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jaxb annotations

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Jody Garnett edited this page Apr 11, 2015 · 8 revisions

Description

Java objects can be binded to XML using annotations. Many frameworks exist and JAXB is one of them. The traditional approach was to keep org.geotools.metadata as plain JavaBeans implementations without annotations, and let peoples generate their own annotated classes in separated modules. Those annotated classes are typically generated automatically from XSD using some processor - JAXB among others can do that. However like every automatically generated code, the result are plain containers with no intelligence in the sense of no method doing non-trivial tasks related to GIS. It is possible to modify the generated classes afterward, but adding intelligence in the generated classes duplicate the core library.

Our proposal is to annotate the existing metadata classes with JAXB, providing that:

  • Those annotations do not add any dependency to the distributed JARs, neither for Java 5 or Java 6 users.
  • Those annotations do not imply any API change. More specifically Collection<SomeInterface> stay unchanged - the metadata module must continue to work with arbitrary interfaces. This is possible through the definition of XmlAdapter.
  • Those annotations must map to ISO 19139 compliant XML - no custom XML.

The first point is possible since JAXB 2.0 is bundled with Java 6 (we will suggest later an approach for Java 5 users). JAXB is the result of JSR-222 with the participation of Sun Microsystems, IBM, Oracle, Novell, BEA and others, so it is as close to a standard as possible in Java world. We would not support dependencies toward external frameworks like Hibernate or XStream in core GeoTools library, but JAXB is special because of its inclusion in the JDK.

Why JAXB annotations

  • JAXB is a JCP standard, bundled in JDK 6 and supported (according JSR-222 member list) by Sun, IBM, etc. Because of its status, JAXB is expected to be more "well known" than other frameworks.
  • JAXB doesn't introduce any dependencies in distributed JARs. Every Java 6 users have JAXB 2.0 out of the box. JAXB is "special" in this regard compared to Hibernate, XStream, etc.
  • Using JAXB doesn't exclude usage of other frameworks. JAXB annotations are invisible in the Javadoc since they are not declared as @Documented, so users who don't care will even not notice that they are there.
  • If we don't add JAXB annotations ourself, users wanting them in their own org.geotools.metadata subclasses will have to edit and rebuild GeoTools themself, since they would need to add @XmlTransient on all super-classes.
  • Users wanting JAXB in a separated set of classes (probably generating them from XSD using JAXB processor) while get a lot of duplication. If they want to implement GeoAPI interfaces, significant hand-work would be required anyway.
  • The hand-written org.geotools.metadata classes offer more flexibility than generated classes. They also contains additional functionalities, like contains(point) methods. Those methods could be added to generated classes, but doing so duplicate the GeoTools core library. It still doable for metadata classes, but would be more difficult for referencing classes.
  • Annotations keep XML binding close to the code. The advantage is similar to javadoc comments which keep documentation close to the code, and thus increase the chance that the binding/documentation is updated in case of code change.
  • Annotations allow usage of Annotation Processing Tools (APT), which may open interresting perspectives in the future. A short term APT could be to compare XML binding against the UML names declared in GeoAPI interfaces.

Inconvenient

  • Increase the metadata JAR size for Java 6 users, since the class files are expanded with annotations. Adapters also introduce new non-public classes (invisible to users).
  • Introduce yet an other XML technology in GeoTools.

Java 5 users

We will configure the build in such a way that Java 5 users will not be affected. First, we will create a new module in the org.geotools.maven group, which will be used at compile time only. This module will contains a copy of JAXB annotations we use, except that their retention policy will be changed as below:

@Retention(RetentionPolicy.SOURCE)

According Javadoc: "Annotations are to be discarded by the compiler". The results for Java 5 build will be as if no annotation have been declared.

The metadata module will depends on this temporary gt2-mock-jaxb module with <scope>provided</scope>, which implies that this dependency is used for compilation only and do not appears in distributed JAR. Other modules will not inherit this dependency.

Java 6 users

Everything described in the previous section do not apply to Java 6 users. The gt2-mock-jaxb module is never declared for Java 6 users, and the metadata classes are compiled with the annotations bundled in the JDK.

Status

This proposal was voted on; added to the 2.5.x series - and then withdrawn due to build issues with Java 5 vs Java 6.

Community support:

Tasks

Cédric Briançon will be doing the following work:

  1. ✅ Add a build/maven (or build/mock, or other name to be determined) module for allowing Java 5 users to get annotation erased.
  2. ✅ Updated modules/library/metadata/pom.xml with Java 5 and Java 6 profiles.
  3. ✅ Add adapters in org.geotools.resources.jaxb or some other package invisible to users.
  4. ✅ Add annotations to the Metadata implementations.
  5. ✅ User document page for the metadata module showing how to generate an XML document.
  6. ✅ User document page for the metadata module showing how to parse an XML document

Progression state

Proposal completed; and withdrawn since the build was not stable between Java 6 and Java 5 JAXB differences.

API Change

The API change consists of the addition of annotations only; as such the functionality of the code has not changed but the information you can determine using reflection has changed. This information is used by jaxb when marshalling and unmarshalling.

Only implementations are annotated.

BEFORE

public class CitationImpl extends MetadataEntity implements Citation {
...
    /**
     * Returns the name by which the cited resource is known.
     */
    public InternationalString getTitle() {
        return title;
    }

    /**
     * Set the name by which the cited resource is known.
     */
    public synchronized void setTitle(final InternationalString newValue) {
        checkWritePermission();
        title = newValue;
    }
...
}

AFTER

@XmlType(propOrder={"title", ...})
@XmlRootElement(name = "CI_Citation")
public class CitationImpl extends MetadataEntity implements Citation {
...
    /**
     * Returns the name by which the cited resource is known.
     */
    @XmlElement(name = "title", required = true, namespace = "http://www.isotc211.org/2005/gmd")
    public InternationalString getTitle() {
        return title;
    }

    /**
     * Set the name by which the cited resource is known.
     */
    public synchronized void setTitle(final InternationalString newValue) {
        checkWritePermission();
        title = newValue;
    }
...
}
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