Alright, so I'm writing this program that essentially batch runs other java programs for me (multiple times, varying parameters, parallel executions, etc).
So far the running part works great. Using ProcessBuilder's .start() method (equivalent to the Runtime.exec() I believe), it creates a separate java process and off it goes.
Problem is I would like to be able to pause/stop these processes once they've been started. With simple threads this is generally easy to do, however the external process doesn't seem to have any inbuilt functionality for waiting/sleeping, at least not from an external point of view.
My question(s) is this: Is there a way to pause a java.lang.Process object? If not, does anyone know of any related exec libraries that do contain this ability? Barring all of that, is extending Process a more viable alternative?
My question(s) is this: Is there a way to pause a java.lang.Process object?
As you've probably discovered, there's no support for this in the standard API. Process for instance provides no suspend() / resume() methods.
If not, does anyone know of any related exec libraries that do contain this ability?
On POSIX compliant operating systems such as GNU/Linux or Mac OS you could use another system call (using Runtime.exec, ProcessBuilder or some natively implemented library) to issue a kill command.
Using the kill command you can send signals such as SIGSTOP (to suspend a process) and SIGCONT (to resume it).
(You will need to get hold of the process id of the external program. There are plenty of questions and answers around that answers this.)
You will need to create a system for sending messages between processes. You might do this by:
Sending signals, depending on OS. (As aioobe notes.)
Having one process occasionally check for presence/absence of a file that another process can create/delete. (If the file is being read/written, you will need to use file locking.)
Have your "main" process listen on a port, and when it launches the children it tells them (via a comamnd-line argument) how to "phone home" as they start up. Both programs alternate between doing work and checking for handling messages.
From what you have described (all Java programs in a complex batch environment) I would suggest #3, TCP/IP communication.
While it certainly involves extra work, it also gives you the flexibility to send commands or information of whatever kind you want between different processes.
A Process represents a separate process running on the machine. Java definitely does not allow you to pause them through java.lang.Process. You can forcibly stop them using Process.destroy(). For pausing, you will need the co-operation of the spawned process.
What sorts of processes are these? Did you write them?
Related
main aim = minimize time of execution process.
Want to create system process with running some programm, and reuse it.
For example
command = "/client.exe -ip=127.0.0.1 -port=1234" + somecommand
execute it
Process(command).lineStream.mkString
Result of execution is very slow.
How can I run client.exe once, and reuse this process. Just send some new commands every time to the existing process client.exe.
Any ideas how to increase speed of execution ?
Thanks.
What you want is actually interprocess communication and/or remote procedure call. You can use several methods to achieve this. Some of them are:
Using REST/HTTP, spray is probably simplest and best solution for this.
Using Akka, Akka supports remote actors, this means you can spawn an actor on the main process and access it from other processes and send/receive messages.
If you are on a *nix system you can use raw sockets.
Use a message queue, check RabbitMQ
if client.exe has sequential execution and it is designed to quit after work done, then You can't do much. Executable should be written to handle interprocess communication.
I've written a Java application that is launched as a daemon (I daemonize redirecting stderr and stdout and closing stdin though bash). However, occasionally I would like to be able to message this application and inform it to change certain parts of its behavior. I need to be able to message the application from a terminal, so anything that requires a graphical utility is a no-go.
The change in behaviour is fairly simple. I need a toggle for the state of one thread in my application, and a way to gracefully close the application.
What are my options in achieving this? I know I could have a thread in the process that listens on a socket of some sort for messages, but this seems like overkill for the needs I have.
I'm not too familiar with Signals on Linux/Unix, so I'd like to ask if I could simply set up a custom handler for some signals, and have my code execute when the process receives a signal.
Are there any other options that I simply don't know about or haven't thought about?
Signals may be the easiest. You have SIGUSR1 and SIGUSR2 available for application use and you can write a handler for them to manage your toggle. Generally you don't want to do much in a handler so you can set a switch and your main loop (or whatever) needs to check the switch and act accordingly. Similarly the receipt of the signal could be programmed to read a config file that you changed.
Beyond that you can use any available IPC (fifos, sockets, MQs) but you are going to either have thread blocking on them or somehow incorporate them a select statement (or whatever the java equivalent of that is).
You basically need a small client that communicates with your application (server). So the default solution should be to use some IPC mechanism. Putting an IPC mechanism is a one time effort and is worth the trouble because it scales well with the requirements. Using signals for IPC is not recommended. I think sockets is a good way to go.
Personally, I like to use OS signals for this.
Use java.lang.Runtime.addShutdownHook and send a SIGTERM, SIGINT or SIGHUP.
Be aware that some signals are reserved for internal use, check out the docs for the JVM you are using.
The SignalHandler is not part of the supported, public interface, so your mileage may vary.
If you are instantiating a JVM from C code, simply set signal handlers (in C) before the JVM and you will be fine.
Is it possible to stop an application in windows using java code ?
I'd like to create a program which is like a shortcut to stop an application on my pc. Is it possible using ProcessBuilder ? Thank you
Plain Java, without native access? No.
ProcessBuilder lets you control processes that you started from within the Java application, but it doesn't give you control to processes that were started by other processes.
