一個例子讓你瞭解Java反射機制
阿新 • • 發佈:2018-12-23
本文來自:blog.csdn.net/ljphhj
JAVA反射機制:
通俗地說,反射機制就是可以把一個類,類的成員(函式,屬性),當成一個物件來操作,希望讀者能理解,也就是說,類,類的成員,我們在執行的時候還可以動態地去操作他們.
理論的東東太多也沒用,下面我們看看實踐 Demo ~
Demo:
- package cn.lee.demo;
- import java.lang.reflect.Constructor;
- import java.lang.reflect.Field;
- import java.lang.reflect.InvocationTargetException;
- import java.lang.reflect.Method;
- import java.lang.reflect.Modifier;
- import java.lang.reflect.TypeVariable;
- publicclass Main {
- /**
- * 為了看清楚Java反射部分程式碼,所有異常我都最後丟擲來給虛擬機器處理!
- * @param args
- * @throws ClassNotFoundException
- * @throws InstantiationException
- * @throws IllegalAccessException
- * @throws InvocationTargetException
- * @throws IllegalArgumentException
- * @throws NoSuchFieldException
- * @throws SecurityException
- * @throws NoSuchMethodException
- */
- publicstaticvoid main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, SecurityException, NoSuchFieldException, NoSuchMethodException {
- // TODO Auto-generated method stub
- //Demo1. 通過Java反射機制得到類的包名和類名
- Demo1();
- System.out.println("===============================================");
- //Demo2. 驗證所有的類都是Class類的例項物件
- Demo2();
- System.out.println("===============================================");
- //Demo3. 通過Java反射機制,用Class 建立類物件[這也就是反射存在的意義所在],無參構造
- Demo3();
- System.out.println("===============================================");
- //Demo4: 通過Java反射機制得到一個類的建構函式,並實現構造帶參例項物件
- Demo4();
- System.out.println("===============================================");
- //Demo5: 通過Java反射機制操作成員變數, set 和 get
- Demo5();
- System.out.println("===============================================");
- //Demo6: 通過Java反射機制得到類的一些屬性: 繼承的介面,父類,函式資訊,成員資訊,型別等
- Demo6();
- System.out.println("===============================================");
- //Demo7: 通過Java反射機制呼叫類中方法
- Demo7();
- System.out.println("===============================================");
- //Demo8: 通過Java反射機制獲得類載入器
- Demo8();
- System.out.println("===============================================");
- }
- /**
- * Demo1: 通過Java反射機制得到類的包名和類名
- */
- publicstaticvoid Demo1()
- {
- Person person = new Person();
- System.out.println("Demo1: 包名: " + person.getClass().getPackage().getName() + ","
- + "完整類名: " + person.getClass().getName());
- }
- /**
- * Demo2: 驗證所有的類都是Class類的例項物件
- * @throws ClassNotFoundException
- */
- publicstaticvoid Demo2() throws ClassNotFoundException
- {
- //定義兩個型別都未知的Class , 設定初值為null, 看看如何給它們賦值成Person類
- Class<?> class1 = null;
- Class<?> class2 = null;
- //寫法1, 可能丟擲 ClassNotFoundException [多用這個寫法]
- class1 = Class.forName("cn.lee.demo.Person");
- System.out.println("Demo2:(寫法1) 包名: " + class1.getPackage().getName() + ","
- + "完整類名: " + class1.getName());
- //寫法2
- class2 = Person.class;
- System.out.println("Demo2:(寫法2) 包名: " + class2.getPackage().getName() + ","
- + "完整類名: " + class2.getName());
- }
- /**
- * Demo3: 通過Java反射機制,用Class 建立類物件[這也就是反射存在的意義所在]
- * @throws ClassNotFoundException
- * @throws IllegalAccessException
