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感谢GY丶L粉丝的提问:属性描述器PropertyDescriptor是干嘛用的?
本来我也没有仔细了解过 描述符 这一块的知识,不过粉丝问了,我就抽周末的时间看看,顺便学习一下,粉丝问的刚好是PropertyDescriptor这个属性描述符,我看了下源码。
/** * A PropertyDescriptor describes one property that a Java Bean * exports via a pair of accessor methods. */ public class PropertyDescriptor extends FeatureDescriptor { //... }
emmmm,假装自己英语能厉害的说,属性描述符描述了一个属性,即Java Bean 通过一对访问器方法来导出。(没错,他确实是存在于java.beans包下的)
通过类关系图,可以知道,我们应该提前了解一下 FeatureDescriptor 才行了。很好,起码目前还没有设计抽象类或者接口。
/** * The FeatureDescriptor class is the common baseclass for PropertyDescriptor, * EventSetDescriptor, and MethodDescriptor, etc. * <p> * It supports some common information that can be set and retrieved for * any of the introspection descriptors. * <p> * In addition it provides an extension mechanism so that arbitrary * attribute/value pairs can be associated with a design feature. */ public class FeatureDescriptor { //... }
okay,这是很合理的设计方式,FeatureDescriptor为类似PropertyDescriptor、EvebtSetDescriptor、MethodDescriptor的描述符提供了一些共用的常量信息。同时它也提供一个扩展功能,方便任意属性或键值对可以于设计功能相关联。
这里简单的说下,在我大致看了一下源码后(可能不够详细,最近有点忙,时间较赶),FeatureDescriptor主要是针对一下属性的一些get/set,同时这些属性都是基本通用于PropertyDescriptor、EvebtSetDescriptor、MethodDescriptor。
private boolean expert; // 专有 private boolean hidden; // 隐藏 private boolean preferred; // 首选 private String shortDescription; //简单说明 private String name; // 编程名称 private String displayName; //本地名称 private Hashtable<String, Object> table; // 属性表
其实该类还有另外几个方法,比如深奥的构造函数等等,这里就不深入探讨了。
那么我们大致知道了 FeatureDescriptor ,接下来就可以来深入了解看看这个属性描述符 PropertyDescriptor
。
说到属性,大家一定会想到的就是get/set这个些基础的东西,当我打开 PropertyDescriptor
源码的时候,我也看到了一开始猜想的点。
private final MethodRef readMethodRef = new MethodRef(); private final MethodRef writeMethodRef = new MethodRef(); private String writeMethodName; private String readMethodName;
这里的代码是我从源码中抽离的一部分,起码我们这样看可以大致理解,是分为写和读的步骤,那么就和我们初学java的get/set是一致的。
同时我还看到了,这个,及其注释。
// The base name of the method name which will be prefixed with the // read and write method. If name == "foo" then the baseName is "Foo" private String baseName;
这好像可以解释,为什么我们的属性在生成get/set的时候,第一个字母变成大写?!注释好像确实是这样写的。
由于可能需要一个Bean对象,所以我以前在案例中先创建了一个 Cat
类。
public class Cat { private String name; private String describe; private int age; private int weight; public String getName() { return name; } public void setName(String name) { this.name = name; } public String getDescribe() { return describe; } public void setDescribe(String describe) { this.describe = describe; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public int getWeight() { return weight; } public void setWeight(int weight) { this.weight = weight; } }
起码目前,我还不知道我应该怎么使用它,那么我们就一步一步来吧,我看到它有好几个构造函数,这是一个有趣而且有难度的事情,我们先试着创建一个 PropertyDescriptor
吧。
/** * Constructs a PropertyDescriptor for a property that follows * the standard Java convention by having getFoo and setFoo * accessor methods. Thus if the argument name is "fred", it will * assume that the writer method is "setFred" and the reader method * is "getFred" (or "isFred" for a boolean property). Note that the * property name should start with a lower case character, which will * be capitalized in the method names. * * @param propertyName The programmatic name of the property. * @param beanClass The Class object for the target bean. For * example sun.beans.OurButton.class. * @exception IntrospectionException if an exception occurs during * introspection. */ public PropertyDescriptor(String propertyName, Class<?> beanClass) throws IntrospectionException { this(propertyName, beanClass, Introspector.IS_PREFIX + NameGenerator.capitalize(propertyName), Introspector.SET_PREFIX + NameGenerator.capitalize(propertyName)); }
这个好像是参数最少的,它只需要我们传入一个属性字符串,还有对应的类就好了,其实它也是调用了另一个构造函数,只是它会帮我们默认生成读方法和写方法。方法中的 Introspector.IS_PREFIX + NameGenerator.capitalize(propertyName)
其实就是自己拼出一个默认的get/set方法,大家有兴趣可以去看看源码。
那么对应的实现内容,我想大家应该都想到了。
public static void main(String[] args) throws Exception { PropertyDescriptor CatPropertyOfName = new PropertyDescriptor("name", Cat.class); System.out.println(CatPropertyOfName.getPropertyType()); System.out.println(CatPropertyOfName.getPropertyEditorClass()); System.out.println(CatPropertyOfName.getReadMethod()); System.out.println(CatPropertyOfName.getWriteMethod()); }
