從 註解和繼承 到 JAXB中的注意事項

從 註解和繼承 到 JAXB中的注意事項

註解在繼承中的行爲

若是一個父類添加了一個註解，子類是否能取到這個註解呢？以下

package inheritance;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

public class Main {
    public static void main(String[] args) {
        Type t = Son.class.getAnnotation(Type.class);
        if (t != null) {
            System.out.println(t.name());
        }
    }
}

@Type(name = "Father")
class Father {
}

class Son extends Father {
}

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface Type {
    String name();
}

如上代碼，註解不會被子類繼承。若是想註解也被子類繼承，該怎麼辦呢？

只須要在註解定義裏修改一下，添加@Inherited

@Inherited
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface Type {
    String name();
}

關於@Inherited須要注意的：

• 只能做用於繼承，不能做用於接口實現

JAXB 的註解

@XmlAccessorType用於標註該類的成員是否用於綁定到XML，例如XmlAccessType.PUBLIC_MEMBER表示，全部public字段都會被綁定(除去@XmlElement和@XmlTransient的標記，他們優先級更高)。
這個註解就是標記了@Inherited。

咱們知道序列化順序能夠由@XmlType(propOrder)去設置，那麼繼承後是什麼樣子呢？

package inheritance;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;
import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlType;

public class Main {
    public static void main(String[] args) throws Exception {
        JAXBContext ctx = JAXBContext.newInstance(Son.class);
        Marshaller ms = ctx.createMarshaller();
        ms.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        ms.marshal(new Son(), System.out);
    }
}

@XmlRootElement(name = "Father")
@XmlAccessorType(XmlAccessType.PUBLIC_MEMBER)
@XmlType(propOrder = { "two", "one" })
class Father {
    @XmlElement
    private String one = "one";
    @XmlElement
    public String two = "two";
}

@XmlRootElement(name = "Son")
@XmlAccessorType(XmlAccessType.NONE)
@XmlType(propOrder = { "four", "three" })
class Son extends Father {
    @XmlElement
    public String three = "three";
    @XmlElement
    public String four = "four";
}

上述代碼會輸出

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<Son>
    <two>two</two>
    <one>one</one>
    <four>four</four>
    <three>three</three>
</Son>

即父類成員先序列化，再子類成員，順序由各自類的@XmlType設置的順序決定。

那麼若是子類必定想控制父類成員的序列化順序，並且不一樣的子類還想各自定義父類成員的序列化順序怎麼辦？(CNM, 屁事真多)

好吧，JAXB仍是能夠知足這種屁事兒的。

package inheritance;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;
import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlTransient;
import javax.xml.bind.annotation.XmlType;

public class Main {
    public static void main(String[] args) throws Exception {
        go(Son.class);
        go(Daughter.class);
    }

    private static void go(Class<?> clz) throws Exception {
        JAXBContext ctx = JAXBContext.newInstance(clz);
        Marshaller ms = ctx.createMarshaller();
        ms.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        ms.marshal(clz.newInstance(), System.out);
    }
}

@XmlTransient
class Father {
    @SuppressWarnings("unused")
    private String one = "one";
    public String two = "two";
}

@XmlRootElement(name = "Son")
@XmlAccessorType(XmlAccessType.PUBLIC_MEMBER)
@XmlType(propOrder = { "four", "three", "two" })
class Son extends Father {
    @XmlElement
    public String three = "three";
    @XmlElement
    public String four = "four";
}

@XmlRootElement(name = "Daughter")
@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(propOrder = { "one", "two", "three", "four" })
class Daughter extends Father {
    @XmlElement
    public String three = "three";
    @XmlElement
    public String four = "four";
}

上述代碼將輸出

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<Son>
    <four>four</four>
    <three>three</three>
    <two>two</two>
</Son>
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<Daughter>
    <one>one</one>
    <two>two</two>
    <three>three</three>
    <four>four</four>
</Daughter>

解釋以下：

• 要想每一個子類去控制父類成員，必須把父類標記成@XmlTransient，並且子類@XmlType的propOrder要負責全部本身要序列化的成員。
• 類Son的@XmlAccessorType標註爲XmlAccessType.PUBLIC_MEMBER，意味着Son只序列化Public字段。加上父類的public成員，Son共有3個public成員，因此@XmlType(propOrder)寫了三個成員的順序。
• 類Daughter的@XmlAccessorType標註爲XmlAccessType.FIELD，意味着全部父類和本身的成員都會序列化，因而標記四個成員的順序。

忽略dtd

import java.io.FileReader;
import javax.xml.XMLConstants;
import javax.xml.bind.JAXBContext;
import javax.xml.bind.Unmarshaller;
import javax.xml.parsers.SAXParserFactory;
import javax.xml.transform.sax.SAXSource;

import org.xml.sax.InputSource;
import org.xml.sax.XMLReader;

public class Demo2 {

    public static void main(String[] args) throws Exception {

        JAXBContext jc = JAXBContext.newInstance(MyBean.class);

        SAXParserFactory spf = SAXParserFactory.newInstance();
        spf.setFeature("http://apache.org/xml/features/nonvalidating/load-external-dtd", false);
        spf.setFeature("http://xml.org/sax/features/validation", false);
        spf.setNamespaceAware(true);

        XMLReader xmlReader = spf.newSAXParser().getXMLReader();
        InputSource inputSource = new InputSource(
                new FileReader("myfile.xml"));
        SAXSource source = new SAXSource(xmlReader, inputSource);

        Unmarshaller unmarshaller = jc.createUnmarshaller();
        MyBean foo = (MyBean) unmarshaller.unmarshal(source);
    }
}