PropertyEditorRegistry与PropertyAccessor的学习

PropertyEditorRegister

简介

PropertyEditorRegistry 是 Spring 框架中的一个接口，它提供了注册和管理 PropertyEditor 的功能。PropertyEditor 是 Java Bean 规范的一部分，用于在不同类型的属性值和字符串之间进行转换。

PropertyEditorRegistry 接口定义了两个主要的方法：

registerCustomEditor(Class requiredType, PropertyEditor propertyEditor): 这个方法允许你注册一个自定义的 PropertyEditor，用于转换特定类型的属性。当 Spring 需要将一个字符串值转换为一个 JavaBean 属性时，它会查找已注册的 PropertyEditor 来执行转换。
findCustomEditor(Class requiredType, PropertyEditorRegistry registry): 这个方法用于查找与给定类型匹配的自定义 PropertyEditor。通常，这个方法不是由应用程序代码直接调用的，而是由 Spring 的内部机制在需要时调用。

在 Spring 的上下文中，PropertyEditorRegistry 接口通常与数据绑定和类型转换相关。例如，在 Spring MVC 中，当请求参数需要绑定到 JavaBean 时，Spring 会使用已注册的 PropertyEditor 来转换字符串参数为 JavaBean 属性的适当类型。

PropertyEditorRegistry 的一个典型实现是 CustomEditorConfigurer，它允许你在 Spring 配置中声明自定义的 PropertyEditor。这样，你可以为特定的 Java 类型指定自定义的转换逻辑。

源码

/*** Encapsulates methods for registering JavaBeans {@link PropertyEditor PropertyEditors}.* This is the central interface that a {@link PropertyEditorRegistrar} operates on.** <p>Extended by {@link BeanWrapper}; implemented by {@link BeanWrapperImpl}* and {@link org.springframework.validation.DataBinder}.** @author Juergen Hoeller* @since 1.2.6* @see java.beans.PropertyEditor* @see PropertyEditorRegistrar* @see BeanWrapper* @see org.springframework.validation.DataBinder* 和DataBinder可以一起使用*/
public interface PropertyEditorRegistry {/*** Register the given custom property editor for all properties of the given type.* @param requiredType the type of the property* @param propertyEditor the editor to register*  注册一个自定义类型转换器(PropertyEditor)主要是将字符串转成我们需要的数据类型*/void registerCustomEditor(Class<?> requiredType, PropertyEditor propertyEditor);/*** Register the given custom property editor for the given type and* property, or for all properties of the given type.* <p>If the property path denotes an array or Collection property,* the editor will get applied either to the array/Collection itself* (the {@link PropertyEditor} has to create an array or Collection value) or* to each element (the {@code PropertyEditor} has to create the element type),* depending on the specified required type.* <p>Note: Only one single registered custom editor per property path* is supported. In the case of a Collection/array, do not register an editor* for both the Collection/array and each element on the same property.* <p>For example, if you wanted to register an editor for "items[n].quantity"* (for all values n), you would use "items.quantity" as the value of the* 'propertyPath' argument to this method.* @param requiredType the type of the property. This may be {@code null}* if a property is given but should be specified in any case, in particular in* case of a Collection - making clear whether the editor is supposed to apply* to the entire Collection itself or to each of its entries. So as a general rule:* <b>Do not specify {@code null} here in case of a Collection/array!</b>* @param propertyPath the path of the property (name or nested path), or* {@code null} if registering an editor for all properties of the given type* @param propertyEditor editor to register*/void registerCustomEditor(@Nullable Class<?> requiredType, @Nullable String propertyPath, PropertyEditor propertyEditor);/*** Find a custom property editor for the given type and property.* @param requiredType the type of the property (can be {@code null} if a property* is given but should be specified in any case for consistency checking)* @param propertyPath the path of the property (name or nested path), or* {@code null} if looking for an editor for all properties of the given type* @return the registered editor, or {@code null} if none* 查找自定义的类型转换器*/@NullablePropertyEditor findCustomEditor(@Nullable Class<?> requiredType, @Nullable String propertyPath);}

示例

import org.springframework.beans.factory.config.CustomEditorConfigurer;  
import org.springframework.beans.PropertyEditorRegistrar;  
import org.springframework.beans.PropertyEditorRegistry;  
import org.springframework.context.annotation.Bean;  
import org.springframework.context.annotation.Configuration;  import java.beans.PropertyEditor;  
import java.text.ParseException;  
import java.text.SimpleDateFormat;  @Configuration  
public class AppConfig {  @Bean  public CustomEditorConfigurer customEditorConfigurer() {  CustomEditorConfigurer configurer = new CustomEditorConfigurer();  configurer.setPropertyEditorRegistrars(new PropertyEditorRegistrar() {  @Override  public void registerCustomEditors(PropertyEditorRegistry registry) {  registry.registerCustomEditor(Date.class, new CustomDateEditor());  }  });  return configurer;  }  static class CustomDateEditor extends PropertyEditorSupport {  private final SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");  @Override  public void setAsText(String text) throws IllegalArgumentException {  try {  setValue(dateFormat.parse(text));  } catch (ParseException e) {  throw new IllegalArgumentException("Could not parse date: " + text, e);  }  }  }  
}

在这个例子中，我们创建了一个自定义的日期编辑器 CustomDateEditor，它能够将字符串解析为 Date 对象。然后，我们创建了一个 CustomEditorConfigurer 的 bean，并在其中注册了我们的自定义编辑器。这样，当 Spring 遇到需要将字符串转换为 Date 类型时，它会使用我们的自定义编辑器来完成转换。

