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How do I cleanly determine if a file is not of the type I want?

I am using Kryo to save binary files of user data. The user can open one of their files in my application. I'm not sure if I have a clean approach to detecting whether they tried to open a file of some other type.
Right now, I'm writing a simple FileHeader object to the file before the user's data. The file header has info about what version of the app saved the file.
public void write (UserProject project, File file) throws FileNotFoundException {
OutputStream outputStream = new DeflaterOutputStream(new FileOutputStream(file));
Output output = new Output(outputStream);
kryo.writeObject(output, new FileHeader());
kryo.writeObject(output, project);
output.close();
}
So when I load a file, I can try to deserialize the file header and the user project and catch any Exception that might occur. But doing a catch-all block could hide certain issues I could perhaps react to in a more elegant way that simply showing the user an error no matter the exception. Here's what I'm doing now:
public Project read (File file) throws FileNotFoundException, FileVersionException, UnreadableException {
InputStream inputStream = new InflaterInputStream(new FileInputStream(file));
Input input = new Input(inputStream);
try {
FileHeader fileHeader = kryo.readObject(input, FileHeader.class);
if (fileHeader.fileVersion > CURRENT_FILE_VERSION)
throw new FileVersionException(/* */);
Project project = kryo.readObject(input, Project.class);
return project;
} catch (Exception e){
if (DEBUG) e.printStackTrace();
throw new UnreadableException(e); //caller will show user error msg
} finally {
input.close();
}
}
I suppose there's also a very tiny (infinitesimal?) chance that some file actually loads without throwing an exception, in which case a very unexpected error could happen elsewhere in my application. Not sure if I should worry about this...a user should not expect to open an incorrect file type and have it work correctly.

You could use magic numbers, a set of bytes that describes the type of file. Like .jpg, .pdf, .wav, etc. all have a few bytes at the beginning of each file, so even if these types are saved with different extensions you can check to see if the file's magic number is OK.
Magic Number Description
However, if you're serializing and deserializing you may have to tack on some additional data to the file after serializing and remove it before deserializing.

Open file content in new tab in browser

I want to show the file content in new tab in browser. What i have done is this:
int BUFF_SIZE = 102400;
FileInputStream is = null;
byte[] buffer = new byte[BUFF_SIZE];
int a = -1;
try
{
is = new FileInputStream(file);
ByteArrayOutputStream out = new ByteArrayOutputStream();
while((a = is.read(buffer)) != -1)
{
out.write(buffer);
}
out.flush();
out.close();
ServletOutputStream os = null;
os = response.getOutputStream();
os.write(out.toByteArray());
os.close();
is.close();
}
catch(Exception e)
{
// Exception handling
}
But this is leading to download of the file instead of opening the file-content in new tab.
I am not able to find what i am doing wrong.
Any help would be great!!

Actually, all you should need to do now is add JQuery to your webpage, and use JQUery.get. Once you get the html from the servlet, use jquery or javascript to set the text in your tab.
BTW, you might want to set other details on the servlet output stream, like file type, length etc. Just a thought

You could also try this with the omnifaces library
Faces.sendFile(file, false);//true makes it as an attachment
more information on http://omnifaces.org/docs/javadoc/1.8/org/omnifaces/util/Faces.html#sendFile(java.io.File,%20boolean)

A web application might not even know what is a brower. It receives requests through HTTP protocol and send responses through same protocol. The protocol by itsels knows nothing about browsers and tabs.
You must use javascript for anything that happens at browser level. Other answers adviced you to use jQuery. It is a well known javascript library that hides differences between browsers, but there are others around (dojo, extJs, ...) : Google and make your choice.
By the way, if all you want is open an URL in a new tab, that's one of the very few operations that you can do at HTML level. Just look at this example
from W3Schools.com :
Visit W3Schools!
that opens www.w3schools.com in a new tab (if browser has tabs what is now common) or a new window.