As per user988052's comment, you can use pskill from the Sysinternals Suite to accomplish this through Runtime.exec. pskill uses the TerminateProcess function call. You can also call this function with JNI (or some other native caller, like JNA, NLink, etc.), but you'll need to obtain a handle to the process via the OpenProcess function call.
Is it possible to stop an application in windows using java code ? ... Is it possible using ProcessBuilder ?
Only if you launched the application from the same Java application that you are trying to fill it from.
Other than that, you need to resort to running a Windows-specific command to do the killing ... or something based on JNI to make native Windows library calls.
The other complications are that you may not be able to kill certain processes due to permissions issues, and some process may refuse to be killed.
I believe the JRE does not provide an API for system processes. ProcessBuilder can create system processes, but it cannot provide you with a Process object for a process created outside the JVM. I believe that is a philosophical decision on the part of the Java folks, to avoid certain OS-specific tasks where possible. You might think that every OS has pids, or that they all have some kind of kill message, but Java runs on lots of OSes, some of them quite strange.
You could make native code to do it and wrap that with JNI, but that would be wasteful. As Mr 988052 says, I suggest you execute a system-specific command through Runtime.exec or ProcessBuilder. You would need to decide which OSes you want to support, and be sure to get the commands (and the OS-detection code) right for each one.
What I would like to accomplish is to start a Java program and have it keep running until the user kills it with a control-C. I realize that it is possible to do this by creating a BufferedReader and having it endlessly loop while reading the BufferedReader, but what I am doing involves me backgrounding the Java program (e.g., java -jar app.jar &) which kills standard in so that method would not work. I've read a bit on Java's daemon threads, but I also do not think that is the correct solution in this instance because I want the JVM to stay alive.
Any help would be much appreciated.
Thanks,
Chris
I'm guessing you're launching the java program from a different process perhaps? A potential option may be integrating the Java Service Wrapper ? Depending on what you are doing, licensing may be an issue, and you probably wouldn't kill the process with a ctrl-c, but it's a thought (in addition to the others given above).
I'm trying to develop an application that just before quit has to run a new daemon process to execute the main method of a class.
I require that after the main application quits the daemon process must still be in execution.
It is a Java Stored Procedure running on Oracle DB so I can't use Runtime.exec because I can't locate the java class from the Operating System Shell because it's defined in database structures instead of file system files.
In particular the desired behavior should be that during a remote database session I should be able to
call the first java method that runs the daemon process and quits leaving the daemon process in execution state
and then (having the daemon process up and the session control, because the last call terminated) consequentially
call a method that communicates with the daemon process (that finally quits at the end of the communication)
Is this possible?
Thanks
Update
My exact need is to create and load (reaching the best performances) a big text file into the database supposing that the host doesn't have file transfer services from a Java JDK6 client application connecting to Oracle 11gR1 DB using JDBC-11G oci driver.
I already developed a working solution by calling a procedure that stores into a file the LOB(large database object) given as input, but such a method uses too many intermediate structures that I want to avoid.
So I thought about creating a ServerSocket on the DB with a first call and later connect to it and establish the data transfer with a direct and fast communication.
The problem I encountered comes out because the java procedure that creates the ServerSocket can't quit and leave an executing Thread/Process listening on that Socket and the client, to be sure that the ServerSocket has been created, can't run a separate Thread to handle the rest of the job.
Hope to be clear
I'd be surprised if this was possible. In effect you'd be able to saturate the DB Server machine with an indefinite number of daemon processes.
If such a thing is possible the technique is likely to be Oracle-specific.
Perhaps you could achieve your desired effect using database triggers, or other such event driven Database capabilities.
I'd recommend explaining the exact problem you are trying to solve, why do you need a daemon? My instict is that trying to manage your daemon's life is going to get horribly complex. You may well need to deal with problems such as preventing two instances being launched, unexpected termination of the daemon, taking daemon down when maintenance is needed. This sort of stuff can get really messy.
If, for example, you want to run some Java code every hour then almost certanly there are simpler ways to achieve that effect. Operating systems and databases tend to have nice methods for initiating work at desired times. So having a stored procedure called when you need it is probably a capability already present in your environment. Hence all you need to do is put your desired code in the stored procedure. No need for you to hand craft the shceduling, initiation and management. One quite significant advantage of this approach is that you end up using a tehcnique that other folks in your environment already understand.
Writing the kind of code you're considering is very intersting and great fun, but in commercial environments is often a waste of effort.
Make another jar for your other Main class and within your main application call the jar using the Runtime.getRuntime().exec() method which should run an external program (another JVM) running your other Main class.
The way you start subprocesses in Java is Runtime.exec() (or its more convenient wrapper, ProcessBuilder). If that doesn't work, you're SOL unless you can use native code to implement equivalent functionality (ask another question here to learn how to start subprocesses at the C++ level) but that would be at least as error-prone as using the standard methods.
I'd be startled if an application server like Oracle allowed you access to either the functionality of starting subprocesses or of loading native code; both can cause tremendous mischief so untrusted code is barred from them. Looking over your edit, your best approach is going to be to rethink how you tackle your real problem, e.g., by using NIO to manage the sockets in a more efficient fashion (and try to not create extra files on disk; you'll just have to put in extra elaborate code to clean them up…)