Develop Reference

Android Library thread safe

I've a small android library which handles a serial port, it has basic functionality like open, read, write and close.
I have made an applications that uses this library to write on the serial port and read the responses, within this application there is a thread that periodically opens the serial port asks for the status get the response and close the serial port.
I want to protect the serial communication in a way that if the main thread opens the communication the secondary thread that only checks the status can not open it and wait for the main thread to finish.
class SerialChannel extends Channel
{
private SerialPortUtility serialPortUtility;
private static final String SERIAL_FILE = "/dev/ttyMT2";
private static final String CONTROL_FILE = "/sys/devices/platform/file";
private static final String UNKNOWN_COMMAND = "UNKNOWN COMMAND";
private FileOutputStream fileOutputStream;
private FileInputStream fileInputStream;
#Override
public void open() throws CommunicationException
{
try
{
if (isSerialOpened() != SerialStatus.Open)
{
toggleSerial(SerialStatus.Open.getStatus());
Thread.sleep(100);
}
serialPortUtility = getSerialPortUtility();
fileInputStream = (FileInputStream) serialPortUtility.getInputStream();
fileOutputStream = (FileOutputStream) serialPortUtility.getOutputStream();
currentProcess = Optional.of(Thread.currentThread().getId());
Thread.sleep(500);
}
catch (IOException | InterruptedException e)
{
throw new CommunicationException(e.getMessage());
}
}
#Override
public void close() throws CommunicationException
{
if (serialPortUtility == null)
{
throw new CommunicationException("SerialPort is null");
}
try
{
toggleSerial(SerialStatus.Close.getStatus());
fileOutputStream.close();
fileInputStream.close();
serialPortUtility.close();
fileInputStream = null;
fileOutputStream = null;
serialPortUtility = null;
}
catch (IOException e)
{
throw new CommunicationException(e.getMessage());
}
}
#Override
public void send(byte[] buffer, int timeout, int length) throws CommunicationException
{
if (fileOutputStream == null)
{
throw new CommunicationException("Problem while sending data!");
}
try
{
fileOutputStream.write(buffer);
fileOutputStream.flush();
}
catch (IOException e)
{
throw new CommunicationException(e.getMessage());
}
}
#Override
public byte[] receive(int length, int timeout) throws CommunicationException
{
StringBuilder stringBuilder = new StringBuilder();
byte[] buffer = new byte[length];
int ret;
int totalSize = 0;
if (fileInputStream == null)
{
throw new CommunicationException("FileInputStream is null!");
}
try
{
long millisStart = Calendar.getInstance().getTimeInMillis();
boolean timeoutReached;
while (true)
{
timeoutReached = (Calendar.getInstance().getTimeInMillis() - millisStart > timeout * 1000);
if (fileInputStream.available() <= 0 && timeoutReached)
{
expectingResult = false;
throw new CommunicationException("Error");
}
else if (fileInputStream.available() > 0)
{
break;
}
}
millisStart = Calendar.getInstance().getTimeInMillis();
while (totalSize != length && (ret = fileInputStream.read(buffer)) != -1)
{
String received = new String(buffer);
stringBuilder.append(received);
if(buffer.length == 15 && received.equals(UNKNOWN_COMMAND))
{
break;
}
totalSize += ret;
}
expectingResult = false;
}
catch (IOException e)
{
throw new CommunicationException(e.getMessage());
}
return stringBuilder.toString().getBytes();
}
private SerialPortUtility getSerialPortUtility() throws IOException
{
if (serialPortUtility == null)
{
File file = new File(SERIAL_FILE);
int baudRate = 115200;
return new SerialPortUtility(file, baudRate, 0);
}
return serialPortUtility;
}
private void toggleSerial(String data) throws IOException
{
FileOutputStream fos = new FileOutputStream(new File(CONTROL_FILE));
fos.write(data.getBytes());
fos.flush();
fos.close();
}
private SerialStatus isSerialOpened() throws IOException
{
byte[] buffer = new byte[1];
FileInputStream fis = new FileInputStream(new File(CONTROL_FILE));
int result = fis.read(buffer);
fis.close();
if (result > -1 && buffer[0] == 1)
{
return SerialStatus.Open;
}
return SerialStatus.Close;
}
}
This class extends custom class Channel that implements an interface with the methods open, close, read, send and implements also AutoCloseable.
Now if I make the open method synchronized any thread that enters here will lock, but will lock until it exits the open method, and when the thread moves to the another method let's say read and stay there until it gets a response, the checker thread will come and enters the open method. Using AutoCloseable the close method will execute and close the serial port communication. If I synchronize an object, there still is a window when the object is not synchronized.
How can I tell the checker thread that the communication is already opened and make him wait until the main thread finish.
Checker looks like this, it is within an timer:
try(Channel ch = CommunicationFactory.getInstance().selectChannel(CommunicationType.SERIAL))
{
ch.open();
//do stuff
}
catch (CommunicationException ex)
{
ex.printStackTrace();
}
The "main" thread looks the same only that it is in an AysncTask.
If additional informations are required please let me know!
Thank you in advance for your effort and time!

