稱”) 返回ClassName物件，這裡的類全名稱表示的類必須要存在，會自動匯入相應的包

*   ClassName.get(“包名”，”類名”) 返回ClassName物件，不檢查該類是否存在

因此如果使用JavaPoet的話後續程式碼重構改變類名往往需要格外注意一點

```
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();
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

然後生成

```
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;
  }
}
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

> 構建類的元素

*   方法

方法的修飾，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();
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

這將會生成如下程式碼

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

複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

當然Methods需要和`MethodSpec.Builder`配置來增加方法引數、異常、javadoc、註解等。

*   構造器

這個其實也是個函式方法而已，因此可以使用MethodSpec來生成構造器方法。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();
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

將會生成

```
public class HelloWorld {
  private final String greeting;

public HelloWorld(String greeting) {
    this.greeting = greeting;
  }
}
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

*   引數(重要)

之前我們是透過`addstatement`直接設定引數，其實引數也有自己的一個專用類`ParameterSpec`，我們可以使用`ParameterSpec.builder()`來生成引數，然後MethodSpec的addParameter去使用，這樣更加優雅。

```
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();
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

生成的程式碼

```
void welcomeOverlords(final String android, final String robot) {
}
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

稍微複雜點的型別 比如泛型 、Map之類的，需要了解下JavaPoet定義的幾種專門描述型別的類

![](https://img-blog.csdnimg.cn/img_convert/124bcd9ff21dea12fed15561d77f4d8a.webp?x-oss-process=image/format,png)

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

常見的有

| 分類 | 生成的型別 | JavaPoet 寫法 | 也可以這麼寫 （等效的 Java 寫法） |
| --- | --- | --- | --- |
| 內建型別 | int | TypeName.INT | int.class |
| 陣列型別 | int[] | ArrayTypeName.of(int.class) | int[].class |
| 需要引入包名的型別 | java.io.File | ClassName.get(“[java.io](https://link.juejin.cn?target=http%3A%2F%2Fjava.io "java.io")”, “File”) | java.io.File.class |
| 引數化型別 （ParameterizedType | List | ParameterizedTypeName.get(List.class, String.class) | - |
| 型別變數 （WildcardType） 用於宣告泛型 | T | TypeVariableName.get(“T”) | - |
| 萬用字元型別 | ? extends String | WildcardTypeName.subtypeOf(String.class) | - |

```
/*
 *Build input type, format as :
 *Map<string class="" extends="" iroutegroup="">>
 */
    ParameterizedTypeName inputMapTypeOfRoot = ParameterizedTypeName.get(
            ClassName.get(Map.class),
            ClassName.get(String.class),
            ParameterizedTypeName.get(
                    ClassName.get(Class.class),
                    WildcardTypeName.subtypeOf(ClassName.get(type_IRouteGroup))
            )
    );

/*
     *Map<string routemeta="">
     */
    ParameterizedTypeName inputMapTypeOfGroup = ParameterizedTypeName.get(
            ClassName.get(Map.class),
            ClassName.get(String.class),
            ClassName.get(RouteMeta.class)
    );

/*
     *Build input param name.
     */
    ParameterSpec rootParamSpec = ParameterSpec.builder(inputMapTypeOfRoot, "routes").build();
    ParameterSpec groupParamSpec = ParameterSpec.builder(inputMapTypeOfGroup, "atlas").build();
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

生成引數型別

```
public class ARouter$Root$app implements IRouteRoot {
  @Override
  public void loadInto(Map<string class="" extends="" iroutegroup="">> routes) {
    routes.put("service", ARouter$Group$service.class);
    routes.put("test", ARouter$Group$test.class);
  }
}

public class ARouter$Group$service implements IRouteGroup {
  @Override
  public void loadInto(Map<string routemeta=""> atlas) {
    atlas.put("/service/hello", RouteMeta.build(RouteType.PROVIDER, HelloServiceImpl.class, "/service/hello", "service", null, -1, -2147483648));
    atlas.put("/service/json", RouteMeta.build(RouteType.PROVIDER, JsonServiceImpl.class, "/service/json", "service", null, -1, -2147483648));
    atlas.put("/service/single", RouteMeta.build(RouteType.PROVIDER, SingleService.class, "/service/single", "service", null, -1, -2147483648));
  }
}

複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

*   欄位

可以使用FieldSpec去宣告欄位，然後加到Method中處理

```
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();
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

然後生成程式碼

```
public class HelloWorld {
  private final String android;

private final String robot;
}
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

通常Builder可以更加詳細的建立欄位的內容，比如javadoc、annotations或者初始化欄位引數等，如：

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

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

對應生成的程式碼

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

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

*   介面

介面方法必須是PUBLIC ABSTRACT並且介面欄位必須是PUBLIC STATIC FINAL ，使用`TypeSpec.interfaceBuilder`

如下

