CODEWITHSUNDEEP
javagenericsreflectionjaxb
fIwJlxSzApHEZIl
fIwJlxSzApHEZIl

Reputation: 13340

Combining JAXB, Generics, and Reflection to XML serialize all my Java classes

My code is the following:

    System.out.println("This will save a table to XML data sheet.");
    System.out.println("Please pick a table to save: " + listOfTables.toString());
    command = scan.nextLine();
    if(listOfTables.contains(command))
    {
        System.out.println("successfuly found table to save: " + command);
        try  //Java reflection
        {
            Class<?> myClass = Class.forName(command); // get the class named after their input
            Method listMethod = myClass.getDeclaredMethod("list"); // get the list method from the class
            Object returnType = listMethod.invoke(myClass, new Object[]{}); // run the list method
            ArrayList<Object> objectList = (ArrayList)returnType; // get the arraylist of objects to send to XML
            try 
            {
                JAXBContext jaxbContext = JAXBContext.newInstance(myClass);
                Marshaller marshaller = jaxbContext.createMarshaller();
                marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
                JAXBElement<?> jaxbElement = new JAXBElement<?>(new QName("jaxbdemo", "generated"), myClass, objectList.get(0));
                marshaller.marshal(jaxbElement, System.out);

            } catch (JAXBException e) {}
        }
        catch (ClassNotFoundException | SecurityException | NoSuchMethodException | IllegalAccessException | IllegalArgumentException | InvocationTargetException e) { }

My problem is that neither:

JAXBElement<?> jaxbElement = new JAXBElement<?>(new QName("jaxbdemo", "generated"), myClass, objectList.get(0));

Nor:

JAXBElement<myClass> jaxbElement = new JAXBElement<myClass>(new QName("jaxbdemo", "generated"), myClass, objectList.get(0));

will compile. So what do I need to put between the <> for my JAXBElement type? Btw I'm getting:

The constructor JAXBElement<myClass>(QName, Class<myClass>, myClass) refers to the missing type myClass

and:

Cannot instantiate the type JAXBElement<?>

Upvotes: 2

Views: 2448

Answers (1)

Paul Bellora
Paul Bellora

Reputation: 55233

You need to use a helper method to do this. Here's a rough example:

static <T> void helper(Class<T> myClass) {

    Method listMethod = myClass.getDeclaredMethod("list");
    Object returnType = listMethod.invoke(myClass, new Object[]{});
    @SuppressWarnings("unchecked") // [carefully document why this is okay here]
    ArrayList<T> objectList = (ArrayList<T>)returnType;

    ...

    JAXBElement<T> jaxbElement = new JAXBElement<T>(
            new QName("jaxbdemo", "generated"),
            myClass,
            objectList.get(0)
    );

    ...
}

From that point, you're free to return a JAXBElement<?> or else finish the remaining work within the helper.

As noted, you should document the unchecked cast, explaining why invoking list for a given class represented by a Class<T> is guaranteed to return an ArrayList<T>, as your code assumes. This methodology seems brittle to me at best and I can already spot a mistake when you call invoke:

listMethod.invoke(myClass, new Object[]{});

That method takes the instance on which to invoke the method as its first argument (or null if it's a static method), but you're passing in myClass - that can't possibly be right.

Upvotes: 1

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