反射

在类加载时, 会在内存中生成一份Object.class对象. 该对象为我们访问字节码文件提供了接口.
1. 获取Class对象
    1). Class<Person> clz = Class.forName("com.xxx.Person");
    2). Class<Proson> clz = com.xxx.Person.class;

2. 获取所有已经声明的构造函数, 并根据构造函数创建实例.
    Constructor[] constructors = clz.getDeclaredConstructors();
    constructors[0].setAccessible(true);
    constructors[0].newInstance();          //无参构造函数
    
3. 获取方法, 并调用
    Method m = clz.getMethod("setName",String.class);
    m.invoke(new Person(),"wangwu");
    
4. 获取注解的值
    Method[] methods = clz.getDeclaredMethods();
    for (Method method : methods) {
        if(method.isAnnotationPresent(Name.class)){
            Name name = method.getAnnotation(Name.class);
            System.out.println(name.value());
        }
    }

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Name注解定义
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Name {
    String value() default "xiaoming";
}

Person对象定义
public class Person(){
    public Person(){}
    private String name;
    @Name(value="zhangsan")
    public void setName(String name){
        this.name = name;
    }
}