/** * This constructor takes the name of a simple property, and method * names for reading and writing the property. * * @param propertyName The programmatic name of the property. * @param beanClass The Class object for the target bean. For * example sun.beans.OurButton.class. * @param readMethodName The name of the method used for reading the property * value. May be null if the property is write-only. * @param writeMethodName The name of the method used for writing the property * value. May be null if the property is read-only. * @exception IntrospectionException if an exception occurs during * introspection. */ public PropertyDescriptor(String propertyName, Class<?> beanClass, String readMethodName, String writeMethodName) throws IntrospectionException { if (beanClass == null) { throw new IntrospectionException("Target Bean class is null"); } if (propertyName == null || propertyName.length() == 0) { throw new IntrospectionException("bad property name"); } if ("".equals(readMethodName) || "".equals(writeMethodName)) { throw new IntrospectionException("read or write method name should not be the empty string"); } setName(propertyName); setClass0(beanClass); this.readMethodName = readMethodName; if (readMethodName != null && getReadMethod() == null) { throw new IntrospectionException("Method not found: " + readMethodName); } this.writeMethodName = writeMethodName; if (writeMethodName != null && getWriteMethod() == null) { throw new IntrospectionException("Method not found: " + writeMethodName); } // If this class or one of its base classes allow PropertyChangeListener, // then we assume that any properties we discover are "bound". // See Introspector.getTargetPropertyInfo() method. Class[] args = { PropertyChangeListener.class }; this.bound = null != Introspector.findMethod(beanClass, "addPropertyChangeListener", args.length, args); }
没错,这个构造函数就是第一种构造函数内部二次调用的,所需要的参数很简单,同时我也希望大家可以借鉴这个方法中的一些检测方式。这次的实现方式也是同样的形式。
public static void main(String[] args) throws Exception { PropertyDescriptor CatPropertyOfName = new PropertyDescriptor("name", Cat.class,"getName","setName"); System.out.println(CatPropertyOfName.getPropertyType()); System.out.println(CatPropertyOfName.getPropertyEditorClass()); System.out.println(CatPropertyOfName.getReadMethod()); System.out.println(CatPropertyOfName.getWriteMethod()); }
/** * This constructor takes the name of a simple property, and Method * objects for reading and writing the property. * * @param propertyName The programmatic name of the property. * @param readMethod The method used for reading the property value. * May be null if the property is write-only. * @param writeMethod The method used for writing the property value. * May be null if the property is read-only. * @exception IntrospectionException if an exception occurs during * introspection. */ public PropertyDescriptor(String propertyName, Method readMethod, Method writeMethod) throws IntrospectionException { if (propertyName == null || propertyName.length() == 0) { throw new IntrospectionException("bad property name"); } setName(propertyName); setReadMethod(readMethod); setWriteMethod(writeMethod); }
这个不用传类,因为你需要传递两个实际的方法进来,所以主要三个对应属性的参数既可。看看大致的实现内容
public static void main(String[] args) throws Exception { Class<?> classType = Cat.class; Method CatNameOfRead = classType.getMethod("getName"); Method CatNameOfWrite = classType.getMethod("setName", String.class); PropertyDescriptor CatPropertyOfName = new PropertyDescriptor("name", CatNameOfRead,CatNameOfWrite); System.out.println(CatPropertyOfName.getPropertyType()); System.out.println(CatPropertyOfName.getPropertyEditorClass()); System.out.println(CatPropertyOfName.getReadMethod()); System.out.println(CatPropertyOfName.getWriteMethod()); }
好了,大致介绍了几种构造函数与实现方式,起码我们现在知道它需要什么。
其实在我上面写一些构造函数的时候,我想大家应该已经感受到与反射相关了,起码我感觉上是这样的,所以我一开始想到这样的案例形式,通过反射与这个属性描述类去赋予我的类。
public static void main(String[] args) throws Exception { //获取类 Class classType = Class.forName("com.example.demo.beans.Cat"); Object catObj = classType.newInstance(); //获取Name属性 PropertyDescriptor catPropertyOfName = new PropertyDescriptor("name",classType); //得到对应的写方法 Method writeOfName = catPropertyOfName.getWriteMethod(); //将值赋进这个类中 writeOfName.invoke(catObj,"river"); Cat cat = (Cat)catObj; System.out.println(cat.toString()); }
运行结果还是顺利的。
Cat{name='river', describe='null', age=0, weight=0}
可以看到,我们确实得到了一个理想中的对象。
那么我是不是可以改变一个已经创建的对象呢?
public static void main(String[] args) throws Exception { //一开始的默认对象 Cat cat = new Cat("river","黑猫",2,4); //获取name属性 PropertyDescriptor catPropertyOfName = new PropertyDescriptor("name",Cat.class); //得到读方法 Method readMethod = catPropertyOfName.getReadMethod(); //获取属性值 String name = (String) readMethod.invoke(cat); System.out.println("默认:" + name); //得到写方法 Method writeMethod = catPropertyOfName.getWriteMethod(); //修改值 writeMethod.invoke(cat,"copy"); System.out.println("修改后:" + cat); }
上面的demo是,我先创建了一个对象,然后通过属性描述符读取name值,再进行修改值,最后输出的对象的值也确实改变了。
默认:river
这是一个有趣的API,我想另外两个(EvebtSetDescriptor、MethodDescriptor)应该也差不多,大家可以再通过此方法去探究,只有自己尝试一次才能学到这里面的一些东西,还有一些项目场景的使用方式,不过一般的业务场景应该很少使用到这个API。那么这个东西究竟可以干什么呢?我想你试着敲一次也许有一些答案了。
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