Java Language JAXB Specifying a XmlAdapter instance to (re)use existing data
Example
Sometimes specific instances of data should be used. Recreation is not desired and referencing static data would have a code smell.
It is possible to specify a XmlAdapter instance the Unmarshaller should use, which allows the user to use XmlAdapters with no zero-arg constructor and/or pass data to the adapter.
Example
User class
The following class contains a name and a user's image.
import java.awt.image.BufferedImage;
import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;
@XmlRootElement
public class User {
private String name;
private BufferedImage image;
@XmlAttribute
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@XmlJavaTypeAdapter(value=ImageCacheAdapter.class)
@XmlAttribute
public BufferedImage getImage() {
return image;
}
public void setImage(BufferedImage image) {
this.image = image;
}
public User(String name, BufferedImage image) {
this.name = name;
this.image = image;
}
public User() {
this("", null);
}
}
Adapter
To avoid creating the same image in memory twice (as well as downloading the data again), the adapter stores the images in a map.
Java SE 7
For valid Java 7 code replace the getImage method with
public BufferedImage getImage(URL url) {
BufferedImage image = imageCache.get(url);
if (image == null) {
try {
image = ImageIO.read(url);
} catch (IOException ex) {
Logger.getLogger(ImageCacheAdapter.class.getName()).log(Level.SEVERE, null, ex);
return null;
}
imageCache.put(url, image);
reverseIndex.put(image, url);
}
return image;
}
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.net.URL;
import java.util.HashMap;
import java.util.Map;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.imageio.ImageIO;
import javax.xml.bind.annotation.adapters.XmlAdapter;
public class ImageCacheAdapter extends XmlAdapter<String, BufferedImage> {
private final Map<URL, BufferedImage> imageCache = new HashMap<>();
private final Map<BufferedImage, URL> reverseIndex = new HashMap<>();
public BufferedImage getImage(URL url) {
// using a single lookup using Java 8 methods
return imageCache.computeIfAbsent(url, s -> {
try {
BufferedImage img = ImageIO.read(s);
reverseIndex.put(img, s);
return img;
} catch (IOException ex) {
Logger.getLogger(ImageCacheAdapter.class.getName()).log(Level.SEVERE, null, ex);
return null;
}
});
}
@Override
public BufferedImage unmarshal(String v) throws Exception {
return getImage(new URL(v));
}
@Override
public String marshal(BufferedImage v) throws Exception {
return reverseIndex.get(v).toExternalForm();
}
}
Example XMLs
The following 2 xmls are for Jon Skeet and his earth 2 counterpart, which both look exactly the same and therefore use the same avatar.
<?xml version="1.0" encoding="UTF-8"?>
<user name="Jon Skeet" image="https://www.gravatar.com/avatar/6d8ebb117e8d83d74ea95fbdd0f87e13?s=328&d=identicon&r=PG"/>
<?xml version="1.0" encoding="UTF-8"?>
<user name="Jon Skeet (Earth 2)" image="https://www.gravatar.com/avatar/6d8ebb117e8d83d74ea95fbdd0f87e13?s=328&d=identicon&r=PG"/>
Using the adapter
ImageCacheAdapter adapter = new ImageCacheAdapter();
JAXBContext context = JAXBContext.newInstance(User.class);
Unmarshaller unmarshaller = context.createUnmarshaller();
// specifiy the adapter instance to use for every
// @XmlJavaTypeAdapter(value=ImageCacheAdapter.class)
unmarshaller.setAdapter(ImageCacheAdapter.class, adapter);
User result1 = (User) unmarshaller.unmarshal(Main.class.getResource("user.xml"));
// unmarshal second xml using the same adapter instance
Unmarshaller unmarshaller2 = context.createUnmarshaller();
unmarshaller2.setAdapter(ImageCacheAdapter.class, adapter);
User result2 = (User) unmarshaller2.unmarshal(Main.class.getResource("user2.xml"));
System.out.println(result1.getName());
System.out.println(result2.getName());
// yields true, since image is reused
System.out.println(result1.getImage() == result2.getImage());
