深入理解Java:内省(Introspector)

一些概念:

内省(Introspector) 是Java 语言对 JavaBean 类属性、事件的一种缺省处理方法。

JavaBean是一种特殊的类,主要用于传递数据信息,这种类中的方法主要用于访问私有的字段,且方法名符合某种命名规则。如果在两个模块之间传递信息,可以将信息封装进JavaBean中,这种对象称为“值对象”(Value Object),或“VO”。方法比较少。这些信息储存在类的私有变量中,通过set()、get()获得。

例如类UserInfo :

package com.peidasoft.Introspector;

public class UserInfo {
    
    private long userId;
    private String userName;
    private int age;
    private String emailAddress;
    
    public long getUserId() {
        return userId;
    }
    public void setUserId(long userId) {
        this.userId = userId;
    }
    public String getUserName() {
        return userName;
    }
    public void setUserName(String userName) {
        this.userName = userName;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getEmailAddress() {
        return emailAddress;
    }
    public void setEmailAddress(String emailAddress) {
        this.emailAddress = emailAddress;
    }
    
}

在类UserInfo中有属性 userName, 那我们可以通过 getUserName,setUserName来得到其值或者设置新的值。通过 getUserName/setUserName来访问 userName属性,这就是默认的规则。 Java JDK中提供了一套 API 用来访问某个属性的 getter/setter 方法,这就是内省。

JDK内省类库:

PropertyDescriptor类:

PropertyDescriptor类表示JavaBean类通过存储器导出一个属性。主要方法:
  1. getPropertyType(),获得属性的Class对象;
  2. getReadMethod(),获得用于读取属性值的方法;getWriteMethod(),获得用于写入属性值的方法;
  3. hashCode(),获取对象的哈希值;
  4. setReadMethod(Method readMethod),设置用于读取属性值的方法;
  5. setWriteMethod(Method writeMethod),设置用于写入属性值的方法。

实例代码如下:

package com.peidasoft.Introspector;

import java.beans.BeanInfo;
import java.beans.Introspector;
import java.beans.PropertyDescriptor;
import java.lang.reflect.Method;

public class BeanInfoUtil {  
 
    public static void setProperty(UserInfo userInfo,String userName)throws Exception{
        PropertyDescriptor propDesc=new PropertyDescriptor(userName,UserInfo.class);
        Method methodSetUserName=propDesc.getWriteMethod();
        methodSetUserName.invoke(userInfo, "wong");
        System.out.println("set userName:"+userInfo.getUserName());
    }
  
    public static void getProperty(UserInfo userInfo,String userName)throws Exception{
        PropertyDescriptor proDescriptor =new PropertyDescriptor(userName,UserInfo.class);
        Method methodGetUserName=proDescriptor.getReadMethod();
        Object objUserName=methodGetUserName.invoke(userInfo);
        System.out.println("get userName:"+objUserName.toString());
    }
} 

Introspector类:

将JavaBean中的属性封装起来进行操作。在程序把一个类当做JavaBean来看,就是调用Introspector.getBeanInfo()方法,得到的BeanInfo对象封装了把这个类当做JavaBean看的结果信息,即属性的信息。

getPropertyDescriptors(),获得属性的描述,可以采用遍历BeanInfo的方法,来查找、设置类的属性。具体代码如下:

package com.peidasoft.Introspector;

import java.beans.BeanInfo;
import java.beans.Introspector;
import java.beans.PropertyDescriptor;
import java.lang.reflect.Method;


public class BeanInfoUtil {
        
    public static void setPropertyByIntrospector(UserInfo userInfo,String userName)throws Exception{
        BeanInfo beanInfo=Introspector.getBeanInfo(UserInfo.class);
        PropertyDescriptor[] proDescrtptors=beanInfo.getPropertyDescriptors();
        if(proDescrtptors!=null&&proDescrtptors.length>0){
            for(PropertyDescriptor propDesc:proDescrtptors){
                if(propDesc.getName().equals(userName)){
                    Method methodSetUserName=propDesc.getWriteMethod();
                    methodSetUserName.invoke(userInfo, "alan");
                    System.out.println("set userName:"+userInfo.getUserName());
                    break;
                }
            }
        }
    }
    
