问题
I want to prevent the user from running my java application multiple times in parallel.
To prevent this, I have created a lock file when am opening the application, and delete the lock file when closing the application.
When the application is running, you can not open an another instance of jar. However, if you kill the application through task manager, the window closing event in the application is not triggered and the lock file is not deleted.
How can I make sure the lock file method works or what other mechanism could I use?
回答1:
Similar discussion is at http://www.daniweb.com/software-development/java/threads/83331
Bind a ServerSocket. If it fails to bind then abort the startup. Since a ServerSocket can be bound only once, only single instsances of the program will be able to run.
And before you ask, no. Just because you bind a ServerSocket, does not mean you are open to network traffic. That only comes into effect once the program starts "listening" to the port with accept().
回答2:
I see two options you can try:
- Use a Java shutdown hook
- Have your lock file hold the main process number. The process should exist when you lanuch another instance. If it's not found in your system, you can assume that the lock can be dismissed and overwritten.
回答3:
You could use a FileLock, this also works in environments where multiple users share ports:
String userHome = System.getProperty("user.home");
File file = new File(userHome, "my.lock");
try {
FileChannel fc = FileChannel.open(file.toPath(),
StandardOpenOption.CREATE,
StandardOpenOption.WRITE);
FileLock lock = fc.tryLock();
if (lock == null) {
System.out.println("another instance is running");
}
} catch (IOException e) {
throw new Error(e);
}
Also survives Garbage Collection. The lock is released once your process ends, doesn't matter if regular exit or crash or whatever.
回答4:
You could write the process id of the process that created the lock file into the file. When you encounter an existing lock file, you do not just quit, but you check if the process with that id is still alive. If not, you can go ahead.
回答5:
Creating a server socket, bounds to a specific port with a ServerSocket instance as the application starts is a straight way.
Note that ServerSocket.accept() blocks, so running it in its own thread makes sense to not block the main Thread.
Here is an example with a exception thrown as detected :
public static void main(String[] args) {
assertNoOtherInstanceRunning();
... // application code then
}
public static void assertNoOtherInstanceRunning() {
new Thread(() -> {
try {
new ServerSocket(9000).accept();
} catch (IOException e) {
throw new RuntimeException("the application is probably already started", e);
}
}).start();
}
回答6:
You can create a Server socket like
new ServerSocket(65535, 1, InetAddress.getLocalHost());
at very beginning of your code. Then if AddressAlreadyInUse exception caught in main block you can display the appropriate message.
回答7:
..what other mechanism could I use?
If the app. has a GUI it can be launched using Java Web Start. The JNLP API provided to web-start offers the SingleInstanceService. Here is my demo. of the SingleInstanceService.
回答8:
You can write something like this.
If file exists try to delete it. if it is not able to delete. We can say that application is already running.
Now create the same file again and redirect the sysout and syserr.
This works for me
回答9:
There are already available java methods in File class to achieve the same. The method is deleteOnExit() which ensure the file is automatically deleted when the JVM exits. However, it does not cater to forcible terminations. One should use FileLock in case of forcible termination.
For more details check, https://docs.oracle.com/javase/7/docs/api/java/io/File.html
Thus code snippet which could be used in the main method can be like :
public static void main(String args[]) throws Exception {
File f = new File("checkFile");
if (!f.exists()) {
f.createNewFile();
} else {
System.out.println("App already running" );
return;
}
f.deleteOnExit();
// whatever your app is supposed to do
System.out.println("Blah Blah")
}
回答10:
I struggled with this same problem for a while... none of the ideas presented here worked for me. In all cases, the lock (file, socket or otherwise) did not persist into the 2nd process instance, so the 2nd instance still ran.
So I decided to try an old school approach to simply crate a .pid file with the process id of the first process. Then any 2nd process would quit if it finds the .pid file, and also the process number specified in the file is confirmed to be still running. This approach worked for me.
There is a fair bit of code, which I provide here in full for your use... a complete solution.
package common.environment;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import javax.annotation.Nonnull;
import javax.annotation.Nullable;
import java.io.*;
import java.nio.charset.Charset;
public class SingleAppInstance
{
private static final @Nonnull Logger log = LogManager.getLogger(SingleAppInstance.class.getName());
/**
* Enforces that only a single instance of the given component is running. This
* is resilient to crashes, unexpected reboots and other forceful termination
* scenarios.
