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Java & XML Data Binding by Brett McLaughlin

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Converting to Java

Now comes the fun part: turning these XML documents into Java object instances. I’m going to really take this process step by step, even though the steps are awfully simple. The point of this exercise isn’t to bore you or fill pages; you need to be able to understand exactly what happens so you can track down problems. As a general rule, the higher level the API, the more that happens without your direct intervention. That means that more can go wrong without the casual user being able to do a thing about it. Since you’re not a casual user (at least not after working through this book), you’ll want to be able to dig in and figure out what’s going on.

XML Input

The first step in unmarshalling is getting access to your XML input. I’ve already spent a bit of time detailing the process of creating that XML; now you need to get a handle to it through a Java input method. The easiest way to do this is to wrap the XML data in either an InputStream or a Reader, both from the java.io package. When using JAXB, you’ll need to limit your input format to InputStreams, as Readers aren’t supported (although many other frameworks do support Readers, it is simple enough to convert between the two input formats).

If you know much about Java, there isn’t any special method you need to invoke to open a stream; however, you do need to understand what state the stream is in when returned to you after unmarshalling completes. Specifically, you should be aware of whether the stream you supplied to the unmarshalling process is open or closed when returned from the unmarshal( ) method. The answer with respect to the JAXB framework is that the stream is closed. That effectively ends the use of the stream once unmarshalling occurs. Trying to use the stream after unmarshalling results in an exception like this:

java.io.IOException: Stream closed
        at java.io.BufferedInputStream.ensureOpen(BufferedInputStream.java:123)
        at java.io.BufferedInputStream.reset(BufferedInputStream.java:371)
        at javajaxb.RereadStreamTest.main(RereadStreamTest.java:84)

As a result, you don’t expect to continue using the stream, even through buffering or other I/O tricks. That will save you the hassle of writing lots of I/O code, compiling, and then getting errors at runtime and having to rewrite large chunks of your code. If you do need to get access to input data once it has been unmarshalled, you will need to create a new stream for the data and read from that new stream:[9]

    public static void main(String[] args) {
        try {
            File xmlFile = new File(args[0]);
            FileInputStream inputStream = new FileInputStream(xmlFile);
  
            // Buffer input
            BufferedInputStream bufferedStream = 
                new BufferedInputStream(inputStream);
            bufferedStream.mark(bufferedStream.available(  ));
  
            // Unmarshal
            Movies movies = Movies.unmarshal(bufferedStream);
  
            FileInputStream newInputStream = new FileInputStream(xmlFile);
  
            // Read the stream and output (for testing)
            BufferedReader reader = new BufferedReader(
                new InputStreamReader(newInputStream));
            String line = null;
            while ((line = reader.readLine(  )) != null) {
                System.out.println(line);
            }
        } catch (Exception e) {
            e.printStackTrace(  );
        }
    }

Other than these somewhat rare issues, if you can write a simple InputStream construction statement, you’re ready to turn your XML input into Java output. Be sure to remember that you can use a file, network connection, URL, or any other source for input, and you’re all set.

Java Output

You should still have the generated source files from the movies database (or your own DTD) from the last chapter. Open the top-level object—the one that corresponds to your root element. If you used the movies DTD, this object is Movies.java. Search through the file for the unmarshal( ) methods, which will convert your XML to Java. Here are the signatures for these methods in the Movies object:

public static Movies unmarshal(XMLScanner xs, Dispatcher d)
    throws UnmarshalException;
  
public static Movies unmarshal(XMLScanner xs)
    throws UnmarshalException;
  
public static Movies unmarshal(InputStream in)
                   throws UnmarshalException;
  
public void unmarshal(Unmarshaller u)
    throws UnmarshalException;

Of these four, there’s really only one that I care much about—the third one, which I’ve boldfaced and takes an InputStream as an argument. The reason why the others are less important to common programming is that they involve using specific JAXB constructs; it builds a dependency on JAXB into your application—possibly a specific version of JAXB, which I try to avoid as a general principle. This isn’t because JAXB isn’t a good framework; I recommend it for any data binding framework, especially when you have the option to use a common input parameter like an InputStream (as discussed in the last section).

