In Java, you can load all kinds of resources using the same API but with different URL protocols:
file:///tmp.txt
http://127.0.0.1:8080/a.properties
jar:http://www.foo.com/bar/baz.jar!/COM/foo/Quux.class
This nicely decouples the actual loading of the resource from the application that needs the resource, and since a URL is just a String, resource loading is also very easily configurable.
Is there a protocol to load resources using the current classloader? This is similar to the Jar protocol, except that I do not need to know which jar file or class folder the resource is coming from.
I can do that using Class.getResourceAsStream("a.xml")
, of course, but that would require me to use a different API, and hence changes to existing code. I want to be able to use this in all places where I can specify a URL for the resource already, by just updating a property file.
First up, you're going to need at least a URLStreamHandler. This will actually open the connection to a given URL. Notice that this is simply called Handler
; this allows you to specify java -Djava.protocol.handler.pkgs=org.my.protocols
and it will automatically be picked up, using the "simple" package name as the supported protocol (in this case "classpath").
new URL("classpath:org/my/package/resource.extension").openConnection();
package org.my.protocols.classpath;
import java.io.IOException;
import java.net.URL;
import java.net.URLConnection;
import java.net.URLStreamHandler;
/** A {@link URLStreamHandler} that handles resources on the classpath. */
public class Handler extends URLStreamHandler {
/** The classloader to find resources from. */
private final ClassLoader classLoader;
public Handler() {
this.classLoader = getClass().getClassLoader();
}
public Handler(ClassLoader classLoader) {
this.classLoader = classLoader;
}
@Override
protected URLConnection openConnection(URL u) throws IOException {
final URL resourceUrl = classLoader.getResource(u.getPath());
return resourceUrl.openConnection();
}
}
If you control the code, you can do
new URL(null, "classpath:some/package/resource.extension", new org.my.protocols.classpath.Handler(ClassLoader.getSystemClassLoader()))
and this will use your handler to open the connection.
But again, this is less than satisfactory, as you don't need a URL to do this - you want to do this because some lib you can't (or don't want to) control wants urls...
The ultimate option is to register a URLStreamHandlerFactory
that will handle all urls across the jvm:
package my.org.url;
import java.net.URLStreamHandler;
import java.net.URLStreamHandlerFactory;
import java.util.HashMap;
import java.util.Map;
class ConfigurableStreamHandlerFactory implements URLStreamHandlerFactory {
private final Map<String, URLStreamHandler> protocolHandlers;
public ConfigurableStreamHandlerFactory(String protocol, URLStreamHandler urlHandler) {
protocolHandlers = new HashMap<String, URLStreamHandler>();
addHandler(protocol, urlHandler);
}
public void addHandler(String protocol, URLStreamHandler urlHandler) {
protocolHandlers.put(protocol, urlHandler);
}
public URLStreamHandler createURLStreamHandler(String protocol) {
return protocolHandlers.get(protocol);
}
}
To register the handler, call URL.setURLStreamHandlerFactory()
with your configured factory. Then do new URL("classpath:org/my/package/resource.extension")
like the first example and away you go.
Note that this method may only be called once per JVM, and note well that Tomcat will use this method to register a JNDI handler (AFAIK). Try Jetty (I will be); at worst, you can use the method first and then it has to work around you!
I release this to the public domain, and ask that if you wish to modify that you start a OSS project somewhere and comment here with the details. A better implementation would be to have a URLStreamHandlerFactory
that uses ThreadLocal
s to store URLStreamHandler
s for each Thread.currentThread().getContextClassLoader()
. I'll even give you my modifications and test classes.