[java] URL to load resources from the classpath in Java

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.

I try to avoid the URL class and instead rely on URI. Thus for things that need URL where I would like to do Spring Resource like lookup with out Spring I do the following:

public static URL toURL(URI u, ClassLoader loader) throws MalformedURLException {
    if ("classpath".equals(u.getScheme())) {
        String path = u.getPath();
        if (path.startsWith("/")){
            path = path.substring("/".length());
        }
        return loader.getResource(path);
    }
    else if (u.getScheme() == null && u.getPath() != null) {
        //Assume that its a file.
        return new File(u.getPath()).toURI().toURL();
    }
    else {
        return u.toURL();
    }
}

To create a URI you can use URI.create(..). This way is also better because you control the ClassLoader that will do the resource lookup.

I noticed some other answers trying to parse the URL as a String to detect the scheme. I think its better to pass around URI and use it to parse instead.

I have actually filed an issue a while ago with Spring Source begging them to separate out their Resource code from core so that you don't need all the other Spring stuff.

I've created a class which helps to reduce errors in setting up custom handlers and takes advantage of the system property so there are no issues with calling a method first or not being in the right container. There's also an exception class if you get things wrong:

CustomURLScheme.java:
/*
 * The CustomURLScheme class has a static method for adding cutom protocol
 * handlers without getting bogged down with other class loaders and having to
 * call setURLStreamHandlerFactory before the next guy...
 */
package com.cybernostics.lib.net.customurl;

import java.net.URLStreamHandler;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 * Allows you to add your own URL handler without running into problems
 * of race conditions with setURLStream handler.
 * 
 * To add your custom protocol eg myprot://blahblah:
 * 
 * 1) Create a new protocol package which ends in myprot eg com.myfirm.protocols.myprot
 * 2) Create a subclass of URLStreamHandler called Handler in this package
 * 3) Before you use the protocol, call CustomURLScheme.add(com.myfirm.protocols.myprot.Handler.class);
 * @author jasonw
 */
public class CustomURLScheme
{

    // this is the package name required to implelent a Handler class
    private static Pattern packagePattern = Pattern.compile( "(.+\\.protocols)\\.[^\\.]+" );

    /**
     * Call this method with your handlerclass
     * @param handlerClass
     * @throws Exception 
     */
    public static void add( Class<? extends URLStreamHandler> handlerClass ) throws Exception
    {
        if ( handlerClass.getSimpleName().equals( "Handler" ) )
        {
            String pkgName = handlerClass.getPackage().getName();
            Matcher m = packagePattern.matcher( pkgName );

            if ( m.matches() )
            {
                String protocolPackage = m.group( 1 );
                add( protocolPackage );
            }
            else
            {
                throw new CustomURLHandlerException( "Your Handler class package must end in 'protocols.yourprotocolname' eg com.somefirm.blah.protocols.yourprotocol" );
            }

        }
        else
        {
            throw new CustomURLHandlerException( "Your handler class must be called 'Handler'" );
        }
    }

    private static void add( String handlerPackage )
    {
        // this property controls where java looks for
        // stream handlers - always uses current value.
        final String key = "java.protocol.handler.pkgs";

        String newValue = handlerPackage;
        if ( System.getProperty( key ) != null )
        {
            final String previousValue = System.getProperty( key );
            newValue += "|" + previousValue;
        }
        System.setProperty( key, newValue );
    }
}


CustomURLHandlerException.java:
/*
 * Exception if you get things mixed up creating a custom url protocol
 */
package com.cybernostics.lib.net.customurl;

/**
 *
 * @author jasonw
 */
public class CustomURLHandlerException extends Exception
{

    public CustomURLHandlerException(String msg )
    {
        super( msg );
    }

}

I think this is worth its own answer - if you're using Spring, you already have this with

Resource firstResource =
    context.getResource("http://www.google.fi/");
Resource anotherResource =
    context.getResource("classpath:some/resource/path/myTemplate.txt");

Like explained in the spring documentation and pointed out in the comments by skaffman.

From Java 9+ and up, you can define a new URLStreamHandlerProvider. The URL class uses the service loader framework to load it at run time.

Create a provider:

package org.example;

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

public class ClasspathURLStreamHandlerProvider extends URLStreamHandlerProvider {

    @Override
    public URLStreamHandler createURLStreamHandler(String protocol) {
        if ("classpath".equals(protocol)) {
            return new URLStreamHandler() {
                @Override
                protected URLConnection openConnection(URL u) throws IOException {
                    return ClassLoader.getSystemClassLoader().getResource(u.getPath()).openConnection();
                }
            };
        }
        return null;
    }

}

Create a file called java.net.spi.URLStreamHandlerProvider in the META-INF/services directory with the contents:

org.example.ClasspathURLStreamHandlerProvider

Now the URL class will use the provider when it sees something like:

URL url = new URL("classpath:myfile.txt");