How can I tell the checker thread that the communication is already opened and make him wait until the main thread finish.
I don't fully understand your code but the critical thing with threads and locking is to make sure that all threads are calling code that is synchronized on the same object instance.
If I synchronize an object, there still is a window when the object is not synchronized.
Not if you use the same instance of the object. Making each of the public methods in SerialChannel synchronized will make sure that only 1 thread can be using the object at once.
I suspect that your real problem is not about protecting the SerialChannel object but more about race-conditions between the threads. They need to make multiple calls to the methods and they can block each other or interleave in an improper manner.
You can get around this with a couple of changes. You can make the send(...) and receive(...) methods auto-opening. Threads would just call send() or receive() which in turn would internally call open() if the fileInputStream or fileOutputStream was null. The thread would be inside of a synchronized so this would not be interrupted by another thread.
Another completely different model to consider would be to have one thread reading from the serial port and another writing to it that are dedicated to that task -- they would be built into the SerialChannel object. They would share data with the external threads using a read BlockingQueue and a write BlockingQueue. Then the serial port is opened early in your application which starts the IO threads and the external threads never worry about the IO. They just put() and take() from the queues. I typically do this (for example) when reading and writing to the console.
Hope something here helps.

what is the fastest way to write a byte array to socket outputstream in java

As the title, and assume the size of byte array is no larger than 16 Kbytes.
Currently I am implementing a middleware for MySQL (like MySQL Proxy), which requires high throughput. but the overhead caused by reading data from socket and writing data to socket. For now, I use
in = new DataInputStream(new BufferedInputStream(socket.getInputStream()))
and
out = new DataOutputStream(new BufferedOutputStream(socket.getOutputStream()))
When read data and write, I use
in.read(byte[] b) and out.write(byte[] b, int offset, int len) with out.flush()
Can anyone tell me a better way to do this?

If you're writing byte arrays it doesn't make much difference. The network is the limiting factor, not the API. I think you're already doing it near-optimally. The most significant factor is the size of your socket send buffer in the kernel, and the socket receive buffer at the receiver.
You could investigate NIO and direct buffers, but I doubt you'll see a significant difference. Direct buffers are really for the case where you're just copying between channels, and the rest of NIO is really about scalability rather than performance over an individual channel.

Since you are just forwarding bytes, you could save a little time by not using DataInputStream, and instead just using BufferedInputStream.read() and BufferedOutputStream.write().

