Revision as of 16:52, 7 January 2010

Design Specification: MOXy Support for Dynamic Persistence

ER 296967

Document History

Project Overview

The goal of this feature is to enable users of EclipseLink JAXB to perform typical JAXB operations without having real .class files available for their domain objects. Users will pass in some form of metadata (XML Schema, EclipseLink Deployment XML or External Metadata) to create a DynamicJAXBContext, and this metadata will be used to construct DynamicEntity objects representing the domain classes.

The ultimate purpose of this feature is to support an end-to-end JPA<->JAXB solution. EclipseLink JPA already has some support for Dynamic Persistence and this work will provide a path from Dynamic JPA to JAXB.

The progress of this feature can be broken up into the following parts:

• Bootstrapping from an EclipseLink Deployment XML / Project object
• Bootstrapping from an External Metadata file (eclipselink-oxm.xml)
• Bootstrapping JAXB Context with XML Schema
• Support for adding arbitrary properties/mappings at runtime, after initialization has been performed
• Thread Safety

Some initial groundwork was included in EclipseLink 2.0, more information is available here.

Concepts

Dynamic Persistence

The core of this feature is the Dynamic Persistence support that was initially added for JPA (found in org.eclipse.persistence.dynamic). To obtain a "dynamic" project (i.e. one that maps to classes that were generated in-memory), a "dry" project (one that does not have Java Classes specified, only class names) is passed to the DynamicTypeBuilder, passing in an instance of DynamicClassLoader, which does the work of building the in-memory classes:

InputStream metadata = ...
Project dynamicProject = DynamicTypeBuilder.loadDynamicProject(inputStream, null, new DynamicClassLoader(Thread.currentThread().getContextClassLoader()));

When this project is used to unmarshall XML documents, the objects that are returned by EclipseLink will be subclasses of DynamicEntity. DynamicEntities offer a simple get(propertyName) / set(propertyName, propertyValue) API to manipulate their data:

DynamicEntity dynamicEmployee = (DynamicEntity) jaxbUnmarshaller().unmarshal(instanceDoc);
String firstName = dynamicEmp.get("fname");
DynamicEntity dynamicAddress = dynamicEmp.get("address");
 
...
 
dynamicEmp.set("lname", "Duggar");
address.set("street", "1001 Duggar Ranch Way");

Requirements

The following sections will expand the goals of this project into more concrete requirements.

Design Constraints

Design / Functionality

DynamicJAXBContext

The entry point to this feature is a new class, DynamicJAXBContext, which is a subclass of the existing EclipseLink JAXBContext (which is itself a subclass of Sun's JAXBContext). Because this functionality is proprietary and not part of the JAXB spec, we will not provide creation APIs on JAXBContextFactory.

public DynamicJAXBContext(URL metadataURL, int metadataFormat, ClassLoader classLoader)

• URL metadataURL - A Java URL pointing to the metadata (XML Schema, EclipseLink Deployment XML, or EclipseLink External Metadata)
• int metadataFormat - One of DynamicJAXBContext.XML_SCHEMA, DynamicJAXBContext.DEPLOYMENT_XML, or DynamicJAXBContext.EXT_METADATA
• ClassLoader classLoader - The current class loader; a DynamicClassLoader will be created with this loader as its parent

Additionally, if the user has created an EclipseLink Project in code, and has specified only class names and not actual Class objects, a DynamicJAXBContext can be created directly from the Project:

public DynamicJAXBContext(Project project, ClassLoader classLoader)

Bootstrapping from Deployment XML

A user bootstrapping from Deployment XML can use the following code:

File projectFile = new File("resource/eclipselink/project.xml");
ClassLoader loader = Thread.currentThread().getContextClassLoader();
 
DynamicJAXBContext jaxbContext = new DynamicJAXBContext(projectFile.toURI().toURL(), DynamicJAXBContext.DEPLOYMENT_XML, loader); 
JAXBMarshaller jaxbMarshaller = jaxbContext.createMarshaller();
JAXBUnmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();

Internally, a DynamicClassLoader is created using the supplied ClassLoader as its parent. An InputStream is created from the metadata URL and used to create a Project mapped to Dynamic Entities. Finally, XMLContext and DynamicHelper objects are instantiated and stored.

DynamicClassLoader dynamicClassLoader = new DynamicClassLoader(classLoader);            
InputStream inputStream = metadataURL.openStream();
Project p = DynamicTypeBuilder.loadDynamicProject(inputStream, null, dynamicClassLoader);
 
this.xmlContext = new XMLContext(p);
this.dynamicHelper = new DynamicHelper(xmlContext.getSession(0));

Bootstrapping from XML Schema

A user bootstrapping from an XML Schema can use the following code:

File schemaFile = new File("resource/xsd/employee.xsd");
ClassLoader loader = Thread.currentThread().getContextClassLoader();
 
DynamicJAXBContext jaxbContext = new DynamicJAXBContext(schemaFile.toURI().toURL(), DynamicJAXBContext.XML_SCHEMA, loader); 
JAXBMarshaller jaxbMarshaller = jaxbContext.createMarshaller();
JAXBUnmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();

Internally, we can use APIs from the Java XJC to parse a schema and create Java class definitions (XJC's JCodeModel) in memory, then pass these class definitions to an EclipseLink Generator (org.eclipse.persistence.jaxb.compiler.Generator) to generate an EclipseLink project. After we have created this project, we can use the DynamicTypeBuilder to create a dynamic project, generating Java classes in memory along the way.

