validation 校验框架（二）-字段校验vs非字段校验
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在我们的 -validation.xml文件里面可以有两种方式定义校验器（validator）：
当用其中任何一个是请记住一下几点：
Non-Field-Validator:? <validator>允许我们声明两种类型的校验器（a plain Validator或者 a field-specific Validator）
例如：
<field name="email_address">
<field-validator type="required">
<message>You cannot leave the email address field empty.</message>
</field-validator>
<field-validator type="email">
<message>The email address you entered is not valid.</message>
</field-validator>
</field>
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这两种是等价的：
<field name="email_address">
<field-validator type="required">
<message>You cannot leave the email address field empty.</message>
</field-validator>
<field-validator type="email">
<message>The email address you entered is not valid.</message>
</field-validator>
</field>
<validator type="required">
<param name="fieldName">email_address</param>
<message>You cannot leave the email address field empty.</message>
</validator>
<validator type="email">
<param name="fieldName">email_address</param>
<message>The email address you entered is not valid.</message>
</validator>
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