总结

PropertyEditorRegistry 和相关的类在 Spring 中提供了灵活的方式来管理属性转换逻辑，这对于处理不同类型的数据绑定和类型转换非常有用。

PropertyAccessor

简介

在Spring框架中，PropertyAccessor是一个接口，它提供了一种访问JavaBean属性的通用机制。PropertyAccessor接口定义了一系列方法来读取、写入和查询JavaBean的属性。Spring提供了多个PropertyAccessor的实现，如DirectFieldAccessor、BeanWrapperImpl等，这些实现类提供了对JavaBean属性的不同访问策略。

PropertyAccessor的主要功能包括：

属性读取：通过属性名获取JavaBean的属性值。
属性写入：通过属性名和属性值设置JavaBean的属性。
属性存在性检查：检查JavaBean是否具有某个属性。
属性类型获取：获取JavaBean属性的类型。
属性描述符获取：获取描述属性元数据的PropertyDescriptor对象。

Spring的PropertyAccessor接口通常与数据绑定、类型转换和属性访问相关。例如，在Spring MVC中，当请求参数需要绑定到JavaBean时，Spring会使用PropertyAccessor来读取和设置请求参数的值。

PropertyAccessor接口有几个重要的实现：

BeanWrapperImpl：这是PropertyAccessor的一个常用实现，它提供了对JavaBean属性的完整访问。BeanWrapperImpl使用Java的反射API来访问属性，并支持延迟属性访问（lazy property access），这意味着属性只在首次访问时才会被检索。
DirectFieldAccessor：这个实现类似于BeanWrapperImpl，但它使用Java的字段访问机制（而不是反射API）来直接访问JavaBean的字段。这通常比反射更快，但可能受到Java访问控制限制的影响。
MapWrapper：这个实现将Map对象包装为PropertyAccessor，使得可以通过属性名来访问Map中的键值对。

ListWrapper：类似于MapWrapper，这个实现将List对象包装为PropertyAccessor，允许通过索引访问列表元素。

源码

/*** Common interface for classes that can access named properties* (such as bean properties of an object or fields in an object)* Serves as base interface for {@link BeanWrapper}.** @author Juergen Hoeller* @since 1.1* @see BeanWrapper* @see PropertyAccessorFactory#forBeanPropertyAccess* @see PropertyAccessorFactory#forDirectFieldAccess*/
public interface PropertyAccessor {/*** Path separator for nested properties.* Follows normal Java conventions: getFoo().getBar() would be "foo.bar".*/String NESTED_PROPERTY_SEPARATOR = ".";/*** Path separator for nested properties.* Follows normal Java conventions: getFoo().getBar() would be "foo.bar".*/char NESTED_PROPERTY_SEPARATOR_CHAR = '.';/*** Marker that indicates the start of a property key for an* indexed or mapped property like "person.addresses[0]".*/String PROPERTY_KEY_PREFIX = "[";/*** Marker that indicates the start of a property key for an* indexed or mapped property like "person.addresses[0]".*/char PROPERTY_KEY_PREFIX_CHAR = '[';/*** Marker that indicates the end of a property key for an* indexed or mapped property like "person.addresses[0]".*/String PROPERTY_KEY_SUFFIX = "]";/*** Marker that indicates the end of a property key for an* indexed or mapped property like "person.addresses[0]".*/char PROPERTY_KEY_SUFFIX_CHAR = ']';/***属性是否可读*/boolean isReadableProperty(String propertyName);/***属性是否可写*/boolean isWritableProperty(String propertyName);/*** 获取属性的Class*/@NullableClass<?> getPropertyType(String propertyName) throws BeansException;/*** 获取属性的类型描述符*/@NullableTypeDescriptor getPropertyTypeDescriptor(String propertyName) throws BeansException;/*** 根据属性名获取属性值*/@NullableObject getPropertyValue(String propertyName) throws BeansException;/*** 设置一个PropertyValue对象 存储属性名:属性值*/void setPropertyValue(PropertyValue pv) throws BeansException;/*** 设置一个map集合*/void setPropertyValues(Map<?, ?> map) throws BeansException;/*** 设置一个PropertyValues 多个PropertyValue对象*/void setPropertyValues(PropertyValues pvs) throws BeansException;/*** 设置一个PropertyValues ignoreUnknown:属性在不可写的情况是否抛出异常*/ void setPropertyValues(PropertyValues pvs, boolean ignoreUnknown)throws BeansException;void setPropertyValues(PropertyValues pvs, boolean ignoreUnknown, boolean ignoreInvalid)throws BeansException;}

示例

import org.springframework.beans.BeanWrapper;  
import org.springframework.beans.BeanWrapperImpl;  
import org.springframework.beans.PropertyAccessorFactory;  public class Example {  public static void main(String[] args) {  MyObject myObject = new MyObject();  // 创建BeanWrapperImpl实例  BeanWrapper beanWrapper = PropertyAccessorFactory.forBeanPropertyAccess(myObject);  // 使用BeanWrapper访问属性  beanWrapper.setPropertyValue("propertyName", "propertyValue");  Object propertyValue = beanWrapper.getPropertyValue("propertyName");  // 还可以检查属性是否存在、获取属性类型等  boolean isPropertyEditable = beanWrapper.isPropertyEditable("propertyName");  Class<?> propertyType = beanWrapper.getPropertyType("propertyName");  }  
}  class MyObject {  private String propertyName;  // getter和setter方法  public String getPropertyName() {  return propertyName;  }  public void setPropertyName(String propertyName) {  this.propertyName = propertyName;  }  
}