I'm trying to extend my library for integrating Swing and JPA by making JPA config as automatic (and portable) as can be done, and it means programmatically adding <class> elements. (I know it can be done via Hibernate's AnnotationConfiguration or EclipseLInk's ServerSession, but - portability). I'd also like to avoid using Spring just for this single purpose.
I can create a persistence.xml on the fly, and fill it with <class> elements from specified packages (via the Reflections library). The problem starts when I try to feed this persistence.xml to a JPA provider. The only way I can think of is setting up a URLClassLoader, but I can't think of a way what wouldn't make me write the file to the disk somewhere first, for sole ability to obtain a valid URL. Setting up a socket for serving the file via an URL(localhost:xxxx) seems... I don't know, evil?
Does anyone have an idea how I could solve this problem? I know it sounds like a lot of work to avoid using one library, but I'd just like to know if it can be done.
EDIT (a try at being more clear):
Dynamically generated XML is kept in a String object. I don't know how to make it available to a persistence provider. Also, I want to avoid writing the file to disk.
For purpose of my problem, a persistence provider is just a class which scans the classpath for META-INF/persistence.xml. Some implementations can be made to accept dynamic creation of XML, but there is no common interface (especially for a crucial part of the file, the <class> tags).
My idea is to set up a custom ClassLoader - if you have any other I'd be grateful, I'm not set on this one.
The only easily extendable/configurable one I could find was a URLClassLoader. It works on URL objects, and I don't know if I can create one without actually writing XML to disk first.
That's how I'm setting things up, but it's working by writing the persistenceXmlFile = new File("META-INF/persistence.xml") to disk:
Thread.currentThread().setContextClassLoader(
new URLResourceClassLoader(
new URL[] { persistenceXmlFile.toURI().toURL() },
Thread.currentThread().getContextClassLoader()
)
);
URLResourceClassLoader is URLCLassLoader's subclass, which allows for looking up resources as well as classes, by overriding public Enumeration<URL> findResources(String name).
Maybe a bit late (after 4 years), but for others that are looking for a similar solution, you may be able to use the URL factory I created:
public class InMemoryURLFactory {
public static void main(String... args) throws Exception {
URL url = InMemoryURLFactory.getInstance().build("/this/is/a/test.txt", "This is a test!");
byte[] data = IOUtils.toByteArray(url.openConnection().getInputStream());
// Prints out: This is a test!
System.out.println(new String(data));
}
private final Map<URL, byte[]> contents = new WeakHashMap<>();
private final URLStreamHandler handler = new InMemoryStreamHandler();
private static InMemoryURLFactory instance = null;
public static synchronized InMemoryURLFactory getInstance() {
if(instance == null)
instance = new InMemoryURLFactory();
return instance;
}
private InMemoryURLFactory() {
}
public URL build(String path, String data) {
try {
return build(path, data.getBytes("UTF-8"));
} catch (UnsupportedEncodingException ex) {
throw new RuntimeException(ex);
}
}
public URL build(String path, byte[] data) {
try {
URL url = new URL("memory", "", -1, path, handler);
contents.put(url, data);
return url;
} catch (MalformedURLException ex) {
throw new RuntimeException(ex);
}
}
private class InMemoryStreamHandler extends URLStreamHandler {
@Override
protected URLConnection openConnection(URL u) throws IOException {
if(!u.getProtocol().equals("memory")) {
throw new IOException("Cannot handle protocol: " + u.getProtocol());
}
return new URLConnection(u) {
private byte[] data = null;
@Override
public void connect() throws IOException {
initDataIfNeeded();
checkDataAvailability();
// Protected field from superclass
connected = true;
}
@Override
public long getContentLengthLong() {
initDataIfNeeded();
if(data == null)
return 0;
return data.length;
}
@Override
public InputStream getInputStream() throws IOException {
initDataIfNeeded();
checkDataAvailability();
return new ByteArrayInputStream(data);
}
private void initDataIfNeeded() {
if(data == null)
data = contents.get(u);
}
private void checkDataAvailability() throws IOException {
if(data == null)
throw new IOException("In-memory data cannot be found for: " + u.getPath());
}
};
}
}
}