反射知识点整理

目录(?)[-]

1. 来自CODE的代码片ReflectDemojava
2. 来自CODE的代码片Demo1java
3. 来自CODE的代码片Demo2java
4. 来自CODE的代码片Demo3java

         JAVA反射机制是在运行状态中，对于任意一个类 (class文件)，都能够知道这个类的所有属性和方法； 对于任意一个对象，都能够调用它的任意一个方法和属性；这种动态获取的信息以及动态调用对象的方法的功能称为java语言的反射机制。
         动态获取类中信息，就是java反射 。 可以理解为对类的解剖。 要想要对字节码文件进行解剖，必须要有字节码文件对象.
下面就是创建字节码文件对象的三种方法：

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package cn.itcast.reflect.demo;
import cn.itcast.bean.Person;
/*
 * 
 * 如何获取字节码文件对象呢？
 * 
 */
public class ReflectDemo {
/**
 * @param args
 * @throws ClassNotFoundException 
 */
public static void main(String[] args) throws ClassNotFoundException {
getClassObject_3();
}
/*
 * 方式三：
 * 只要通过给定的类的 字符串名称就可以获取该类，更为扩展。
 * 可是用Class类中的方法完成。
 * 该方法就是forName.
 * 这种方式只要有名称即可，更为方便，扩展性更强。 
 */
public static void getClassObject_3() throws ClassNotFoundException {
String className = "cn.itcast.bean.Person";
Class clazz = Class.forName(className);
System.out.println(clazz);
}
/*
 * 方式二：
 * 2，任何数据类型都具备一个静态的属性.class来获取其对应的Class对象。
 * 相对简单，但是还是要明确用到类中的静态成员。
 * 还是不够扩展。 
 * 
 */
public static void getClassObject_2() {
Class clazz = Person.class;
Class clazz1 = Person.class;
System.out.println(clazz==clazz1);
}
/*
 * 获取字节码对象的方式：
 * 1，Object类中的getClass()方法的。
 * 想要用这种方式，必须要明确具体的类，并创建对象。
 * 麻烦 .
 * 
 */
public static void getClassObject_1(){
Person p = new Person();
Class clazz = p.getClass();
Person p1 = new Person();
Class clazz1 = p1.getClass();
System.out.println(clazz==clazz1);
}
}

来自CODE的代码片

ReflectDemo.java

创建了字节码文件对象之后，就需要使用对象去做一些事情，比如获取指定名称对应类中所体现的对象，获取其中的字段和获取其中的方法

1，获取指定名称对应类中所体现的对象

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/*
 * 当获取指定名称对应类中的所体现的对象时，
 * 而该对象初始化不使用空参数构造该怎么办呢？
 * 既然是通过指定的构造 函数进行对象的初始化，
 * 所以应该先获取到该构造函数。 通过字节码文件对象即可完成。
 * 该方法是：getConstructor(paramterTypes);
 * 
 */
String name = "cn.itcast.bean.Person";
//找寻该名称类文件，并加载进内存，并产生Class对象。
Class clazz = Class.forName(name);
//获取到了指定的构造函数对  象。
Constructor constructor = clazz.getConstructor(String.class,int.class);
//通过该构造器对象的newInstance方法进行对象的初始化。
Object obj = constructor.newInstance("小明",38);
    //早期：new时候，先根据被new的类的名称找寻该类的字节码文件，并加载进内存，
        //并创建该字节码文件对象，并接着创建该字节文件的对应的Person对象.
        //cn.itcast.bean.Person p = new cn.itcast.bean.Person();
//现在：
String name = "cn.itcast.bean.Person";
//找寻该名称类文件，并加载进内存，并产生Class对象。
Class clazz = Class.forName(name);
//如何产生该类的对象呢？
Object obj  = clazz.newInstance();

来自CODE的代码片

Demo1.java

2，获取其中的字段

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/*
 * 获取字节码文件中的字段。
 */
public static void getFieldDemo() throws Exception {
Class clazz = Class.forName("cn.itcast.bean.Person");
Field field = null;//clazz.getField("age");//只能获取公有的，
field = clazz.getDeclaredField("age");//只获取本类，但包含私有。 
//对私有字段的访问取消权限检查。暴力访问。
field.setAccessible(true);
Object obj = clazz.newInstance();
field.set(obj, 89);
Object o = field.get(obj);
System.out.println(o);

来自CODE的代码片

Demo2.java

3，获取其中的方法

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    /*
 * 获取指定Class中的所有公共函数。
 */
public static void getMethodDemo() throws Exception {
Class clazz = Class.forName("cn.itcast.bean.Person");
Method[] methods  = clazz.getMethods();//获取的都是公有的方法。 
methods = clazz.getDeclaredMethods();//只获取本类中所有方法，包含私有。
for(Method method : methods){
System.out.println(method);
}
}
    public static void getMethodDemo_2() throws Exception {
Class clazz = Class.forName("cn.itcast.bean.Person");
Method method = clazz.getMethod("show", null);//获取空参数一般方法。
        //Object obj = clazz.newInstance();
Constructor constructor = clazz.getConstructor(String.class,int.class);
Object obj = constructor.newInstance("小明",37);
method.invoke(obj, null);
}
public static void getMethodDemo_3() throws Exception {
Class clazz = Class.forName("cn.itcast.bean.Person");
Method method = clazz.getMethod("paramMethod", String.class,int.class);
Object obj = clazz.newInstance();
method.invoke(obj, "小强",89);
}

来自CODE的代码片

Demo3.java

以上代码全部来自于毕向东老师讲解的反射