Processing non-root XML elements with Spring Batch StaxEventItemReader - spring

Question

I am trying to read non-root elements from an XML file using Spring Batch.
The batch configuration I am using contains:
a StaxEventItemReader configured to read <dependency> elements
a Jaxb2Marshaller bound to JAXB-generated classes
How do I configure either StAX or JAXB to parse non-root elements as single Spring Batch items?
For example, let's say I need to process <dependency> elements from a Maven POM:
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>...</groupId>
<artifactId>...</artifactId>
<version>...</version>
<packaging>...</packaging>
<dependencies>
<dependency>...</dependency>
<dependency>...</dependency>
<dependency>...</dependency>
...
</dependencies>
</project>
With the following code (I am showing only the relevant parts):
#Configuration
#EnableBatchProcessing
public class BatchConfiguration {
#Bean
public ItemReader<Dependency> reader(Jaxb2Marshaller marshaller) {
return new StaxEventItemReaderBuilder<Dependency>().name("itemReader")
.resource(inputFile)
.addFragmentRootElements("dependency")
.unmarshaller(marshaller)
.build();
}
#Bean
public Jaxb2Marshaller marshaller() {
Jaxb2Marshaller marshaller = new Jaxb2Marshaller();
marshaller.setPackagesToScan("org.apache.maven.pom._4_0");
return marshaller;
}
}
But I am getting the following error:
javax.xml.bind.UnmarshalException: unexpected element (uri:"http://maven.apache.org/POM/4.0.0", local:"dependency"). Expected elements are <{http://maven.apache.org/POM/4.0.0}project>
What am I missing?

Answer 1

I found a solution: I needed to call Jaxb2Marshaller.setMappedClass to enable partial unmarshalling:
#Bean
public Jaxb2Marshaller marshaller() {
Jaxb2Marshaller marshaller = new Jaxb2Marshaller();
marshaller.setPackagesToScan("org.apache.maven.pom._4_0");
marshaller.setMappedClass(Dependency.class); // ADD THIS LINE
return marshaller;
}

Answer 2

The pom defines a custom namespace xmlns="http://maven.apache.org/POM/4.0.0", so you need to prefix the fragment root element name with it in your reader's bean definition:
#Bean
public ItemReader<Dependency> reader(Jaxb2Marshaller marshaller) {
return new StaxEventItemReaderBuilder<Dependency>().name("itemReader")
.resource(inputFile)
.addFragmentRootElements("{http://maven.apache.org/POM/4.0.0}dependency")
.unmarshaller(marshaller)
.build();
}