As EJP mentions, the network is the limiting factor. But that did not stop me trying to make the fastest implementation I could imagine without using NIO. The thing is, you can read from a socket while you write to another/the same socket. One thread cannot do this (either reads or writes) so multiple threads are needed. But without NIO, that requires a lot of threads (mostly sitting idle waiting on I/O though). NIO is a bit more complicated but is very good at using very few threads when there are a lot of connections with low volume (see the summary on this page of the article that Baldy mentions).
Anyway, below a non-NIO test class that you can update and use to see for yourself what is (not) the limiting factor.
public class SocketForwarder {
public static void main(String[] args) {
try {
new SocketForwarder().forward();
} catch (Exception e) {
e.printStackTrace();
}
}
public static final int portNumber = 54321;
public static final int maxSend = 1024 * 1024 * 100; // 100 MB
public static final int bufSize = 16 * 1024;
public static final int maxBufInMem = 128;
private static final SimpleDateFormat df = new SimpleDateFormat("HH:mm:ss.SSS");
private final ExecutorService tp = Executors.newCachedThreadPool();
private final ArrayBlockingQueue<byte[]> bq = new ArrayBlockingQueue<byte[]>(maxBufInMem);
private final CountDownLatch allReceived = new CountDownLatch(1);
private Socket from, to, sender, receiver;
private int bytesSend, bytesReceived;
public void forward() throws Exception {
tp.execute(new Runnable() {
public void run() {
ServerSocket ss = null;
try {
ss = new ServerSocket(portNumber);
from = ss.accept();
to = ss.accept();
} catch (Exception e) {
e.printStackTrace();
} finally {
try { ss.close(); } catch (Exception ignored) {}
}
}
});
sender = new Socket(InetAddress.getLocalHost(), portNumber);
receiver = new Socket(InetAddress.getLocalHost(), portNumber);
// Setup proxy reader.
tp.execute(new Runnable() {
public void run() {
byte[] buf = new byte[bufSize];
try {
InputStream in = from.getInputStream();
int l = 0;
while ((l = in.read(buf)) > 0) {
byte[] bufq = new byte[l];
System.arraycopy(buf, 0, bufq, 0, l);
bq.put(bufq);
}
} catch (Exception e) {
e.printStackTrace();
}
}
});
// Setup proxy writer.
tp.execute(new Runnable() {
public void run() {
try {
OutputStream out = to.getOutputStream();
while (true) {
byte[] bufq = bq.take();
out.write(bufq);
out.flush();
}
} catch (Exception e) {
e.printStackTrace();
}
}
});
// Start receiver.
tp.execute(new Runnable() {
public void run() {
byte[] buf = new byte[bufSize];
try {
InputStream in = receiver.getInputStream();
int l = 0;
while (bytesReceived < maxSend && (l = in.read(buf)) > 0) {
bytesReceived += l;
}
} catch (Exception e) {
e.printStackTrace();
}
System.out.println(df.format(new Date()) + " bytes received: " + bytesReceived);
allReceived.countDown();
}
});
// Start sender.
tp.execute(new Runnable() {
public void run() {
Random random = new Random();
try {
OutputStream out = sender.getOutputStream();
System.out.println(df.format(new Date()) + " start sending.");
while (bytesSend < maxSend) {
byte[] buf = new byte[random.nextInt(bufSize)];
out.write(buf);
out.flush();
bytesSend += buf.length;
}
} catch (Exception e) {
e.printStackTrace();
}
System.out.println("Bytes send: " + bytesSend);
}
});
try {
allReceived.await();
} finally {
close(sender);
close(from);
close(to);
close(receiver);
tp.shutdownNow();
}
}
private static void close(Socket s) {
try { s.close(); } catch (Exception ignored) {}
}
}
It took my computer 2 seconds to transfer 100MB locally, expect a lot less when a network is involved.

For the best throughput you're going to want to use NIO and ByteBuffers. NIO keeps most of the work reading and writing to the sockets in native code and so can be much faster.
It is more complex to write good NIO code but depending on what kind of performance you're looking for, it can be worth the effort.
There are some good NIO examples out there along with some good introductions and comparisons. One resource I've used is http://tutorials.jenkov.com/java-nio/index.html.

Concurrent Modification Exception, despite waiting for finish

Here's a section of my onCreate, which sometimes is causing exception:
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_tilisting);
_context = getApplicationContext();
SDName = Environment.getExternalStorageDirectory();
//listview = (ListView)findViewById(R.id.TIlistview);
String TIdir = new File(SDName, "/TitaniumBackup/").toString();
final ArrayList<String> apps = new ArrayList<String>();
final StringBuffer done = new StringBuffer();
Command command = new Command(0,"ls -a "+TIdir+"/*.properties") {
#Override
public void output(int arg0, String arg1) {
synchronized(apps) {
apps.add(arg1);
if (!done.toString().equals("")) {
done.append("done");//oh no
}
}
}
};
try {
RootTools.getShell(true).add(command).waitForFinish();
String attrLine = "";
int ind;
backups = new ArrayList<TIBackup>();
synchronized(apps) {
for (String app : apps) {
try {
TIBackup bkup = new TIBackup(app);
FileInputStream fstream = new FileInputStream(app);
BufferedReader atts = new BufferedReader(new InputStreamReader(fstream));
while ((attrLine = atts.readLine()) != null) {
ind = attrLine.indexOf('=');
if (ind !=-1 && !attrLine.substring(0,1).equals("#"))
bkup.prop.put(attrLine.substring(0,ind), attrLine.substring(ind+1));
}
backups.add(bkup);
atts.close();
} catch (FileNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
done.append("done");
}
setListAdapter( new StableArrayAdapter(this,backups));
} catch (InterruptedException e) {
//TODO:errors
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} catch (TimeoutException e) {
e.printStackTrace();
}
The for (String app : apps) { is causing the exception, despite the waitforfinish() before it.
This updated code should fix it, adding data from the output, and waiting for any stragglers with the synchronized in the main code, but if you set a breakpoint on the //oh no line above, it is still getting to this point where it tries to add an item after the UI main code ran. So waitforfinish() is not waiting? How do I prevent this race condition?
I also tried the RootTask code below, but it seems to stop at the last readline?
RootTask getProfile = new RootTask() {
#Override
public void onPostExecute(ArrayList<String> result) {
super.onPostExecute(result);
for (String r : result) {
System.out.println(r);
}
}
};
getProfile.execute("ls /data/data/org.mozilla.firefox/files/mozilla/" );
onPostExecute never runs.

