java PropertyDescriptor 应用及源码分析

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