反射

知识基础

• 类
• 对象
• 构造方法
• 重载
• 单例模式

动态加载

JVM在执行的时候，并不是一次性把所有的class加载到内存中的，而是用到谁加载谁。

反射的概述

https://www.cnblogs.com/tech-bird/p/3525336.html

Reflection（反射）是Java被视为动态语言的关键，反射机制允许程序在执行期借助于Reflection API取得任何类的內部信息，并能直接操作任意对象的内部属性及方法

获取class对象的三种方式

1. class.forName(“全类名”)
2. 类名.class
3. 对象.getClass()，如果上下文存在某个实例对象，可以通过getClass获取他的类

public class re {
    public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException {
        Class cls1 = Class.forName("demo.Person");
        Class cls2 = Person.class;
        Person person = new Person();
        Class cls3 = person.getClass();
        System.out.println(cls1);
        System.out.println(cls2);
        System.out.println(cls3);
        System.out.println(cls1==cls2);
        System.out.println(cls3==cls2);
}

类“初始化”执行顺序是什么

package demo;

public class test {
    public static void main(String[] args) {
        Ref ref = new Ref();
    }
}

class Ref{
    static {
        System.out.println("最先执行\r\n");
    }
    {
        System.out.println("第二执行\r\n");
    }

    public Ref(){
        System.out.println("最后执行\r\n");
    }
}

demo1

package demo;

public class test {
    public static void main(String[] args) throws ClassNotFoundException {
        Class.forName("demo.CalcDemo");
    }
}
class CalcDemo {
    static {
        try {
            Runtime rt = Runtime.getRuntime();
            Process pc = rt.exec("calc");
            pc.waitFor();
        } catch (Exception e) {

        }
    }
}

Class方法

获取变量

• Field getField(name)：根据字段名获取某个public的field（包括父类）
• Field getDeclaredField(name)：根据字段名获取当前类的某个field（不包括父类）
• Field[] getFields()：获取所有public的field（包括父类）
• Field[] getDeclaredFields()：获取当前类的所有field（不包括父类）

cls3.getField("aaa");//指定名称的public修饰的
cls3.getFields();//获取所有public修饰的成员变量
cls3.getDeclaredField("aaaa");//获取所有
cls3.getDeclaredFields();

获取构造方法

cls3.getConstructors();
cls3.getConstructor("aaa");
cls3.getDeclaredConstructor("bbb");
cls3.getDeclaredConstructors();

package demo;

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

public class test {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
//        Ref ref = new Ref();
        Class cls = Class.forName("demo.Person");
        Constructor constructor = cls.getConstructor();
        Constructor constructor1 = cls.getConstructor(String.class);
        constructor.newInstance();
        constructor1.newInstance("jl");

    }
}

demo2

package demo;

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.Arrays;
import java.util.List;

public class test {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
        Class cls = Class.forName("java.lang.ProcessBuilder");
        Method methodStart = cls.getMethod("start");
        Constructor constructor = cls.getConstructor(List.class);
        Object obj =  constructor.newInstance(Arrays.asList("calc.exe"));
        methodStart.invoke(obj);
    }
}

demo3-可变长参数

package demo;import java.lang.reflect.Constructor;import java.lang.reflect.InvocationTargetException;import java.lang.reflect.Method;import java.util.Arrays;import java.util.List;public class test {    public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {        Class cls = Class.forName("java.lang.ProcessBuilder");        Method methodStart = cls.getMethod("start");        Constructor constructor = cls.getConstructor(String[].class);        Object obj = constructor.newInstance(new String[][]{{"calc.exe"}});        methodStart.invoke(obj);    }}

获取成员方法

cls3.getMethods();cls3.getMethod("ccc");cls3.getDeclaredMethods();cls3.getDeclaredMethod("ccc");

demo4

package demo;import java.lang.reflect.InvocationTargetException;import java.lang.reflect.Method;public class test {    public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, InvocationTargetException, IllegalAccessException {     Person person = new Person();     Class cls = Class.forName("demo.Person");     Method method = cls.getDeclaredMethod("eat", String.class);     method.setAccessible(true);     method.invoke(person,"emmmm");    }}

demo5

package demo;import java.lang.reflect.InvocationTargetException;import java.lang.reflect.Method;public class test {    public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, InvocationTargetException, IllegalAccessException {        Class cls = Class.forName("java.lang.Runtime");        Method method = cls.getMethod("exec", String.class);        Method method1 = cls.getMethod("getRuntime");        method.invoke(method1.invoke(cls),"calc.exe");    }}

Runtime类就是单例模式，我们只能通过 Runtime.getRuntime() 来获取到 Runtime 对
象。