- * @throws InstantiationException
- */
- publicstaticvoid Demo3() throws ClassNotFoundException, InstantiationException, IllegalAccessException
- {
- Class<?> class1 = null;
- class1 = Class.forName("cn.lee.demo.Person");
- //由於這裡不能帶引數,所以你要例項化的這個類Person,一定要有無參建構函式哈~
- Person person = (Person) class1.newInstance();
- person.setAge(20);
- person.setName("LeeFeng");
- System.out.println("Demo3: " + person.getName() + " : " + person.getAge());
- }
- /**
- * Demo4: 通過Java反射機制得到一個類的建構函式,並實現建立帶參例項物件
- * @throws ClassNotFoundException
- * @throws InvocationTargetException
- * @throws IllegalAccessException
- * @throws InstantiationException
- * @throws IllegalArgumentException
- */
- publicstaticvoid Demo4() throws ClassNotFoundException, IllegalArgumentException, InstantiationException, IllegalAccessException, InvocationTargetException
- {
- Class<?> class1 = null;
- Person person1 = null;
- Person person2 = null;
- class1 = Class.forName("cn.lee.demo.Person");
- //得到一系列建構函式集合
- Constructor<?>[] constructors = class1.getConstructors();
- person1 = (Person) constructors[0].newInstance();
- person1.setAge(30);
- person1.setName("leeFeng");
- person2 = (Person) constructors[1].newInstance(20,"leeFeng");
- System.out.println("Demo4: " + person1.getName() + " : " + person1.getAge()
- + " , " + person2.getName() + " : " + person2.getAge()
- );
- }
- /**
- * Demo5: 通過Java反射機制操作成員變數, set 和 get
- *
- * @throws IllegalAccessException
- * @throws IllegalArgumentException
- * @throws NoSuchFieldException
- * @throws SecurityException
- * @throws InstantiationException
- * @throws ClassNotFoundException
- */
- publicstaticvoid Demo5() throws IllegalArgumentException, IllegalAccessException, SecurityException, NoSuchFieldException, InstantiationException, ClassNotFoundException
- {
- Class<?> class1 = null;
- class1 = Class.forName("cn.lee.demo.Person");
- Object obj = class1.newInstance();
- Field personNameField = class1.getDeclaredField("name");
- personNameField.setAccessible(true);
- personNameField.set(obj, "胖虎先森");
- System.out.println("Demo5: 修改屬性之後得到屬性變數的值:" + personNameField.get(obj));
- }
- /**
- * Demo6: 通過Java反射機制得到類的一些屬性: 繼承的介面,父類,函式資訊,成員資訊,型別等
- * @throws ClassNotFoundException
- */
- publicstaticvoid Demo6() throws ClassNotFoundException
- {
- Class<?> class1 = null;
- class1 = Class.forName("cn.lee.demo.SuperMan");
- //取得父類名稱
- Class<?> superClass = class1.getSuperclass();
- System.out.println("Demo6: SuperMan類的父類名: " + superClass.getName());
- System.out.println("===============================================");
- Field[] fields = class1.getDeclaredFields();
- for (int i = 0; i < fields.length; i++) {
- System.out.println("類中的成員: " + fields[i]);
- }
- System.out.println("===============================================");
- //取得類方法
- Method[] methods = class1.getDeclaredMethods();
- for (int i = 0; i < methods.length; i++) {
- System.out.println("Demo6,取得SuperMan類的方法:");
- System.out.println("函式名:" + methods[i].getName());
