指定 XmlAdapter 实例以(重新)使用现有数据
有时应使用特定的数据实例。不需要重新创建,并且引用 static
数据会产生代码味道。
可以指定 Unmarshaller
应该使用的 XmlAdapter
实例,这允许用户使用没有零参数构造函数的 XmlAdapter
s 和/或将数据传递给适配器。
例
用户类
以下类包含名称和用户图像。
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);
}
}
适配器
为避免在内存中创建相同的图像两次(以及再次下载数据),适配器将图像存储在地图中。
Version <= Java SE 7
对于有效的 Java 7 代码,请替换 getImage
方法
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();
}
}
示例 XML
以下 2 个 xmls 用于 Jon Skeet 和他的地球 2 对应,两者看起来完全相同,因此使用相同的化身。
<?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"/>
使用适配器
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());