注解和反射笔记

Dear 丶 2021-09-28 12:20 413阅读 0赞

*  自定义两个注解

@Target({ElementType.CONSTRUCTOR, ElementType.TYPE})
    @Retention(RetentionPolicy.RUNTIME)//运行时加载Annotation 到JVM 中，只有此状态可通过反射获取注解的信息。
    @interface A {
    	String value() default "默认构造方法";//成员
    }
    
    @Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER})
    @Retention(RetentionPolicy.RUNTIME)
    @interface B {
    	String describe();
    	Class<?> type() default void.class;
    }

*  把以上两个注解来注解一个类

@A
    public class Record {
    	@B(describe="编号", type=int.class)
    	int id;
    	
    	@B(describe="姓名", type=String.class)
    	String name;
    	
    	@A()
    	public Record() {
    		
    	}
    	
    	@A("立即初始化构造函数")
    	public Record(
    			@B(describe="编号", type=int.class) int id,
    			@B(describe="姓名", type=String.class) String name) {
    		this.id = id;
    		this.name = name;
    	}
    	
    	@B(describe="获取id", type=int.class)
    	public int getId() {
    		return id;
    	}
    
    	@B(describe="设置id", type=int.class)
    	public void setId(@B(describe="编号", type=int.class) int id) {
    		this.id = id;
    	}
    
    	@B(describe="获取姓名", type=String.class)
    	public String getName() {
    		return name;
    	}
    
    	@B(describe="设置姓名", type=String.class)
    	public void setName(@B(describe="姓名", type=String.class) String name) {
    		this.name = name;
    	}		
    	
    }

*  测试类

public class TestAnnotation {
    	public static void main(String[] args) throws Exception {
    		Class<?> clazz = Record.class;
    		
    		System.out.println("类上是否有注解：" + clazz.isAnnotationPresent(A.class));
    		A a = clazz.getAnnotation(A.class);
    		System.out.println("A注解的value值：" + a.value());
    		
    		System.out.println("---------");
    		System.out.println("属性上的注解：");
    		Field[] fields = clazz.getDeclaredFields();
    		for(Field f: fields) {
    			System.out.println(f.getName() + ": ");
    			Annotation[] annotations = f.getAnnotations();//与注解本身名字有何不同呢？其实就是多态
    			for(Annotation annotation: annotations) {
    				System.out.println("     " + annotation);
    			}
    		}
    		
    		System.out.println("---------");
    		System.out.println("构造方法上的注解：");
    		Constructor<?>[] constructors = clazz.getDeclaredConstructors();
    		for (Constructor<?> constructor : constructors) {
    			constructor.setAccessible(true);
    			System.out.println(constructor);
    			Annotation[] annotations = constructor.getAnnotations();
    			for (Annotation annotation : annotations) {
    				System.out.println("     " + annotation);
    			}
    		}
    		
    		System.out.println("---------");
    		System.out.println("方法上的注解：");
    		Method[] methods = clazz.getDeclaredMethods();
    		for(Method m: methods) {
    			System.out.println(m.getName() + ":");
    			Annotation[] annotations = m.getDeclaredAnnotations();
    			for(Annotation annotation: annotations) {
    				System.out.println("     " + annotation);
    			}
    		}
    	}
    }

*  运行结果

类上是否有注解：true
    A注解的value值：默认构造方法
    ---------
    属性上的注解：
    id: 
         @com.qhf.annotation.B(type=int, describe=编号)
    name: 
         @com.qhf.annotation.B(type=class java.lang.String, describe=姓名)
    ---------
    构造方法上的注解：
    public com.qhf.annotation.Record()
         @com.qhf.annotation.A(value=默认构造方法)
    public com.qhf.annotation.Record(int,java.lang.String)
         @com.qhf.annotation.A(value=立即初始化构造函数)
    ---------
    方法上的注解：
    getName:
         @com.qhf.annotation.B(type=class java.lang.String, describe=获取姓名)
    getId:
         @com.qhf.annotation.B(type=int, describe=获取id)
    setName:
         @com.qhf.annotation.B(type=class java.lang.String, describe=设置姓名)
    setId:
         @com.qhf.annotation.B(type=int, describe=设置id)