Scala Library: scala.sys
scala.sys
The package object scala.sys
contains methods for reading and altering core
aspects of the virtual machine as well as the world outside of it.
- Source
- Version
- 2.9
- Since
- 2.9
Type Members
trait BooleanProp extends Prop[Boolean]
A few additional conveniences for Boolean properties.
trait Prop[+T] extends AnyRef
A lightweight interface wrapping a property contained in some unspecified map. Generally it’ll be the system properties but this is not a requirement.
See scala.sys.SystemProperties
for an example usage.
- Source
- Version
- 2.9
- Since
- 2.9
class ShutdownHookThread extends Thread
A minimal Thread wrapper to enhance shutdown hooks. It knows how to unregister itself.
- Source
- Version
- 2.9
- Since
- 2.9
class SystemProperties extends AbstractMap[String, String] with Map[String, String]
A bidirectional map wrapping the java System properties. Changes to System properties will be immediately visible in the map, and modifications made to the map will be immediately applied to the System properties. If a security manager is in place which prevents the properties from being read or written, the AccessControlException will be caught and discarded.
Value Members
object BooleanProp
object Prop
object ShutdownHookThread
object SystemProperties
The values in SystemProperties can be used to access and manipulate designated
system properties. See scala.sys.Prop
for particulars.
Example:
package process
This package handles the execution of external processes. The contents of this package can be divided in three groups, according to their responsibilities:
- Indicating what to run and how to run it.
- Handling a process input and output.
- Running the process.
For simple uses, the only group that matters is the first one. Running an
external command can be as simple as "ls".!
, or as complex as building a
pipeline of commands such as this:
We describe below the general concepts and architecture of the package, and then take a closer look at each of the categories mentioned above.
Concepts and Architecture
The underlying basis for the whole package is Java’s Process
and
ProcessBuilder
classes. While there’s no need to use these Java classes, they
impose boundaries on what is possible. One cannot, for instance, retrieve a _
process id_ for whatever is executing.
When executing an external process, one can provide a command’s name, arguments
to it, the directory in which it will be executed and what environment variables
will be set. For each executing process, one can feed its standard input through
a java.io.OutputStream
, and read from its standard output and standard error
through a pair of java.io.InputStream
. One can wait until a process finishes
execution and then retrieve its return value, or one can kill an executing
process. Everything else must be built on those features.
This package provides a DSL for running and chaining such processes, mimicking Unix shells ability to pipe output from one process to the input of another, or control the execution of further processes based on the return status of the previous one.
In addition to this DSL, this package also provides a few ways of controlling input and output of these processes, going from simple and easy to use to complex and flexible.
When processes are composed, a new ProcessBuilder
is created which, when run,
will execute the ProcessBuilder
instances it is composed of according to the
manner of the composition. If piping one process to another, they’ll be executed
simultaneously, and each will be passed a ProcessIO
that will copy the output
of one to the input of the other.
What to Run and How
The central component of the process execution DSL is the
scala.sys.process.ProcessBuilder trait. It is ProcessBuilder
that implements
the process execution DSL, that creates the scala.sys.process.Process that will
handle the execution, and return the results of such execution to the caller. We
can see that DSL in the introductory example: #|
, #&&
and #!!
are methods
on ProcessBuilder
used to create a new ProcessBuilder
through composition.
One creates a ProcessBuilder
either through factories on the
scala.sys.process.Process ‘s companion object, or through implicit conversions
available in this package object itself. Implicitly, each process is created
either out of a String
, with arguments separated by spaces – no escaping of
spaces is possible – or out of a scala.collection.Seq, where the first element
represents the command name, and the remaining elements are arguments to it. In
this latter case, arguments may contain spaces.
To further control what how the process will be run, such as specifying the directory in which it will be run, see the factories on scala.sys.process.Process ‘s object companion.
Once the desired ProcessBuilder
is available, it can be executed in different
ways, depending on how one desires to control its I/O, and what kind of result
one wishes for:
- Return status of the process (
!
methods) - Output of the process as a
String
(!!
methods) - Continuous output of the process as a
Stream[String]
(lines
methods) - The
Process
representing it (run
methods)
Some simple examples of these methods:
We’ll see more details about controlling I/O of the process in the next section.
Handling Input and Output
In the underlying Java model, once a Process
has been started, one can get
java.io.InputStream
and java.io.OutputStream
representing its output and
input respectively. That is, what one writes to an OutputStream
is turned into
input to the process, and the output of a process can be read from an
InputStream
– of which there are two, one representing normal output, and
the other representing error output.
This model creates a difficulty, which is that the code responsible for actually running the external processes is the one that has to take decisions about how to handle its I/O.
This package presents an alternative model: the I/O of a running process is
controlled by a scala.sys.process.ProcessIO object, which can be passed to the
code that runs the external process. A ProcessIO
will have direct access to
the java streams associated with the process I/O. It must, however, close these
streams afterwards.
Simpler abstractions are available, however. The components of this package that handle I/O are:
- scala.sys.process.ProcessIO : provides the low level abstraction.
- scala.sys.process.ProcessLogger : provides a higher level abstraction for output, and can be created through its object companion
- scala.sys.process.BasicIO : a library of helper methods for the creation of
ProcessIO
. - This package object itself, with a few implicit conversions.
Some examples of I/O handling:
Instances of the java classes java.io.File
and java.net.URL
can both be used
directly as input to other processes, and java.io.File
can be used as output
as well. One can even pipe one to the other directly without any intervening
process, though that’s not a design goal or recommended usage. For example, the
following code will copy a web page to a file:
More information about the other ways of controlling I/O can be looked at in the scaladoc for the associated objects, traits and classes.
Running the Process
Paradoxically, this is the simplest component of all, and the one least likely to be interacted with. It consists solely of scala.sys.process.Process, and it provides only two methods:
exitValue()
: blocks until the process exit, and then returns the exit value. This is what happens when one uses the!
method ofProcessBuilder
.-
destroy()
: this will kill the external process and close the streams associated with it. - Source
Value Members From scala.sys
def allThreads(): IndexedSeq[Thread]
Returns all active thread in the current thread’s thread group and subgroups.
- returns
- an IndexedSeq containing the threads.
(defined at scala.sys)
def addShutdownHook(body: ⇒ Unit): ShutdownHookThread
Register a shutdown hook to be run when the VM exits. The hook is automatically registered: the returned value can be ignored, but is available in case the Thread requires further modification. It can also be unregistered by calling ShutdownHookThread#remove().
Note that shutdown hooks are NOT guaranteed to be run.
- body
- the body of code to run at shutdown
- returns
- the Thread which will run the shutdown hook.
- See also
- scala.sys.ShutdownHookThread
(defined at scala.sys)
def env: Map[String, String]
An immutable Map representing the current system environment.
- returns
- a Map containing the system environment variables.
(defined at scala.sys)
def error(message: String): Nothing
Throw a new RuntimeException with the supplied message.
- returns
- Nothing.
(defined at scala.sys)
def exit(status: Int): Nothing
Exit the JVM with the given status code.
- returns
- Nothing.
(defined at scala.sys)
def props: SystemProperties
A bidirectional, mutable Map representing the current system Properties.
- returns
- a SystemProperties.
- See also
- scala.sys.SystemProperties (defined at scala.sys)
Full Source:
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