[java]JAXB解析XML时默认值处理

今天药忘吃喽~ 2022-08-26 08:23 178阅读 0赞

package test.xml;
    
    import java.io.StringReader;
    
    import javax.xml.bind.JAXBContext;
    import javax.xml.bind.Unmarshaller;
    import javax.xml.bind.annotation.XmlElement;
    import javax.xml.bind.annotation.XmlRootElement;
    
    /**
     * <h1>Element default values and unmarshalling</h1> <br>
     * When a class has an element property with the default value, and if the
     * document you are reading is missing the element, then the unmarshaller does
     * not fill the field with the default value. Instead, the unmarshaller fills in
     * the field when the element is present but the content is missing.
     * 
     */
    public class JAXBTest {
    
    	@XmlRootElement
    	static class Foo {
    		private String a = "java default";
    
    		public String getA() {
    			return a;
    		}
    
    		@XmlElement(defaultValue = "jaxb default")
    		public void setA(String a) {
    			this.a = a;
    		}
    
    	}
    
    	static String xml1 = "<foo/>";
    	static String xml2 = "<foo><a/></foo>";
    	static String xml3 = "<foo><a>hello</a></foo>";
    
    	public static void main(String[] args) throws Exception {
    		JAXBContext jc = JAXBContext.newInstance(Foo.class);
    		Unmarshaller unmarshaller = jc.createUnmarshaller();
    
    		Foo foo1 = (Foo) unmarshaller.unmarshal(new StringReader(xml1));
    		System.out.println(foo1.a); // "java default"
    
    		Foo foo2 = (Foo) unmarshaller.unmarshal(new StringReader(xml2));
    		System.out.println(foo2.a); // "jaxb default". The default kicked in.
    
    		Foo foo3 = (Foo) unmarshaller.unmarshal(new StringReader(xml3));
    		System.out.println(foo3.a); // "hello". Read from the instance.
    	}
    
    }