Java教程

【Java】注解与反射（三）——创建运行时类的对象

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✨获取类的运行时结构

package com.example.reflection;

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

//获取类的运行时结构
public class Test07 {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException, NoSuchMethodException {
        Class<?> c1 = Class.forName("com.example.reflection.User");

//        获得类的名字
        System.out.println(c1.getName());
        System.out.println(c1.getSimpleName());

        User user = new User();
        Class<? extends User> c2 = user.getClass();
        System.out.println(c2.getName());
        System.out.println(c2.getSimpleName());

//        获得类的属性
        System.out.println("==============================");
        // 只能找到public属性
        Field[] fields = c1.getFields();
        for (Field field : fields) {
            System.out.println(field);
        }
        System.out.println("==============================");
        // 能找到全部属性
        Field[] declaredFields = c1.getDeclaredFields();
        for (Field declaredField : declaredFields) {
            System.out.println(declaredField);
        }

//        获取指定属性
        System.out.println("==============================");
        Field name = c1.getDeclaredField("name");
        System.out.println(name);

//        获得类的方法
        System.out.println("==============================");
//         获得本类及其父类的全部public方法
        Method[] methods = c1.getMethods();
        System.out.println("-----");
        for (Method method : methods) {
            System.out.println("[getMethods()]" + method);
        }
//         获得本类的所有方法
        Method[] declaredMethods = c1.getDeclaredMethods();
        System.out.println("-----");
        for (Method declaredMethod : declaredMethods) {
            System.out.println("[getDeclaredMethods()]" + declaredMethod);
        }

//        获得指定方法
//        根据parameterTypes获得不同的方法（重载）
        System.out.println("==============================");
        Method getName = c1.getMethod("getName", null);
        Method setName = c1.getMethod("setName", String.class);
        System.out.println(getName);
        System.out.println(setName);

//        获得构造器
        System.out.println("==============================");
//        获得public构造器
        Constructor<?>[] constructors = c1.getConstructors();
//        获得全部构造器
        Constructor<?>[] declaredConstructors = c1.getDeclaredConstructors();

        for (Constructor<?> constructor : constructors) {
            System.out.println("[getConstructors()]" + constructor);
        }

        for (Constructor<?> declaredConstructor : declaredConstructors) {
            System.out.println("[declaredConstructor()]"+ declaredConstructor);
        }
//        获得指定构造器
        System.out.println("-----");
        Constructor<?> constructor = c1.getConstructor(null);
        System.out.println(constructor);
    }
}

✨动态创建对象执行方法

package com.example.reflection;

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

//动态创建对象执行方法
public class Test08 {
    public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {
//        获得Class对象
        Class<?> c1 = Class.forName("com.example.reflection.User");

//        构造对象
//        调用了无参构造器
        User user = (User) c1.newInstance();
        System.out.println(user);

//        通过构造器创建对象
        Constructor<?> constructor = c1.getDeclaredConstructor(int.class, String.class, int.class);
        User user2 = (User) constructor.newInstance(1, "username", 66);
        System.out.println(user2);

//        通过反射调用方法
        User user3 = (User) c1.newInstance();
//        通过反射获取一个方法
        Method setName = c1.getMethod("setName", String.class);
//        invoke 激活（调用）
//        (对象实例，参数)
        setName.invoke(user3, "通过反射调用方法");
        System.out.println(user3.getName());

//        通过反射操作属性
        User user4 = (User) c1.newInstance();
        Field name = c1.getDeclaredField("name");

//        不能直接操作private属性 需要关闭检测
//        属性/方法.setAccessible(true)
        name.setAccessible(true);
        name.set(user4, "通过反射操作属性");
        System.out.println(user4.getName());
    }
}

✨性能分析对比

package com.example.reflection;

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

//性能分析对比
public class Test09 {

//    普通方式调用
    public static void test01(){
        User user = new User();
        long startTime = System.currentTimeMillis();
        for (int i = 0; i < 1000000000; i++) {
            user.getName();
        }
        long endTime = System.currentTimeMillis();
        System.out.println("[普通方式调用10亿次]" + (endTime - startTime) + "ms");
    }

//    反射方式调用
    public static void test02() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        User user = new User();
        Class<? extends User> c1 = user.getClass();
        Method getName = c1.getMethod("getName", null);
        long startTime = System.currentTimeMillis();
        for (int i = 0; i < 1000000000; i++) {
            getName.invoke(user, null);
        }
        long endTime = System.currentTimeMillis();
        System.out.println("[反射方式调用10亿次]" + (endTime - startTime) + "ms");
    }

//    反射方式调用（关闭检测）
    public static void test03() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        User user = new User();
        Class<? extends User> c1 = user.getClass();
        Method getName = c1.getMethod("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("[反射方式调用（关闭检测）10亿次]" + (endTime - startTime) + "ms");
    }

    public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException {
        test01();
        test02();
        test03();

//        [普通方式调用10亿次]6ms
//        [反射方式调用10亿次]6879ms
//        [反射方式调用（关闭检测）10亿次]2122ms

    }
}

✨获取泛型信息

package com.example.reflection;

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

//获取泛型信息
public class Test10 {

    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;
    }

    public static void main(String[] args) throws NoSuchMethodException {
        Method method = Test10.class.getMethod("test01", Map.class, List.class);
        Type[] genericParameterTypes = method.getGenericParameterTypes();
        for (Type genericParameterType : genericParameterTypes) {
            System.out.println(genericParameterType);
            if (genericParameterType instanceof ParameterizedType){
                Type[] actualTypeArguments = ((ParameterizedType) genericParameterType).getActualTypeArguments();
                for (Type actualTypeArgument : actualTypeArguments) {
                    System.out.println(actualTypeArgument);
                }
            }
        }

        System.out.println("==============================");
        method = Test10.class.getMethod("test02", null);
        Type genericReturnType = method.getGenericReturnType();
        if(genericReturnType instanceof ParameterizedType){
            Type[] actualTypeArguments = ((ParameterizedType) genericReturnType).getActualTypeArguments();
            for (Type actualTypeArgument : actualTypeArguments) {
                System.out.println(actualTypeArgument);
            }
        }
    }
}

✨获取注解信息

package com.example.reflection;

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

//获取注解信息
public class Test11 {

    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
        Class<?> c1 = Class.forName("com.example.reflection.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
        TableAnnotation tableAnnotation = c1.getAnnotation(TableAnnotation.class);
        System.out.println(tableAnnotation.value());

//        获得类指定的的注解
        Field name = c1.getDeclaredField("name");
        FieldAnnotation nameAnnotation = name.getAnnotation(FieldAnnotation.class);
        System.out.println(nameAnnotation.columnName());
        System.out.println(nameAnnotation.type());
        System.out.println(nameAnnotation.length());
    }

}

@TableAnnotation("db_student")
class Student2{
    @FieldAnnotation(columnName = "db_id", type = "int", length = 10)
    private int id;
    @FieldAnnotation(columnName = "db_age", type = "int", length = 10)
    private int age;
    @FieldAnnotation(columnName = "db_name", type = "varchar", length = 10)
    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 + '\'' +
                '}';
    }
}

//类名注解
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface TableAnnotation{
    String value();
}

//属性注解
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface FieldAnnotation{
    String columnName();
    String type();
    int length();
}

✨课程链接

【狂神说Java】注解和反射_哔哩哔哩_bilibili


⭐转载请注明出处

本文作者：双份浓缩馥芮白

原文链接：https://www.cnblogs.com/Flat-White/p/15168719.html

版权所有，如需转载请注明出处。

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