- System.out.println("函式返回型別:" + methods[i].getReturnType());
- System.out.println("函式訪問修飾符:" + Modifier.toString(methods[i].getModifiers()));
- System.out.println("函式程式碼寫法: " + methods[i]);
- }
- System.out.println("===============================================");
- //取得類實現的介面,因為介面類也屬於Class,所以得到介面中的方法也是一樣的方法得到哈
- Class<?> interfaces[] = class1.getInterfaces();
- for (int i = 0; i < interfaces.length; i++) {
- System.out.println("實現的介面類名: " + interfaces[i].getName() );
- }
- }
- /**
- * Demo7: 通過Java反射機制呼叫類方法
- * @throws ClassNotFoundException
- * @throws NoSuchMethodException
- * @throws SecurityException
- * @throws InvocationTargetException
- * @throws IllegalAccessException
- * @throws IllegalArgumentException
- * @throws InstantiationException
- */
- publicstaticvoid Demo7() throws ClassNotFoundException, SecurityException, NoSuchMethodException, IllegalArgumentException, IllegalAccessException, InvocationTargetException, InstantiationException
- {
- Class<?> class1 = null;
- class1 = Class.forName("cn.lee.demo.SuperMan");
- System.out.println("Demo7: \n呼叫無參方法fly():");
- Method method = class1.getMethod("fly");
- method.invoke(class1.newInstance());
- System.out.println("呼叫有參方法walk(int m):");
- method = class1.getMethod("walk",int.class);
- method.invoke(class1.newInstance(),100);
- }
- /**
- * Demo8: 通過Java反射機制得到類載入器資訊
- *
- * 在java中有三種類類載入器。[這段資料網上擷取]
- 1)Bootstrap ClassLoader 此載入器採用c++編寫,一般開發中很少見。
- 2)Extension ClassLoader 用來進行擴充套件類的載入,一般對應的是jre\lib\ext目錄中的類
- 3)AppClassLoader 載入classpath指定的類,是最常用的載入器。同時也是java中預設的載入器。
- *
- * @throws ClassNotFoundException
- */
- publicstaticvoid Demo8() throws ClassNotFoundException
- {
- Class<?> class1 = null;
- class1 = Class.forName("cn.lee.demo.SuperMan");
- String nameString = class1.getClassLoader().getClass().getName();
- System.out.println("Demo8: 類載入器類名: " + nameString);
- }
- }
- /**
- *
- * @author xiaoyaomeng
- *
- */
- class Person{
- privateint age;
- private String name;
- public Person(){
- }
- public Person(int age, String name){
- this.age = age;
- this.name = name;
- }
- publicint getAge() {
- return age;
- }
- publicvoid setAge(int age) {
- this.age = age;
- }
- public String getName() {
- return name;
- }
- publicvoid setName(String name) {
- this.name = name;
- }
- }
- class SuperMan extends Person implements ActionInterface
- {
- privateboolean BlueBriefs;
- publicvoid fly()
- {
- System.out.println("超人會飛耶~~");
- }
- publicboolean isBlueBriefs() {
- return BlueBriefs;
- }
- publicvoid setBlueBriefs(boolean blueBriefs) {
- BlueBriefs = blueBriefs;
- }
- @Override
- publicvoid walk(int m) {
- // TODO Auto-generated method stub
- System.out.println("超人會走耶~~走了" + m + "米就走不動了!");
- }
- }
- interface ActionInterface{
- publicvoid walk(int m);
- }
package cn.lee.demo;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.lang.reflect.TypeVariable;
public class Main {
/**
* 為了看清楚Java反射部分程式碼,所有異常我都最後丟擲來給虛擬機器處理!
