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PD4ML: Custom Resource Loaders
In most of the situations PD4ML is able to resolve relative or absolute
resource references (to images, to CSS etc) and load them. However there
are special cases, when the default mechanisms are not sufficient.
For example:
- The resources are stored in an unusual place, i.e. in a database.
- The resources are referenced by an "exotic" or non-standard protocol.
For instance, Weblogic and WebSphere SSL implementations are not derived
from the standard JDK SSL classes PD4ML relies on, so it causes ClassCastException.
As a workaround PD4ML allows you to create your own resource loader, which
takes an URL as a parameter, and should return the loaded resource as an
array of bytes.
import java.io.BufferedInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.URL;
import java.net.URLConnection;
import org.zefer.cache.ResourceProvider;
public class CustomFileResourceProvider extends ResourceProvider {
public byte[] getResourceAsBytes(String resource, boolean debugOn) throws IOException {
ByteArrayOutputStream fos = new ByteArrayOutputStream();
byte buffer[] = new byte[2048];
InputStream is = null;
resource = "http://old.pd4ml.com/i/" + resource;
URL src = new URL(resource);
URLConnection urlConnect = src.openConnection();
try {
urlConnect.connect();
} catch (Throwable e) {
return new byte[0];
}
is = urlConnect.getInputStream();
BufferedInputStream bis = new BufferedInputStream(is);
int read;
do {
read = is.read(buffer, 0, buffer.length);
if (read > 0) { // something to put down
fos.write(buffer, 0, read);
}
} while (read > -1);
fos.close();
bis.close();
is.close();
return fos.toByteArray();
}
}
In order to enable the loader, you may either pass to PD4ML the class name
with an environment variable (add the following to JVM command line):
-Dpd4ml.extra.resource.loaders=CustomFileResourceProvider
or via API call:
HashMap map = new HashMap();
map.put( "pd4ml.extra.resource.loaders", "CustomFileResourceProvider" );
pd4ml.setDynamicParams(map);
It may also be a coma-separated list of multiple resource loaders.
Note: a custom resource loader
can be called twice for each resource reference: first time it assumes
the resource URL is a relative path, so it sets resource
parameter to an URL, resolved according to a document base. If the first
attempt has been responded with null, it is called with the
original string from SRC attribute of <img> tag, for
example. If the second time the resource loader responded with null,
PD4ML tries to load the reference resource the regular way.
Custom resource loaders topic discussion is
here.
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