JVM daemon

daemonjava

I'm curious, is there a JVM which runs as an OS daemon, and forks when a new process should be started in it ( i.e. like init, or zygote process on Android )? If no, why not? I am not talking about a daemon thread within the JVM.

The reason why I'm asking, is because the JVM startup time is quite slow, and incurs a heavy toll on scripting.

Best Answer

JVM startup time is quite slow, and incurs a heavy toll on scripting . . . huge optimization for java process startup time.

The other answers answer the question with respect to generic app startup, which are useful considerations.

But seems your primary question is Java application startup performance re overhead of starting a jvm. This has also been my question. I found this:

If willing to sacrifice user-separation security, check out Nailgun, an Insanely Fast Java:

Nailgun is a client, protocol, and server for running Java programs from the command line without incurring the JVM startup overhead. Programs run in the server (which is implemented in Java), and are triggered by the client (written in C), which handles all I/O.

Related Solutions

Is there something like a per-user daemon

systemd allows users to run their own systemd instances to manage private daemons.

If you already have systemd installed, all you have to do is launch systemd --user and manage your services by running systemctl --user. User services will be searched for in ~/.config/systemd/user.

By default systemd will kill user services on logout (as you requested). This behavior can be altered by enabling lingering for a user with the loginctl --enable-linger $USER command.

Mor information can be found on the ArchWiki page.

Configure java daemon with systemd

Here's some minor modifications:

Since it listens on a network socket, make it a dependency of network.target.
nohup is not needed since systemd will daemonize the executable for you.
I think a separate shell script would be an overkill, so just merge it into the service file.
Redirection (< /dev/null and so forth) isn't needed since systemd sets up an appropriate standard I/O context. Indeed, if you take the redirection out systemd will log anything sent to standard output by the Java program in its journal, with no special logging mechanism required.
Running asynchonously from the invoking shell (&) isn't needed or appropriate.
There's a specific behaviour pattern required by Type=forking, and if it isn't followed by the dæmon things go wrong. So try for Type=simple (or Type=notify).

So the service file looks like this:

[Unit]
Description=Some job
After=network.target

[Service]
WorkingDirectory=/home/user/tmp/testout
SyslogIdentifier=SocketTest
ExecStart=/bin/sh -c "exec java -jar /home/user/programming/tests/java/core/SocketTest/SocketTest.jar"
User=dlt
Type=simple

[Install]
WantedBy=multi-user.target

Notes:

You cannot just use java as the name of the program to run. systemd doesn't search PATH for executables, and the name of the executable given to ExecStart must be absolute. So if you want path searching you have to invoke via a shell or /usr/bin/env. We choose /bin/sh here.
Because this is Type=simple the shell must exec Java, not run it as a child process. systemd controls the service through the main process, and that needs to be Java, not a parent shell process.
Because this isn't invoking the Java executable directly, systemd will put the name sh in its journal as the service name. See How to avoid /usr/bin/env being marked in systemd logs as the executable for more on this.

As far as I know, there is no special caveat of running Java application with Systemd.

Best Answer

Related Solutions

Is there something like a per-user daemon

Configure java daemon with systemd

Related Question