深入理解Java:内省(Introspector)

长平狐 发布于 2013/06/17 12:52
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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:peidaset userName:wongget userName:wongset userName:alanget userName:alanjava.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:peidaget userName:peidaset age:18 get age:18 get userName type:java.lang.Stringget age type:java.lang.String8 java.lang.IntegerException 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。

参考资料:

 


1.http://www.cnblogs.com/avenwu/archive/2012/02/28/2372586.html

2.http://blog.csdn.net/zhuruoyun/article/details/8219333


原文链接:http://www.cnblogs.com/peida/archive/2013/06/03/3090842.html
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