*
* @param componentName = Name of this component, for disambiguation with other
* components that may run simultaneously with this one.
* @return = true if the program is the only instance and is allowed to run.
*/
public static boolean isOnlyInstanceOf(@Nonnull String componentName)
{
boolean result = false;
// Make sure the directory exists
String dirPath = getHomePath();
try
{
FileUtil.createDirectories(dirPath);
}
catch (IOException e)
{
throw new RuntimeException(String.format("Unable to create directory: [%s]", dirPath));
}
File pidFile = new File(dirPath, componentName + ".pid");
// Try to read a prior, existing pid from the pid file. Returns null if the file doesn't exist.
String oldPid = FileUtil.readFile(pidFile);
// See if such a process is running.
if (oldPid != null && ProcessChecker.isStillAllive(oldPid))
{
log.error(String.format("An instance of %s is already running", componentName));
}
// If that process isn't running, create a new lock file for the current process.
else
{
// Write current pid to the file.
long thisPid = ProcessHandle.current().pid();
FileUtil.createFile(pidFile.getAbsolutePath(), String.valueOf(thisPid));
// Try to be tidy. Note: This won't happen on exit if forcibly terminated, so we don't depend on it.
pidFile.deleteOnExit();
result = true;
}
return result;
}
public static @Nonnull String getHomePath()
{
// Returns a path like C:/Users/Person/
return System.getProperty("user.home") + "/";
}
}
class ProcessChecker
{
private static final @Nonnull Logger log = LogManager.getLogger(io.cpucoin.core.platform.ProcessChecker.class.getName());
static boolean isStillAllive(@Nonnull String pidStr)
{
String OS = System.getProperty("os.name").toLowerCase();
String command;
if (OS.contains("win"))
{
log.debug("Check alive Windows mode. Pid: [{}]", pidStr);
command = "cmd /c tasklist /FI \"PID eq " + pidStr + "\"";
}
else if (OS.contains("nix") || OS.contains("nux"))
{
log.debug("Check alive Linux/Unix mode. Pid: [{}]", pidStr);
command = "ps -p " + pidStr;
}
else
{
log.warn("Unsupported OS: Check alive for Pid: [{}] return false", pidStr);
return false;
}
return isProcessIdRunning(pidStr, command); // call generic implementation
}
private static boolean isProcessIdRunning(@Nonnull String pid, @Nonnull String command)
{
log.debug("Command [{}]", command);
try
{
Runtime rt = Runtime.getRuntime();
Process pr = rt.exec(command);
InputStreamReader isReader = new InputStreamReader(pr.getInputStream());
BufferedReader bReader = new BufferedReader(isReader);
String strLine;
while ((strLine = bReader.readLine()) != null)
{
if (strLine.contains(" " + pid + " "))
{
return true;
}
}
return false;
}
catch (Exception ex)
{
log.warn("Got exception using system command [{}].", command, ex);
return true;
}
}
}
class FileUtil
{
static void createDirectories(@Nonnull String dirPath) throws IOException
{
File dir = new File(dirPath);
if (dir.mkdirs()) /* If false, directories already exist so nothing to do. */
{
if (!dir.exists())
{
throw new IOException(String.format("Failed to create directory (access permissions problem?): [%s]", dirPath));
}
}
}
static void createFile(@Nonnull String fullPathToFile, @Nonnull String contents)
{
try (PrintWriter writer = new PrintWriter(fullPathToFile, Charset.defaultCharset()))
{
writer.print(contents);
}
catch (IOException e)
{
throw new RuntimeException(String.format("Unable to create file at %s! %s", fullPathToFile, e.getMessage()), e);
}
}
static @Nullable String readFile(@Nonnull File file)
{
try
{
try (BufferedReader fileReader = new BufferedReader(new FileReader(file)))
{
StringBuilder result = new StringBuilder();
String line;
while ((line = fileReader.readLine()) != null)
{
result.append(line);
if (fileReader.ready())
result.append("\n");
}
return result.toString();
}
}
catch (IOException e)
{
return null;
}
}
}
To use it, simply invoke it like this:
if (!SingleAppInstance.isOnlyInstanceOf("my-component"))
{
// quit
}
I hope you find this helpful.
来源：https://stackoverflow.com/questions/7036108/prevent-launching-multiple-instances-of-a-java-application