Packages

package app

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  1. trait App extends Closable with CloseAwaitably

    A composable application trait that includes flag parsing as well as basic application lifecycle (pre- and post- main).

    A composable application trait that includes flag parsing as well as basic application lifecycle (pre- and post- main). Flag parsing is done via com.twitter.app.Flags, an instance of which is defined in the member flag. Applications should be constructed with modularity in mind, and common functionality should be extracted into mixins.

    Flags should only be constructed in the constructor, and should only be read in the premain or later, after they have been parsed.

    object MyApp extends App {
      val n = flag("n", 100, "Number of items to process")
    
      def main() {
        for (i <- 0 until n()) process(i)
      }
    }

    Note that a missing main is OK: mixins may provide behavior that does not require defining a custom main method.

  2. class Flag[T] extends AnyRef

    A single command-line flag, instantiated by a com.twitter.app.Flags instance.

    A single command-line flag, instantiated by a com.twitter.app.Flags instance.

    The current value can be extracted via apply(), get, and getWithDefault. Local-scoped modifications of their values, which can be useful for tests, can be done by calls to let and letClear.

    Using a String-typed Flag, myFlag, which is unset and has a default value of "DEFAULT" as an example:

    myFlag.isDefined // => false
    myFlag.get // => None
    myFlag.getWithDefault // => Some("DEFAULT")
    myFlag() // => "DEFAULT"
    
    myFlag.let("a value") {
      myFlag.isDefined // => true
      myFlag.get // => Some("a value")
      myFlag.getWithDefault // => Some("a value")
      myFlag() // => "a value"
    
      myFlag.letClear {
        myFlag.isDefined // => false
        myFlag.get // => None
        myFlag.getWithDefault // => Some("DEFAULT")
        myFlag() // => "DEFAULT"
      }
    }
    See also

    com.twitter.app.GlobalFlag

    com.twitter.app.Flags for information on how flags can be set by the command line.

  3. case class FlagParseException(message: String, cause: Throwable = null) extends Exception with Product with Serializable

    Exception thrown upon flag-parsing failure.

    Exception thrown upon flag-parsing failure. Should typically lead to process death, since continued execution would run the risk of unexpected behavior on account of incorrectly-interpreted or malformed flag values.

    message

    A string name of the flag for which parsing failed.

    cause

    The underlying java.lang.Throwable that caused this exception.

  4. class FlagUndefinedException extends Exception
  5. case class FlagUsageError(usage: String) extends Exception with Product with Serializable
  6. class FlagValueRequiredException extends Exception
  7. abstract class Flaggable[T] extends AnyRef

    A type class providing evidence for parsing type T as a flag value.

    A type class providing evidence for parsing type T as a flag value.

    Any class that is to be provided as a flaggable value must have an accompanying implicit Flaggable (contained within a companion object of the class in question) for converting a string to an object of that type. For instance, to make a hypothetical type called Foo flaggable:

    class Foo {
      ...
    }
    
    object Foo {
      implicit val flagOfFoo = new Flaggable[Foo] {
        def parse(v: String): Foo = {
          ...
        }
      }
    }

    For simple implicit definitions based on existing String => T functions, use the Flaggable.mandatory function:

    object Foo {
      def parse(v: String): Foo = {
         ...
      }
    
      implicit val ofFoo = Flaggable.mandatory(Foo.parse(_))
    }

    [1] http://en.wikipedia.org/wiki/Type_class

  8. class Flags extends AnyRef

    A simple flags implementation.

    A simple flags implementation. We support only two formats:

    for flags with optional values (e.g. booleans): -flag, -flag=value for flags with required values: -flag[= ]value

    That's it. These can be parsed without ambiguity.

    There is no support for mandatory arguments: That is not what flags are for.

    Flags.apply adds a new flag to the flag set, so it is idiomatic to assign instances of Flags to a singular flag val:

    val flag = new Flags("myapp")
    val i = flag("i", 123, "iteration count")

    Global flags, detached from a particular Flags instance but accessible to all, are defined by com.twitter.app.GlobalFlag.

  9. abstract class GlobalFlag[T] extends Flag[T]

    Subclasses of GlobalFlag (that are defined in libraries) are "global" in the sense that they are accessible by any application that depends on that library.

    Subclasses of GlobalFlag (that are defined in libraries) are "global" in the sense that they are accessible by any application that depends on that library. Regardless of where in a library a GlobalFlag is defined, a value for it can be passed as a command-line flag by any binary that includes the library. The set of defined GlobalFlags can be enumerated (via GlobalFlag.getAll) by the application.

    A GlobalFlag must be declared as an object (see below for Java):

    import com.twitter.app.GlobalFlag
    
    object myFlag extends GlobalFlag[String]("default value", "this is my global flag")

    All such global flag declarations in a given classpath are visible to and used by com.twitter.app.App.

    A flag's name (as set on the command line) is its fully-qualified classname. For example, the flag

    package com.twitter.server
    
    import com.twitter.app.GlobalFlag
    
    object port extends GlobalFlag[Int](8080, "the TCP port to which we bind")

    is settable by the command-line flag -com.twitter.server.port=8080.

    Global flags may also be set by Java system properties with keys named in the same way. However, values supplied by flags override those supplied by system properties.

    Annotations
    @GlobalFlagVisible()
  10. class GlobalFlagVisible extends Annotation with Annotation with ClassfileAnnotation
  11. abstract class JavaGlobalFlag[T] extends GlobalFlag[T]

Value Members

  1. object App
  2. object Flag
  3. object Flaggable

    Default Flaggable implementations.

  4. object Flags
  5. object GlobalFlag
  6. object LoadService

    Load classes in the manner of java.util.ServiceLoader.

    Load classes in the manner of java.util.ServiceLoader. It is more resilient to varying Java packaging configurations than ServiceLoader.

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