<!DOCTYPE 根元素名[]
    元素描述
>
XML文档主体部分

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE messages[
    <!ELEMENT messages (message)*>
    <!ELEMENT message (title,content,hashcode)>
    <!ELEMENT title (#PCDATA)>
    <!ELEMENT content (#PCDATA)>
    <!ELEMENT hashcode (#PCDATA)>
]>
<messages>
    <message>
        <title>hello,world</title>
        <content>hello</content>
        <hashcode>2kljfosaf7912</hashcode>
    </message>
</messages>