示列——使用注解反射实例化类

1.先构建注解：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface Bean {
  String name() default "";
}

2.根据包名拿到路径下的class文件，同时在根据指定的注解去实例化

public class SpringDemo {
        public static void main(String[] args)throws IOException {
            String packageName="com.springdemo";
            Map excuten = excuten(packageName);
            System.out.println(excuten);
        }
        /*由包名拿到class文件的全路径名*/
    public static String getpaths(String packageName){
        ClassLoader loader = SpringDemo.class.getClassLoader();
        URL resource = loader.getResource(packageName.toString().replace('.', '/'));
        String path = resource.getPath();
        File file = new File(path);
        String absolutePath = file.getAbsolutePath();
        return absolutePath;
    }
    /*字符串的截取，截取为包名的形式 */
    public static String packageName(Path path){
        String s1 = path.toString();
        int com = s1. lastIndexOf("com");
        int coms= s1.lastIndexOf(".class");
        String packageClassName = s1.substring(com, coms);
        String replace = packageClassName.replace('\\', '.');
        return replace;
    }
    //遍历文件目录那到class 文件，并通过反射拿到类中的注解类型。同时创建class 对象
    public static Map<String,? extends Object> excuten(String packageName) throws IOException {
           Map   map = new HashMap<>();
             String getpaths = getpaths(packageName);
            Path path = Paths.get(getpaths);
            Files.walkFileTree(path, new SimpleFileVisitor<Path>(){
                @Override
                public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {
                    if(file.toString().endsWith(".class")){
                        String packageName = packageName(file);//拿到包路径
                        try {
                            Class<?> aClass = Class.forName(packageName);
                            Bean bean = aClass.getAnnotation(Bean.class);
                            if (bean!=null){
                                String name = bean.name();
                                if (name.length()<1){
                                       name=aClass.getSimpleName();
                                 map.put(name,Class.forName(packageName).newInstance());
                                }else{
                                    map.put(name,Class.forName(packageName).newInstance());
                                }
                            }
                        }catch (Exception e){
                            e.printStackTrace();
                        }
                    }
                    return FileVisitResult.CONTINUE;
                }
            });
            return map;
        }
}