反射入门

 面试常问编译->加载阶段，应用常问运行->加载阶段

re.properties(src包下)

classfullpath=Reflection.Cat
//method=hi
method=cry

Cat

package Reflection;


public class Cat {
    private String name="招财猫";
    public  int age=1;
    public Cat(){}
    public Cat(String name){
        this.name=name;
    }
    public void hi(){
        System.out.println("我是"+name);
    }
    public void cry(){
        System.out.println("你是"+name);
    }
}

Ocp

package Reflection;

import java.io.FileInputStream;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.Properties;


public class Ocp {
    public static void main(String[] args) throws Exception {
        //使用该方法读取配置文件
        Properties p = new Properties();
        p.load(new FileInputStream("src\re.properties"));
        String classfullpath = p.get("classfullpath").toString();
        String method = p.get("method").toString();

        //反射机制
        //(1)加载类，返回Class类型的对象cls
        Class cls = Class.forName(classfullpath);
        //(2)通过cls得到你加载的类com.wzg.Cat的对象实例
        Object o = cls.newInstance();
        System.out.println("o的运行类型"+o.getClass());
        //(3)通过cls得到你加载的类com.wzg.Cat的method"hi"的方法对象
        //   即在反射中可以视方法为对象
        Method method1 = cls.getMethod(method);
        //(4)通过method1调用方法即通过方法对象来实现调用方法
        //   传统：对象.方法() 反射：方法.invoke(对象)
        method1.invoke(o);
        //(5)获取指定类的成员变量
        //   传统：对象.成员变量 反射：成员变量对象.get(对象)
        Field age = cls.getField("age");
        System.out.println("你的年龄是:"+age.get(o));
        //(6)获取指定类的构造器,如果()里为空则为无参构造器,int类的class对象
        Constructor constructor = cls.getConstructor(String.class);
        System.out.println(constructor);
    }
}

反射与传统方法相比，调优

package Reflection;

import java.lang.reflect.Method;


public class Tuning {
    public static void main(String[] args) throws Exception {
        t1();
        t2();
        t3();
    }
    //传统方法
    public static void t1(){
        Cat cat = new Cat();
        long start =System.currentTimeMillis();
        for (int i = 0; i < 900000000; i++) {
            cat.hi();
        }
        long end = System.currentTimeMillis();
        System.out.println("传统方法耗时:"+(end-start));
    }
    //反射(未调优)
    public static void t2() throws Exception {
        Class cls = Class.forName("Reflection.Cat");
        Object o = cls.newInstance();
        Method hi = cls.getMethod("hi");

        long start =System.currentTimeMillis();
        for (int i = 0; i < 900000000; i++) {
            hi.invoke(o);
        }
        long end = System.currentTimeMillis();
        System.out.println("反射(未调优)耗时:"+(end-start));
    }
    //反射(调优后)
    public static void t3() throws Exception {
        Class cls = Class.forName("Reflection.Cat");
        Object o = cls.newInstance();
        Method hi = cls.getMethod("hi");
        //在反射调用方法时取消检查
        hi.setAccessible(true);
        long start =System.currentTimeMillis();
        for (int i = 0; i < 900000000; i++) {
            hi.invoke(o);
        }
        long end = System.currentTimeMillis();
        System.out.println("反射(调优后)耗时:"+(end-start));
    }
}