将XML解组为Java对象JAXB

Question

我的XML文档如下，

<Configuration>
<Shop>
    <Level>1</Level>
    <Block>
        <Items>
            <Item>Basic</Item>
        </Items>
    </Block>
</Shop>
<Shop>
    <Level>2</Level>
    <Block>
        <Items>
            <Item>Basic</Item>
            <Item>Pin</Item>
        </Items>
    </Block>
</Shop></Configuration>

并有如下课程，

@XmlRootElement(name = "Configuration")
class ShopConfig {
private List<Shop> shops;

@XmlElementWrapper(name = "Configuration", nillable = false)
@XmlElement(name = "Shop", nillable = false)
public List<Shop> getShops() {
    return shops;
}
public void setShops(List<Shop> shops) {
    this.shops = shops;
}
public void print(){
    for(Shop shop: shops){
        List<String> l = shop.getItemNames();
        for(String s : l){
            System.out.println(s);

        }
    }
}
}

public class Shop {
private String level;
private List<String> itemNames;
public String getLevel() {
    return level;
}
@XmlElement(name = "Level")
public void setLevel(String level) {
    this.level = level;
}
@XmlElementWrapper(name = "Items", nillable = false)
@XmlElement(name = "Item", nillable = false)
public List<String> getItemNames() {
    return itemNames;
}
public void setItemNames(List<String> itemNames) {
    this.itemNames = itemNames;
}
}

我按照以下方式解组xml，

private static ShopConfig initShopConfig() throws JAXBException {
    String configPath = "Config.xml";
    File file = new File(configPath);
    JAXBContext ctx = JAXBContext.newInstance(ShopConfig.class);
    Unmarshaller um = ctx.createUnmarshaller();
    return  (ShopConfig) um.unmarshal(file);
}

但是当我尝试打印时，当我尝试调用print方法时，会给出NPE，说shop变量为null。这意味着它没有正确地解散。我在这做什么错？

 public static void main(String args[]){
    ShopConfig l = (ShopConfig)ConfigLoader.getInstance().getConfig();
    l.print();
}

该例外不具备信息，

Exception in thread "main" java.lang.NullPointerException
at config.ShopConfig.print(ShopConfig.java:42)
at config.Test.main(Test.java:9)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)