java PropertyDescriptor 应用及源码分析

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1. 概述

PropertyDescriptor描述Java Bean中通过一对存储器方法（getter / setter）导出的一个属性。我们可以通过该PropertyDescriptor对bean中的该属性进行读取和写入操作，也可以设置其getter / setter。

2. 关键接口及内部属性

public PropertyDescriptor(String name, Class<?> beanClass) throws IntrospectionException  public PropertyDescriptor(String name, Class<?> beanClass, String getMethodName, String setMethodName) throws IntrospectionException  public PropertyDescriptor(String name, Method readMethod, Method writeMethod) throws IntrospectionException  public Class<?> getPropertyType()  public Method getReadMethod()  public Method getWriteMethod()  public void setReadMethod(Method readMethod) throws IntrospectionException  public void setWriteMethod(Method writeMethod)  public boolean equals(Object o)

相关的PropertyDescriptor内部属性如下：
Class<?> propertyType; //该属性的类型
Method getMethod; //getter
Method setMethod; //setter
还有继承自其父类FeatureDescriptor的功能，用于指定该属性的编程名称

3. 简单应用

现有Person类如下：

package com.cwind.property;  public class Person {          private String name ;          private int age ;          public Person(){ this.name = ""; this.age = 0; }          public Person(String name, int age) { super(); this.name = name; this. age = age; }          public String getName() { return name; }          public void setName(String name) { this. name = name; }          public int getAge() { return age; }          public void setAge(int age) { this. age = age; }          public String getNameInUpperCase(){                 return this .name .toUpperCase();         }          public void setNameToLowerCase(String name){                 this.name = name.toLowerCase();         }  }

该类中除了name和age两个属性的标准getter和setter之外，还有增加了一个获取大写name的get方法和一个将name设置为小写的set方法。
在测试类中，首先获得这两个方法对象。

Class personClass = Class.forName("com.cwind.property.Person");  Method read = personClass.getMethod("getNameInUpperCase", null);  Method write = personClass.getMethod("setNameToLowerCase", String.class );  //然后可以通过两种方式构造PropertyDescriptor  PropertyDescriptor prop1 = new PropertyDescriptor( "name", Person.class );     //使用其标准getter和setter  PropertyDescriptor prop2 = new PropertyDescriptor( "name", read, write);     //使用read和write两个方法对象所自定义的getter和setter  //下面构建一个Person对象  Person person = new Person("Kobe" , 36);  System. out.println(prop1.getReadMethod().invoke(person, null));     // --实际调用Person.getName(), result: Kobe  System. out.println(prop2.getReadMethod().invoke(person, null));     // --实际调用Person.getNameInUpperCase(), result: KOBE  prop1.getWriteMethod().invoke(person, "James");     // --实际调用Person.setName(), person.name被设置为James  prop2.getWriteMethod().invoke(person, "James");     // --实际调用Person.setNameToLowerCase(), person.name被设置为james

4. 源码分析

构造函数1：

public PropertyDescriptor(String name, Class<?> beanClass)          throws IntrospectionException {          setName(name);     //设置属性编程名，本例中即'name'          if (name.length() == 0){              throw new IntrospectionException("empty property name");       // 编程名为空则抛出异常          }          String caps = Character.toUpperCase(name.charAt(0)) + name.substring(1);       // 标准getter应为getName()或isName(), 先将首字母大写          findMethods(beanClass, "is" + caps, "get" + caps, "set" + caps);       // 参数依次为：类类型，可能的getter函数名1，可能的getter函数名2，setter函数名          if (getMethod == null){   // findMethods()设置PropertyDescriptor的getMethod和setMethod属性              throw new IntrospectionException(                  "Cannot find a is" + caps + " or get" + caps + " method");          }          if (setMethod == null){              throw new IntrospectionException(                  "Cannot find a " + caps + " method" );          }          propertyType = checkMethods(getMethod, setMethod);       // checkMethods()函数用来检测getMethod得到的类型与setMethod的参数类型是否匹配，若匹配则置propertyType为该类型      }