Webapplication: Delete files during runtime

I am developing a webapp (for mobile phones). There is one xhtml page, where I want to show a picture, which is stored locally on my hard drive (for example: D:\pictures\test.jpg).
Since browsers block images when they are located on a local harddrive, I wrote a method in my javabean, where the picture, stored on the localHD, is copied to the webApp directory, when the user enters the xhtml page. After the user leaves the page, the copied file inside the webapp should be deleted.
So when I'm running my app, copying works perfectly and the pictures are displayed correctly. However, when the files should get deleted, I get this errormessage:
java.nio.file.FileSystemException: D:\WebAppPath\src\main\webapp\resources\pics\test.jpg:
The process cannot be accessed because the file is being used by another process.
Strangely enough, after stopping and restarting the application I can delete the same image if it is still in the webApp directory. (But Only once; after re-copying it, I get the error message again.)
Also if I want to delete the file manually, by using Windows explorer, I get the error message that the file can't be deleted because it is used by Java(TM) Platform SE Binary.
So to delete the file (manually or via the bean) I have to wait for a restart of the application, which of course is not an acceptable solution for the end user.
I'm using JSF2.0 with Primefaces and Primefaces Mobile components. My IDE is Netbeans and I use Spring Webflow framework to navigate and trigger actions/methods between the xhtml pages.
Here's the code for the copying method in my JavaBean:
public void copyFotoToLocalhost() {
if (fotoList.size() > 0) {
for (int i = 0; i < fotoList.size(); i++) {
Foto tempPic = fotoList.get(i);
String tempItemName = tempPic.getItemName();
String originalFile = "D:\\localFilepath\\" + tempItemName;
String tempFileName = "D:\\WebAppPath\\src\\main\\webapp\\resources\\pics\\" + tempItemName;
File existTest = new File(tempFileName);
if (existTest.exists() == false) {
try {
File orFile = new File(originalFile);
File tempFile = new File(tempFileName);
InputStream in = new FileInputStream(orFile);
OutputStream out = new FileOutputStream(tempFile);
byte[] buf = new byte[8192];
int len;
while ((len = in.read(buf)) > 0) {
out.write(buf, 0, len);
}
in.close();
out.close();
tempFile.setWritable(true);
System.out.println("File copied.");
} catch (FileNotFoundException ex) {
System.out.println(ex.getMessage() + " in the specified directory.");
System.exit(0);
} catch (IOException e) {
System.out.println(e.getMessage());
}
}
}
}
}
Here's the code for the delete method:
public void deleteFotos() {
if (fotoList.size() > 0) {
for (int i = 0; i < fotoList.size(); i++) {
Foto tempPic = fotoList.get(i);
String tempItemName = tempPic.getItemName();
Path tempLocation = Paths.get("D:\\webAppPath\\src\\main\\webapp\\resources\\pics\\" + tempItemName);
fotoList.remove(i);
i--;
try {
Files.deleteIfExists(tempLocation);
System.out.println("sucessfully deleted" + tempPic.getItemName());
} catch (IOException ex) {
Logger.getLogger(WundDokuBean.class.getName()).log(Level.SEVERE, null, ex);
System.out.println("Fail # " + tempPic.getItemName());
}
}
fotoList.clear();
}
Do you have an idea, how to fix this?
I hope you understand my problem, if not please tell me which information you need, I'll try to provide it.