* @param args
* @throws ClassNotFoundException
* @throws InstantiationException
* @throws IllegalAccessException
* @throws InvocationTargetException
* @throws IllegalArgumentException
* @throws NoSuchFieldException
* @throws SecurityException
* @throws NoSuchMethodException
*/
public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, SecurityException, NoSuchFieldException, NoSuchMethodException {
// TODO Auto-generated method stub
//Demo1. 通過Java反射機制得到類的包名和類名
Demo1();
System.out.println("===============================================");
//Demo2. 驗證所有的類都是Class類的例項物件
Demo2();
System.out.println("===============================================");
//Demo3. 通過Java反射機制,用Class 建立類物件[這也就是反射存在的意義所在],無參構造
Demo3();
System.out.println("===============================================");
//Demo4: 通過Java反射機制得到一個類的建構函式,並實現構造帶參例項物件
Demo4();
System.out.println("===============================================");
//Demo5: 通過Java反射機制操作成員變數, set 和 get
Demo5();
System.out.println("===============================================");
//Demo6: 通過Java反射機制得到類的一些屬性: 繼承的介面,父類,函式資訊,成員資訊,型別等
Demo6();
System.out.println("===============================================");
//Demo7: 通過Java反射機制呼叫類中方法
Demo7();
System.out.println("===============================================");
//Demo8: 通過Java反射機制獲得類載入器
Demo8();
System.out.println("===============================================");
}
/**
* Demo1: 通過Java反射機制得到類的包名和類名
*/
public static void Demo1()
{
Person person = new Person();
System.out.println("Demo1: 包名: " + person.getClass().getPackage().getName() + ","
+ "完整類名: " + person.getClass().getName());
}
/**
* Demo2: 驗證所有的類都是Class類的例項物件
* @throws ClassNotFoundException
*/
public static void Demo2() throws ClassNotFoundException
{
//定義兩個型別都未知的Class , 設定初值為null, 看看如何給它們賦值成Person類
Class<?> class1 = null;
Class<?> class2 = null;
//寫法1, 可能丟擲 ClassNotFoundException [多用這個寫法]
class1 = Class.forName("cn.lee.demo.Person");
System.out.println("Demo2:(寫法1) 包名: " + class1.getPackage().getName() + ","
+ "完整類名: " + class1.getName());
//寫法2
class2 = Person.class;
System.out.println("Demo2:(寫法2) 包名: " + class2.getPackage().getName() + ","
+ "完整類名: " + class2.getName());
}
/**
* Demo3: 通過Java反射機制,用Class 建立類物件[這也就是反射存在的意義所在]
* @throws ClassNotFoundException
* @throws IllegalAccessException
* @throws InstantiationException
*/
public static void Demo3() throws ClassNotFoundException, InstantiationException, IllegalAccessException
{
Class<?> class1 = null;
class1 = Class.forName("cn.lee.demo.Person");
//由於這裡不能帶引數,所以你要例項化的這個類Person,一定要有無參建構函式哈~
Person person = (Person) class1.newInstance();
person.setAge(20);
person.setName("LeeFeng");
System.out.println("Demo3: " + person.getName() + " : " + person.getAge());
}
/**
* Demo4: 通過Java反射機制得到一個類的建構函式,並實現建立帶參例項物件
* @throws ClassNotFoundException
* @throws InvocationTargetException
* @throws IllegalAccessException
* @throws InstantiationException
* @throws IllegalArgumentException
*/
public static void Demo4() throws ClassNotFoundException, IllegalArgumentException, InstantiationException, IllegalAccessException, InvocationTargetException
{
Class<?> class1 = null;
Person person1 = null;
Person person2 = null;
class1 = Class.forName("cn.lee.demo.Person");
//得到一系列建構函式集合
Constructor<?>[] constructors = class1.getConstructors();
person1 = (Person) constructors[0].newInstance();
person1.setAge(30);
person1.setName("leeFeng");
person2 = (Person) constructors[1].newInstance(20,"leeFeng");
System.out.println("Demo4: " + person1.getName() + " : " + person1.getAge()
+ " , " + person2.getName() + " : " + person2.getAge()
);
}
/**
* Demo5: 通過Java反射機制操作成員變數, set 和 get
*
* @throws IllegalAccessException
* @throws IllegalArgumentException
* @throws NoSuchFieldException
* @throws SecurityException
* @throws InstantiationException
* @throws ClassNotFoundException
*/
