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JDK-18 foreign functions: How to work with multiple functions that have the same name, but come from different native libraries?

Let's say I loaded a native library gg3 that has a function get_gg(), and would like to load another native library gg4 that also has a get_gg() function - how can I look one up in a "library aware" manner (for example, use the get_gg() from gg4 on certain occasions, and otherwise from gg3)?
I tried CLinker.systemCLinker().lookup() but that seems to just fetch the first symbol that is found in the loaded native libraries.
After searching in the internal classes, I found SystemLookup.libLookup() which seemed promising but isn't public, so I implemented a similar method to lookup a loaded native lib:
private static SymbolLookup libLookup(Function<NativeLibraries, NativeLibrary> loader) {
NativeLibrary lib = loader.apply(NativeLibraries.rawNativeLibraries(SystemLookup.class, false));
return name -> {
Objects.requireNonNull(name);
try {
long addr = lib.lookup(name);
return addr == 0 ?
Optional.empty() :
Optional.of(NativeSymbol.ofAddress(name, MemoryAddress.ofLong(addr), ResourceScope.globalScope()));
} catch (NoSuchMethodException e) {
return Optional.empty();
}
};
}
However, this apparently is prohibited, because I cannot load the library from another class loader (and I don't know what class loader was used originally):
java.lang.UnsatisfiedLinkError: Native Library already loaded in another classloader
So how can I either find & use the appropriate class loader, or manage to find the library without re-loading it, so I can look for a symbol specifically from it?

FWIW, the API needed to do this was made public in 19
Before that, the advice was to manually implement this by binding dlopen, dlclose, and dlsym (or the Windows equivalents) to load the library and look up symbols.
e.g. on my Windows machine:
public class Main {
static final MethodHandle MH_LoadLibraryA;
static final MethodHandle MH_FreeLibrary;
static final MethodHandle MH_GetProcAddress;
static {
System.loadLibrary("Kernel32");
SymbolLookup lookup = SymbolLookup.loaderLookup();
CLinker linker = CLinker.systemCLinker();
MH_LoadLibraryA = linker.downcallHandle(
lookup.lookup("LoadLibraryA").orElseThrow(),
FunctionDescriptor.of(ADDRESS, ADDRESS));
MH_FreeLibrary = linker.downcallHandle(
lookup.lookup("FreeLibrary").orElseThrow(),
FunctionDescriptor.of(JAVA_INT, ADDRESS));
MH_GetProcAddress = linker.downcallHandle(
lookup.lookup("GetProcAddress").orElseThrow(),
FunctionDescriptor.of(ADDRESS, ADDRESS, ADDRESS));
}
static MemoryAddress LoadLibraryA(String name) {
try (ResourceScope scope = ResourceScope.newConfinedScope()) {
SegmentAllocator allocator = SegmentAllocator.nativeAllocator(scope);
return (MemoryAddress) MH_LoadLibraryA.invokeExact((Addressable) allocator.allocateUtf8String(name));
} catch (Throwable t) {
throw new RuntimeException(t);
}
}
static boolean FreeLibrary(MemoryAddress handle) {
try {
return ((int) MH_FreeLibrary.invokeExact((Addressable) handle)) != 0;
} catch (Throwable t) {
throw new RuntimeException(t);
}
}
static MemoryAddress GetProcAddress(MemoryAddress handle, String name) {
try (ResourceScope scope = ResourceScope.newConfinedScope()) {
SegmentAllocator allocator = SegmentAllocator.nativeAllocator(scope);
return (MemoryAddress) MH_GetProcAddress.invokeExact((Addressable) handle, (Addressable) allocator.allocateUtf8String(name));
} catch (Throwable t) {
throw new RuntimeException(t);
}
}
static SymbolLookup libraryLookup(String libraryName, ResourceScope scope) {
MemoryAddress handle = LoadLibraryA(libraryName);
if (handle == MemoryAddress.NULL) {
throw new IllegalArgumentException("Cannot find library: " + libraryName);
}
scope.addCloseAction(() -> FreeLibrary(handle));
return name -> {
var addr = GetProcAddress(handle, name);
return addr == MemoryAddress.NULL ?
Optional.empty() : Optional.of(NativeSymbol.ofAddress(name, addr, scope));
};
}
public static void main(String[] args) throws Throwable {
try (ResourceScope scope = ResourceScope.newConfinedScope()) {
SymbolLookup lookup = libraryLookup("gg", scope);
MethodHandle handle = CLinker.systemCLinker().downcallHandle(
lookup.lookup("get_gg").orElseThrow(),
FunctionDescriptor.of(JAVA_INT));
int x = (int) handle.invoke();
System.out.println(x);
}
}
}

Error in creating new workspace in Eclipse RCP application

Problem in creating a new workspace in an Eclipse RCP application.
When the RCP application is launched, a dialog is prompted to ask the workspace location.
When the location is given, then there is error saying "Could not launch the product because the specified workspace cannot be created.
The specified workspace directory is either invalid or read-only".
I have followed the code from IDEApplication.java from eclipse, but still I am facing same issue.
Application code:
#SuppressWarnings("restriction")
public class MyRcpApplication implements IApplication
{
private static final String METADATA_PROJECTS_PATH = "/.plugins/org.eclipse.core.resources/.projects";
private static final String METADATA_ROOT = ".metadata";
private static final String COMMAND_ARG = "--container";
private static final String SYSTEM_PROPERTY_EXIT_CODE = "eclipse.exitcode";
private static final String WORKSPACE_VERSION_KEY = "org.eclipse.core.runtime";
private static final String VERSION_FILENAME = "version.ini";
private static final String WORKSPACE_VERSION_VALUE = "1"; //$NON-NLS-1$
public static final String METADATA_FOLDER = ".metadata"; //$NON-NLS-1$
private Shell shell;
/*
* (non-Javadoc)
*
* #see org.eclipse.equinox.app.IApplication#start(org.eclipse.equinox.app.IApplicationContext)
*/
#Override
public Object start(final IApplicationContext context)
{
Display display = PlatformUI.createDisplay();
Shell shell = display.getActiveShell();
try
{
// for backward compatibility we need to clear the workspace before start also
cleanUpTheWorkSpace();
boolean instanceLocationCheck = checkInstanceLocation(shell, context.getArguments());
if (!instanceLocationCheck)
{
MessageDialog.openError(shell, IDEWorkbenchMessages.IDEApplication_workspaceInUseTitle,
"Could not launch the product because the associated workspace is currently in use by another My Application.");
return IApplication.EXIT_OK;
}
int returnCode = PlatformUI.createAndRunWorkbench(display, new MyApplicationWorkbenchAdvisor());
if (returnCode == PlatformUI.RETURN_RESTART)
{
// eclipse.exitcode system property may be set to re-launch
if (IApplication.EXIT_RELAUNCH.equals(Integer.getInteger(SYSTEM_PROPERTY_EXIT_CODE)))
{
return IApplication.EXIT_RELAUNCH;
}
return IApplication.EXIT_RESTART;
}
// if application return code is exit clean up the workspace
// cleanUpTheWorkSpace();
return IApplication.EXIT_OK;
}
finally
{
if (display != null)
{
display.dispose();
}
Location instanceLoc = Platform.getInstanceLocation();
if (instanceLoc != null)
{
instanceLoc.release();
}
}
}
#SuppressWarnings("rawtypes")
private boolean checkInstanceLocation(final Shell shell, final Map arguments)
{
Location instanceLoc = Platform.getInstanceLocation();
if (instanceLoc == null)
{
MessageDialog.openError(shell, "Workspace is Mandatory", "IDEs need a valid workspace.");
return false;
}
// -data "/valid/path", workspace already set
if (instanceLoc.isSet())
{
// make sure the meta data version is compatible (or the user has
// chosen to overwrite it).
try
{
// Used to check whether are we launching My application from development environment or not
if (isDevLaunchMode(arguments))
{
return true;
}
// Used to check instance location is locked or not
if (instanceLoc.isLocked())
{
return false;
}
// we failed to create the directory.
// Two possibilities:
// 1. directory is already in use
// 2. directory could not be created
File workspaceDirectory = new File(instanceLoc.getURL().getFile());
if (workspaceDirectory.exists())
{
if (isDevLaunchMode(arguments))
{
return true;
}
MessageDialog.openError(
shell,
"Workspace Cannot Be Locked",
"Could not launch the product because the associated workspace at '" + workspaceDirectory.getAbsolutePath()
+ "' is currently in use by another Eclipse application");
}
else
{
MessageDialog
.openError(
shell,
"Workspace Cannot Be Created",
"Could not launch the product because the specified workspace cannot be created. The specified workspace directory is either invalid or read-only.");
}
}
catch (IOException e)
{
MessageDialog.openError(shell, "Internal Error", e.getMessage());
}
}
else
{
try
{
// -data #noDefault or -data not specified, prompt and set
ChooseWorkspaceData launchData = new ChooseWorkspaceData(instanceLoc.getDefault());
boolean force = false;
while (true)
{
URL workspaceUrl = promptForWorkspace(shell, launchData, force);
if (workspaceUrl == null)
{
return false;
}
// if there is an error with the first selection, then force the
// dialog to open to give the user a chance to correct
force = true;
try
{
// the operation will fail if the url is not a valid
// instance data area, so other checking is unneeded
if (instanceLoc.set(workspaceUrl, false))
{
launchData.writePersistedData();
writeWorkspaceVersion(workspaceUrl);
return true;
}
}
catch (IllegalStateException e)
{
MessageDialog
.openError(
shell,
IDEWorkbenchMessages
.IDEApplication_workspaceCannotBeSetTitle,
IDEWorkbenchMessages
.IDEApplication_workspaceCannotBeSetMessage);
return false;
}
// by this point it has been determined that the workspace is
// already in use -- force the user to choose again
MessageDialog.openError(shell, IDEWorkbenchMessages
.IDEApplication_workspaceInUseTitle,
IDEWorkbenchMessages.
IDEApplication_workspaceInUseMessage);
}
}
catch (IllegalStateException | IOException e)
{
}
}
return true;
}
private static void writeWorkspaceVersion(final URL defaultValue)
{
Location instanceLoc = Platform.getInstanceLocation();
if (instanceLoc.isReadOnly())
{
// MessageDialog.openError(shell,"Read-Only Dialog", "Location was read-only");
System.out.println("Instance Got Locked......");
}
if ((instanceLoc == null) || instanceLoc.isReadOnly())
{
return;
}
File versionFile = getVersionFile(instanceLoc.getURL(), true);
if (versionFile == null)
{
return;
}
OutputStream output = null;
try
{
String versionLine = WORKSPACE_VERSION_KEY + '=' + WORKSPACE_VERSION_VALUE;
output = new FileOutputStream(versionFile);
output.write(versionLine.getBytes("UTF-8")); //$NON-NLS-1$
}
catch (IOException e)
{
IDEWorkbenchPlugin.log("Could not write version file", //$NON-NLS-1$
StatusUtil.newStatus(IStatus.ERROR, e.getMessage(), e));
}
finally
{
try
{
if (output != null)
{
output.close();
}
}
catch (IOException e)
{
// do nothing
}
}
}
/*
* (non-Javadoc)
*
* #see org.eclipse.equinox.app.IApplication#stop()
*/
#Override
public void stop()
{
if (!PlatformUI.isWorkbenchRunning())
{
return;
}
final IWorkbench workbench = PlatformUI.getWorkbench();
final Display display = workbench.getDisplay();
display.syncExec(new Runnable()
{
#Override
public void run()
{
if (!display.isDisposed())
{
workbench.close();
}
}
});
}
private URL promptForWorkspace(final Shell shell, final ChooseWorkspaceData launchData, boolean force)
{
URL url = null;
do
{
new ChooseWorkspaceDialog(shell, launchData, false, force).prompt(force);
String instancePath = launchData.getSelection();
if (instancePath == null)
{
return null;
}
// the dialog is not forced on the first iteration, but is on every
// subsequent one -- if there was an error then the user needs to be
// allowed to
force = true;
// create the workspace if it does not already exist
File workspace = new File(instancePath);
if (!workspace.exists())
{
workspace.mkdir();
}
try
{
// Don't use File.toURL() since it adds a leading slash that Platform does not
// handle properly. See bug 54081 for more details.
String path = workspace.getAbsolutePath().replace(File.separatorChar, '/');
url = new URL("file", null, path); //$NON-NLS-1$
}
catch (MalformedURLException e)
{
MessageDialog
.openError(
shell,
IDEWorkbenchMessages
.IDEApplication_workspaceInvalidTitle,
IDEWorkbenchMessages
.IDEApplication_workspaceInvalidMessage);
continue;
}
}
while (!checkValidWorkspace(shell, url));
return url;
}
private boolean checkValidWorkspace(final Shell shell, final URL url)
{
String version = readWorkspaceVersion(url);
// if the version could not be read, then there is not any existing
// workspace data to trample, e.g., perhaps its a new directory that
// is just starting to be used as a workspace
if (version == null)
{
return true;
}
final int ide_version = Integer.parseInt(WORKSPACE_VERSION_VALUE);
int workspace_version = Integer.parseInt(version);
// equality test is required since any version difference (newer
// or older) may result in data being trampled
if (workspace_version == ide_version)
{
return true;
}
// At this point workspace has been detected to be from a version
// other than the current ide version -- find out if the user wants
// to use it anyhow.
String title = "My App Titile"; //$NON-NLS-1$
String message = "My App Message";
MessageBox mbox = new MessageBox(shell, SWT.OK | SWT.CANCEL
| SWT.ICON_WARNING | SWT.APPLICATION_MODAL);
mbox.setText(title);
mbox.setMessage(message);
return mbox.open() == SWT.OK;
}
private static String readWorkspaceVersion(final URL workspace)
{
File versionFile = getVersionFile(workspace, false);
if ((versionFile == null) || !versionFile.exists())
{
return null;
}
try
{
// Although the version file is not spec'ed to be a Java properties
// file, it happens to follow the same format currently, so using
// Properties to read it is convenient.
Properties props = new Properties();
FileInputStream is = new FileInputStream(versionFile);
try
{
props.load(is);
}
finally
{
is.close();
}
return props.getProperty(WORKSPACE_VERSION_KEY);
}
catch (IOException e)
{
IDEWorkbenchPlugin.log("Could not read version file", new Status( //$NON-NLS-1$
IStatus.ERROR, IDEWorkbenchPlugin.IDE_WORKBENCH,
IStatus.ERROR,
e.getMessage() == null ? "" : e.getMessage(), //$NON-NLS-1$,
e));
return null;
}
}
private static File getVersionFile(final URL workspaceUrl, final boolean create)
{
if (workspaceUrl == null)
{
return null;
}
try
{
// make sure the directory exists
File metaDir = new File(workspaceUrl.getPath(), METADATA_FOLDER);
if (!metaDir.exists() && (!create || !metaDir.mkdir()))
{
return null;
}
// make sure the file exists
File versionFile = new File(metaDir, VERSION_FILENAME);
if (!versionFile.exists()
&& (!create || !versionFile.createNewFile()))
{
return null;
}
return versionFile;
}
catch (IOException e)
{
// cannot log because instance area has not been set
return null;
}
}
#SuppressWarnings("rawtypes")
private static boolean isDevLaunchMode(final Map args)
{
// see org.eclipse.pde.internal.core.PluginPathFinder.isDevLaunchMode()
if (Boolean.getBoolean("eclipse.pde.launch"))
{
return true;
}
return args.containsKey("-pdelaunch"); //$NON-NLS-1$
}
/**
* Deletes all the available projects in the workspace
*/
private void cleanUpTheWorkSpace()
{
// this will be the
String[] commands = Platform.getCommandLineArgs();
if (commands != null)
{
List<String> args = Arrays.asList(commands);
if (args.contains(COMMAND_ARG))
{
// if project is in the root delete it.. it will delete associated metadata
IProject[] projects = ResourcesPlugin.getWorkspace().getRoot().getProjects();
if (projects != null)
{
for (IProject project : projects)
{
try
{
project.delete(true, new NullProgressMonitor());
}
catch (CoreException e)
{
// msgHandler.post(MsgSeverity.ERROR, "Unable to clear the workspace");
}
}
}
// if project is not in the root but if its in the workspace delete the metadata too
File[] workSpaceFiles = Platform.getLocation().toFile().listFiles();
for (File file : workSpaceFiles)
{
if (METADATA_ROOT.equals(file.getName()))
{
File projectMeta = new File(file.getPath() + METADATA_PROJECTS_PATH);
if ((projectMeta != null) && projectMeta.exists())
{
File[] children = projectMeta.listFiles();
for (File child : children)
{
FileUtils.deleteQuietly(child);
}
}
}
/*
* else { FileUtils.deleteQuietly(file); }
*/
}
}
}
}
}

Try run Eclipse as administrator.

Your code is just calling
if (instanceLoc.isLocked())
{
return false;
}
to check if the workspace is locked, but is doing nothing to make the workspace locked so this will always fall through to the error code.
IDEApplication does this:
if (instanceLoc.lock()) {
writeWorkspaceVersion();
return null;
}
you need to do something similar.

Find the directory for a FileStore

I'm trying to find a way to detect when a flash drive has been plugged into my computer. So far, the solution I found was to poll FileSystem#getFileStores for changes. This does indeed tell me when the flash drive has been inserted, but as far as I can tell there is no way to retrieve the location for it. FileStore#type and FileStore#name both seem highly unreliable as their return value is implementation specific, but they appear to be the only methods that might return any relevant information that might help find the directory for the FileStore.
With that in mind, the following code:
public class Test {
public static void main(String[] args) throws IOException {
for (FileStore store : FileSystems.getDefault().getFileStores()) {
System.out.println(store);
System.out.println("\t" + store.name());
System.out.println("\t" + store.type());
System.out.println();
}
}
}
Gave me this output:
/ (/dev/sda5)
/dev/sda5
ext4
/* snip */
/media/TI103426W0D (/dev/sda2)
/dev/sda2
fuseblk
/media/flashdrive (/dev/sdb1)
/dev/sdb1
vfat
As it turns out, FileStore#type returns the format of the drive and FileStore#name returns the location of the device file for the drive. As far as I can tell, the only method which has the location of the drive is the toString method, but extracting the path name out of it seems dangerous because I'm not sure how well that particular solution would hold up on other operating systems and future versions of Java.
Is there something I'm missing here or is this simply not possible purely with Java?
System Information:
$ java -version
java version "1.7.0_03"
OpenJDK Runtime Environment (IcedTea7 2.1.1pre) (7~u3-2.1.1~pre1-1ubuntu2)
OpenJDK Client VM (build 22.0-b10, mixed mode, sharing)
$ uname -a
Linux jeffrey-pc 3.2.0-24-generic-pae #37-Ubuntu SMP Wed Apr 25 10:47:59 UTC 2012 i686 athlon i386 GNU/Linux

Here's a temporary work around until a better solution is found:
public Path getRootPath(FileStore fs) throws IOException {
Path media = Paths.get("/media");
if (media.isAbsolute() && Files.exists(media)) { // Linux
try (DirectoryStream<Path> stream = Files.newDirectoryStream(media)) {
for (Path p : stream) {
if (Files.getFileStore(p).equals(fs)) {
return p;
}
}
}
} else { // Windows
IOException ex = null;
for (Path p : FileSystems.getDefault().getRootDirectories()) {
try {
if (Files.getFileStore(p).equals(fs)) {
return p;
}
} catch (IOException e) {
ex = e;
}
}
if (ex != null) {
throw ex;
}
}
return null;
}
As far as I know, this solution will only work for Windows and Linux systems.
You have to catch the IOException in the Windows loop because if there is no CD in the CD drive an exception is thrown when you try to retrieve the FileStore for it. This might happen before you iterate over every root.

This is what I have ended up doing. This is limited to Windows + UNIX but avoids using external tools or additional library calls. It steals the information Java already has in the FileStore objects
LinuxFileStore definitely extends UnixFileStore, so it will work. Same deal for Solaris. Since Mac OS X is UNIX, it probably works there but I'm not sure because I couldn't see its subclass in any place I was looking.
public class FileStoreHacks {
/**
* Stores the known hacks.
*/
private static final Map<Class<? extends FileStore>, Hacks> hacksMap;
static {
ImmutableMap.Builder<Class<? extends FileStore>, Hacks> builder =
ImmutableMap.builder();
try {
Class<? extends FileStore> fileStoreClass =
Class.forName("sun.nio.fs.WindowsFileStore")
.asSubclass(FileStore.class);
builder.put(fileStoreClass, new WindowsFileStoreHacks(fileStoreClass));
} catch (ClassNotFoundException e) {
// Probably not running on Windows.
}
try {
Class<? extends FileStore> fileStoreClass =
Class.forName("sun.nio.fs.UnixFileStore")
.asSubclass(FileStore.class);
builder.put(fileStoreClass, new UnixFileStoreHacks(fileStoreClass));
} catch (ClassNotFoundException e) {
// Probably not running on UNIX.
}
hacksMap = builder.build();
}
private FileStoreHacks() {
}
/**
* Gets the path from a file store. For some reason, NIO2 only has a method
* to go in the other direction.
*
* #param store the file store.
* #return the path.
*/
public static Path getPath(FileStore store) {
Hacks hacks = hacksMap.get(store.getClass());
if (hacks == null) {
return null;
} else {
return hacks.getPath(store);
}
}
private static interface Hacks {
Path getPath(FileStore store);
}
private static class WindowsFileStoreHacks implements Hacks {
private final Field field;
public WindowsFileStoreHacks(Class<?> fileStoreClass) {
try {
field = fileStoreClass.getDeclaredField("root");
field.setAccessible(true);
} catch (NoSuchFieldException e) {
throw new IllegalStateException("file field not found", e);
}
}
#Override
public Path getPath(FileStore store) {
try {
String root = (String) field.get(store);
return FileSystems.getDefault().getPath(root);
} catch (IllegalAccessException e) {
throw new IllegalStateException("Denied access", e);
}
}
}
private static class UnixFileStoreHacks implements Hacks {
private final Field field;
private UnixFileStoreHacks(Class<?> fileStoreClass) {
try {
field = fileStoreClass.getDeclaredField("file");
field.setAccessible(true);
} catch (NoSuchFieldException e) {
throw new IllegalStateException("file field not found", e);
}
}
#Override
public Path getPath(FileStore store) {
try {
return (Path) field.get(store);
} catch (IllegalAccessException e) {
throw new IllegalStateException("Denied access", e);
}
}
}
}

I've not really explored this area of java, but I found this, which seems to be related. It uses File.listRoots()
There also seems to be a number of related questions linked there too.

This works for Windows:
public Path getFileStoreRootPath(FileStore fs) throws Exception {
for (Path root : FileSystems.getDefault().getRootDirectories()) {
if (Files.isDirectory(root) && Files.getFileStore(root).equals(fs)) {
return root;
}
}
throw new RuntimeException("Root directory for filestore " + fs + " not found");
}
Basically, by filtering by condition Files.isDirectory(root) we are excluding all CD/DVD drives which will throw IOException when compact-disc is not inserted.

getSnapshot not supported on Blackberry

I'm having problem when taking a picture using VideoControl.getSnapshot() method. It always throw the exception: getSnapshot not Supported. I'm using JRE 5.0.0 with Eclipse and BlackBerry® Java® SDK 5.0 Plugin.
What I do first is to list the encoding supported by Blackberry SmartPhone selected (bold 9700) with the command System.getProperty("video.snapshot.encodings") and select one encoding from the list and pass it as the getSnapshot argument.
I've tested on several Blackberry and the same exception is thrown.
Part of the code:
mPlayer = Manager.createPlayer("capture://video?encoding=video/3gpp");
mPlayer.realize();
mPlayer = Manager.createPlayer("capture://video?encoding=video/3gpp");
mPlayer.start();
videoControl = (VideoControl)mPlayer.getControl("VideoControl");
Field cameraView = (Field) videoControl.initDisplayMode(VideoControl.USE_GUI_PRIMITIVE, "net.rim.device.api.ui.Field");
Thread.sleep(1000);
UiApplication.getUiApplication().pushScreen(new TempScreen(cameraView));
byte[] snapShot = videoControl.getSnapshot("encoding=jpeg&width=480&height=360&quality=superfine");
Bitmap image = Bitmap.createBitmapFromBytes(snapShot, 0, snapShot.length, 1);
UiApplication.getUiApplication().pushScreen(new TempScreen(image));
}catch (MediaException e){
UiApplication.getUiApplication().pushScreen(new TempScreen("Exception: " + e.getMessage())); }
catch (IOException e){
UiApplication.getUiApplication().pushScreen(new TempScreen("IO Exception: " + e.getMessage()));
}
catch (InterruptedException e){UiApplication.getUiApplication().pushScreen(new TempScreen("Interrupted Exception: "+ e.getMessage()));}

Not sure is my answer is actual after more than a half of year, but may be it will be useful.
You may try to use Thread.sleep(1000); before getSnapshot() call.
The problem may be related with that fact: "viewfinder must actually be visible on the screen prior to calling getSnapShot()."
So if you call getSnapshot immediately after UiApplication.getUiApplication().pushScreen(new TempScreen(cameraView));
the camera isn't prepared for the next shot.
Also are you really sure that getSnapshot() API is supported exactly on your device? Some manufacturers may not support it, despite the API defines this method. Did you run System.getProperty("video.snapshot.encodings") exactly on the same device where you test getSnapshot()?

Player _p;
VideoControl _vc ;
RecordControl _rc ;
String PATH;
FileConnection fileconn;
Object canvas= new Object();
public static boolean SdcardAvailabulity() {
String root = null;
Enumeration e = FileSystemRegistry.listRoots();
while (e.hasMoreElements()) {
root = (String) e.nextElement();
if( root.equalsIgnoreCase("sdcard/") ) {
return true;
}else if( root.equalsIgnoreCase("store/") ) {
return false;
}
}
class MySDListener implements FileSystemListener {
public void rootChanged(int state, String rootName) {
if( state == ROOT_ADDED ) {
if( rootName.equalsIgnoreCase("sdcard/") ) {
}
} else if( state == ROOT_REMOVED ) {
}
}
}
return true;
}
protected boolean invokeAction(int action){
boolean handled = super.invokeAction(action);
if(SdcardAvailabulity()){
PATH = System.getProperty("fileconn.dir.memorycard.videos")+"Video_"+System.currentTimeMillis()+".3gpp";//here "str" having the current Date and Time;
} else {
PATH = System.getProperty("fileconn.dir.videos")+"Video_"+System.currentTimeMillis()+".3gpp";
}
if(!handled){
if(action == ACTION_INVOKE){
try{
if(_p!=null)
_p.close();
}catch(Exception e){
}
}
}
return handled;
}
public MyScreen(){
setTitle("Video recording demo");
ButtonField AddPhoto = new ButtonField("push",ButtonField.FOCUSABLE | ButtonField.FIELD_HCENTER | ButtonField.FIELD_VCENTER | DrawStyle.HCENTER | ButtonField.NEVER_DIRTY | Field.USE_ALL_WIDTH);
FieldChangeListener PhotoListener = new FieldChangeListener() {
public void fieldChanged(Field field, int context) {
ButtonField Button = (ButtonField) field;
if (Button.getLabel().equals("push")){
}
}
};
AddPhoto.setChangeListener(PhotoListener);
add(AddPhoto);
}
}

Browser Java Plugin Detection

What is the preferred method to determine if the Sun Java Plugin is installed in the browser?

java deployment toolkit
script src="http://java.com/js/deployJava.js"
if (deployJava.versionCheck('1.6'))
{
alert("1.6 installed")
}

You may also consider PluginDetect script.

This isn't an answer for your exact question but is offered as a solution for determining the browser itself. Don't be too harsh, this is really old code that I wrote some time ago.
import java.applet.*;
public class BrowserDetector extends Applet {
public void init() {
if (isNetscape()) {
System.out.println("This browser is a Netscape Browser.");
}
if (isMicrosoft()) {
System.out.println("This browser is a Microsoft Browser.");
}
System.out.println("VM Type: " + getVMType());
}
public static boolean isNetscape() {
try {
Class.forName("netscape.applet.MozillaAppletContext");
} catch (ClassNotFoundException e) {
System.out.println("This browser is not a Netscape Browser.");
return false;
}
return true;
}
public static boolean isMicrosoft() {
try {
Class.forName("com.ms.applet.GenericAppletContext");
} catch (ClassNotFoundException e) {
System.out.println("This browser is not a Microsoft Browser.");
return false;
}
return true;
}
public String getVMType() {
String theBrowser = "No VM";
String appletContext = getAppletContext().toString();
if (appletContext.startsWith("sun.applet.AppletViewer"))
theBrowser = "APPLETVIEWER";
else if (appletContext.startsWith("netscape.applet."))
theBrowser = "NETSCAPE";
else if (appletContext.startsWith("com.ms.applet."))
theBrowser = "MICROSOFT";
else if (appletContext.startsWith("sunw.hotjava.tags.TagAppletPanel"))
theBrowser = "HOTJAVA";
else if (appletContext.startsWith( "sun.plugin.navig.win32.AppletPlugin"))
theBrowser = "NETSCAPEPLUGIN";
else if (appletContext.startsWith( "sun.plugin.ocx.ActiveXApplet"))
theBrowser = "MICROSOFTPLUGIN";
else if (appletContext.startsWith( "sun.plugin.viewer.context.IExplorerAppletContext"))
theBrowser = "MICROSOFTPLUGINJRE1.4";
return theBrowser;
}
}