Open Declaration Element org.w3c.dom.Document.getElementById(String elementId)


Returns the Element that has an ID attribute with the given value. If no such element exists, this returns null . If more than one element has an ID attribute with that value, what is returned is undefined. 
The DOM implementation is expected to use the attribute Attr.isId to determine if an attribute is of type ID. 

Note: Attributes with the name "ID" or "id" are not of type ID unless so defined.

Parameters:
elementId The unique id value for an element.
Returns:
The matching element or null if there is none.
Since:
DOM Level 2

demo:

        <dependency>
            <groupId>org.xmlunit</groupId>
            <artifactId>xmlunit-matchers</artifactId>
            <version>2.3.0</version>
        </dependency>
import java.io.StringReader;

import javax.xml.transform.stream.StreamSource;

import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.xmlunit.transform.Transformation;

public class TT {

    public static void main(String[] args) {
        String xmlStr = "<table><tr id="aaa">这是一行</tr></table>";
//        String xmlStr = "<!DOCTYPE table [<!ATTLIST tr id ID #REQUIRED>]><table><tr id="aaa">这是一行</tr></table>";
          
        
        Transformation transformation = new Transformation(new StreamSource(new StringReader(xmlStr)));
        Document doc = transformation.transformToDocument();
        Element tr = doc.getElementById("aaa");
        System.out.println(tr);
    }
}

输出结果是：null

原因见红色字内容

如何解决：

为该文档声明DTD

其中有一种内部声明方法如下：

<?xml version="1.0"?>
<!DOCTYPE note [
<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
]>
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend</body>
</note>

还有一种外部声明方法：

<?xml version="1.0"?>
<!DOCTYPE note SYSTEM "note.dtd">
<note>
  <to>Tove</to>
  <from>Jani</from>
  <heading>Reminder</heading>
  <body>Don't forget me this weekend!</body>
</note>

其中内部声明方法的解释是：

https://www.w3schools.com/xml/xml_dtd_elements.asp

本例的解决方案：

<!DOCTYPE table [<!ATTLIST tr id ID #REQUIRED>]>

语法：

<!ATTLIST element-name attribute-name attribute-type attribute-value>

https://www.w3schools.com/xml/xml_dtd_intro.asp

https://www.w3schools.com/xml/xml_dtd_attributes.asp