使用SPI加载时消失的jar条目

Question

So in my application I have a plugin system that I've written using SPI (service provider interface), so I have a jar file called scripts.jar that I keep somewhere (note: it's not on the classpath), then I load a script from it using this method:

/**
 * Loads a script from the given name, regardless of case.
 * @param name script name
 * @return the loaded Script
 */
public static Script loadScript(Client c, String name) {
    Script script = null;
    try {
        URLClassLoader loader = new URLClassLoader(new URL[]{new File(Config.CONF_DIR, "scripts.jar").toURI().toURL()});
        ServiceLoader serviceLoader = ServiceLoader.load(Script.class, loader);
        serviceLoader.reload();
        Iterator<Script> scripts = serviceLoader.iterator();
        while(scripts.hasNext()) {
            Script cur = scripts.next();
            if(cur.getClass().getSimpleName().equalsIgnoreCase(name)) {
                script = cur;
                break;
            }
        }
    } catch(Exception e) {
        log.warn("Error loading script "+name, e);
    }
    return script;
}

Now, this works fantastically when I start my program. I can load any script in the jar with ease.

The problem arises when I reload the script - ie change the file, and overwrite scripts.jar, then try to load any script. I get this error:

    Exception in thread "anjin_san:test" java.util.ServiceConfigurationError: org.stork.script.Script: Error reading configuration file
        at java.util.ServiceLoader.fail(ServiceLoader.java:207)
        at java.util.ServiceLoader.parse(ServiceLoader.java:284)
        at java.util.ServiceLoader.access$200(ServiceLoader.java:164)
        at java.util.ServiceLoader$LazyIterator.hasNext(ServiceLoader.java:332)
        at java.util.ServiceLoader$1.hasNext(ServiceLoader.java:415)
        at org.stork.script.Script.loadScript(Script.java:408)
        at org.stork.Client$2.run(Client.java:95)
        at java.lang.Thread.run(Thread.java:619)
Caused by: java.io.FileNotFoundException: JAR entry META-INF/services/org.stork.script.Script not found in C:\Users\stork\Documents\bot-sama\etc\scripts.jar
        at sun.net.www.protocol.jar.JarURLConnection.connect(JarURLConnection.java:122)
        at sun.net.www.protocol.jar.JarURLConnection.getInputStream(JarURLConnection.java:132)
        at java.net.URL.openStream(URL.java:1010)
        at java.util.ServiceLoader.parse(ServiceLoader.java:279)
        ... 6 more

So it says the jar entry can't be found, but I can check it's there fairly easily:

    C:\Users\stork\Documents\bot-sama\etc>jar tvf scripts.jar
     0 Thu Sep 03 16:58:00 BST 2009 META-INF/
   102 Thu Sep 03 16:57:58 BST 2009 META-INF/MANIFEST.MF
     0 Thu Sep 03 16:35:20 BST 2009 META-INF/services/
   482 Thu Sep 03 16:57:52 BST 2009 META-INF/services/org.stork.script.Script
   ...

And it's the right size, too. The problem happens when I try to load any script, not just the one that was modified.

I'm thinking this might be a bug, any input?

Answer 1

Check the file permissions. When the original JAR is placed there with the rest of the application, it is readable by the program. Perhaps its being updated by a different user—without granting read permissions to other users—and the program can no longer open the file.

I can't tell which version of Windows you are running, but the security properties dialog or, on XP Home, the cacls command should detail the permissions before and after.

---

Update: Re-reading the ServiceLoader documentation, I noticed an instance method, reload . Although the code creates a new ServiceLoader each time it's needed, it may be that the ServiceLoader class is keeping a cache that is used by all instances. Rather than creating new loaders, try keeping one around, and using the reload method to check for new scripts.

Answer 2

As a comment in another answer says, disabling the caching in the URLconnection worked for me, but in my solution I don't call the openConnection() method. Instead, I call following method:

private void disableURLConnectionCache() {

    new URLConnection(null) {
        @Override
        public void connect() throws IOException {}
    }.setDefaultUseCaches(false);
}