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

class.forName("全类名")
类名.class
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.getConstructors();cls3.getConstructor("aaa");cls3.getDeclaredConstructor("bbb");cls3.getDeclaredConstructors();

demo4

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");
    }}

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");    }}