There is one xhtml page, where I want to show a picture, which is stored locally on my hard drive (for example: D:\pictures\test.jpg). Since browsers block images when they are located on a local harddrive (...)
I want to clear out a conceptual misunderstanding first: You seem to expect that it would work fine when the browser wouldn't have blocked it. This is completely untrue. You seem to expect that images are inlined in the HTML output. No, they are downloaded individually and independently from the HTML page. If you had continued to use local disk file system paths, then it would have worked only and only if your webpage visitor has also exactly the same file at exactly the same location at their disk file system. In reality, this is obviously not the case. It would only work if both the webbrowser and webserver runs at physically the same machine.
Coming back to your concrete problem of being unable to delete the file, it's is caused because the servletcontainer usually locks the files in expanded WAR folder. I can't tell the exact reason, but that's not relevant here as this whole approach is wrong anyway. This approach would fail when the deployed WAR file is not expanded on disk file system, but instead in server's memory. Also, hardcoding environment-specific disk file system paths is a bad idea. You'd need to edit, rewrite, recompile, rebuild the whole WAR everytime you change the environment. In other words, your webapp is not portable.
You need to keep the files there where they originally are and make them publicly available by a real URL. This can be achieved in 2 general ways:
Add a virtual host to the server config, pointing to D:\localFilepath\. How to achieve that depends on the server used. You didn't tell anything about the server make/version used, but using Spring suggests that you're not being able to use full Java EE stack and are likely using a barebones JSP/Servlet container such as Tomcat. In that case, it's a matter of adding the following line to its /conf/server.xml:
<Context docBase="D:\localFilepath" path="/fotos" />
This way they are available by http://localhost:8080/fotos/*.
Create a servlet which reads files from D:\localFilepath and writes to HTTP response. With Servlet 3.0 and Java 7 it's really a breeze. Here's a kickoff example (nullchecks/file-exist-checks/doHead()/caching/resuming omitted for brevity):
#WebServlet("/fotos/*")
public class FotosServlet extends HttpServlet {
#Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletExcpetion, IOException {
File file = new File("D:/localFilepath", request.getPathInfo().substring(1));
response.setHeader("Content-Type", getServletContext().getMimeType(file.getName()));
response.setHeader("Content-Length", String.valueOf(file.length()));
Files.copy(file.toPath(), response.getOutputStream());
}
}
That's basically it. This way they're available on http://localhost:8080/contextname/fotos/*.

PDF export printing in Internet Explorer

protected static byte[] exportReportToPdf(JasperPrint jasperPrint)
throws JRException {
JRPdfExporter exporter = new JRPdfExporter();
ByteArrayOutputStream baos = new ByteArrayOutputStream();
exporter.setParameter(JRExporterParameter.JASPER_PRINT, jasperPrint);
exporter.setParameter(JRExporterParameter.OUTPUT_STREAM, baos);
exporter.setParameter(JRPdfExporterParameter.PDF_JAVASCRIPT,
"this.print({bUI: true,bSilent: false,bShrinkToFit: true});");
exporter.exportReport();
return baos.toByteArray();
}
We are using code like this to export a PDF document from a Jasper application.
The line
exporter.setParameter(JRPdfExporterParameter.PDF_JAVASCRIPT,
"this.print({bUI: true,bSilent: false,bShrinkToFit: true});");
adds JavaScript to send the PDF document directly to the printer.
The expected behavior is that a print dialog will come up with a preview of the PDF document.
This works fine most of the time - except I am having problems about one out of every 5-6 times in Internet Explorer 8 and Firefox.
What happens is - the print preview dialog with the PDF document does not appear or it appears with a blank document in the preview window.
-I've tried a number of different JavaScripts (different params to this.print() via exporter.setParameter
-I've tried setting different response headers such as
response.setContentType("application/pdf");
response.setHeader("Content-disposition","inline; filename=\""
+ reportName
+ "\"");
response.setContentLength(baos.size());
these did not seem to help
This seems to be an IE and FF issue. Has anyone ever dealt with this problem? I need to get it to work across all browsers 100% of the time. Perhaps a different approach to accomplish the goal of sending the PDF document export directly to the printer? or a third party library that will work across browsers?

