I am writing a Java applet that downloads images from a web server and displays them to the user. It works fine in Java 1.6.0_3 and later, but on older versions it will completely crash the process about once every 20 page views. There are no error messages in the Java console, because the process is completely frozen. I've waited for almost 15 minutes sometimes, but it never un-freezes.
I added a debug message after every line of code, and determined that the line that is causing the crash is this: InputStream data = urlConn.getInputStream().
urlConn is a URLConnection object that is pointed at the image I want to load. I've tried every combination of options that I can think of, but nothing helps. I haven't been able to find anything in the Java bug database or the release notes for 1.6.0_3.
Has anyone encountered this problem before? Any idea how to fix it?
To determine if it really is the whole JVM process that's frozen, or something else:
(1) get a java stack dump (sigquit/ctrl-break/jstack)
(2) have another background thread doing something you can observe; does it stop?
(3) check if another process (browser/etc) can contact server during freeze? (There's a chance the real problem is server connection depletion)
Is it randomly once-in-every-20-fetches (for example, 5% of the time, sometimes the first fetch in the JVM run), or always after about 20 fetches? If the latter, it sounds like something isn't being closed properly.
If on Linux you can use 'netstat -t' or 'lsof' (with certain options or grepped to show only some lines) to see open sockets; if after each fetch, one more is open, and the count never goes down, you're not closing things properly.
If so, calling close() on the stream you get back and/or disconnect() on the HttpUrlConnection after each try may help. (There may also be more severe limits on the number of connections an applet can leave open, so you're hitting this more quickly than you would in a standalone app.)
The fact that it 'works' in later Javas is also suggestive that some sort of automatic cleanup might be happening more effectively/regularly by finalization/GC. It's best to close things up cleanly yourself but you could also try forcing a GC/runFinalization in the earlier Javas showing the problem.
I'm unsure the cause of the problem you are facing, but I use the following code successfully for synchronously loading images from within applets (loads from either jar file or the server):
public Image loadImage(String imageName) {
// get the image
Image image = getImage(getCodeBase(), imageName);
// wait for it to fully load
MediaTracker tracker = new MediaTracker(this);
tracker.addImage(image, 0);
boolean interrupted = false;
try {
tracker.waitForID(0);
} catch (InterruptedException e) {
interrupted = true;
}
int status = tracker.statusID(thisImageTrackerID, false);
if (status != MediaTracker.COMPLETE) {
throw new RuntimeException("Failed to load " + imageName + ", interrupted:" + interrupted + ", status:" + status);
}
return image;
}
Related
I am trying to read a message from RFID reader connected via USB to windows 10pro machine with usb4java library.
I have managed to claim the interface, opened pipe and registered listener for the data, however the listener is never triggered. The reader acts as keyboard and whatever it reads ends up in active application, such as IDE i have open, instead of in listener.
UsbInterface usbInterface = activeInteface(device);
// there is only one endpoint in the list
UsbEndpoint endpoint = (UsbEndpoint)usbInterface.getUsbEndpoints().get(0);
UsbPipe pipe = endpoint.getUsbPipe();
try {
usbInterface.claim();
// true
System.out.println("claimed usb interface: " + usbInterface.isClaimed());
pipe.open();
// true
System.out.println("pipe open: " + pipe.isOpen());
pipe.addUsbPipeListener(new MessageListener());
// true
System.out.println("pipe active: " + pipe.isActive());
// keep main thread alive, async call should be done from another thread i guess
Thread.sleep(15000);
}
catch (Exception any) {System.out.println(any);}
}
And the listener:
private static class MessageListener implements UsbPipeListener {
#Override
public void errorEventOccurred(UsbPipeErrorEvent event) {
System.out.println(event.toString() + " , " +event.getUsbException());
}
#Override
public void dataEventOccurred(UsbPipeDataEvent event) {
// this code block never triggers
System.out.println("listener ...);
int actualLength = event.getActualLength();
System.out.println("length: " + actualLength);
byte[] data = event.getData();
System.out.println("data length " + data.length);
}
}
i have also tried synchronous read instead of asynchronous in the block above, like this:
byte[] buffer = new data[8];
// this fails on its own, don't even need to read something with RFID reader
int received = pipe.syncSubmit(buffer);
fails with:
USB error1: Transfer error on interrupt endpoint: Input/Output error
There is some windows specific property that library supports: org.usb4java.javax.useUSBDK = true
but this fails when i try to set it with an exception.
