java反射

2008-11-12
Java反射经典实例 Java Reflection Cookbook
关键字: 反射
http://orangewhy.iteye.com/blog/56011





Java提供了一套机制来动态执行方法和构造方法，以及数组操作等，这套机制就叫——反射。反射机制是如今很多流行框架的实现基础，其中包括Spring、Hibernate等。原理性的问题不是本文的重点，接下来让我们在实例中学习这套精彩的机制。

1. 得到某个对象的属性

public Object getProperty(Object owner, String fieldName) throws Exception {      
    Class ownerClass = owner.getClass();                                          
                                                                                  
    Field field = ownerClass.getField(fieldName);                                 
                                                                                  
    Object property = field.get(owner);                                           
                                                                                  
    return property;                                                              
}                                                                                 


Class ownerClass = owner.getClass()：得到该对象的Class。

Field field = ownerClass.getField(fieldName)：通过Class得到类声明的属性。

Object property = field.get(owner)：通过对象得到该属性的实例，如果这个属性是非公有的，这里会报IllegalAccessException。



2. 得到某个类的静态属性


public Object getStaticProperty(String className, String fieldName)    
            throws Exception {                                         
    Class ownerClass = Class.forName(className);                       
                                                                       
    Field field = ownerClass.getField(fieldName);                      
                                                                       
    Object property = field.get(ownerClass);                           
                                                                       
    return property;                                                   
}                                                                      


Class ownerClass = Class.forName(className) ：首先得到这个类的Class。

Field field = ownerClass.getField(fieldName)：和上面一样，通过Class得到类声明的属性。

Object property = field.get(ownerClass) ：这里和上面有些不同，因为该属性是静态的，所以直接从类的Class里取。



3. 执行某对象的方法

public Object invokeMethod(Object owner, String methodName, Object[] args) throws Exception {   
    Class ownerClass = owner.getClass();   
    Class[] argsClass = new Class[args.length];   
    for (int i = 0, j = args.length; i < j; i++) {   
        argsClass[i] = args[i].getClass();   
    }    
    Method method = ownerClass.getMethod(methodName, argsClass);      
    return method.invoke(owner, args);    
}  

Class owner_class = owner.getClass() ：首先还是必须得到这个对象的Class。

3～6行：配置参数的Class数组，作为寻找Method的条件。

Method method = ownerClass.getMethod(methodName, argsClass)：通过Method名和参数的Class数组得到要执行的Method。

method.invoke(owner, args)：执行该Method，invoke方法的参数是执行这个方法的对象，和参数数组。返回值是Object，也既是该方法的返回值。


4. 执行某个类的静态方法

public Object invokeStaticMethod(String className, String methodName,            
            Object[] args) throws Exception {                                    
    Class ownerClass = Class.forName(className);                                 
                                                                                 
    Class[] argsClass = new Class[args.length];                                  
                                                                                 
    for (int i = 0, j = args.length; i < j; i++) {                               
        argsClass[i] = args[i].getClass();                                       
    }                                                                            
                                                                                 
    Method method = ownerClass.getMethod(methodName, argsClass);                 
                                                                                 
    return method.invoke(null, args);                                            
}                                                                                

基本的原理和实例3相同，不同点是最后一行，invoke的一个参数是null，因为这是静态方法，不需要借助实例运行。



5. 新建实例

public Object newInstance(String className, Object[] args) throws Exception {    
    Class newoneClass = Class.forName(className);                                
                                                                                 
    Class[] argsClass = new Class[args.length];                                  
                                                                                 
    for (int i = 0, j = args.length; i < j; i++) {                               
        argsClass[i] = args[i].getClass();                                       
    }                                                                            
                                                                                 
    Constructor cons = newoneClass.getConstructor(argsClass);                    
                                                                                 
    return cons.newInstance(args);                                               
                                                                                 
}                                                                                

这里说的方法是执行带参数的构造函数来新建实例的方法。如果不需要参数，可以直接使用newoneClass.newInstance()来实现。

Class newoneClass = Class.forName(className)：第一步，得到要构造的实例的Class。

第6～第10行：得到参数的Class数组。

Constructor cons = newoneClass.getConstructor(argsClass)：得到构造子。

cons.newInstance(args)：新建实例。


6. 判断是否为某个类的实例

public boolean isInstance(Object obj, Class cls) {        
    return cls.isInstance(obj);                           
}                                                         


7. 得到数组中的某个元素

public Object getByArray(Object array, int index) {       
    return Array.get(array,index);                        
}                                                         


附完整源码：

import java.lang.reflect.Array;   
import java.lang.reflect.Constructor;   
import java.lang.reflect.Field;   
import java.lang.reflect.Method;   
  
  
/**  
* Java Reflection Cookbook  
*  
* @author Michael Lee  
* @since 2006-8-23  
* @version 0.1a  
*/  
  
public class Reflection {   
    /**  
     * 得到某个对象的公共属性  
     *  
     * @param owner, fieldName  
     * @return 该属性对象  
     * @throws Exception  
     *  
     */  
    public Object getProperty(Object owner, String fieldName) throws Exception {   
        Class ownerClass = owner.getClass();   
  
        Field field = ownerClass.getField(fieldName);   
  
        Object property = field.get(owner);   
  
        return property;   
    }   
  
    /**  
     * 得到某类的静态公共属性  
     *  
     * @param className   类名  
     * @param fieldName   属性名  
     * @return 该属性对象  
     * @throws Exception  
     */  
    public Object getStaticProperty(String className, String fieldName)   
            throws Exception {   
        Class ownerClass = Class.forName(className);   
  
        Field field = ownerClass.getField(fieldName);   
  
        Object property = field.get(ownerClass);   
  
        return property;   
    }   
  
  
    /**  
     * 执行某对象方法  
     *  
     * @param owner  
     *            对象  
     * @param methodName  
     *            方法名  
     * @param args  
     *            参数  
     * @return 方法返回值  
     * @throws Exception  
     */  
    public Object invokeMethod(Object owner, String methodName, Object[] args)   
            throws Exception {   
  
        Class ownerClass = owner.getClass();   
  
        Class[] argsClass = new Class[args.length];   
  
        for (int i = 0, j = args.length; i < j; i++) {   
            argsClass[i] = args[i].getClass();   
        }   
  
        Method method = ownerClass.getMethod(methodName, argsClass);   
  
        return method.invoke(owner, args);   
    }   
  
  
      /**  
     * 执行某类的静态方法  
     *  
     * @param className  
     *            类名  
     * @param methodName  
     *            方法名  
     * @param args  
     *            参数数组  
     * @return 执行方法返回的结果  
     * @throws Exception  
     */  
    public Object invokeStaticMethod(String className, String methodName,   
            Object[] args) throws Exception {   
        Class ownerClass = Class.forName(className);   
  
        Class[] argsClass = new Class[args.length];   
  
        for (int i = 0, j = args.length; i < j; i++) {   
            argsClass[i] = args[i].getClass();   
        }   
  
        Method method = ownerClass.getMethod(methodName, argsClass);   
  
        return method.invoke(null, args);   
    }   
  
  
  
    /**  
     * 新建实例  
     *  
     * @param className  
     *            类名  
     * @param args  
     *            构造函数的参数  
     * @return 新建的实例  
     * @throws Exception  
     */  
    public Object newInstance(String className, Object[] args) throws Exception {   
        Class newoneClass = Class.forName(className);   
  
        Class[] argsClass = new Class[args.length];   
  
        for (int i = 0, j = args.length; i < j; i++) {   
            argsClass[i] = args[i].getClass();   
        }   
  
        Constructor cons = newoneClass.getConstructor(argsClass);   
  
        return cons.newInstance(args);   
  
    }   
  
  
       
    /**  
     * 是不是某个类的实例  
     * @param obj 实例  
     * @param cls 类  
     * @return 如果 obj 是此类的实例，则返回 true  
     */  
    public boolean isInstance(Object obj, Class cls) {   
        return cls.isInstance(obj);   
    }   
       
    /**  
     * 得到数组中的某个元素  
     * @param array 数组  
     * @param index 索引  
     * @return 返回指定数组对象中索引组件的值  
     */  
    public Object getByArray(Object array, int index) {   
        return Array.get(array,index);   
    }   
}