This was partially caused by a design flaw in RootTools. I believe the crux of the issue is that the operation that you are performing on the shell is taking longer than the default timeout that is set for shell commands. When the timeout occurs it simply returns the command as completed which is where the design flaw lies.
I have provided a new jar to use as well as some more information on this. I have also deprecated waitForFinish() as I agree that it was, and is, a poor solution.
https://code.google.com/p/roottools/issues/detail?id=35
Please let me know if you have any questions or problems :)

Output() is to be called during waitForFinish() waits. Something is wrong in the code implementing Command execution.
Most likely: the command executor (RootTools ?) runs the command on shell, gets a bunch of output lines, notifies the calling thread from waiting, and then calls output() of command for each line it got as output. I think it should notify the command thread after output() has been called on command object, for all output lines.
Still you can wrap the list modifying code and list iterating code in synchronized(<some common object>){}.
Update:
So waitForFinish() is not waiting? How do I prevent this race condition?
It does wait, but not for your code. Synchronized keyword merely made sure that output() of Command object is not called at the same time when you are iterating the apps collection. It does not schedule the two threads to run in a particular sequence.
IMHO, waitForFinish() is not a good pattern, making calling thread waiting defeats the point of a separate executor. It better be formulated like an AsyncTask or accept an event listener for each Command object.
Just a rough example, this class:
public class RootTask extends AsyncTask<String,Void,List<String>> {
private boolean mSuccess;
public boolean isSuccess() {
return mSuccess;
}
#Override
protected List<String> doInBackground(String... strings) {
List<String> lines = new ArrayList<String>();
try {
Process p = Runtime.getRuntime().exec("su");
InputStream is = p.getInputStream();
OutputStream os = p.getOutputStream();
os.write((strings[0] + "\n").getBytes());
BufferedReader rd = new BufferedReader(new InputStreamReader(is));
String line;
while ((line = rd.readLine()) != null){
lines.add(line);
}
mSuccess = true;
os.write(("exit\n").getBytes());
p.destroy();
} catch (IOException e) {
mSuccess = false;
e.printStackTrace();
}
return lines;
}
}
can be used as:
RootTask listTask = new RootTask{
#Override
public void onPostExecute(List<String> result){
super.onPostExecute();
apps.addAll(result);
//-- or process the results strings--
}
};
listTask.execute("ls -a "+TIdir+"/*.properties");

