Question

Java: Create a File Object from URL (without downloading)

Asked by: noblends

Hi.  I need a function that does the following:

public static File getFileObjectFromURL(URL url) {
     f = new File();
     //get File Object from URL
     .
     .
     .
     return f;
}

This function is going to be used in an applet so there can be no writing of files or creating temporary files on a local machine.  I've trolled the internet for days on this one . . . good luck!

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Asked On
2008-04-10 at 08:44:06ID23312221
Tags

Java

Topics

Java Application Servers

,

WebApplications

,

Java Programming Language

Participating Experts
2
Points
500
Comments
30

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Answers

 

by: CEHJPosted on 2008-04-10 at 08:47:38ID: 21326091

URL.getFile

 

by: CEHJPosted on 2008-04-10 at 08:48:50ID: 21326102

 

by: noblendsPosted on 2008-04-10 at 08:49:53ID: 21326115

that returns a string - not a File Object

http://java.sun.com/j2se/1.4.2/docs/api/java/net/URL.html

 

by: ksivananthPosted on 2008-04-10 at 08:52:10ID: 21326148

File is a reference of the resource in the local file system and URL is a reference of a resource in remote or local system. So you will be able to create URL from File but the reverse is not possible unless you write the URL resource locally and then create File reference of it!

 

by: noblendsPosted on 2008-04-10 at 08:59:22ID: 21326235

Thanks for the clarification ksivanath.

So, if i have an *applet* running on www.whatever.com, trying to create a File object from www.whatever.com/images/thisImage.jpg - this isn't possible?

 

by: CEHJPosted on 2008-04-10 at 09:01:55ID: 21326258

>>that returns a string - not a File Object

Well only a String is meaningful. The file(File) resides elsewhere.

What is your objective?

 

by: ksivananthPosted on 2008-04-10 at 09:02:19ID: 21326263

nope, u need to write that locally and then refer it as File!

 

by: ksivananthPosted on 2008-04-10 at 09:02:56ID: 21326270

locally  --> local disk!

 

by: CEHJPosted on 2008-04-10 at 09:04:57ID: 21326298

>>trying to create a File object from ...

Why do you *want* to create a File object?

 

by: CEHJPosted on 2008-04-10 at 09:07:01ID: 21326321

Also

>>So, if i have an *applet* running on www.whatever.com

the applet won't be running there (probably). The applet runs on the machine of whomever's looking at it

 

by: noblendsPosted on 2008-04-10 at 09:10:33ID: 21326358

CEHJ:

Objective: Metaphile (http://sourceforge.net/projects/metaphile) is a Java package that basically reads metadata from image files.  It requires a File object.  I want to use it in a web-based environment, in an applet.  All images are "local" - local to the server where the applet was launched.  like i wrote earlier, if the applet is running on www.whatever.com, all images are in www.whatever.com/images.  I need to pass it a File object from within an applet . . .

 

by: noblendsPosted on 2008-04-10 at 09:13:17ID: 21326385

CEHJ:

(sorry, you're right, the applet isn't "running on" www.whatever.com - it was launched from there (or whatever the appropriate vocabulary is).  anyway, the project involves reading metadata, using metaphile, in an applet environment.)

 

by: CEHJPosted on 2008-04-10 at 09:17:18ID: 21326443

Reading metadata from a remote URL is a piece of cake if that's what you want to do...

 

by: CEHJPosted on 2008-04-10 at 09:18:54ID: 21326465

... and you can do that without using a file e.g.

InputStream metaDataIn = getClass().getResourceAsStream("metadata.xml");

                                              
1:

Select allOpen in new window

 

by: ksivananthPosted on 2008-04-10 at 09:26:24ID: 21326559

>>getClass().getResourceAsStream("metadata.xml");

CEHJ, I think, the asker is using a openesourced java called metafile which reads metadata from image file if you pass the image as File instasnce to them... in such case, this is not appropriate. your code will be useful if he has a file in the jar and want to read!