I have 0 experience with USB devices so not sure how to proceed from here. Is there something wrong with the code, do i need USBDK or device does not support libUSB driver ? Sadly this is not my device and i don't have access to documentation of the device so cannot be sure if it is device driver issue.
I know that this is 2 years old, but i've had similar issue and this was one of the first questions that i ran into looking for solution, which took me hours.
So, basically, windows doesn't let to read/write keyboard devices directly, to do so, you have to override it's driver (That's why you're getting Input/Output error, and it's written in the hid4java's FAQ).
First way to override device driver is described in libusb wiki.
As far as i know you would have to install a new driver every time you connect the device to a new USB port, which is why i recommend you to read further.
Second way is what you've already mentioned, which is using UsbDk (Usb Drivers Development Kit for Windows). It makes the device accessible for you by detaching the kernel driver and reattaching it back after you're done playing with it.
In order to use it, you need to do two things:
Set the org.usb4java.javax.useUSBDK = true in you javax.usb.properties file as stated in the manual (this can also be done manually in low-level usb4java, see OPTION_USE_USBDK and setOption(Context, int)).
Download and install UsbDk on your system (simplest way is to download x64 or x86 version msi installer which has GUI and is fully automated), which is sadly not in the manual (maybe it's obvious for some people, but took me amount of time that i am not proud of to realize).
Im guessing that the lack of second step is why OP has been getting an exception.
Hope that this will help someone, knowing all this two days ago would save me a lot of headache.
RFID readers operate in keyboard emulation mode by default.
You can normally get a tool from the manufacturer's website to configure the RFID reader.
This will allow you to change the reader to HID mode.
This should resolve your issues.
Sorry for the late response but I hope it helps others.
Let me get straight to an example to explain further.
final var socket = new java.net.ServerSocket(1234);
for (;;)
{
try (final var client = socket.accept())
{
client.getOutputStream().write("HTTP/1.1 200 OK\r\n\r\n".concat(java.time.Instant.now().toString()).getBytes());
}
}
When I now open my browser of choice (Firefox cough) I'll receive the current time and date. The question now is how I can write to that socket at a later point in time.
hypothetical solution
Here's something I already tried, but doesn't work at all.
final var socket = new java.net.ServerSocket(1234);
for (;;)
{
try (final var client = socket.accept())
{
client.getOutputStream().write("HTTP/1.1 200 OK\r\n\r\n".concat(java.time.Instant.now().toString()).getBytes());
client.getOutputStream().flush();
Thread.sleep(1000L);
client.getOutputStream().write("And another paragraph.".getBytes());
}
}
The result is a web page loading for approximately a single second, printing out the following result (may vary due to different date and time on your end).
2019-01-19T18:19:15.607192500Z
And another paragraph.
Instead I would like the see something like that:
print out the current time and date.
wait a second without the content of the web page changing.
print out the next paragraph.
How would I go about implementing that?
Is it possible for the server to write text into a web page after it is loaded? Yes it definitely is, but these days I suspect it it is rarely done. I started web development in the 1990s and back then that was a pretty common technique. We used it to write live chat messages to browsers with no Javascript. These days Javascript is ubiquitous and powerful, so using client-side Javascript to update a page will be the best option in most cases.
That said, the technologies we used for writing server-side updates back then should still work now. I suspect the reason you don't see updates in your browser is because it doesn't know it should start displaying the page before everything is loaded. Using chunked transfer encoding, a 1990s technology still supported by modern browsers, should resolve that. It it allows the server to indicate when a 'chunk' of data is complete and browsers will generally process each chunk immediately rather than wait for all the chunks to arrive.