Java Object returning to default values

I have the following code structure.
A transaction handler of type Transaction which is a field in a Client Handler class, which talks to a Server. (the client handler and the server are collocated), the client talks to the client handler via serialized object messages.
When a new transaction request comes in from the client, (comes on thread using the readObject() method of an object input stream), I then do a series of trx_handler.setFoo(trx.getFoo))). This works fine, I can handle the first request. But when a subsequent request comes in (which only starts getting executed after the first request finished due to the loop structure, I find that the trx handler has been reinitialised to its default values, the object is still there, but all the values inside are the defaut ones. What can cause this problem?
My first guess would be garbage collection, but in my Client Handler class, there is always a pointer to this trx_handler.
The code below illustrates what happens. A statement would first be of type start, so the trx_handler will be correctly initialised. handle_statement will then be called. Subsequent statements should then be received, but at this point the trx_handler has been reinitialised to its default settings, so the access_set field is null, the session id as well, and none of the modification made to the object in hande_statement are visible
Thanks
public class Handler {
private Statement trx_handler;
/* Constructor initialises trx_handler to new Statement(); */
public ClientHandler(final Socket socket, long uid, Server server, ObjectInputStream ois) throws IOException, Exception {
LOGGER.info("Constructing Handler");
this.uid = uid;
this.server = server;
this.socket = socket;
this.database = server.getDB();
this.trx_sys = database.getTransactionManager();
create_listening(socket, ois);
out = socket.getOutputStream();
oos = new ObjectOutputStream(out);
this.trx_handler = new Statement(false);
}
private void create_incoming(final Socket socket, final ObjectInputStream stream) {
Thread incoming = new Thread() {
#Override
public void run() {
ObjectInputStream ois = stream;
InputStream in = null;
while (true) {
Object statement = null;
try {
statement = ois.readObject();
execute_stat(statement, socket, null);
LOGGER.info("Ready to execute next ");
} catch (SocketException e) {
LOGGER.severe("Connection Closed");
return;
} catch (IOException e) {
LOGGER.severe("Connection Closed");
return;
} catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (Exception e) {
e.printStackTrace();
String error_message = e.getMessage();
send_error(socket, error_message);
}
}
}
};
incoming.setDaemon(true);
incoming.start();
}
private synchronized void execute_stat(Statement trx) {
if (trx.getTransactionState() == Consts.trx_end) {
trx_sys.commitTransaction(trx_handler);
return;
} else if (trx.getTransactionState() == Consts.trx_start) {
try {
trx_handler.setAccessSet(trx.getAccessSet());
trx_handler.setSession_id(trx.getSession_id());
trx_sys.startTransaction(trx_handler);
handle_statement(socket, trx_handler);
/* TEST HERE THAT FIELDS IN TRX_HANDLER ARE CORRECTLY SET (INCLUDING SOME MODIFIED IN
handle_statement and they are correctly set */
return;
} catch (Exception ex) {
Logger.getLogger(ClientHandler.class.getName()).log(Level.SEVERE, null, ex);
}
}
try {
LOGGER.info("Execute Trx: stat");
/* Can't see modifications made in the start case */
Statement stats = trx.getStatement();
trx_handler.setStatement(stats);
handle_statement(stats, socket, trx_handler);
} catch (Exception e) {
e.printStackTrace();
}
return;
}

You need to either send a brand new object for each transaction, use ObjectOutputStream.writeUnshared(), or else call ObjectOutputStream.reset() between sends.

How to make sure that only a single instance of a Java application is running?

I want my application to check if another version of itself is already running.
For example, demo.jar started, user clicks to run it again, but the second instance realizes "oh wait, there is already a demo.jar running." and quits with a message.

Enforce one instance of a program running with a ServerSocket Lock
Java Code. Put this into a file called Main.java:
import java.net.*;
import java.io.*;
public class Main{
public static void main(String args[]){
ServerSocket socket = null;
try {
socket = new ServerSocket(34567);
System.out.println("Doing hard work for 100 seconds");
try{ Thread.sleep(100000); } catch(Exception e){ }
socket.close();
}
catch (IOException ex) {
System.out.println("App already running, exiting...");
}
finally {
if (socket != null)
try{ socket.close(); } catch(Exception e){}
}
}
}
Compile and run it
javac Main.java
java Main
Test it in a normal case:
Run the program. You have 100 seconds to run the program again in another terminal, it will fall through saying its already running. Then wait 100 seconds, it should allow you to run it in the 2nd terminal.
Test it after force halting the program with a kill -9
Start the program in terminal 1.
kill -9 that process from another terminal within 100 seconds.
Run the program again, it is allowed to run.
Conclusion:
The socket occupation is cleaned up by the operating system when your program is no longer operating. So you can be sure that the program will not run twice.
Drawbacks
If some sneaky person, or some naughty process were to bind all of the ports, or just your port, then your program will not run because it thinks its already running.

What you are looking for can probably best be accomplished with a lock file. By lock file I simply mean a file that will have a predefined location and whose existence is your mutex.
Test if that file exists when your program starts, if it does, exit immediately. Create a file in a known location. If your program exits normally, delete the lock file.
Probably best is if you can also populate the file with a pid (process id) so that you can detect abnormal exits that didn't delete the file but this get OS specific.

