spring xml中的String值是怎么转化成我们所需要类型的

在Spring的配置中，使用配置类还稍微好理解一点，毕竟直接使用的就是相对应的对象，但是使用XML时就稍稍有点难理解，在XML解析时里面的像这样的子元素

<property name="" value=""/>

value值统统都会转化成String值，我们有时候对应的property对应的类型是int将其转换成int也就罢了，但有时候不是int，而是像Date,Resource,File等等这些类型，他又是怎么转化过去的呢？

比如：

import org.springframework.core.io.Resource;

public class Test {
    Resource resource;
    public Test(Resource resource){
        this.resource=resource;
    }
}

这里边有个Resource属性

<bean id="test" class="Helison.Test">
    <constructor-arg index="0" value="classpath:resources"/>
 </bean>

在XML文件里面将会这么配置，运行完全成功，不报错，为什么呢，value明明是String啊，此时将涉及到PropertyEditor（属性编辑器）以及相关的类了

spring在为bean的相关属性进行赋值的时候，会在其beanFactory里面寻找相对应的属性编辑器，其作用是将解析到的String类型的值转化成我们所需要的其他类型的值，比如Resource,Date等等，因此你需要提供一种映射机制去做这件事；

相关Spring的源码如下：

protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {
   // Tell the internal bean factory to use the context's class loader etc.
   beanFactory.setBeanClassLoader(getClassLoader());
   beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));
	//关键是这句话
   beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));

   // Configure the bean factory with context callbacks.
   beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
   beanFactory.ignoreDependencyInterface(EnvironmentAware.class);
   beanFactory.ignoreDependencyInterface(EmbeddedValueResolverAware.class);
   beanFactory.ignoreDependencyInterface(ResourceLoaderAware.class);
   beanFactory.ignoreDependencyInterface(ApplicationEventPublisherAware.class);
   beanFactory.ignoreDependencyInterface(MessageSourceAware.class);
   beanFactory.ignoreDependencyInterface(ApplicationContextAware.class);

   // BeanFactory interface not registered as resolvable type in a plain factory.
   // MessageSource registered (and found for autowiring) as a bean.
   beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory);
   beanFactory.registerResolvableDependency(ResourceLoader.class, this);
   beanFactory.registerResolvableDependency(ApplicationEventPublisher.class, this);
   beanFactory.registerResolvableDependency(ApplicationContext.class, this);

   // Register early post-processor for detecting inner beans as ApplicationListeners.
   beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(this));

   // Detect a LoadTimeWeaver and prepare for weaving, if found.
   if (beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
      beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
      // Set a temporary ClassLoader for type matching.
      beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
   }

   // Register default environment beans.
   if (!beanFactory.containsLocalBean(ENVIRONMENT_BEAN_NAME)) {
      beanFactory.registerSingleton(ENVIRONMENT_BEAN_NAME, getEnvironment());
   }
   if (!beanFactory.containsLocalBean(SYSTEM_PROPERTIES_BEAN_NAME)) {
      beanFactory.registerSingleton(SYSTEM_PROPERTIES_BEAN_NAME, getEnvironment().getSystemProperties());
   }
   if (!beanFactory.containsLocalBean(SYSTEM_ENVIRONMENT_BEAN_NAME)) {
      beanFactory.registerSingleton(SYSTEM_ENVIRONMENT_BEAN_NAME, getEnvironment().getSystemEnvironment());
   }
}

ResourceEditorRegistrar相关源码：

注册相关的PropertyEditor

public void registerCustomEditors(PropertyEditorRegistry registry) {
    ResourceEditor baseEditor = new ResourceEditor(this.resourceLoader, this.propertyResolver);
    this.doRegisterEditor(registry, Resource.class, baseEditor);
    this.doRegisterEditor(registry, ContextResource.class, baseEditor);
    this.doRegisterEditor(registry, InputStream.class, new InputStreamEditor(baseEditor));
    this.doRegisterEditor(registry, InputSource.class, new InputSourceEditor(baseEditor));
    this.doRegisterEditor(registry, File.class, new FileEditor(baseEditor));
    if (pathClass != null) {
        this.doRegisterEditor(registry, pathClass, new PathEditor(baseEditor));
    }

    this.doRegisterEditor(registry, Reader.class, new ReaderEditor(baseEditor));
    this.doRegisterEditor(registry, URL.class, new URLEditor(baseEditor));
    ClassLoader classLoader = this.resourceLoader.getClassLoader();
    this.doRegisterEditor(registry, URI.class, new URIEditor(classLoader));
    this.doRegisterEditor(registry, Class.class, new ClassEditor(classLoader));
    this.doRegisterEditor(registry, Class[].class, new ClassArrayEditor(classLoader));
    if (this.resourceLoader instanceof ResourcePatternResolver) {
        this.doRegisterEditor(registry, Resource[].class, new ResourceArrayPropertyEditor((ResourcePatternResolver)this.resourceLoader, this.propertyResolver));
    }

}

上面源码注册已经为我们注册了一些常用的PropertyEditor，像Resource,File,Url等等，but,我们自己注册的那些在哪呢？？？

继续探索源码，发现：

public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName) throws BeansException {
    Object result = existingBean;
    Iterator var4 = this.getBeanPostProcessors().iterator();

    do {
        if (!var4.hasNext()) {
            return result;
        }

        BeanPostProcessor beanProcessor = (BeanPostProcessor)var4.next();
        result = beanProcessor.postProcessBeforeInitialization(result, beanName);
    } while(result != null);

    return result;
}

在进行为beanFactory进行配置后会调用上述方法，而

public class CustomEditorConfigurer implements BeanFactoryPostProcessor, Ordered 

CustomEditorConfigurer实现了BeanFactoryPostProcessor(注，BeanFactoryPostProcessor是个很重要的接口)将会被容器发现，并注册

因此我们只需要在配置文件中声明一个CustomEditorConfigurer，就好了，如下:

<bean  class="org.springframework.beans.factory.config.CustomEditorConfigurer">
    <property name="propertyEditorRegistrars">
        <list>
            <bean class="Helison.DatePropertyEditor"/>
        </list>
    </property>
</bean>

而

public class DatePropertyEditor implements PropertyEditorRegistrar {
    @Override
    public void registerCustomEditors(PropertyEditorRegistry propertyEditorRegistry) {
        propertyEditorRegistry.registerCustomEditor(Date.class,new CustomDateEditor(new SimpleDateFormat("yyyy-MM-dd"),true));
    }
}

此时就可以放心的进行使用了