# 10.2.3. 使用BeanDefinitionParser

如果`NamespaceHandler`遇到已映射到特定 bean 定义解析器（在本例中为日期格式）的类型的 XML 元素，则使用 `BeanDefinitionParser`。换句话说，`BeanDefinitionParser` 负责解析模式中定义的一个不同的顶级 XML 元素。在解析器中，我们可以访问 XML 元素（因此也可以访问其子元素），以便可以解析自定义 XML 内容，如以下示例所示：

```java
package org.springframework.samples.xml;

import org.springframework.beans.factory.support.BeanDefinitionBuilder;
import org.springframework.beans.factory.xml.AbstractSingleBeanDefinitionParser;
import org.springframework.util.StringUtils;
import org.w3c.dom.Element;

import java.text.SimpleDateFormat;

public class SimpleDateFormatBeanDefinitionParser extends AbstractSingleBeanDefinitionParser { 

    protected Class getBeanClass(Element element) {
        return SimpleDateFormat.class; 
    }

    protected void doParse(Element element, BeanDefinitionBuilder bean) {
        // this will never be null since the schema explicitly requires that a value be supplied
        String pattern = element.getAttribute("pattern");
        bean.addConstructorArgValue(pattern);

        // this however is an optional property
        String lenient = element.getAttribute("lenient");
        if (StringUtils.hasText(lenient)) {
            bean.addPropertyValue("lenient", Boolean.valueOf(lenient));
        }
    }

}
```


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