Solution with registering URLStreamHandlers is most correct, of course, but sometimes the simplest solution is needed. So, I use the following method for that:

/**
 * Opens a local file or remote resource represented by given path.
 * Supports protocols:
 * <ul>
 * <li>"file": file:///path/to/file/in/filesystem</li>
 * <li>"http" or "https": http://host/path/to/resource - gzipped resources are supported also</li>
 * <li>"classpath": classpath:path/to/resource</li>
 * </ul>
 *
 * @param path An URI-formatted path that points to resource to be loaded
 * @return Appropriate implementation of {@link InputStream}
 * @throws IOException in any case is stream cannot be opened
 */
public static InputStream getInputStreamFromPath(String path) throws IOException {
    InputStream is;
    String protocol = path.replaceFirst("^(\\w+):.+$", "$1").toLowerCase();
    switch (protocol) {
        case "http":
        case "https":
            HttpURLConnection connection = (HttpURLConnection) new URL(path).openConnection();
            int code = connection.getResponseCode();
            if (code >= 400) throw new IOException("Server returned error code #" + code);
            is = connection.getInputStream();
            String contentEncoding = connection.getContentEncoding();
            if (contentEncoding != null && contentEncoding.equalsIgnoreCase("gzip"))
                is = new GZIPInputStream(is);
            break;
        case "file":
            is = new URL(path).openStream();
            break;
        case "classpath":
            is = Thread.currentThread().getContextClassLoader().getResourceAsStream(path.replaceFirst("^\\w+:", ""));
            break;
        default:
            throw new IOException("Missed or unsupported protocol in path '" + path + "'");
    }
    return is;
}

I dont know if there is one already, but you can make it yourself easilly.

That different protocols example looks to me like a facade pattern. You have a common interface when there are different implementations for each case.

You could use the same principle, make a ResourceLoader class which takes the string from your properties file, and checks for a custom protocol of ours

myprotocol:a.xml
myprotocol:file:///tmp.txt
myprotocol:http://127.0.0.1:8080/a.properties
myprotocol:jar:http://www.foo.com/bar/baz.jar!/COM/foo/Quux.class

strips the myprotocol: from the start of the string and then makes a decision of which way to load the resource, and just gives you the resource.

You can also set the property programmatically during startup:

final String key = "java.protocol.handler.pkgs";
String newValue = "org.my.protocols";
if (System.getProperty(key) != null) {
    final String previousValue = System.getProperty(key);
    newValue += "|" + previousValue;
}
System.setProperty(key, newValue);

Using this class:

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 {

    @Override
    protected URLConnection openConnection(final URL u) throws IOException {
        final URL resourceUrl = ClassLoader.getSystemClassLoader().getResource(u.getPath());
        return resourceUrl.openConnection();
    }
}

Thus you get the least intrusive way to do this. :) java.net.URL will always use the current value from the system properties.

(Similar to Azder's answer, but a slightly different tact.)

I don't believe there is a predefined protocol handler for content from the classpath. (The so-called classpath: protocol).

However, Java does allow you to add your own protocols. This is done through providing concrete implementations java.net.URLStreamHandler and java.net.URLConnection.

This article describes how a custom stream handler can be implemented: http://java.sun.com/developer/onlineTraining/protocolhandlers/.

In a Spring Boot app, I used the following to get the file URL,

Thread.currentThread().getContextClassLoader().getResource("PromotionalOfferIdServiceV2.wsdl")

An extension to Dilums's answer:

Without changing code, you likely need pursue custom implementations of URL related interfaces as Dilum recommends. To simplify things for you, I can recommend looking at the source for Spring Framework's Resources. While the code is not in the form of a stream handler, it has been designed to do exactly what you are looking to do and is under the ASL 2.0 license, making it friendly enough for re-use in your code with due credit.

If you have tomcat on the classpath, it's as simple as:

TomcatURLStreamHandlerFactory.register();

This will register handlers for "war" and "classpath" protocols.

URL url = getClass().getClassLoader().getResource("someresource.xxx");

That should do it.

Inspire by @Stephen https://stackoverflow.com/a/1769454/980442 and http://docstore.mik.ua/orelly/java/exp/ch09_06.htm

To use

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

just create this class into sun.net.www.protocol.classpath package and run it into Oracle JVM implementation to work like a charm.

package sun.net.www.protocol.classpath;

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

public class Handler extends URLStreamHandler {

    @Override
    protected URLConnection openConnection(URL u) throws IOException {
        return Thread.currentThread().getContextClassLoader().getResource(u.getPath()).openConnection();
    }
}

In case you are using another JVM implementation set the java.protocol.handler.pkgs=sun.net.www.protocol system property.

FYI: http://docs.oracle.com/javase/7/docs/api/java/net/URL.html#URL(java.lang.String,%20java.lang.String,%20int,%20java.lang.String)