Maybe it isn't getting a chance to update the UI. The following code delays the print perhaps giving it the chance it needs. I didn't test as I don't have your environment.
exporter.setParameter(JRPdfExporterParameter.PDF_JAVASCRIPT,
"app.setTimeOut('this.print({bUI: true,bSilent: false,bShrinkToFit: true});',200);")

Problem sending XML via HTTP

I want to have an application which parses various RSS feeds and send the information to a remote server. The information is sent in xml format via http. At first I tried to deploy this application on my own server, so I send the xml using the method shown in this tutorial by Java Tips. Here is my code which is replicated from the example:
First Method
String strURL = "http://localhost/readme/readme_xml";
String strXMLFilename = "output.xml";
File input = new File(strXMLFilename);
PostMethod post = new PostMethod(strURL);
post.setRequestEntity(new InputStreamRequestEntity(
new FileInputStream(input), input.length()));
post.setRequestHeader(
"Content-type", "text/xml; charset=ISO-8859-1");
HttpClient httpclient = new HttpClient();
try {
int result = httpclient.executeMethod(post);
System.out.println("Response status code: " + result);
System.out.println("Response body: ");
System.out.println(post.getResponseBodyAsString());
} finally {
post.releaseConnection();
}
This works perfectly (I even tested using a remote server outside the localhost). Then, somehow I cant use my own server to deploy this application, so I decided to migrate to Google Apps Engine. One thing about it, as we know it, is that not all libraries are allowed in the environment. So I try another method shown in ExampleDepot.com (I can't find where the exact url though) as below:
Second Method
try {
/* fill up this url with the remote server url */
URL url = new URL("http://localhost/readme/readme_xml");
FileReader fr = new FileReader("output.xml");
char[] buffer = new char[1024*10];
int len = 0;
if ((len = fr.read(buffer)) != -1){
/* send http request to remote server */
URLConnection conn = url.openConnection();
conn.setRequestProperty("Content-Type","text/xml;charset=ISO-8859-1"); /* need to specify the content type */
conn.setDoOutput(true);
conn.setDoOutput(true);
PrintWriter pw = new PrintWriter(conn.getOutputStream());
pw.write(buffer, 0, len);
pw.flush();
/* receive response from remote server*/
BufferedReader bf = new BufferedReader(new InputStreamReader(conn.getInputStream()));
String input = null;
while ((input = bf.readLine()) != null){
System.out.println(input);
}
}
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
The second method though, doesn't work and gives the following error (I use SimpleXMLElement (php) object to parse xml in the remote hosting):
Error message from remote server
Here's the php code from the remote server (In here, I just want the SimpleXMLElement to parse the xml without doing anything else fancy for now)
$xml = new SimpleXMLElement('php://input', NULL, TRUE);
foreach ($xml -> attributes() as $name => $val){
echo "[".$name."] = ".$val."\n";
}
I thought the cause of this problem is the malfunction xml file (because the eclipse IDE indicates there's error of "invalid byte 1 of 1-byte utf-8 sequence"). Then I use the same exact input xml file to the first method, but it still works perfectly.
So is there any adjustment that I need to make to the second method? Or is there any other method that I can use to send xml file to remote server? Let me know if I need to add some other details. Thanks for your help.
NOTE: I actually solved this problem by using the solution given in the comments. I didn't use approaches suggested in the answers, even though those answers are pretty useful. So, I didn't select the best answer out of those answers given. Nonetheless, I still appreciate all of your helps, thus deserve my upvote. Cheers!

I guess you need to change the content type to multipart/form-data. See an already answered question in detailed. The file upload is discussed at the bottom of this example

I would, as the first answer suggest, read the file with an InputStream. Converting from byte to char and back again is unnecessary and a source of error. Also, verify that the input file really is using the ISO-8859-1 encoding.
UPDATE:
When using a FileReader, you accept the default encoding (i.e. how to make chars from bytes). This encoding must match the encoding used for the input file, otherwise there's a great risk that the result is corrupted. The default Java encoding is different for different platforms, so it is generally not a good idea to rely on it.
In your second example, there's no reason to read the file as characters, since it will be sent on the wire as bytes anyway. Using byte streams all the way also avoids the encoding issue (apart from the information in the content-type header).

never read a file as chars unless you are reading a text file. xml is not text, it is a binary format. copy the file using normal InputStreams and byte[]s.
also, as #beny23 suggested in his comment, make sure you always copy streams using a loop, not a single read() (even if your buffer is big enough, it is not guaranteed that the InputStream will give you all the bytes in one call, even for a FileInputStream).