getClassLoader().getResource(filepath) returns a n

问题:

I'm using a method to generate XML files dynamically for a research project, they get put into a loader that reads from a file path, I don't have any control over how the loader handles things (otherwise I'd pass the internal XML representation instead of monkeying with temp files), I'm using this code to save the file:

File outputs = File.createTempFile("lvlFile", ".tmp.xml");

FileWriter fw = new FileWriter(outputs);

fw.write(el.asXML());

fw.close();

// filenames is my list of file paths which gets returned and passed around

filenames.add(outputs.getAbsolutePath());

Now, I'm sure that the file in question is written to directly. If I print outputs.getAbsolutePath() and navigate there via terminal to check the files, everything is generated and written properly, so everything is correct on the filesystem. However, this code:

URL url = this.getClass().getClassLoader().getResource(_levelFile);

Where _levelFile is one of my filenames generated above, causes url to be null. The path isn't getting corrupted or anything, printing verifies that _levelFile points to the correct path. The same code has succeeded for other files. Further, the bug doesn't seem related to whether or not I use getPath(), getCanonicalPath(), or getAbsolutePath(), further setting outputs.isReadable(true) doesn't do anything.

Any ideas? Please don't offer alternatives to the Url url = structure, I don't have any control over this code*, I'm obligated to change my code so that the url is set correctly.

(*) At least without SIGNIFICANT effort rewriting a large section of the framework I'm working with, even though the current code succeeds in all other cases.

Edit:

Again, I can't use an alternative to the URL code, it's part of a loader that I can't touch. Also, the loading fails even if I set the path of the temp file to the same directory that my successfully loaded files come from.

回答1:

I assume that the ClassLoader will only look for resources within the class path - which probably doesn't include /tmp. I'm not sure if it actually supports absolute path names. It might just interpret them as relative to the root of the individual class path.

How about using _levelFile.toURI().toURL() instead?

回答2:

Your are creating file in file system and then trying to read it as a resource. Resource is where JVM takes its classes, i.e. the classpath. So this operation will work only if your are writing file into your classpath.

And even if this is correct be careful: if for example you are running from eclipse your process will not probably "see" the new resource until you refresh your workspace.

Now my question is: Are your really sure that you want to read files as resources. It seems that your just should create new FileInputStream(_levelFile) and read from it.

Edit

@Anonymouse is right. You are creating temporary file using 2-arg version of createTempFile(), so your file is created in your temporary directory. The chance that it is into your classpath is very low... :) So, if you want to read it then you have to get its path or just use it when creating your input stream:

File outputs = File.createTempFile("lvlFile", ".tmp.xml");
..........................
InputStream in = new FileInputStream(ouptuts);
// now read from this stream.