Simple yet powerful tested solution.
static File file;
static FileChannel fileChannel;
static FileLock lock;
static boolean running = false;
#SuppressWarnings("resource")
public static boolean checkIfAlreadyRunning() throws IOException {
file = new File(FilePath.FILEPATH + "az-client.lock");
if (!file.exists()) {
file.createNewFile();
running = true;
} else {
file.delete();
}
fileChannel = new RandomAccessFile(file, "rw").getChannel();
lock = fileChannel.tryLock();
if (lock == null) {
fileChannel.close();
return true;
}
ShutdownHook shutdownHook = new ShutdownHook();
Runtime.getRuntime().addShutdownHook(shutdownHook);
return running;
}
public static void unlockFile() {
try {
if (lock != null)
lock.release();
fileChannel.close();
file.delete();
running = false;
} catch (IOException e) {
e.printStackTrace();
}
}
static class ShutdownHook extends Thread {
public void run() {
unlockFile();
}
}
Put these methods in some Util class and before launching your main class just check that if already exists then show some dialog to user otherwise launch an application. It works even if you abnormally shutdown java process or what ever you do. It is robust and efficient, no need to set up DataGram listeners or whatever...

If you use a Mutex, logically that Mutex would need to be accessible from any JVM which was running a copy of "the program". In C programming, this might be accomplished via shared memory, but Java doesn't have such a thing by default.
With that understanding, there are plenty of ways to implement what you want. You could open a server socket on a designated port (the operating system assures that only one process is the recipient of the server socket, and subsequent opens fail).
You could use a "lock file" but it is a bit complicated, as the file you would need to use would really be a directory (and it becomes heavily dependent on whether directory creation is atomic for your file system, even though most directory creations are). If a sysadmin decides to run you via NFS, then things get even harder (if not impossible).
You can also do a number of nifty tricks with JVMs and debugging / JMI, provided you can somehow assure youself that all relevant JVMs are launched with the same configurations (in time, an impossible task).
Other people have used the exec facility to run the equivalent of a process listing, but it is a bit tricky due to the possibility of race condition (two processes simultaneously check, and fail to see each other).
In the end, the server socket route is probably the most stable, as it is guaranteed to only bind to one process by the TCP/IP stack (and is mediated by the operating system). That said, you will have to flush the socket of incoming messages, and it opens up the possibility of other security issues.

If your application is running on Windows, you can call CreateMutex through JNI.
jboolean ret = FALSE;
HANDLE hMutex = CreateMutex(NULL, FALSE, mutexName);
ret = TRUE;
if(WAIT_TIMEOUT == WaitForSingleObject(hMutex, 10))
{
ret = FALSE;
}
else if(GetLastError() != 0)
{
ret = FALSE;
}
This returns true if nobody else is using this mutex, false otherwise.
You could provide "myApplication" as a mutex name or "Global\MyApplication" if you want your mutex to be shared by all Windows sessions.
Edit: It's not as complicated as it looks :) and I find it clean.

The strategy of this code is to keep the PID around from the last run in the registry, if that PID is found running on the system, don't start. If you finish, reset.
The preferences are stored on Windows Registry in HKEY_LOCAL_MACHINE\SOFTWARE\JavaSoft\Prefs
import java.io.*;
import java.util.prefs.Preferences;
public class JavaApplication3 {
public static void main(String[] args){
if(isRunning()){
System.out.println("Two instances of this program cannot " +
"be running at the same time. Exiting now");
}
else{
onStart();
epicHeavyWorkGoesHere();
onFinish();
}
}
public static void epicHeavyWorkGoesHere(){
try {
Thread.sleep(15000);
} catch (InterruptedException ex) {}
}
public static void onStart(){
Preferences prefs = Preferences.systemRoot().node("JavaApplication3");
prefs.put("RUNNINGPID", getCurrentPID());
}
public static void onFinish(){
Preferences prefs = Preferences.systemRoot().node("JavaApplication3");
prefs.put("RUNNINGPID", "");
}
public static boolean isRunning(){
Preferences prefs = Preferences.systemRoot().node("JavaApplication3");
if (prefs.get("RUNNINGPID", null) == null || prefs.get("RUNNINGPID", null).equals(""))
return false;
if (isProcessIdRunningOnWindows(Integer.parseInt(prefs.get("RUNNINGPID", null))))
return true;
return false;
}
public static String getCurrentPID(){
//This function should work with Windows, Linux and Mac but you'll have to
//test to make sure. If not then get a suitable getCurrentPID function replacement.
try{
java.lang.management.RuntimeMXBean runtime = java.lang.management.ManagementFactory.getRuntimeMXBean();
java.lang.reflect.Field jvm = runtime.getClass().getDeclaredField("jvm");
jvm.setAccessible(true);
sun.management.VMManagement mgmt = (sun.management.VMManagement) jvm.get(runtime);
java.lang.reflect.Method pid_method = mgmt.getClass().getDeclaredMethod("getProcessId");
pid_method.setAccessible(true);
return pid_method.invoke(mgmt) + "";
}
catch(Exception e){
throw new RuntimeException("Cannot get the current PID");
}
}
public static boolean isProcessIdRunningOnWindows(int pid){
//This Function only works for windows, if you want it to work on linux
//or mac, you will have to go find a replacement method that
//takes the processID as a parameter and spits out a true/false
//if it is running on the operating system.
try {
Runtime runtime = Runtime.getRuntime();
String cmds[] = {"cmd", "/c", "tasklist /FI \"PID eq " + pid + "\""};
Process proc = runtime.exec(cmds);
InputStream inputstream = proc.getInputStream();
InputStreamReader inputstreamreader = new InputStreamReader(inputstream);
BufferedReader bufferedreader = new BufferedReader(inputstreamreader);
String line;
while ((line = bufferedreader.readLine()) != null) {
if (line.contains(" " + pid + " ")){
return true;
}
}
return false;
}
catch (Exception ex) {
throw new RuntimeException("Cannot run the tasklist command to query if a pid is running or not");
}
}
}
If the program is hung and the pid remains in the task list this will be blocked. You could add an additional registry key that will store the last successful run time, and if the run time becomes too great, the stored PID is killed, and the program re-run.

