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) 


  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:


  获得属性的值,例如,BeanUtils.getProperty(userInfo,"userName"),返回字符串
  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。    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();
}
}
}


  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。

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