注解和反射

注解和反射

image-20240806090340858

元注解

package lesson05;

import java.lang.annotation.*;

//定义元注解

public class Test01 {
    @MyAnnotation
    public void test(){}
}

//定义一个注解
//Target表示注解可以用在哪些地方
@Target(value = {ElementType.METHOD,ElementType.TYPE})
//Retention表示我们的注解在什么地方还有效
//runtime>class>source
//Documented表示是否将注解生成在javadoc中
@Documented
//Inherited 子类可以继承父类的注解
@Inherited
@Retention(value = RetentionPolicy.RUNTIME)
@interface MyAnnotation{

}

自定义注解

package lesson05;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

//自定义注解
public class Test02 {
    //注解可以显示赋值，如果没有默认值，就必须赋值
    @MyAnnotation2(schools = "大学")
    public void test(){}

    @MyAnnotation3("jf")
    public void test2(){}
}

@Target({ElementType.TYPE,ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@interface MyAnnotation2{
    //注解的参数 参数类型 +参数名();
    String name() default "";
    int age() default 0;
    int id() default -1;//如果默认值为-1，代表不存在,indexof,如果找不到就返回-1.
    String[] schools();
}

@Target({ElementType.TYPE,ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@interface MyAnnotation3{
    String value();
}

测试Class类的创建方式有哪些

package lesson05;

//测试Class类的创建方式有哪些
public class Test04 {
    public static void main(String[] args) throws ClassNotFoundException {
        Person person = new Student();
        System.out.println("这个人是："+person.name);
        //方式一：通过对象获得
        Class c1 = person.getClass();
        System.out.println(c1.hashCode());
        //方式二：forname获得
        Class c2 = Class.forName("lesson05.Student");
        System.out.println(c2.hashCode());
        //方式三：通过类名.class获得
        Class c3 = Student.class;
        System.out.println(c3.hashCode());
        //方式四：基本内置类型的包装类都有一个Type属性
        Class c4 = Integer.TYPE;
        System.out.println(c4);
        //获得父类类型
        Class c5 = c1.getSuperclass();
        System.out.println(c5);
    }
}

class Person{
   public String name;

    public Person() {
    }

    public Person(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                '}';
    }
}

class Student extends Person{
    public Student(){
        this.name="学生";
    }
}
class Teacher extends Person{
    public Teacher(){
        this.name="老师";
    }
}

所有类型的Class

package lesson05;

import java.lang.annotation.ElementType;
import java.lang.annotation.Target;

//所有类型的Class
public class Test05 {
    public static void main(String[] args) {
        Class c1 = Object.class;//类
        Class c2 = Comparable.class;//接口
        Class c3 = String[].class;//一维数组
        Class c4 = int[][].class;//二维数组
        Class c5 = Override.class;//注解
        Class c6 = ElementType.class;//枚举
        Class c7 = Target.class;//基本数据类型
        Class c8 = void.class;
        Class c9 = Class.class;

        System.out.println(c1);
        System.out.println(c2);
        System.out.println(c3);
        System.out.println(c4);
        System.out.println(c5);
        System.out.println(c6);
        System.out.println(c7);
        System.out.println(c8);
        System.out.println(c9);

        //只要元素类型与维度一样，就是同一个Class
        int[] a = new int[10];
        int[] b = new int[100];
        System.out.println(a.getClass().hashCode());
        System.out.println(b.getClass().hashCode());

    }
}

类加载器

package lesson05;

public class Test06 {
    static {
        System.out.println("Main类被加载");
    }

    public static void main(String[] args) throws ClassNotFoundException {
        //主动引用
       // Son son =new Son();
        //反射也会产生主动引用
        //Class.forName("lesson05.Son");
        //不会产生类的引用的方法
        //System.out.println(Son.b);
        //Son[] array=new Son[5];
        System.out.println(Son.M);
    }

}

class Father {
    static int b=2;
    static {
        System.out.println("父类被加载");
    }
}

class Son extends Father{
    static {
        System.out.println("子类被加载");
    }
    static int m=100;
    static final int M=1;
}