构造函数2：
public PropertyDescriptor(String name, Class

private void findMethods(Class beanClass, String getMethodName1, String getMethodName2, String setMethodName) throws IntrospectionException {          try {              // 首先查找getMethodName1指定的getter (isXXX)              if (getMethodName1 != null) {                  try {                      getMethod = beanClass.getMethod(getMethodName1, new Class[0]);                  }                  catch (NoSuchMethodException e)                  {}              }              // 若失败，则查找getMethodName2指定的getter (getXXX)              if (getMethod == null && getMethodName2 != null) {                  try {                      getMethod = beanClass.getMethod(getMethodName2, new Class[0]);                  }                  catch (NoSuchMethodException e)                  {}              }              if (setMethodName != null) {                  if (getMethod != null) {                      // 如果得到了getMethod，则通过其返回值类型决定setMethod的参数类型                      Class propertyType = getMethod.getReturnType();                      if (propertyType == Void.TYPE) {   // 若getter的返回值为Void类型则抛出异常                          String msg = "The property's read method has return type 'void'";                          throw new IntrospectionException(msg);                      }                      Class[] setArgs = new Class[] { propertyType };                       try {                          setMethod = beanClass.getMethod(setMethodName, setArgs);   // 通过函数名和参数类型获得setMethod                      }                      catch (NoSuchMethodException e)                      {}                  }                  else if (getMethodName1 == null && getMethodName2 == null) {                      // getMethodName1和2均为空，则此属性为只写属性，此时遍历bean中的函数，返回第一个名称与setMethodName一致且返回类型为Void的单参数函数                      Method[] methods = beanClass.getMethods();                      for (int i = 0; i < methods.length; i++) {                          if (methods[i].getName().equals(setMethodName)                              && methods[i].getParameterTypes().length == 1                              && methods[i].getReturnType() == Void.TYPE) {                              setMethod = methods[i];                              break;                          }                      }                  }              }          }          catch (SecurityException e) {              String msg = "SecurityException thrown on attempt to access methods.";     // 作者在纠结要不要修改异常类型              throw new IntrospectionException(msg);          }      }

checkMethods方法

private Class<?> checkMethods(Method readMethod, Method writeMethod) throws IntrospectionException {          Class<?> newPropertyType = propertyType;           // 合法的read方法应该无参同时带有一个非空的返回值类型          if (readMethod != null) {              if (readMethod.getParameterTypes().length > 0) {                  throw new IntrospectionException("read method has unexpected parameters");              }              newPropertyType = readMethod.getReturnType();               if (newPropertyType == Void.TYPE) {                  throw new IntrospectionException("read method return type is void");              }          }           // 合法的write方法应该包含一个类型相同的参数          if (writeMethod != null) {              if (writeMethod.getParameterTypes().length != 1) { // 参数不能超过一个                  String msg = "write method does not have exactly one parameter" ;                  throw new IntrospectionException(msg);              }              if (readMethod == null) {                  // 若无read方法，属性类型就应为writeMethod的参数类型                  newPropertyType = writeMethod.getParameterTypes()[0];              }              else {                  // 检查read方法的返回值类型是否与write方法的参数类型相匹配                  if (newPropertyType != null                      && !newPropertyType.isAssignableFrom(                          writeMethod.getParameterTypes()[0])) {                       throw new IntrospectionException("read and write method are not compatible");                  }              }          }           return newPropertyType;      }

最后提一句PropertyDescriptor.equals(), 只有当属性类型、标志、读写方法和PropertyEditorClass均相同时才认为两个PropertyDescriptor相等

return samePropertyType                  && sameFlags                  && sameReadMethod                  && sameWriteMethod                  && samePropertyEditorClass;

转载自：http://cwind.iteye.com/blog/2028208?utm_source=tuicool
参考：http://blog.csdn.net/z69183787/article/details/8443777

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