The returned object on this method, as well as the other three, is an instance of the Movies class. This shouldn’t be surprising, as you want the data in the supplied input stream to be converted into Java object instances, and this is the topmost object of interest. You can then use this object like any other:

System.out.println("*** Movie Database ***");
  
List movies = movies.getMovie(  );
for (Iterator i = movies.iterator(); i.hasNext(  ); ) {
    Movie movie = (Movie)i.next(  );
    System.out.println("  * " + movie.getTitle(  ));
}

Here, you’d get a list like this:

*** Movie Database ***
  * Pitch Black
  * Memento

I’ll leave the rest of the discussion of result object use for the next main section, where it can be covered more thoroughly.

Finally, notice that the unmarshal( ) methods are all static. This makes sense, as there is no object instance to operate upon until after the method is invoked. Here’s how you would turn an XML document into a Java object:

try {
    // Get XML input
    File xmlFile = new File("movies.xml");
    FileInputStream inputStream = new FileInputStream(xmlFile);
  
    // Convert to Java
    Movies movies = Movies.unmarshal(inputStream);
} catch (Exception e) {
    // Handle errors
}

I know that probably seems a bit simple after all this talk and detail, but that’s really it. What is interesting is how the objects are used and where the XML data comes from. I’ll take a slight detour into JAXB’s inner workings and then address that very topic (JAXB usage) next.

Intermediate Objects

I want to talk briefly about the “in-between” of the JAXB unmarshalling process—in other words, what happens between XML input and Java output. The key classes involved in unraveling this process in JAXB are javax.xml.bind.Unmarshaller, javax.xml.marshal.XMLScanner, and javax.xml.bind.Dispatcher. The Unmarshaller class is the centerpiece of the framework and relies heavily on the XMLScanner mechanism for parsing. The Dispatcher class takes care of mapping XML structures to Java ones. Here’s the basic rundown:

First, the JAXB framework presupposes that a full XML parser is not required. The assumption is that because all the XML data is derived from a set of constraints, basic well-formedness rules (like start tags matching end tags) and validity are assured before parsing begins. This hearkens back to my earlier admonition to validate your XML content before using it in a data binding context. Because of these assumptions, an XMLScanner instance can operate much like a SAX parser. However, it ignores some basic error checking, as well as XML structures like comments, which are not needed in data-bound classes. Of course, the whole point of this class is to improve the performance issues surrounding parsing data specifically for use in data-bound classes.

Second, JAXB uses a Dispatcher to handle name conversion. For every Dispatcher instance, there exists a map of XML names and a map of Java class names. The XML names have mappings from XML element names to Java class names (attributes and so forth are not relevant here). The Java class names map from Java classes to user-defined subclasses, in the case that users define their own classes to unmarshal and marshal data into. This class, then, provides several lookup methods, allowing the unmarshalling or marshalling processes to supply an XML element name and get a Java class name (or to supply a Java class name and get a user-defined subclass name).

Finally, the unmarshalling process, through an Unmarshaller instance, is accomplished by invoking an unmarshal( ) method on a Dispatcher instance. The current XMLScanner instance is examined, the current data being parsed is converted to Java (looking up the appropriate name using the Dispatcher instance), and the result is one or more Java object instances. Then the scanner continues through the XML input stream and the process repeats. Over and over, XML data is turned into Java data, until the end of the XML input stream is reached. Finally, the root-level object is returned to the invoking program and you get to operate on this object. This is the tale of a JAXB unmarshaller. This process is illustrated more completely in Figure 4-4.

The JAXB unmarshalling process in detail

Figure 4-4. The JAXB unmarshalling process in detail

While it’s not mandatory that you understand this process, or even know about it, it can help you understand where performance problems creep in (and turn into a bona fide JAXB guru).



[9] This fragment is available as a complete Java source file from the web site, as ch04/src/java/javajaxb/RereadStreamTest.java.

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