How to read non repeating elements while implementing an Item Reader? - spring-batch

Question

I have a scenario where I have to read an xml like this:
<MovieList language="English">
<Movie>..<Movie>
<Movie>..<Movie>
</MovieList>
I have to read the Movie Tag which is a complex object(tag) and insert the details into a movie table. I have set the fragmentRootElementName as Movie and able to read the Movie tag completely. How ever, I am not able to read the language attribute, which is not a repeating tag.
How should I be fetching the non repeating tag details? Should I parse the XML myself to read it? or Should I write one more fragmentRootElementName just to read the language attribute?
The configuration for item reader is as below:
<bean id="movieReader" class="org.springframework.batch.item.xml.StaxEventItemReader">
<property name="unmarshaller" ref="marshaller" />
<property name="fragmentRootElementName" value="Movie" />
<property name="resource" value="file:#{jobParameters['inputFile']}" />
</bean>

Answer 1

Ok, the simple way would be to define your fragmentRootElementName = MovieList but i guess this is not a good idea if your MovieList can contains millions of movies!!
I had a similar problem where i needed to knew the parent tag of my fragmentRootElementName.
So we created a CustomStaxEventItemReader that extends the original StaxEventItemReader.
We added a property parentElement that you can set in your config. and we override the method moveCursorToNextFragment() and doRead() to be able to deal with this problem!
Now the code i have don't do exactly what you need, but i modified it and it look like it works!!!
protected boolean moveCursorToNextFragment(XMLEventReader reader) {
try {
while (true) {
while (reader.peek() != null && !reader.peek().isStartElement()) {
reader.nextEvent();
}
if (reader.peek() == null) {
return false;
}
XMLEvent ev = reader.peek();
QName startElementName = ((StartElement) ev).getName();
// Take note of current parent element. Must be one of
// ParentTags
String tmp = startElementName.getLocalPart();
for (ParentTags aTag : ParentTags.values()) {
if (aTag.toString().equals(tmp)) {
currentParent = tmp;
Attribute attr = ((StartElement) ev)
.getAttributeByName(new QName("Language"));
if (null != attr) {
parentAttribute = attr.getValue();
}
break;
}
}
if (startElementName.getLocalPart().equals(
fragmentRootElementName)) {
if ((fragmentRootElementNameSpace == null && parentElement
.equals(currentParent))
|| startElementName.getNamespaceURI().equals(
fragmentRootElementNameSpace)) {
return true;
}
}
reader.nextEvent();
}
} catch (XMLStreamException e) {
throw new DataAccessResourceFailureException(
"Error while reading from event reader", e);
}
}
So basically, if you look at the code of the original StaxEventReader, you will see that it pass thru all the element of the xml file. Each time it get an element with the name = to your rootElement, it return true and the doRead unmarshall it and return the object associated.
Now, we only add some code to also find a given parent element. I have used an Enum called ParentTags because my XML was a bit more complexe, but you could only compare the name of the new parentElement defined in your config.
So if the actual element isEquals to your parentElement, you simply assign it to currentParent and try to get your attribute. If not null, assign it to parentAttribute property.
then in your doRead() method, you can access the parentAttribute property and set it in your domain object!
protected T doRead() throws Exception {
if (noInput) {
return null;
}
T item = null;
if (moveCursorToNextFragment(fragmentReader)) {
fragmentReader.markStartFragment();
#SuppressWarnings("unchecked")
T mappedFragment = (T) unmarshaller.unmarshal(StaxUtils
.createStaxSource(fragmentReader));
item = mappedFragment;
logger.info("Item read : " + item);
currentIndex = cIndex.getAndIncrement();
T p = (T) item;
if (p instanceof myDomainObj) {
myDomainObj pp = (myDomainObj) p;
logger.info(pp);
logger.info("attribute parent = " + parentAttribute);
pp.setLanguage(parentAttribute);
}
fragmentReader.markFragmentProcessed();
}
return item;
}
I hope this is clear enough!
Good luck and regards