    public static void getPropertyByIntrospector(UserInfo userInfo,String userName)throws Exception{
        BeanInfo beanInfo=Introspector.getBeanInfo(UserInfo.class);
        PropertyDescriptor[] proDescrtptors=beanInfo.getPropertyDescriptors();
        if(proDescrtptors!=null&&proDescrtptors.length>0){
            for(PropertyDescriptor propDesc:proDescrtptors){
                if(propDesc.getName().equals(userName)){
                    Method methodGetUserName=propDesc.getReadMethod();
                    Object objUserName=methodGetUserName.invoke(userInfo);
                    System.out.println("get userName:"+objUserName.toString());
                    break;
                }
            }
        }
    }
    
}

通过这两个类的比较可以看出,都是需要获得PropertyDescriptor,只是方式不一样:前者通过创建对象直接获得,后者需要遍历,所以使用PropertyDescriptor类更加方便。

使用实例:

package com.peidasoft.Introspector;

public class BeanInfoTest {

    /**
     * @param args
     */
    public static void main(String[] args) {
        UserInfo userInfo=new UserInfo();
        userInfo.setUserName("peida");
        try {
            BeanInfoUtil.getProperty(userInfo, "userName");
            
            BeanInfoUtil.setProperty(userInfo, "userName");
            
            BeanInfoUtil.getProperty(userInfo, "userName");
            
            BeanInfoUtil.setPropertyByIntrospector(userInfo, "userName");            
            
            BeanInfoUtil.getPropertyByIntrospector(userInfo, "userName");
            
            BeanInfoUtil.setProperty(userInfo, "age");
            
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }

}

输出:

get userName:peida
set userName:wong
get userName:wong
set userName:alan
get userName:alan
java.lang.IllegalArgumentException: argument type mismatch
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at com.peidasoft.Introspector.BeanInfoUtil.setProperty(BeanInfoUtil.java:14)
    at com.peidasoft.Introspector.BeanInfoTest.main(BeanInfoTest.java:22)

说明:BeanInfoUtil.setProperty(userInfo, "age");报错是应为age属性是int数据类型,而setProperty方法里面默认给age属性赋的值是String类型。所以会爆出argument type mismatch参数类型不匹配的错误信息。

BeanUtils工具包:

由上述可看出,内省操作非常的繁琐,所以所以Apache开发了一套简单、易用的API来操作Bean的属性——BeanUtils工具包。
  BeanUtils工具包:下载:http://commons.apache.org/beanutils/ 注意:应用的时候还需要一个logging包 http://commons.apache.org/logging/
  使用BeanUtils工具包完成上面的测试代码:

package com.peidasoft.Beanutil;

import java.lang.reflect.InvocationTargetException;

import org.apache.commons.beanutils.BeanUtils;
import org.apache.commons.beanutils.PropertyUtils;

import com.peidasoft.Introspector.UserInfo;

public class BeanUtilTest {
    public static void main(String[] args) {
        UserInfo userInfo=new UserInfo();
         try {
            BeanUtils.setProperty(userInfo, "userName", "peida");
            
            System.out.println("set userName:"+userInfo.getUserName());
            
            System.out.println("get userName:"+BeanUtils.getProperty(userInfo, "userName"));
            
            BeanUtils.setProperty(userInfo, "age", 18);
            System.out.println("set age:"+userInfo.getAge());
            
            System.out.println("get age:"+BeanUtils.getProperty(userInfo, "age"));
             
            System.out.println("get userName type:"+BeanUtils.getProperty(userInfo, "userName").getClass().getName());
            System.out.println("get age type:"+BeanUtils.getProperty(userInfo, "age").getClass().getName());
            