 

by: noblendsPosted on 2008-04-10 at 09:29:00ID: 21326590

the metadata is embedded in the image . . . does that make a difference?  i've attached a file if you'd like to try.  the XMP metadata is as follows:

- CopyrightNotice: 2006
- Keywords: book, Senior Library 2006, bird, orange
- Document Title: To Kill a Mockingbird
- Author: Richard Smith
- Description: To Kill a Mockingbird book cover

  • test.jpg
    • 51 KB

    JPG with embedded XMP metadata

    JPG with embedded XMP metadata
 

by: CEHJPosted on 2008-04-10 at 09:35:15ID: 21326674

Just get the stream as i mentioned and you'll be able to get the contents, including this sort of stuff:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
        <key>com.apple.print.PageFormat.PMHorizontalRes</key>
................................................

                                              
1:
2:
3:
4:
5:
6:

Select allOpen in new window

 

by: noblendsPosted on 2008-04-10 at 09:35:29ID: 21326680

if you dl the image and open it in photoshop you get something like the attached image.  getting at this data is no simple task, hence rather robust libraries like metaphile.  all images in www.whatever.com/images contain embedded metadata, which i need to access through an applet . . . and metaphile only takes File objects . . .

 

by: CEHJPosted on 2008-04-10 at 09:42:54ID: 21326765

>>and metaphile only takes File objects

Unfortunate and bad design. Do you have a link to this API?

 

by: noblendsPosted on 2008-04-10 at 09:47:16ID: 21326808

Sourceforge project: http://sourceforge.net/projects/metaphile

code examples: http://www.miniturismo.co.uk/metaphile/example01.php
     - also see (on the same site) example02.php, example03.php, example04.php, example05.php


 

by: CEHJPosted on 2008-04-10 at 10:15:07ID: 21327098

Since they haven't seen fit to give you an InputStream ctor, you'll have to create a temporary file. Web Start would be less strict than an applet.

If you can get hold of the source (can't see it there) you can fix it easily enough. Or get them to do it ;-)

 

by: ksivananthPosted on 2008-04-10 at 10:36:48ID: 21327350

whats the problem for you to write the file locally from applet?

 

by: noblendsPosted on 2008-04-10 at 10:47:38ID: 21327437

from http://java.sun.com/sfaq/#read

unless i'm missing something:

What are applets prevented from doing?
In general, applets loaded over the net are prevented from reading and writing files on the client file system, and from making network connections except to the originating host.

How do I let an applet write a file?
Applets loaded into a Java-enabled browser can't write files.

I suppose all this prevents me from getting access to a File object . . .

 

by: ksivananthPosted on 2008-04-10 at 10:52:00ID: 21327472

all you have to sign the applet jar files and the client user has to accept the certificate while running the applet!

here is how you sign the applet,

http://www.doc.ic.ac.uk/csg/java/signed-applets/

 

by: ksivananthPosted on 2008-04-10 at 10:55:02ID: 21327496

here is a complete glossary of signed applet...

http://mindprod.com/jgloss/signedapplets.html

 

by: noblendsPosted on 2008-04-10 at 10:57:28ID: 21327519

CEHJ:

i've managed to track down the source in a CVS repository off sourceforge.  here's the constructor i'd need to change - any ideas?

public JpegFile(File f) throws IOException, JpegException {
        try {
            this.file = f;

            fileLog.log(Level.INFO, "Creating JpegFile based on file " + f.getAbsolutePath());

            fileStream = new BufferedInputStream(new FileInputStream(file));
            this.loadHeader();
        } finally {
            if (fileStream != null) {
                fileStream.close();
            }
        }
    }

 

by: ksivananthPosted on 2008-04-10 at 11:00:17ID: 21327551

just add another constructor as below

public JpegFile(InputStream is) throws IOException, JpegException {
        try {

            fileStream = new BufferedInputStream( is );
            this.loadHeader();
        } finally {
            if (fileStream != null) {
                fileStream.close();
            }
        }
    }

 

by: ksivananthPosted on 2008-04-10 at 11:02:04ID: 21327571

and call,

new JpegFile( url.openStream() ) ;

 

by: CEHJPosted on 2008-04-10 at 11:04:32ID: 21327601

Yes, but put an appropriate loggin line in there too

 

by: noblendsPosted on 2008-04-11 at 14:49:51ID: 31447773

ksivananth - it took me a little while to reconfigure my java project to work off the modified source code, but thank you - your solution works perfectly.

awesome.

20120131-EE-VQP-002

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