获取类的加载器

package lesson05;

public class Test07 {
    public static void main(String[] args) throws ClassNotFoundException {
        //获取系统类的加载器
        ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();
        System.out.println(systemClassLoader);
        //获取系统类的加载器的父类加载器--》扩展类加载器
        ClassLoader parent = systemClassLoader.getParent();
        System.out.println(parent);
        //获取扩展类加载器的父类加载器-->根加载器（c++）
        ClassLoader parent1 = parent.getParent();
        System.out.println(parent1);
        //测试当前类是哪个加载器加载的
        ClassLoader classLoader = Class.forName("lesson05.Test07").getClassLoader();
        System.out.println(classLoader);
        //测试JDK内置类是谁加载的
        classLoader = Class.forName("java.lang.Object").getClassLoader();
        System.out.println(classLoader);
        //如何获得系统类加载器可以加载的路径
        System.out.println(System.getProperty("java.class.path"));
    }
}

获取类的运行时结构

package lesson05;

import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;

public class Test08 {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException, NoSuchMethodException {
        Class c1 = Class.forName("lesson05.User");
        //获得类的名字
        System.out.println(c1.getName());//获得包名+类名
        System.out.println(c1.getSimpleName());//获得类名
        //获得类的属性
        Field[] fields = c1.getFields();//只能找到public属性
        fields=c1.getDeclaredFields();//找到全部属性
        for (Field field : fields) {
            System.out.println(field);
        }
        //获得指定属性的值
        Field name = c1.getDeclaredField("name");
        System.out.println(name);
        //获得类的方法
        Method[] methods = c1.getMethods();//获得本类及其父类的全部public方法
        for (Method method : methods) {
            System.out.println("正常的："+method);
        }
        methods=c1.getDeclaredMethods();//获得本类的所有方法
        for (Method method : methods) {
            System.out.println("getDeclaredMethods:"+method);
        }
        //获得指定方法
        Method getname = c1.getMethod("getName",null);
        Method setname = c1.getMethod("setName", String.class);
        System.out.println(getname);
        System.out.println(setname);
        //获得指定的构造器
        System.out.println("===============================");
        Constructor[] constructors = c1.getConstructors();
        for (Constructor constructor : constructors) {
            System.out.println(constructor);
        }
        constructors = c1.getDeclaredConstructors();
        for (Constructor constructor : constructors) {
            System.out.println("#"+constructor);
        }
        //获得指定的构造器
        Constructor declaredConstructor = c1.getDeclaredConstructor(String.class, int.class);
        System.out.println("指定的："+declaredConstructor);
    }
}

通过反射获得方法和操作属性

package lesson05;


import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

//动态的创建对象，通过反射
public class Test09 {
    public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {
        //获得class对象
        Class c1 = Class.forName("lesson05.User");

        //构造一个对象
        //User user = (User)c1.newInstance();//本质是调用了无参构造器
        //System.out.println(user);
        //通过构造器创建对象
//        Constructor constructor = c1.getDeclaredConstructor(String.class, int.class);
//        User user2 = (User) constructor.newInstance("jf", 90);
//        System.out.println(user2);

        //通过反射调用普通方法
        User user3 = (User)c1.newInstance();
        //通过反射获取一个方法
        Method setName = c1.getDeclaredMethod("setName",String.class);
        //invoke：激活的意思
        //(对象，“方法的值”)
        setName.invoke(user3,"jf");
        System.out.println(user3.getName());
        //通过反射操作属性
        User user4 = (User)c1.newInstance();
        Field name = c1.getDeclaredField("name");
        name.setAccessible(true);//关闭私有属性安全检测，不能直接操作私有属性
        name.set(user4,"jf");
        System.out.println(user4.getName());

    }
}

性能对比分析

package lesson05;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

//分析性能问题
public class Test10 {
    //普通方式调用
    public static void test1(){
        User user = new User();
        long startTime= System.currentTimeMillis();
        for (int i = 0; i < 1000000000; i++) {
            user.getName();
        }
        long endTime= System.currentTimeMillis();
        System.out.println("普通方式执行10亿次:"+(endTime-startTime)+"ms");
    }