Following solution work in two deadly scenerio too.
1> Even your launched exe scheduled as javaw.exe in task manager.
2> You can install your application at two location and from launching both location it also works.
String tempDir = System.getProperty("java.io.tmpdir");// dependent to OS find any tem dir.
String filePath = tempDir + "lockReserverd.txt";
try {
final File file = new File(filePath);
if(file.exists())
return false;
final RandomAccessFile randomAccessFile = new RandomAccessFile(file, "rw");
final FileLock fileLock = randomAccessFile.getChannel().tryLock();
if (fileLock != null) {
Runtime.getRuntime().addShutdownHook(new Thread() {
public void run() {
try {
fileLock.release();
randomAccessFile.close();
file.delete();
} catch (Exception e) {
//log.error("Unable to remove lock file: " + lockFile, e);
}
}
});
return true;
}
} catch (Exception e) {
//log.Error("Unable to create and/or lock file");
}
return false
or
This will work if your application.exe is listed in task manager
"tasklist /FI \"IMAGENAME eq "+MyApplication+".exe

Contrary to several other answers, the most reliable method is to create a ServerSocket on a fixed port known only to you, way up in the paint cards. It will automatically be released when your application exits, unlike any lock file, and its prior existence via a BindException is a pretty infallible sign that another instance is already running.

Here is one method that uses an automatically named lock file in the user's home directory.
The name is based on where the jar is being ran from.
import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.channels.FileChannel;
public class SingleInstance {
#SuppressWarnings("resource")
public static boolean isAlreadyRunning() {
File file;
FileChannel fileChannel;
File userDir = new File(System.getProperty("user.home"));
file = new File(userDir, myLockName());
if (!file.exists()) {
try {
file.createNewFile();
file.deleteOnExit();
} catch (IOException e) {
throw new RuntimeException("Unable to create Single Instance lock file!", e);
}
}
try {
fileChannel = new RandomAccessFile(file, "rw").getChannel();
} catch (FileNotFoundException e) {
throw new RuntimeException("Single Instance lock file vanished!", e);
}
try {
if (fileChannel.tryLock() != null) {
return false;
}
} catch (Exception e) {
}
try {
fileChannel.close();
} catch (IOException e1) {
}
return true;
}
private static String myLockName() {
return "." + SingleInstance.class.getProtectionDomain().getCodeSource().getLocation().getPath()
.replaceAll("[^a-zA-Z0-9_]", "_");
}
}

This is also a good solution if your Appication can schedued in task manager with a unique name
"tasklist /FI \"IMAGENAME eq "+MyApplication+".exe

