How to create a new java.io.File in memory?

How can I create new File (from java.io) in memory, not on the hard disk?

I am using the Java language. I don't want to save the file on the hard drive.

I'm faced with a bad API (java.util.jar.JarFile). It's expecting File file of String filename. I have no file (only byte[] content) and can create temporary file, but it's not beautiful solution. I need to validate the digest of a signed jar.

byte[] content = getContent();
File tempFile = File.createTempFile("tmp", ".tmp");
FileOutputStream fos = new FileOutputStream(tempFile);
fos.write(archiveContent);
JarFile jarFile = new JarFile(tempFile);
Manifest manifest = jarFile.getManifest();

Any examples of how to achieve getting manifest without creating a temporary file would be appreciated.

Solution

It is not possible to create a java.io.File that holds its content in (Java heap) memory *.

Instead, normally you would use a stream. To write to a stream, in memory, use:

OutputStream out = new ByteArrayOutputStream();
out.write(...);

But unfortunately, a stream can't be used as input for java.util.jar.JarFile, which as you mention can only use a File or a String containing the path to a valid JAR file. I believe using a temporary file like you currently do is the only option, unless you want to use a different API.

If you are okay using a different API, there is conveniently a class in the same package, named JarInputStream you can use. Simply wrap your archiveContent array in a ByteArrayInputStream, to read the contents of the JAR and extract the manifest:

byte[] content = getContent(); // from OP's code

try (JarInputStream stream = new JarInputStream(new ByteArrayInputStream(content))) {
     Manifest manifest = stream.getManifest();
}

*) It's obviously possible to create a full file-system that resides in memory, like a RAM-disk, but that would still be "on disk" (and not in Java heap memory) as far as the Java process is concerned.