How to Load Resources from the Classpath Using a Custom URL Protocol in Java?

URL to load resources from the classpath in Java

In Java, resources can be loaded using various URL protocols, enabling separation between resource loading and the application using it. A URL is represented as a simple string, making resource loading highly configurable.

Problem Statement

Is there a protocol to load resources using the current classloader? This is similar to the Jar protocol, but without specifying a specific jar or class folder.

Solution

This can be achieved using a custom URLStreamHandler, which opens a connection to a given URL. The Handler is named simply as "Handler," allowing automatic pickup when specified using java -Djava.protocol.handler.pkgs=org.my.protocols.

Basic Implementation:

package org.my.protocols.classpath;

import java.io.IOException;
import java.net.URL;
import java.net.URLConnection;
import java.net.URLStreamHandler;

public class Handler extends URLStreamHandler {
    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();
    }
}

Usage:

new URL("classpath:org/my/package/resource.extension").openConnection();

Additional Considerations:

Manual Code Handler Specification:

If the code is controlled, the handler can be specified manually:

new URL(null, "classpath:some/package/resource.extension", new org.my.protocols.classpath.Handler(ClassLoader.getSystemClassLoader()))

JVM Handler Registration:

The most comprehensive solution is to register a URLStreamHandlerFactory that handles 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<>();
        addHandler(protocol, urlHandler);
    }

    public void addHandler(String protocol, URLStreamHandler urlHandler) {
        protocolHandlers.put(protocol, urlHandler);
    }

    public URLStreamHandler createURLStreamHandler(String protocol) {
        return protocolHandlers.get(protocol);
    }
}

Then call URL.setURLStreamHandlerFactory() with the configured factory to register the handler.

JVM Handler Registration Issue:

This method can only be called once per JVM, and Tomcat may attempt to register its JNDI handler. Using Jetty or a custom URLStreamHandlerFactory with a ThreadLocal approach is recommended.

License:

The solution is released to the public domain, with a request to start an OSS project for modifications.

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