Java 《注解篇》编译时注解

梦里梦外; 2022-05-24 13:58 259阅读 0赞

## 基本概念 ##

<table> 
 <thead> 
  <tr> 
   <th>注解类型</th> 
   <th>特点</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>RetentionPolicy.SOURCE</td> 
   <td>注解只保留在源文件，当Java文件编译成class文件的时候，注解被遗弃；</td> 
  </tr> 
  <tr> 
   <td>RetentionPolicy.CLASS</td> 
   <td>注解被保留到class文件，但jvm加载class文件时候被遗弃，这是默认的生命周期；</td> 
  </tr> 
  <tr> 
   <td>RetentionPolicy.RUNTIME</td> 
   <td>注解不仅被保存到class文件中，jvm加载class文件之后，仍然存在</td> 
  </tr> 
 </tbody> 
</table>

## 参考文章 ##

https://segmentfault.com/a/1190000009756015  
http://www.jcodecraeer.com/a/anzhuokaifa/2017/0803/8336.html

## APT ##

> 注解处理器 (Annotation Processor) 是Javac的一个工具

*  每一个处理器都是继承于 AbstractProcessor

### 先了解下一些类的概念 ###

<table> 
 <thead> 
  <tr> 
   <th>ProcessingEnvironment</th> 
   <th align="center">很重要的工具类</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>Elements getElementUtils</td> 
   <td align="center">返回一些实用的代码实现</td> 
  </tr> 
  <tr> 
   <td>Types getTypeUtils</td> 
   <td align="center">获取源代码的类型信息</td> 
  </tr> 
  <tr> 
   <td>Filer getFiler</td> 
   <td align="center">创建文件时使用</td> 
  </tr> 
 </tbody> 
</table>

## Demo ##

> 工程名 pro

### 定义注解类 ###

@Retention(RetentionPolicy.SOURCE) // 注解只在源码中保留
    @Target(ElementType.TYPE) // 用于修饰类
    public @interface Hello { 
        String name() default "";
    }

### AbstractProcessor ###

public class HelloProcessor extends AbstractProcessor { 
        // 计数器, 用于计算 process() 方法运行了几次
        private int count = 1;
    
        // 用于写文件
        private Filer filer;
        private Types typeUtils;
    
        @Override
        public synchronized void init(ProcessingEnvironment processingEnv) { 
            super.init(processingEnv);
            this.typeUtils = processingEnv.getTypeUtils();
            this.filer = processingEnv.getFiler();
        }
    
        /** * 源码级别 * @return */
        @Override
        public SourceVersion getSupportedSourceVersion() { 
            return SourceVersion.RELEASE_8;
        }
    
        /** * 处理的注解类型 * @return */
        @Override
        public Set<String> getSupportedAnnotationTypes() { 
            Set<String> types = new LinkedHashSet<>();
            types.add(Hello.class.getCanonicalName());
            return types;
        }
    
        // 处理编译时注解的方法
        @Override
        public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) { 
    
            System.out.println("start process, count = " + count++);
            // 获得所有类
            Set<? extends Element> rootElements = roundEnv.getRootElements();
            System.out.println("all class:");
            // 输出类型
            for (Element rootElement : rootElements) { 
                System.out.println("root： " + rootElement.getSimpleName());
            }
    
            // 获得有注解的元素, 这里 Hello 只能修饰类, 所以只有类
            Set<? extends Element> elementsAnnotatedWith = roundEnv.getElementsAnnotatedWith(Hello.class);
            System.out.println("annotated class:");
            for (Element element : elementsAnnotatedWith) { 
                TypeMirror elementType = element.asType();
                String name = typeUtils.erasure(elementType).toString();
                System.out.println("bf： " + name);
                //
                String className = element.getSimpleName().toString();
                System.out.println(" " + className);
    
                String output = element.getAnnotation(Hello.class).name();
                // 产生的动态类的名字
                String newClassName = className + "_New";
                // 写 java 文件
                createFile(newClassName, output);
            }
            return true;
        }
    
        private void createFile(String className, String output) { 
            StringBuilder cls = new StringBuilder();
            cls.append("package apt;\n\npublic class ")
                    .append(className)
                    .append(" {\n public static void main(String[] args) {\n")
                    .append(" System.out.println(\"")
                    .append(output)
                    .append("\");\n }\n}");
            try { 
                JavaFileObject sourceFile = filer.createSourceFile("apt." + className);
                Writer writer = sourceFile.openWriter();
                writer.write(cls.toString());
                writer.flush();
                writer.close();
            } catch (IOException e) { 
                e.printStackTrace();
            }
        }
    }

### 常用命令行编译 ###

javac -encoding UTF-8 -d out/production src/main/java/apt/HelloProcessor.java src/main/java/apt/Hello.java 
    
    
    javac -encoding UTF-8 -cp out/production/ -processor apt.HelloProcessor -d out/production -s src/ src/main/java/apt/*.java
    
    
     java -cp out/production apt/Player_New

> 输出结果

start process, count = 1
    all class:
    root：  Hello
    root：  HelloProcessor
    root：  Ignored
    root：  Player
    annotated class:
    bf：  apt.Player
      Player
    start process, count = 2
    all class:
    root：  Player_New
    annotated class:
    start process, count = 3
    all class:
    annotated class: