Java反射经典实例 Java Reflection Cookbook

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);  
    }  
}