Check PID and file lock technique
We can write the process id of the process that created the lock file into the file. When we encounter an existing lock file, we do not just quit, but we check if the process with that id is still alive. If not, then create a new application instance. I think MongoDB use this technique.
static File file;
static FileChannel fileChannel;
static FileLock lock;
static boolean running = false;
static String currentPID = null;
static String lockFilePID = null;
public static final String USER_DIR = System.getProperty("user.dir");
public static final String LOCK_FILE = "az-client.lock";
public static boolean checkInstance() {
try {
file = new File(USER_DIR + File.separator + LOCK_FILE);
currentPID = Integer.toString(getCurrentPID());
if (!file.exists()) {
file.createNewFile();
writePID(currentPID);
lockFile();
addShudDownHook();
running = true;
return running;
} else {
if (isFileLocked()) {
syso("App already running");
System.exit(0);
} else {
lockFilePID = getPIDFromLockFile();
if (isProcessIdRunningOnWindows(Integer.parseInt(lockFilePID))) {
lockFile();
addShudDownHook();
running = true;
return running;
} else {
file.delete();
file.createNewFile();
writePID(currentPID);
lockFile();
addShudDownHook();
running = true;
return running;
}
}
}
} catch (Exception e) {
syso(e + "App already running");
System.exit(0);
}
return running;
}
/**
*
* #return
* #throws IOException
*/
#SuppressWarnings("resource")
private static boolean isFileLocked() throws IOException {
fileChannel = new RandomAccessFile(file, "rw").getChannel();
lock = fileChannel.tryLock();
if (lock == null) {
fileChannel.close();
fileChannel = null;
return true;
} else {
lock.release();
fileChannel.close();
fileChannel = null;
}
return false;
}
public static int getCurrentPID() {
// This function should work with Windows, Linux and Mac but you'll have
// to
// test to make sure. If not then get a suitable getCurrentPID function
// replacement.
try {
java.lang.management.RuntimeMXBean runtime = java.lang.management.ManagementFactory.getRuntimeMXBean();
java.lang.reflect.Field jvm = runtime.getClass().getDeclaredField("jvm");
jvm.setAccessible(true);
sun.management.VMManagement mgmt = (sun.management.VMManagement) jvm.get(runtime);
java.lang.reflect.Method pid_method = mgmt.getClass().getDeclaredMethod("getProcessId");
pid_method.setAccessible(true);
return (int) pid_method.invoke(mgmt);
} catch (Exception e) {
throw new RuntimeException("Cannot get the current PID");
}
}
public static boolean isProcessIdRunningOnWindows(int pid) {
// This Function only works for windows, if you want it to work on linux
// or mac, you will have to go find a replacement method that
// takes the processID as a parameter and spits out a true/false
// if it is running on the operating system.
try {
Runtime runtime = Runtime.getRuntime();
String cmds[] = { "cmd", "/c", "tasklist /FI \"PID eq " + pid + "\"" };
Process proc = runtime.exec(cmds);
InputStream inputstream = proc.getInputStream();
InputStreamReader inputstreamreader = new InputStreamReader(inputstream);
BufferedReader bufferedreader = new BufferedReader(inputstreamreader);
String line;
while ((line = bufferedreader.readLine()) != null) {
if (line.contains(" " + pid + " ")) {
return true;
}
}
return false;
} catch (Exception ex) {
throw new RuntimeException("Cannot run the tasklist command to query if a pid is running or not");
}
}
/**
* This method write PID to Lock file
*
* #param pid
* #throws Exception
*/
private static void writePID(String pid) throws Exception {
try {
// To Do write PID to LockFile
} catch (Exception e) {
syso(e);
throw e;
}
}
/**
* This method return PID from Lock File
*
* #return
* #throws Exception
*/
private static String getPIDFromLockFile() throws Exception {
try {
return //To Do getPID from File
} catch (Exception e) {
syso(e);
throw e;
}
}
private static void addShudDownHook() {
try {
ShutdownHook shutdownHook = new ShutdownHook();
Runtime.getRuntime().addShutdownHook(shutdownHook);
} catch (Exception e) {
LogWriter.logger.error(e);
}
}
private static void unlockFile() {
try {
if (lock != null) {
lock.release();
}
fileChannel.close();
file.delete();
running = false;
} catch (IOException e) {
syso(e);
}
}
private static void lockFile() {
try {
fileChannel = new RandomAccessFile(file, "rw").getChannel();
lock = fileChannel.tryLock();
if (lock == null) {
fileChannel.close();
fileChannel = null;
}
} catch (IOException e) {
syso(e);
}
}
static class ShutdownHook extends Thread {
public void run() {
unlockFile();
}
}