First, we use XJC API to parse an XSD and generate a JCodeModel. Note that this code stops short of actually generating .java files, it only generates an in-memory representation of the Java files that would be generated.

// Use XJC API to parse the schema and generate its JCodeModel
SchemaCompiler sc = XJC.createSchemaCompiler();
InputSource inputSource = new InputSource(metadataURL.toExternalForm());
sc.parseSchema(inputSource);
S2JJAXBModel model = sc.bind();
JCodeModel jCodeModel = model.generateCode(new Plugin[0], null);

We can then wrap these XJC classes in our own implentations of EclipseLink's JAXB JavaModel classes. This will allow us to use the Generator to create an EclipseLink project and mappings. The JavaModel interfaces define "wrappers" for the various Java language constructs that make up the domain model (e.g. classes, methods, constructors, annotations, packages, etc). For example:

public class XJCJavaFieldImpl implements JavaField {
 
    // XJC's definition of a Field
    protected JFieldVar xjcField;
 
    ...
 
    public int getModifiers() {
        return xjcField.mods().getValue();
    }
 
    public String getName() {
        return xjcField.name();
    }
 
    ...
}

Creating the JavaModel classes:

// Create EclipseLink JavaModel objects for each of XJC's JDefinedClasses
ArrayList<JDefinedClass> classesToProcess = new ArrayList<JDefinedClass>();
Iterator<JPackage> packages = jCodeModel.packages();
while (packages.hasNext()) {
   JPackage pkg = packages.next();
   Iterator<JDefinedClass> classes = pkg.classes();
   while (classes.hasNext()) {
      JDefinedClass cls = classes.next();
         if (!cls.name().equals("ObjectFactory")) {
            classesToProcess.add(cls);
         }
   }
}   
JavaClass[] jotClasses = createClassModelFromXJC(classesToProcess, jCodeModel);

At this point, we can instantiate a Generator and obtain a "dry" EclipseLink project, which we can then turn into a Dynamic Project with generated in-memory classes. Finally, we create and store an XMLContext and a DynamicHelper:

// Use the JavaModel to setup a Generator to generate an EclipseLink project 
XJCJavaModelImpl javaModel = new XJCJavaModelImpl(Thread.currentThread().getContextClassLoader(), jCodeModel);
XJCJavaModelInputImpl javaModelInput = new XJCJavaModelInputImpl(jotClasses, javaModel);
Generator g = new Generator(javaModelInput);
Project p = g.generateProject();
 
// Make a Dynamic Project from this project, because these classes do not exist on the classpath
DynamicClassLoader dynamicClassLoader = new DynamicClassLoader(classLoader);
Project dp = DynamicTypeBuilder.loadDynamicProject(p, null, dynamicClassLoader);
 
this.xmlContext = new XMLContext(dp);
this.dynamicHelper = new DynamicHelper(xmlContext.getSession(0));

Dynamic Type Lookup

Since, from the user's perspective, no concrete Java classes actually exist, the user must have a way to create new instances. This is done using a DynamicHelper, and is exposed through the newDynamicEntity(String) method on DynamicJAXBContext. For example:

DynamicEntity employee = ((DynamicJAXBContext) jaxbContext).newDynamicEntity("org.acme.Employee");
employee.set("firstName", "Bob");
employee.set("lastName", "Barker");
 
DynamicEntity address = ((DynamicJAXBContext) jaxbContext).newDynamicEntity("org.acme.Address");
address.set("street", "999 Studio City Blvd.");
...

Adding Properties/Mappings at Runtime

One of the main aspects of this feature is the ability to add new mappings to an existing running system (aligning with JPA's support for this scenario).

• Mappings must be fully initialized before they are added to the Descriptor, for thread-safety.
• Mapping customizations will be isolated to the current XMLContext, i.e. changes made to a Descriptor will only be visible in that Descriptor's XMLContext, even if multiple XMLContexts were created from the same initial metadata.

Testing

API

GUI

No GUI is required for this feature.

Config Files

A new metadata file may be needed to specify and persist new mapping information. (See Open Issues below)

Documentation

Open Issues

This section lists the open issues that are still pending that must be decided prior to fully implementing this project's requirements.

Decisions

This section lists decisions made. These are intended to document the resolution of open issues or constraints added to the project that are important.

Future Considerations

During the research for this project the following items were identified as out of scope but are captured here as potential future enhancements. If agreed upon during the review process these should be logged in the bug system.