    //反射方式调用
    public static void test2() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        User user = new User();
        Class c1 = user.getClass();
        Method getName = c1.getDeclaredMethod("getName", null);

        long startTime= System.currentTimeMillis();
        for (int i = 0; i < 1000000000; i++) {
            getName.invoke(user,null);
        }
        long endTime= System.currentTimeMillis();
        System.out.println("普通方式执行10亿次:"+(endTime-startTime)+"ms");
    }
    //反射方式调用,关闭检测
    public static void test3() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        User user = new User();
        Class c1 = user.getClass();
        Method getName = c1.getDeclaredMethod("getName", null);
        getName.setAccessible(true);

        long startTime= System.currentTimeMillis();
        for (int i = 0; i < 1000000000; i++) {
            getName.invoke(user,null);
        }
        long endTime= System.currentTimeMillis();
        System.out.println("普通方式执行10亿次:"+(endTime-startTime)+"ms");
    }

    public static void main(String[] args) throws InvocationTargetException, NoSuchMethodException, IllegalAccessException {
        test1();
        test2();
        test3();
    }
}

反射操作泛型

package lesson05;

import java.lang.reflect.Method;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.List;
import java.util.Map;

public class Test11 {
    public void test01(Map<String, User> map, List<User> list) {
        System.out.println("test01");
    }

    public Map<String, User> test02() {
        System.out.println("test02");
        return null;
    }

    //通过注解获得泛型
    public static void main(String[] args) throws NoSuchMethodException {
        Method method = Test11.class.getMethod("test01", Map.class, List.class);
        Type[] genericParameterTypes = method.getGenericParameterTypes();//获得泛型的信息
        for (Type genericParameterType : genericParameterTypes) {
            System.out.println("#" + genericParameterType);
            if (genericParameterType instanceof ParameterizedType) {//属于参数化类型
                Type[] actualTypeArguments = ((ParameterizedType) genericParameterType).getActualTypeArguments();//真实参数化信息
                for (Type actualTypeArgument : actualTypeArguments) {
                    System.out.println(actualTypeArgument);
                }
            }
        }
        method = Test11.class.getMethod("test02", null);
        Type genericReturnType = method.getGenericReturnType();
        if (genericReturnType instanceof ParameterizedType) {//属于参数化类型
            Type[] actualTypeArguments = ((ParameterizedType) genericReturnType).getActualTypeArguments();//真实参数化信息
            for (Type actualTypeArgument : actualTypeArguments) {
                System.out.println(actualTypeArgument);
            }
        }
    }
}

通过反射操作注解

package lesson05;

import java.lang.annotation.*;
import java.lang.reflect.Field;

//练习反射操作注解
public class Test12 {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
        Class c1 = Class.forName("lesson05.Student2");
        //通过反射获得注解
        Annotation[] annotations = c1.getAnnotations();
        for (Annotation annotation : annotations) {
            System.out.println(annotation);
        }
        //获得注解value的值
        Table01 table01 = (Table01)c1.getAnnotation(Table01.class);
        String value = table01.value();
        System.out.println(value);
        //获得类指定的注解
        Field f = c1.getDeclaredField("name");
        Field01 annotation = f.getAnnotation(Field01.class);
        System.out.println(annotation.columName());
        System.out.println(annotation.type());
        System.out.println(annotation.length());


    }

}

@Table01("db_student")
class Student2{
    @Field01(columName = "db_id",type="int",length = 10)
    private int id;
    @Field01(columName = "db_age",type="int",length = 10)
    private int age;
    @Field01(columName = "db_name",type="varchar",length = 3)
    private String name;

    public Student2() {
    }

    public Student2(int id, int age, String name) {
        this.id = id;
        this.age = age;
        this.name = name;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getName() {
        return name;
    }

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

    @Override
    public String toString() {
        return "Student2{" +
                "id=" + id +
                ", age=" + age +
                ", name='" + name + '\'' +
                '}';
    }
}

//类名的注解
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface Table01{
    String value();
}

//属性的注解
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface Field01{
    String columName();
    String type();
    int length();
}
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