public static void Demo5() throws IllegalArgumentException, IllegalAccessException, SecurityException, NoSuchFieldException, InstantiationException, ClassNotFoundException
{
Class<?> class1 = null;
class1 = Class.forName("cn.lee.demo.Person");
Object obj = class1.newInstance();
Field personNameField = class1.getDeclaredField("name");
personNameField.setAccessible(true);
personNameField.set(obj, "胖虎先森");
System.out.println("Demo5: 修改屬性之後得到屬性變數的值:" + personNameField.get(obj));
}
/**
* Demo6: 通過Java反射機制得到類的一些屬性: 繼承的介面,父類,函式資訊,成員資訊,型別等
* @throws ClassNotFoundException
*/
public static void Demo6() throws ClassNotFoundException
{
Class<?> class1 = null;
class1 = Class.forName("cn.lee.demo.SuperMan");
//取得父類名稱
Class<?> superClass = class1.getSuperclass();
System.out.println("Demo6: SuperMan類的父類名: " + superClass.getName());
System.out.println("===============================================");
Field[] fields = class1.getDeclaredFields();
for (int i = 0; i < fields.length; i++) {
System.out.println("類中的成員: " + fields[i]);
}
System.out.println("===============================================");
//取得類方法
Method[] methods = class1.getDeclaredMethods();
for (int i = 0; i < methods.length; i++) {
System.out.println("Demo6,取得SuperMan類的方法:");
System.out.println("函式名:" + methods[i].getName());
System.out.println("函式返回型別:" + methods[i].getReturnType());
System.out.println("函式訪問修飾符:" + Modifier.toString(methods[i].getModifiers()));
System.out.println("函式程式碼寫法: " + methods[i]);
}
System.out.println("===============================================");
//取得類實現的介面,因為介面類也屬於Class,所以得到介面中的方法也是一樣的方法得到哈
Class<?> interfaces[] = class1.getInterfaces();
for (int i = 0; i < interfaces.length; i++) {
System.out.println("實現的介面類名: " + interfaces[i].getName() );
}
}
/**
* Demo7: 通過Java反射機制呼叫類方法
* @throws ClassNotFoundException
* @throws NoSuchMethodException
* @throws SecurityException
* @throws InvocationTargetException
* @throws IllegalAccessException
* @throws IllegalArgumentException
* @throws InstantiationException
*/
public static void Demo7() throws ClassNotFoundException, SecurityException, NoSuchMethodException, IllegalArgumentException, IllegalAccessException, InvocationTargetException, InstantiationException
{
Class<?> class1 = null;
class1 = Class.forName("cn.lee.demo.SuperMan");
System.out.println("Demo7: \n呼叫無參方法fly():");
Method method = class1.getMethod("fly");
method.invoke(class1.newInstance());
System.out.println("呼叫有參方法walk(int m):");
method = class1.getMethod("walk",int.class);
method.invoke(class1.newInstance(),100);
}
/**
* Demo8: 通過Java反射機制得到類載入器資訊
*
* 在java中有三種類類載入器。[這段資料網上擷取]
1)Bootstrap ClassLoader 此載入器採用c++編寫,一般開發中很少見。
2)Extension ClassLoader 用來進行擴充套件類的載入,一般對應的是jre\lib\ext目錄中的類
3)AppClassLoader 載入classpath指定的類,是最常用的載入器。同時也是java中預設的載入器。
*
* @throws ClassNotFoundException
*/
public static void Demo8() throws ClassNotFoundException
{
Class<?> class1 = null;
class1 = Class.forName("cn.lee.demo.SuperMan");
String nameString = class1.getClassLoader().getClass().getName();
System.out.println("Demo8: 類載入器類名: " + nameString);
}
}
/**
*
* @author xiaoyaomeng
*
*/
class Person{
private int age;
private String name;
public Person(){
}
public Person(int age, String name){
this.age = age;
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
class SuperMan extends Person implements ActionInterface
{
private boolean BlueBriefs;
public void fly()
{
System.out.println("超人會飛耶~~");
}
public boolean isBlueBriefs() {
return BlueBriefs;
}
public void setBlueBriefs(boolean blueBriefs) {
BlueBriefs = blueBriefs;
}
@Override
public void walk(int m) {
// TODO Auto-generated method stub
System.out.println("超人會走耶~~走了" + m + "米就走不動了!");
}
}
interface ActionInterface{
public void walk(int m);
}
個人覺得使用反射機制的一些地方:
1.工廠模式:Factory類中用反射的話,添加了一個新的類之後,就不需要再修改工廠類Factory了
2.資料庫JDBC中通過Class.forName(Driver).來獲得資料庫連線驅動
3.分析類檔案:畢竟能得到類中的方法等等
4.訪問一些不能訪問的變數或屬性:破解別人程式碼