The easiest way to use chunked transfer encoding is to use an HTTP library like Apache HttpComponents, then wrap your output stream in the appropriate class:
final var socket = new java.net.ServerSocket(1234);
for (;;)
{
try (final var client = socket.accept())
{
var outputStream = new ChunkedOutputStream(client.getOutputStream());
outputStream.write("HTTP/1.1 200 OK\r\n\r\n".concat(java.time.Instant.now().toString()).getBytes());
outputStream.flush();
Thread.sleep(1000L);
outputStream.write("And another paragraph.".getBytes());
}
}
I'm using jsoup in my android app but the problem is, the html source takes too much time to download. Here is my code:
long t = System.currentTimeMillis();
String url = "http://www.stackoverflow.com/";
Document doc = null;
try {
Connection c = Jsoup.connect(url);
doc = c.get();
System.out.println(System.currentTimeMillis() - t);
} catch (IOException e) {
e.printStackTrace();
}
Executing this code takes 1.265 seconds which feels really weird because i can download the whole website (with images and all that good stuff) using web browser in less than a 0.5 seconds on the same device. Did I do something wrong? Or maybe there is a faster way for getting html source of website? Thanks in advance.
Where are you trying this code on? Your device? If you are using the LTE/3G network it wouldn't be too much off.
The other reason that I could think is that your wireless router is not situated in the best place from your device in case you are using Wifi.
From that code I don't see anything that could cause more delay. 1.2 secs may not be that bad if you dont have the host DNS entry cached and the server is far away from you.
Also, try setting the Agent to the same as your browser when comparing times. It may happen that the server gives different priorities based on the user agent. In this case you are using the default Java user agent.
I am working on a relatively simple packet capture application, and I am using the Jpcap library. Everything has been working until I added in the save feature to my program.
My write function
public void write() {
try {
writer = JpcapWriter.openDumpFile(captor, fileName);
} catch (IOException e) {e.printStackTrace();}
for (Packet packet : this.packets) {
writer.writePacket(packet);
}
writer.close();
}
It correctly gets the captor and filepath, loops through all the packets successfully, but when it tries to write at the end of the code block, the JVM crashes.
My question is, why does my application crash when trying to close the JpcapWriter?
UPDATE: The weird thing I just discovered, is it IS actually writing to the file. It just crashes after the write. I added a print statement after the close, and it never reaches it.
I found several other people with the same issue as me. I'm not sure why but removing the call to close the writer fixed my problem. The file now writes correctly, and has no issues. For now, I am content with it working, but I may come back to this issue at a later date.
UPDATE: It turns out the the file closes when a call is made to stop the capture. When the capture thread is closed it closes the captor, which in turn closes the writer for me. It essence, I was trying to tell it to close the writer, as the writer was already closing, which caused the JVM to crash.
I am really struggling with this issue as it seems to occur randomly for me. When I call,
Desktop.browse("some url");
Internet Explorer will not display. The exception message is as follows,
The requested lookup key was not found in any active activation context.
When it occurs it occurs consistently until I restart the machine, but it eventually occurs again.
The workstations that seem to have this problem are running Windows XP with Internet Explorer 8 set as the default browser.
EDIT: I forgot to mention that if I open up Internet Explorer directly and navigate to the URL in question then it will work fine.
EDIT2: This seems to happen if Desktop.browse is invoked and then is called again at least 15 minutes later. Restarting the application now seems to fix the problem.
I narrowed down the problem and discovered what was TRULY causing this, it had nothing to do with the time after all.
java.awt.Desktop.browse("some url"); was throwing this error because in a previous step in the application an ActiveXObject was opened programmatically using the JACOB framework.
The developer that wrote this code using this ActiveXObject neglected to bother releasing his resources at all. For some reason, this ActiveXObject in memory was preventing or screwing with the Dispatch call to the default OS browser in java.awt.Desktop class. I suppose this makes sense.
I fixed this by declaring a JACOB transaction, and by releasing all resources in a finally block like so:
ActiveXObject ao1 = null;
ActiveXObject ao2 = null;
ComThread.initMTA();
try {
ao1 = new ActiveXObject("blaa.blaa");
ao2 = new ActiveXObject("haa.haa");
// business logic
} finally {
if (ao1 != null) {
ao1.safeRelease();
ao1 = null;
}
if (ao2 != null) {
ao2.safeRelease();
ao2 = null;
}
ComThread.Release();
}