[Java知识库]Java---注解与反射 （反射篇） 反射最强大的操作

获取运行时类的完整结构

package com.kuang.reflection;

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

//获得类的信息
public class Test08 {
    public static void main(String[] args) throws Exception {
        Class c1 = Class.forName("com.kuang.reflection.User");

        User user = new User();
        c1 = user.getClass();
        //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

        System.out.println("=============================");

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

        //获得指定的属性
        Field name = c1.getDeclaredField("name");
        System.out.println(name);

        //获得类的方法
        System.out.println("=============================");
        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);
        }

        //获得指定的方法
        //有重载
        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);
        }

        Constructor declaredConstructor = c1.getDeclaredConstructor(String.class, int.class, int.class);
        System.out.println("指定:"+declaredConstructor);

    }
}

动态创建对象执行方法

获取到对象之后能做什么？

package com.kuang.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，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("com.kuang.reflection.User");

        //构造一个对象
        //User user = (User) c1.newInstance(); //本质是调用了类的无参构造器
        //System.out.println(user);

        //通过个构造器创建对象
        //Constructor constructor = c1.getDeclaredConstructor(String.class, int.class, int.class);
        //User user2 = (User) constructor.newInstance("香蕉", 001, 18);
        //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,"狂神");
        System.out.println(user3.getName());

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

        //不能直接操作私有属性，需要关闭程序的安全监测，属性或者方法的setAccessible(true)
        name.setAccessible(true);   //取消安全检测
        name.set(user4,"狂神2");
        System.out.println(user4.getName());

    }
}

性能对比分析

package com.kuang.reflection;

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

//分析性能问题
public class Test10 {
    public static void main(String[] args) throws InvocationTargetException, NoSuchMethodException, IllegalAccessException {
        test01();
        test02();
        test03();
    }

    //普通方式调用
    public static void test01(){
        User user = new User();

        long startTime = System.currentTimeMillis();

        for (int i = 0; i < 10_0000_0000; 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 test02() 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 < 10_0000_0000; i++) {
            getName.invoke(user,null);
        }

        long endTime = System.currentTimeMillis();

        System.out.println("反射方式执行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 c1 = user.getClass();

        Method getName = c1.getDeclaredMethod("getName", null);
        getName.setAccessible(true);
        long startTime = System.currentTimeMillis();

        for (int i = 0; i < 10_0000_0000; i++) {
            getName.invoke(user,null);
        }

        long endTime = System.currentTimeMillis();

        System.out.println("关闭检测执行10亿次："+(endTime-startTime)+"ms");

    }
}

通过反射操作泛型

package com.kuang.reflection;

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

//通过反射获取泛型
public class Test11 {

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

    public Map<String,User> test02(){
        return null;
    }

    public static void main(String[] args) throws NoSuchMethodException {
        Method method = Test11.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);
                }
            }
        }
        method = Test11.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

模拟一个ORM操作

package com.kuang.reflection;

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

//反射操作注解
public class Test12 {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
        Class c1 = Class.forName("com.kuang.reflection.Student2");

        //通过反射获得注解
        Annotation[] annotations = c1.getAnnotations();
        for (Annotation annotation : annotations) {
            System.out.println(annotation);
        }

        //获得注解的value的值。
        Tablekuang tablekuang = (Tablekuang) c1.getAnnotation(Tablekuang.class);
        String value = tablekuang.value();
        System.out.println(value);

        //获得类指定的注解
        Field f = c1.getDeclaredField("name");
        Fieldkuang annotation = f.getAnnotation(Fieldkuang.class);
        System.out.println(annotation.columnName());
        System.out.println(annotation.type());
        System.out.println(annotation.length());


    }
}

@Tablekuang("db_student")
class Student2{
    @Fieldkuang(columnName = "db_id",type = "int",length = 10)
    private int id;
    @Fieldkuang(columnName = "db_age",type = "int",length = 10)
    private int age;
    @Fieldkuang(columnName = "db_name",type = "varchar",length = 10)
    private String name;

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

    public Student2() {
    }

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


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

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

所有图片均来自：【狂神说Java】注解和反射