Frequently Asked Questions
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## Frequently Asked Questions

1. JAXB 2.0
  Q: Which version of J2SE does JAXB 2.0 require?
  Q: Can I run my existing JAXB 1.0.x applications on the JAXB 2.0 runtime?
  Q: What if I want to port my JAXB 1.0.x application to JAXB 2.0?
  Q: Are the JAXB runtime API's thread safe?
  Q: Why can't I cast the unmarshalled object into the generated type.
  Q: Which jar files do I need to distribute with my application that uses the JAXB RI?
  Q: How can I cause the Marshaller to generate CDATA blocks?
  Q: Can I access <xs:any/> as a DOM node?
  Q: How do I find out which version of the JAXB RI I'm using?

### 1. JAXB 2.0
Q: | Which version of J2SE does JAXB 2.0 require?
A: | Java SE 6 or higher.
Q: | Can I run my existing JAXB 1.0.x applications on the JAXB 2.0 runtime?
A: | This is no longer supported. However, you should be able to deploy `http://search.maven.org/remotecontent?filepath=com/sun/xml/bind/jaxb1-impl/2.2.5-1/jaxb1-impl-2.2.5-1.jar` with your with your application application.
Q: | What if I want to port my JAXB 1.0.x application to JAXB 2.0?
A: | You need to recompile your schema with the newer JAXB 2.0 xjc and modify your application code to work with the new bindings.
Q: | Are the JAXB runtime API's thread safe?
A: | The JAXB Specification currently does not address the thread safety of any of the runtime classes. In the case of the Oracle JAXB RI, the `JAXBContext` class is thread safe, but the `Marshaller`, `Unmarshaller`, and `Validator` classes are not thread safe. For example, suppose you have a multi-thread server application that processes incoming XML documents by JAXB. In this case, for the best performance you should have just one instance of `JAXBContext` in your whole application like this:
    class MyServlet extends HttpServlet {
        static final JAXBContext context = initContext();

        private static JAXBContext initContext() {
            return JAXBContext.newInstance("....", MyServlet.class.getClassLoader());
        }
    }
And each time you need to unmarshal/marshal/validate a document. Just create a new `Unmarshaller`/`Marshaller`/`Validator` from this context, like this:
    public void doGet(HttpServletRequest req, HttpServletResponse resp) {
        Unmarshaller u = context.createUnmarshaller();
        u.unmarshal(...);
    }
This is the simplest safe way to use the JAXB RI from multi-threaded applications. If you really care about the performance, and/or your application is going to read a lot of small documents, then creating `Unmarshaller` could be relatively an expensive operation. In that case, consider pooling `Unmarshaller` objects. Different threads may reuse one `Unmarshaller` instance, as long as you don't use one instance from two threads at the same time.
Q: | Why can't I cast the unmarshalled object into the generated type.
A: | When you invoke `JAXBContext.newInstance("aaa.bbb.ccc")`, it tries to load classes and resources using the same classloader used to load the `JAXBContext` class itself. This classloader may be different from the classloader which was used to load your application (see the picture Parent/Child classloader). In this case, you'll see the above error. This problem is often seen with application servers, J2EE containers, Ant, JUnit, and other applications that use sophisticated class loading mechanisms. Figure 1. Parent/Child classloader
With some applications, things get even more complicated when the JAXB-generated code can be loaded by either classloader. In this case, `JAXBContext.newInstance("aaa.bbb.ccc")` will work but the JVM ends up loading two copies of the generated classes for each class loader. As a result, unmarshalling works but an attempt to cast the returned object into the expected type will fail, even though its `getClass().getName()` returns the expected name. The solution for both situations is to pass your curent class loader like this:
    JAXBContext.newInstance("aaa.bbb.ccc", this.getClass().getClassLoader());
In general, if you are writing code that uses JAXB, it is always better to explicitly pass in a class loader, so that your code will work no matter where it is deployed.
Q: | Which jar files do I need to distribute with my application that uses the JAXB RI?
A: | For JAXB 2.2.x:
    $JAXB_HOME/lib/jaxb-api.jar
    $JAXB_HOME/lib/jaxb-impl.jar
Q: | How can I cause the `Marshaller` to generate `CDATA` blocks?
A: | This functionality is not available from JAXB directly, but you can configure an Apache Xerces-J `XMLSerializer` to produce `CDATA` blocks. Please review the JaxbCDATASample.java sample app for more detail.
Q: | Can I access <xs:any/> as a DOM node?
A: | In JAXB 2.0, <xs:any/> is handled correctly without any customization. If it's `strict`, it will map to `Object` or `List<Object>` and when you unmarshal documents, you'll get objects that map to elements (such as `JAXBElements` or classes that are annotated with `XmlRootElement`). If it's `skip`, it will map to `org.w3c.dom.Element` or `List<Element>` and when you unmarshal documents, you'll get DOM elements. If it's `lax`, it will map to the same as with `strict`, and when you unmarshal documents, you'll get either: `JAXBElement`s classes that are annotated with `XmlRootElement` DOM elements
Q: | How do I find out which version of the JAXB RI I'm using?
A: | Run the following command
    $ java -jar jaxb-xjc.jar -version
Alternatively, each JAXB jar has version information in its `META-INF/MANIFEST.MF`, such as this:
    Manifest-Version: 1.0
    Ant-Version: Apache Ant 1.8.2
    Created-By: 1.6.0_29-b11 (Sun Microsystems Inc.)
    Specification-Title: Java Architecture for XML Binding
    Specification-Version: 2.2.6
    Specification-Vendor: Oracle Corporation
    Implementation-Title: JAXB Reference Implementation
    Implementation-Version: 2.2.5-SNAPSHOT
    Implementation-Vendor: Oracle Corporation
    Implementation-Vendor-Id: com.sun
    Extension-Name: com.sun.xml.bind
    Build-Id: 02/09/2012 01:42PM (hudson)
    Class-Path: jaxb-api.jar

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