Abstract: This document specifies the behavior and life cycle of an abstract Java™ daemon, in relation to its native container. In addition it defines a mechanism for controlling a daemon, and its interaction with the native OS process in which its existance is confined.
Since 1994, the Java™ programming language evolved and became a
valid tool to develop, other than applets and client applications,
reliable and performant server applications. The major disadvantage of
the Java™ platform is that still today the only portable way to
start a Java™ applcation relies on a single point of entry: the
public static void main(String[])
method.
Having a single-point of entry is a valid solution for client
applications, where interactively a user can command to the application
to quit (which can terminate the Virtual Machine process at calling the
System.exit(int)
method), but in those cases where the application is not interactive
(server applications) there is currently no portable way to notify
the Virtual Machine of its imminent shutdown.
A server application written in Java might have to perform several tasks before being able to shutdown the Virtual Machine process. For example in the case of a Servlet container, before the VM process is shut down, sessions might need to be serialized to disk, and web applications need to be destroyed.
One common solution to this problem is to create (for example) a
ServerSocket and wait for a particular
message to be issued. When the message is received, all operations
required to shut down the server applications are performed and at the
end the System.exit method is called
to terminate the Virtual Machine process. This method, however, implies
several disadvantages and risks: for example in case of a system-wide
shutdown, it might happen that the Virtual Machine process will be shut
down directly by the operating system, without notifying the running
server application. Or, for example, if an attacker finds out what is
the required message to send to the server, and discovers a way to send
this message to the running server application, he can easily interrupt
the operation of a server, bypassing all the security restrictions
implemented in the operating system.
Most multi-user operating systems already have a way in which server applications are started and stopped, under Unix based operating systems non interactive server applications are called daemons and are controlled by the operating system with a set of specified signals. Under Windows such programs are called daemons and are controlled by appropriate calls to specific functions defined in the application binary, but although the ways of dealing with the problem are different, in both cases the operating system can notify a server application of its imminent shutdown, and the application has the ability to perform certain tasks before its process of execution is destroyed.
The scope of this specification is to define an API in line with the
current Java™ Platform APIs to support an alternative invocation
mechanism which could be used instead of the above mentioned
public static void main(String[])
method. This specification cover the behavior and life cycle of what
we define as "Java ™ daemons", or, in other words,
non interactive Java™ applications.
This specification does not cover how the container of a Java™
daemon must be implemented, or how to build a native liaison between
the operating system and the Daemon
interface, but defines the relation between the an operating system
process and the Daemon implementation
life cycle. It should be trivial for implementors to build a native
liaison and container for Java™ daemons.
This specification, together with the related API documentation, can be used by software deveopers to build portable non interactive applications based on the Java™ platform.
The Daemon interface (defined as
follows) is the main interface representing a Java™ daemon:
package org.apache.commons.daemon;
public interface Daemon
extends Runnable {
public void init(DaemonContext context)
throws Exception;
public void run();
public void die();
public void destroy();
}
A daemon is managed through a well defined life cycle that defines how
it is loaded, initialized, started, stopped and destroyed. This life
cycle is expressed in the API by the init(...),
run(), die() and destroy() methods
of the Daemon interface.
The daemon container is responsible for instantiating daemons. Because
of this, concrete implementations of the Daemon
interface must always expose a public void constructor, which will be
used by the daemon container to create instances of the class.
For example:
package mypackage;
import org.apache.commons.daemon.*;
public class MyDaemon
implements Daemon {
/** This constructor must be here. */
public MyDaemon() {
super();
...
}
...
}
Once the Virtual Machine process is started in the underlying operating
system, and the Virtual Machine itself is created and configured, the
daemon container associated with its native liaison constructs a new
instance of the concrete class implementing the
Daemon instance. It is not defined by
this specification how the instance is constructed, or how many instances
of one or more concrete daemons implementation can coexist in the same
Virtual Machine.
As a side note, in case of multiple daemons running within the scope of the same virtual machine, developers should not rely on static methods and variables for proper operation, as it is advisable for security reasons to load each different daemon instance in a separate class loader.
After the daemon instance has been constructed, the container must initialize it before it can be started. Initialization is provided so that a daemon can read persisten configuration data, access native resources, and perform other one-time activities.
Under certain operating systems (typically Unix based operating systems),
and if the native liaison of the container supports and it is configured
accordingly, this method might be called with super-user privileges.
For example, within the scope of this method, it might be wise to create
all required ServerSocket instances (as
under several operating systems this requires super-user privileges).
That said, it is very important that within the scope of this method
"untrusted" data cannot be processed (for example, starting
an acceptor thread on a ServerSocket
which receives and processes data from unknown sources), as this would
serious security hazards.
The daemon container must guarantee that the init(...)
method is called at least once, and only once on an instance of a
daemon object, and that it will be called before any access to the
run(), die() or destroy()
methods.
During initialization, the daemon instance can throw any
Exception preventing a the successful
completion of the init(...) method. In this case the daemon
instance must not be started, and the destroy() method must
not be called before the instance is released by the container to allow
garbage collecting.
As the Daemon interface extends the
Runnable interface, to start the
operation of a daemon, the container calls the run() method
within the scope of a Thread. A
daemon is said to be "running" until the thread
Author Pier Fumagalli
<pier.fumagalli@sun.com>
Copyright © 2001, The Apache Software Foundation
<http://www.apache.org/>
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