人生若只如初见，何事秋风悲画扇。
概述
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UserInfo
package com.niocoder.test.introspector;
/**
*
*/
public class UserInfo {
private String userName;
private Integer age;
private String webSite;
public Integer getAge() {
return age;
}
public void setAge(Integer age) {
this.age = age;
}
public String getUserName() {
return userName;
}
public void setUserName(String userName) {
this.userName = userName;
}
public String getWebSite() {
return webSite;
}
public void setWebSite(String webSite) {
this.webSite = webSite;
}
}
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
PropertyDescriptor类表示JavaBean类通过存储器导出一个属性。主要方法：
- getPropertyType()，获得属性的Class对象;
- 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;
- hashCode()，获取对象的哈希值;
- setReadMethod(Method readMethod)，设置用于读取属性值的方法;
- setWriteMethod(Method writeMethod)，设置用于写入属性值的方法。
package com.niocoder.test.introspector;
import java.beans.PropertyDescriptor;
import java.lang.reflect.Method;
public class BeanInfoUtil {
/**
* @param userInfo 实例
* @param propertyName 属性名
* @throws Exception
*/
public static void setProperty(UserInfo userInfo, String propertyName) throws Exception {
PropertyDescriptor propDesc = new PropertyDescriptor(propertyName, UserInfo.class);
Method methodSetUserName = propDesc.getWriteMethod();
methodSetUserName.invoke(userInfo, "郑龙飞");
System.out.println("set userName:" + userInfo.getUserName());
}
/**
* @param userInfo 实例
* @param propertyName 属性名
* @throws Exception
*/
public static void getProperty(UserInfo userInfo, String propertyName) throws Exception {
PropertyDescriptor proDescriptor = new PropertyDescriptor(propertyName, UserInfo.class);
Method methodGetUserName = proDescriptor.getReadMethod();
Object objUserName = methodGetUserName.invoke(userInfo);
System.out.println("get userName:" + objUserName.toString());
}
}
Introspector
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.niocoder.test.introspector;
import java.beans.BeanInfo;
import java.beans.Introspector;
import java.beans.PropertyDescriptor;
import java.lang.reflect.Method;
public class BeanInfoUtil {
/**
* @param userInfo 实例
* @param propertyName 属性名
* @throws Exception
*/
public static void setPropertyByIntrospector(UserInfo userInfo, String propertyName) 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(propertyName)) {
Method methodSetUserName = propDesc.getWriteMethod();
methodSetUserName.invoke(userInfo, "niocoder");
System.out.println("set userName:" + userInfo.getUserName());
break;
}
}
}
}
/**
* @param userInfo 实例
* @param propertyName 属性名
* @throws Exception
*/
public static void getPropertyByIntrospector(UserInfo userInfo, String propertyName) 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(propertyName)) {
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。
Test
BeanInfoTest
public class BeanInfoTest {
public static void main(String[] args) {
UserInfo userInfo = new UserInfo();
userInfo.setUserName("merryyou");
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:merryyou
set userName:郑龙飞
get userName:郑龙飞
set userName:niocoder
get userName:niocoder
Disconnected from the target VM, address: '127.0.0.1:65243', transport: 'socket'
java.lang.IllegalArgumentException: argument type mismatch
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.niocoder.test.introspector.BeanInfoUtil.setProperty(BeanInfoUtil.java:13)
at com.niocoder.test.introspector.BeanInfoTest.main(BeanInfoTest.java:19)
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
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.getProperty(Object bean, String propertyName)
- BeanUtils.setProperty(Object bean, String propertyName, Object value)
BeanUtilTest
public class BeanUtilTest {
public static void main(String[] args) {
UserInfo userInfo = new UserInfo();
try {
BeanUtils.setProperty(userInfo, "userName", "郑龙飞");
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());
// 特殊 age属性为Integer类型
PropertyUtils.setProperty(userInfo, "age", "8");
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (InvocationTargetException e) {
e.printStackTrace();
} catch (NoSuchMethodException e) {
e.printStackTrace();
}
}
}
输出
set userName:郑龙飞
get userName:郑龙飞
set age:18
get age:18
get userName type:java.lang.String
get age type:java.lang.String
8
java.lang.Integer
Disconnected from the target VM, address: '127.0.0.1:50244', transport: 'socket'
Exception in thread "main" java.lang.IllegalArgumentException: Cannot invoke com.niocoder.test.introspector.UserInfo.setAge on bean class 'class com.niocoder.test.introspector.UserInfo' - argument type mismatch - had objects of type "java.lang.String" but expected signature "java.lang.Integer"
at org.apache.commons.beanutils.PropertyUtilsBean.invokeMethod(PropertyUtilsBean.java:2195)
at org.apache.commons.beanutils.PropertyUtilsBean.setSimpleProperty(PropertyUtilsBean.java:2108)
at org.apache.commons.beanutils.PropertyUtilsBean.setNestedProperty(PropertyUtilsBean.java:1914)
at org.apache.commons.beanutils.PropertyUtilsBean.setProperty(PropertyUtilsBean.java:2021)
at org.apache.commons.beanutils.PropertyUtils.setProperty(PropertyUtils.java:896)
at com.niocoder.test.introspector.BeanUtilTest.main(BeanUtilTest.java:28)
Caused by: java.lang.IllegalArgumentException: argument type mismatch
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.apache.commons.beanutils.PropertyUtilsBean.invokeMethod(PropertyUtilsBean.java:2127)
... 5 more
Process finished with exit code 1