            PropertyUtils.setProperty(userInfo, "age", 8);
            System.out.println(PropertyUtils.getProperty(userInfo, "age"));
            
            System.out.println(PropertyUtils.getProperty(userInfo, "age").getClass().getName());
                  
            PropertyUtils.setProperty(userInfo, "age", "8");   
        } 
         catch (IllegalAccessException e) {
            e.printStackTrace();
        } 
         catch (InvocationTargetException e) {
            e.printStackTrace();
        }
        catch (NoSuchMethodException e) {
            e.printStackTrace();
        }
    }
}

运行结果:

set userName:peida
get userName:peida
set age:18
get age:18
get userName type:java.lang.String
get age type:java.lang.String
8
java.lang.Integer
Exception in thread "main" java.lang.IllegalArgumentException: Cannot invoke com.peidasoft.Introspector.UserInfo.setAge 
on bean class 'class com.peidasoft.Introspector.UserInfo' - argument type mismatch - had objects of type "java.lang.String" 
but expected signature "int"
    at org.apache.commons.beanutils.PropertyUtilsBean.invokeMethod(PropertyUtilsBean.java:2235)
    at org.apache.commons.beanutils.PropertyUtilsBean.setSimpleProperty(PropertyUtilsBean.java:2151)
    at org.apache.commons.beanutils.PropertyUtilsBean.setNestedProperty(PropertyUtilsBean.java:1957)
    at org.apache.commons.beanutils.PropertyUtilsBean.setProperty(PropertyUtilsBean.java:2064)
    at org.apache.commons.beanutils.PropertyUtils.setProperty(PropertyUtils.java:858)
    at com.peidasoft.orm.Beanutil.BeanUtilTest.main(BeanUtilTest.java:38)
Caused by: java.lang.IllegalArgumentException: argument type mismatch
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at org.apache.commons.beanutils.PropertyUtilsBean.invokeMethod(PropertyUtilsBean.java:2170)
    ... 5 more

说明:

1.获得属性的值,例如,BeanUtils.getProperty(userInfo,"userName"),返回字符串
  2.设置属性的值,例如,BeanUtils.setProperty(userInfo,"age",8),参数是字符串或基本类型自动包装。设置属性的值是字符串,获得的值也是字符串,不是基本类型。   3.BeanUtils的特点:
    1). 对基本数据类型的属性的操作:在WEB开发、使用中,录入和显示时,值会被转换成字符串,但底层运算用的是基本类型,这些类型转到动作由BeanUtils自动完成。
    2). 对引用数据类型的属性的操作:首先在类中必须有对象,不能是null,例如,private Date birthday=new Date();。操作的是对象的属性而不是整个对象,例如,BeanUtils.setProperty(userInfo,"birthday.time",111111);

package com.peidasoft.Introspector;
import java.util.Date;

public class UserInfo {

    private Date birthday = new Date();
    
    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }
    public Date getBirthday() {
        return birthday;
    }      
}
package com.peidasoft.Beanutil;

import java.lang.reflect.InvocationTargetException;
import org.apache.commons.beanutils.BeanUtils;
import com.peidasoft.Introspector.UserInfo;

public class BeanUtilTest {
    public static void main(String[] args) {
        UserInfo userInfo=new UserInfo();
         try {
            BeanUtils.setProperty(userInfo, "birthday.time","111111");  
            Object obj = BeanUtils.getProperty(userInfo, "birthday.time");  
            System.out.println(obj);          
        } 
         catch (IllegalAccessException e) {
            e.printStackTrace();
        } 
         catch (InvocationTargetException e) {
            e.printStackTrace();
        }
        catch (NoSuchMethodException e) {
            e.printStackTrace();
        }
    }
}

3.PropertyUtils类和BeanUtils不同在于,运行getProperty、setProperty操作时,没有类型转换,使用属性的原有类型或者包装类。由于age属性的数据类型是int,所以方法PropertyUtils.setProperty(userInfo, "age", "8")会爆出数据类型不匹配,无法将值赋给属性。
  

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