java反射

java反射

案例

1.创建一个Car类

package com.hh.test;
public class Car {

    private String brand;
    private String color;
    private int maxSpeed;

    public String getBrand() {
        return brand;
    }

    public void setBrand(String brand) {
        this.brand = brand;
    }

    public String getColor() {
        return color;
    }

    public void setColor(String color) {
        this.color = color;
    }

    public int getMaxSpeed() {
        return maxSpeed;
    }

    public void setMaxSpeed(int maxSpeed) {
        this.maxSpeed = maxSpeed;
    }

    public Car() {
    }

    public Car(String brand, String color, int maxSpeed) {
        this.brand = brand;
        this.color = color;
        this.maxSpeed = maxSpeed;
    }

    public void introduce(){
        System.out.println("brand:"+brand+";color:"+color+";maxSpeed:"+maxSpeed);
    }
}

2.通过一般思维创建Car实例

package com.hh.test;
public class CarTest {
    public static void main(String[] args) {
        Car car = new Car();
        car.setBrand("宝马");
        car.setColor("黑色");
        car.setMaxSpeed(100);
        car.introduce();

    }
}

3.通过Java反射机制创建

package com.hh.test;
import java.lang.reflect.Constructor;
import java.lang.reflect.Method;
public class ReflectTest {
    public static Car initByDefaultConst() throws Exception {
        //通过类转载器获取Car类对象
        ClassLoader loader = Thread.currentThread().getContextClassLoader();
        Class<?> clazz = loader.loadClass("com.hh.test.Car");

        //获取类的默认构造器对象并通过它实例化Car
        Constructor<?> cons = clazz.getDeclaredConstructor((Class[]) null);
        Car car = (Car)cons.newInstance();

        //通过反射方法设置属性
        Method setBrand = clazz.getMethod("setBrand", String.class);
        setBrand.invoke(car,"宝马");
        Method setColor = clazz.getMethod("setColor", String.class);
        setColor.invoke(car,"黑色");
        Method setMaxSpeed = clazz.getMethod("setMaxSpeed", int.class);
        setMaxSpeed.invoke(car,100);
        return car;
    }

    public static void main(String[] args) throws Exception {
        Car car = initByDefaultConst();
        car.introduce();
    }
}

4.两种运行结果都一样，如下：

类装载器

JVM在运行时会产生3个ClassLoader:根装载器，ExtClassLoader(扩展类装载器),AppClassLoader(应用类装载器)。

package com.hh.test;

public class ClassLoaderTest {
    public static void main(String[] args) {
        ClassLoader loader = Thread.currentThread().getContextClassLoader();
        System.out.println("current loader"+loader);
        System.out.println("parent loader:"+loader.getParent());
        System.out.println("grandparent loader:"+loader.getParent().getParent());
    }
}

Java反射机制可以绕过private,protected进行调用

1.PrivateCar类

package com.hh.test;

public class PrivateCar {
    //private成员变量：使用传统的类实例调用方式，只能在本类中访问
    private String color;
    //protected方法：使用传统的类实例调用方式，只能在子类和本包中访问
    protected void driver(){
        System.out.println("drive private car! the color is:"+color);
    }
}

2.PrivateCarReflect类

package com.hh.test;

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

public class PrivateCarReflect {
    public static void main(String[] args) throws Exception {
        ClassLoader loader = Thread.currentThread().getContextClassLoader();
        Class<?> clazz = loader.loadClass("com.hh.test.PrivateCar");
        PrivateCar pCar = (PrivateCar) clazz.newInstance();
        Field color = clazz.getDeclaredField("color");

        //取消Java语言访问检查以访问private变量
        color.setAccessible(true);
        color.set(pCar,"黑色");

        Method driverMethod = clazz.getDeclaredMethod("driver",(Class[])null);

        //取消java语言访问检查以访问protected方法
        driverMethod.setAccessible(true);
        driverMethod.invoke(pCar,(Object[])null);
    }
}

3.运行结果：

4.如果注释掉 color.setAccessible(true);这行代码，则运行结果如下：

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