从类路径中的任何地方加载资源

Question

I have a simple java application that loads a properties file from the current package.

this.getClass().getResourceAsStream("props.properties");

This works fine when the property file I want is in the current package. However, I want to package this application as a JAR and define and override with a new properties file where I use it. Is there a way to load the first resource named "props.properties" that is on the classpath?

I want it to be as easy to override the properties file via command line:

java.exe -classpath props.properties;myJar.jar com.test.MyApp

I don't want to have to unpack the JAR and modify the properties file to change something. I feel like I'm missing something obvious...

Answer 1

The javadoc for Class.getResourceAsStream() documents the lookup logic:

If the name begins with a '/' ( '\/' ), then the absolute name of the resource is the portion of the name following the '/'.

Otherwise, the absolute name is of the following form:
modified_package_name/name

Where the modified_package_name is the package name of this object with '/' substituted for '.' ( '\.' ).

So in other words, the resource name passed to the method should look like /com/package/p2/props.properties if the props.properties is stored in the com.package.p2 package instead of the current class's.

Answer 2

I'm sure it's too late for the answer but it could be interesting for googlers this small code snippet helpers to load a properties file from any where in the Classpath.

ClassLoader cl = ClassLoader.getSystemClassLoader();
    if (cl != null) {
        URL url = cl.getResource(CONF_PROPERTIES);
        if (url == null) {
            url = cl.getResource("/" + CONF_PROPERTIES);
        }
        if (url != null) {
            try {
                InputStream in = url.openStream();
                props = new Properties();
                props.load(in);
            } catch (IOException e) {
                // Log the exception
            } finally {
               // close opened resources
            }

        }
    }

Answer 3

If all else fails you could use two different file names, say props-default.properties inside myJar.jar and props.properties to override on the command-line. In your code, you'd try loading the props.properties file first and fallback to props-default.properties if it wasn't found.

Answer 4

I'm not sure, but maybe: ClassLoader.getResourceAsStream()

EDIT:

I don't think this is significantly different to this.getClass().getResourceAsStream() from the question, since as mentioned you still have to get the ClassLoader you want to use to load the resource.

Since you provide the resource in the -classpath in your example, it should be available from the same class loader as your "main" class (in the SUN JVM, that's sun.misc.Launcher$AppClassLoader , not sure if this can/does vary for other JVM implementations).