注解相关知识回顾

概述

（1.）注解可以理解为一种使用相关信息来绑定Java文件元素的方式
（2.）Java中常见的注解

• @Override: 表示必须对方法重写
• @Deprecated : 表示方法过时
• @SuppresWarning : 表示忽略警告
• @FunctionalInterface : 表示函数式接口
  （3.）按照运行机制分类:
• 源码注解: 注解只在源码中存在，当编译生成.class文件的时候，就不存在了
• 编译时注解: 注解在源码和.class文件中存在，当程序运行的时候它就不存在了
• 运行时注解: 在运行的时候还能够起作用的注解，它会动态影响到程序执行逻辑
• 元注解: 对注解进行注解
  元注解:
  @Target:标识它所标识的注解能够作用在什么元素上
  如果一个注解的@Target上面的注解范围显示了ANNOTATION_TYPE，那么表示该注解是元注解
  @Retention: 表示它所标识的注解的生命周期
  RetentionPolicy.SOURCE:表示该注解只在源码中存在
  RetentionPolicy.CLASS: 表示该注解只在源码和编译时存在
  RetentionPolicy.RUNTIME: 表示该注解在源码和编译时以及运行时存在
  @Inherited: 表示该注解可以被继承
  @Document: 表示该注解可以在生成API文档的时候显示在文档上

通过反射可以获取到字节码文件对象，通过字节码文件对象就能够获取到所有的有关该类的元素 然后注解也是类元素的一种，所以可以获取到对应的注解，但是这个注解必须是运行时注解

• A getAnnotation(Class annotationClass)
• Annotation[] getAnnotations()
• Annotation[] getDeclaredAnnotations()
• boolean isAnnotationPresent(Class annotationClass)

public class AnnotationDemo01 {
	public static void main(String[] args) throws ClassNotFoundException {
		Class<?> c = Class.forName("com.xt.annotationdemo02.Student");
		
		// 判断该字节码文件中是否存在@MyAnnotation注解
		boolean isAnnotationPresent = c.isAnnotationPresent(MyAnnotation.class);
		
		if (isAnnotationPresent) {
			MyAnnotation annotation = c.getDeclaredAnnotation(MyAnnotation.class);
			System.out.println(annotation.value());
		}
		
		System.out.println("----------成员方法-------------");
		Method[] methods = c.getMethods();
		for (Method m : methods) {
			boolean present = m.isAnnotationPresent(MyAnnotation.class);
			if (present) {
				MyAnnotation declaredAnnotation = m.getDeclaredAnnotation(MyAnnotation.class);
				System.out.println(declaredAnnotation.value());
			}
		}
		
		System.out.println("---------构造方法--------------");
		Constructor<?>[] constructors = c.getConstructors();
		for (Constructor<?> constructor : constructors) {
			boolean present = constructor.isAnnotationPresent(MyAnnotation.class);
			if (present) {
				MyAnnotation declaredAnnotation = constructor.getDeclaredAnnotation(MyAnnotation.class);
				System.out.println(declaredAnnotation.value());
			}
		}
		
		System.out.println("---------成员变量--------------");
		Field[] fields = c.getDeclaredFields();
		for (Field field : fields) {
			boolean present = field.isAnnotationPresent(MyAnnotation.class);
			if (present) {
				MyAnnotation declaredAnnotation = field.getDeclaredAnnotation(MyAnnotation.class);
				System.out.println(declaredAnnotation.value());
			}
		}
		
		System.out.println("---------toString--------------");
		System.out.println("获取tostring方法");
		boolean annotationPresent = c.isAnnotationPresent(Override.class);
		if (annotationPresent) {
			MyAnnotation annotation = c.getDeclaredAnnotation(MyAnnotation.class);
			System.out.println(annotation.value());
		}
		
		System.out.println("---------获取PrimaryStudent上的注解--------------");
		Class<?> c2 = Class.forName("com.xt.annotationdemo02.PrimaryStudent");
		boolean result = c2.isAnnotationPresent(MyAnnotation.class);
		System.out.println(result);
	}
}

@MyAnnotation("T_STU")
class Student{
	@MyAnnotation("NAME")
	private String name;
	@MyAnnotation("AGE")
	private int age;
	
	@MyAnnotation("STU")
	public Student() {
		super();
	}
	public Student(String name, int age) {
		super();
		this.name = name;
		this.age = age;
	}
	@MyAnnotation("getName")
	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public int getAge() {
		return age;
	}