```
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();
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

生成的程式碼如下

```
public interface HelloWorld {
  String ONLY_THING_THAT_IS_CONSTANT = "change";

void beep();
}
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```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

*   繼承父類 實現介面

介面程式碼

```
package com.test.javapoet;
public interface TestInterface<t> {
    void test(T testPara);
}
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

父類程式碼

```
public class TestExtendesClass {

}
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```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

使用javapoet實現介面並且繼承父類

```
final ClassName  InterfaceName = ClassName.get("com.test.javapoet","TestInterface");

ClassName superinterface = ClassName.bestGuess("com.test.javapoet.TestClass");
    //ClassName superinterface = ClassName.get("com.test.javapoet","aa");

TypeSpec.Builder spec = TypeSpec.classBuilder("TestImpl")
            .addModifiers(Modifier.PUBLIC)
            // 新增介面，ParameterizedTypeName的引數1是介面，引數2是介面的泛型
            .addSuperinterface(ParameterizedTypeName.get(InterfaceName, superinterface)) 
            //使用ClassName.bestGuess會自動匯入包
            .superclass(ClassName.bestGuess("com.zs.javapoet.test.TestExtendesClass"));

MethodSpec.Builder methodSpec = MethodSpec.methodBuilder("test")
            .addAnnotation(Override.class)
            .returns(TypeName.VOID)
            .addParameter(superinterface, "testPara")
            .addStatement("System.out.println(hello)" );

TypeSpec typeSpec = spec.addMethod(methodSpec.build()).build();

JavaFile file = JavaFile.builder("com.zs.javapoet", typeSpec).build();
    file.writeTo(System.out);

複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

生成程式碼

```
package com.test.javapoet;

import com.zs.javapoet.test.TestExtendesClass;
    import java.lang.Override;

public class TestImpl extends TestExtendesClass implements TestInterface<testclass> {
      @Override
      void test(TestClass testPara) {
        System.out.println(hello);
      }
    }
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

*   列舉型別

使用`TypeSpec.enumBuilder`來建立，使用`addEnumConstant`來新增

```
TypeSpec helloWorld = TypeSpec.enumBuilder("Roshambo")
    .addModifiers(Modifier.PUBLIC)
    .addEnumConstant("ROCK")
    .addEnumConstant("SCISSORS")
    .addEnumConstant("PAPER")
    .build();
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

生成的程式碼

```
public enum Roshambo {
  ROCK,

SCISSORS,

PAPER
}
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

更復雜的型別也可以支援，如重寫、註解等

```
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();
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

生成程式碼

```
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;
  }
}
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

*   匿名內部類

需要使用`Type.anonymousInnerClass("")`,通常可以使用$L佔位符來指代

```
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();
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

生成程式碼

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

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

定義匿名內部類的一個特別棘手的問題是引數的構造。在上面的程式碼中我們傳遞了不帶引數的空字串。TypeSpec.anonymousClassBuilder("")。

*   註解

註解使用起來比較簡單

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

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

生成程式碼

```
  @Override
  public String toString() {
    return "Hoverboard";
  }
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

透過`AnnotationSpec.builder()` 可以對註解設定屬性：

```
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();
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

程式碼生成如下

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

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

註解同樣可以註解其他註解，透過$L引用如

```
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", "$S", "application/json; charset=utf-8")
            .build())
        .addMember("value", "$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();
複製程式碼
```

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

生成程式碼

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

![](data:image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== "點選並拖拽以移動")

> 註釋

*   javadoc

## 0x03 後續

在javapoet之前有javawriter，但javapoet有著更強大的程式碼模型，並且對類的理解更加到位，因此推薦使用javapoet

## 參考文章

*   [juejin.cn/post/684490…](https://juejin.cn/post/6844903475776585741 "juejin.cn/post/684490…")

*   [cloud.tencent.com/developer/a…](https://link.juejin.cn?target=https%3A%2F%2Fcloud.tencent.com%2Fdeveloper%2Farticle%2F1006210 "cloud.tencent.com/developer/a…")

*   [blog.csdn.net/qq_26376637…](https://link.juejin.cn?target=https%3A%2F%2Fblog.csdn.net%2Fqq_26376637%2Farticle%2Fdetails%2F52374063 "blog.csdn.net/qq_26376637…")

作者：渡口一艘船
連結：https://juejin.cn/post/6844903456629587976
來源：稀土掘金
著作權歸作者所有。商業轉載請聯絡作者獲得授權，非商業轉載請註明出處。

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Java中的屠龍之術（二）：如何方便快捷地生成.class檔案

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