java的反射机制

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一：玩反射：

1：

在运行状态中，任意一个class文件，都可以找到这个类的所有属性和方法：

对任意一个对象，都可以调用它的任意一个方法和属性；

动态调用类中的信息，就是反射：

可以理解为对类的解剖；

2：

要想对字节码文件进行解剖，必须要有字节码文件对象；

下面看第一种方法，怎么样拿到类；

测试代码：Person：

package test;

import java.io.Serializable;

public class Person implements Serializable {

private String name;

private int age;

public Person() {

super();

System.out.println("我是无参构造");

}

public Person(String name, int age) {

super();

this.name = name;

this.age = age;

System.out.println("我是有参构造");

}

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

public int getAge() {

return age;

}

public void setAge(int age) {

this.age = age;

}

public void show() {

System.out.println("我要展示L：");

}

public void fun() {

System.out.println(name + ".....我是" + name);

}

package test;

public class Test1 {

public static void main(String[] args) {

// TODO Auto-generated method stub

getClassObject();

}

public static void getClassObject(){

Person p1 = new Person();

Class class1 = p1.getClass();

Person p2 = new Person();

Class class2 = p2.getClass();

System.out.println(p1==p2);

System.out.println(class1==class2);

//我是无参构造

//false

//true

}

这种方法的缺点就是说：必须要知道具体的类，比如上面这个Person类，

下面是第二种方法：

.class来获取对应的Class对象；

package test;

public class Test1 {

public static void main(String[] args) {

// TODO Auto-generated method stub

getClassObject();

}

public static void getClassObject(){

Class class1 = Person.class;

Class class2 = Person.class;

System.out.println(class1==class2);//true

}

第二种方法还是要知道类的名字：还是要用到类中的静态成员；

第三种方法：只要给定类的字符串名称就可以获取该类；

注意：如果不用static 呢：则是这样写：new Test1().getClassObject();

Class在java.lang包中；Class.forName();

package test;

public class Test1 {

public static void main(String[] args) throws ClassNotFoundException {

// TODO Auto-generated method stub

new Test1().getClassObject();

}

public void getClassObject() throws ClassNotFoundException {

Class className = Class.forName("Person");

System.out.println(className);

}

//Exception in thread "main" java.lang.ClassNotFoundException: Person

}

为什么呢？想想：

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

package test;

public class Test1 {

public static void main(String[] args) throws ClassNotFoundException {

// TODO Auto-generated method stub

new Test1().getClassObject();

}

public void getClassObject() throws ClassNotFoundException {

Class className = Class.forName("test.Person");

System.out.println(className);//class test.Person

}

New 跟反射中的newInstanceOf()的区别：new 省事，但后者更容易扩展；

package test;

public class Test1 {

public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException {

// TODO Auto-generated method stub

new Test1().getClassObject();

}

public void getClassObject() throws ClassNotFoundException, InstantiationException, IllegalAccessException {

Person person1 = new Person();

System.out.println(person1);

//结果是：我是无参构造

// test.Person@42d73fb7

//实际上new开辟空间，同时调用了无参构造；

}

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

package test;

public class Test1 {

public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException {

// TODO Auto-generated method stub

new Test1().getClassObject();

}

public void getClassObject() throws ClassNotFoundException, InstantiationException, IllegalAccessException {

String name="test.Person";

Class class1= Class.forName(name);

Object person2=class1.newInstance();

System.out.println(person2);

//我是无参构造

//test.Person@25964fe8

}

结果是一样的。

那个new ，是先根据被new的类的名称找到该类的字节码文件，加载到内存，

并创建该字节码文件对象，接着创建字节码文件对应的Person对象；

而反射中的newInstance()呢？是找到该名称的类文件，加载到内存，产生Class对象；

从而产生该类的对象；

造出对象：

Person p1 = new Person(“xiaoqiang”,25);

相当于：

Class class1= Class.forName("test.Person");

Constructor con= class1.getConstructor(String.class,int.class);

Object obj =con.newInstance("xiaoqiang",25);

String p1=obj.toString();

System.out.println(p1);

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

getDeclaredFields()的用法：

Class class1= Class.forName("test.Person");

Field[] field =class1.getDeclaredFields();

for(Field f:field){

System.out.println(f);//private java.lang.String test.Person.name

//private int test.Person.age

}

说白了，就是查看Person类里面的所有属性成员；

而另外一个呢，getFields()只能访问公有的属性；

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Class class1=Class.forName("test.Person");

Field f=class1.getDeclaredField("age");

Field f1 = class1.getDeclaredField("name");

f.setAccessible(true);//暴力访问f属性

f1.setAccessible(true); //暴力访问f1属性

Object obj =class1.newInstance();

f.set(obj, 10);

f1.set(obj, "xiaoqiang");

System.out.println(obj);//我是无参构造 xiaoqiang 10

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

getDeclarMethod();

说白了，加上declar,就是访问所有的；包括私有；

Class class1= Class.forName("test.Person");

Method method =class1.getDeclaredMethod("show", null);

Object obj = class1.newInstance();

method.invoke(obj, null); 我是无参构造

我要展示L：

2；

Class class1= Class.forName("test.Person");

Method method =class1.getDeclaredMethod("show", null);

Constructor con = class1.getConstructor(String.class,int.class);

Object obj = con.newInstance("xiaoqiang",25);

method.invoke(obj, null); 我是有参构造

我要展示L：

带有参数的：

Class class1= Class.forName("test.Person");

Method method =class1.getMethod("fun", String.class);

Constructor con = class1.getConstructor(String.class,int.class);

Object obj = con.newInstance("xiaoqiang",25);

method.invoke(obj, "yes");

//我是有参构造 yes.....我是yes

-------------------------------------------------------------------------

用配置文件来学习反射：

在src下面新建一个配置文件：test.properties;

File file = new File(“test.properties”);

Property property = new Property();

FileInputStream in = new FileInputStream(file);

Property.load(in);

File file = new File("fi.properties");

Properties pro = new Properties();

FileInputStream in = new FileInputStream(file);

pro.load(in);

for(int x =0;x<pro.size()-1;x++){

System.out.println(pro.getProperty("user1"));

System.out.println(pro.getProperty("user2"));

}

结果是：

//zhangsan

//lisi

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