	@MyAnnotation("setAge")
	public void setAge(int age) {
		this.age = age;
	}
	
	@MyAnnotation("I'm a toString method")
	@Override
	public String toString() {
		return "Student [name=" + name + ", age=" + age + "]";
	}	
}
class PrimaryStudent extends Student{	
}
@Inherited
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE,ElementType.FIELD,ElementType.METHOD,ElementType.CONSTRUCTOR})
@interface MyAnnotation{
	String value();
}

使用注解编写工具类实现Bean和Map之间的转换

/*

• 书写一个工具类
• 将 Map --> JavaBean
• JavaBean --> Map
  */

public class AnnotationTest {
	public static void main(String[] args) throws InstantiationException, IllegalAccessException, NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException {
		// Map map = new HashMap();
		// map.put("name", "张三");
		// map.put("age", 18);
		//
		// User user = PojoMappingUtils.mapToBean(map, User.class);
		// System.out.println(user);
		//
		// Map map2 = new HashMap();
		// map2.put("name", "张三老师");
		// map2.put("age", 18);
		// map2.put("address", "北京西路");
		// map2.put("tid", "1001");
		// Teacher t = PojoMappingUtils.mapToBean(map2, Teacher.class);
		// System.out.println(t);
		
		Teacher t = new Teacher();
		t.setTid("1001");
		t.setName("老师");
		t.setAge(18);
		t.setAddress("北京西路");
		
		User user = new User();
		user.setName("zhangsan");
		user.setAge(18);
		Map<String, Object> map = PojoMappingUtils.beanToMap(user);
		Set<String> keys = map.keySet();
		for (String key : keys) {
			System.out.println(key + "=" + map.get(key));
		}
	}
}

@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface Column {
	String value();
}

/** * 这是一个利用 反射 + 注解 + 泛型 封装的map和javabean相互转换的一个工具类 * @author xyr */
public class PojoMappingUtils {
	private PojoMappingUtils() throws IllegalAccessException {
	}
	
	// User Emlopyee 外界告诉我这个方法 你到底希望我转成什么 bean
	public static <T> T mapToBean(Map<String, Object> map, Class<T> clazz) throws InstantiationException, IllegalAccessException, NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException {
		T t = clazz.newInstance();
		Field[] fields = clazz.getDeclaredFields();
		for (Field field : fields) {
			if (field.isAnnotationPresent(Column.class)) {
				Column columnAnnotation = field.getAnnotation(Column.class);
				if (columnAnnotation != null) {
					Object o = map.get(columnAnnotation.value());
					if (o != null) {
						String fieldName = field.getName();
						Method method = clazz.getMethod("set" + fieldName.substring(0, 1).toUpperCase() + fieldName.substring(1), o.getClass());
						method.invoke(t, o);
					}
				}
			}
		}
		return t;
	}
	
	@Test
	public void testBeanToMap() throws NoSuchMethodException, SecurityException, IllegalAccessException, IllegalArgumentException, InvocationTargetException {
		User user = new User();
		user.setName("zhangsan");
		user.setAge(18);
		Map<String, Object> map = beanToMap(user);
		System.out.println(map);
	}
	
	public static <T> Map<String, Object> beanToMap(T t) throws NoSuchMethodException, SecurityException, IllegalAccessException, IllegalArgumentException, InvocationTargetException{
		// 创建容器来装载bean中的数据
		Map<String, Object> map = new HashMap<String, Object>();
		// 获取bean多对应的字节码文件对象
		Class<?> c = t.getClass();
		// 获取到bean中所有的成员变量对象
		Field[] fields = c.getDeclaredFields();
		for (Field field : fields) {
			// 获取到每一个成员变量对象 name 变量
			if (field.isAnnotationPresent(Column.class)) {
				Column columnAnnotation = field.getAnnotation(Column.class);
				if (columnAnnotation != null) {
					String value = columnAnnotation.value();  // name字符串
					// 获取字段名称 name
					String fieldName = field.getName();
					// 获取getName方法
					Method method = c.getMethod("get" + fieldName.substring(0, 1).toUpperCase() + fieldName.substring(1));
					Object getValue = method.invoke(t);
					map.put(value, getValue);
				}
			}
		}
		return map;
	}
	
}

public class User {
	
	@Column("name")
	private String name;
	@Column("age")
	private Integer age;
	
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public int getAge() {
		return age;
	}
	public void setAge(Integer age) {
		this.age = age;
	}
	@Override
	public String toString() {
		return "User [name=" + name + ", age=" + age + "]";
	}	
}