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Flag Command Line Library for Java

  • Author: Kenny Yu
  • Package: me.kennyyu.flags


This library is a simple lightweight framework for easily creating and using command line flags. All it takes is a simple annotation with @FlagInfo and an invocation to Flags.parse to create and use a flag! The library uses the Reflections library to scan Flag objects at runtime.

This library supports parsing primitive wrapper class types, java.util.Collection types such as List, Set, and Map, and enumerations.


  • Guava 12.0
  • Reflections 0.9.8
  • JUnit 4.0

How to Install

To install the library, you must have maven installed.

For Maven Users

To use this flag library, include this in your pom.xml:


Manual Library Install

If you would rather include the jar manually, first clone the this repository:

$ git clone git://

Next, build the maven package with all of the necessary dependencies:

$ cd flags
flags$ mvn assembly:single

This should create a flags-1.0-jar-with-dependencies.jar inside the target directory:

flags$ ls target
flags-1.0-jar-with-dependencies.jar ...

Include the jar in your classpath when compiling your application.

How to Create a Flag

To create a new flag, create a new static me.kennyyu.flags.Flag field. The parameter type of the field will be the type of the flag. Then annotate the field with me.kennyyu.flags.FlagInfo and provide the necessary fields. Example:

@FlagInfo(help = "maximum number of threads to use", altName = "n")
private static final Flag<Integer> maxNumThreads = Flags.valueOf(4);

This example declares a new flag indicating the maximum number of threads to use. On the right hand side, you may provide a default value for the flag.

me.kennyyu.flags.FlagInfo takes several parameters:

  • String help : (required) help message for this flag
  • String altName : (optional) short name for this flag. Only one dash is needed when using the alternate name.
  • String environment : (optional) environment of this flag. Different flag environments may be loaded for different use cases. The default environment is the empty string and will be loaded by default when no environment is provided to Flags.parse.

Parsing Command Line Arguments

Once you have defined the flag, you must call either Flags.parse(String, String...) or Flags.parseWithExceptions(String, String...). The difference between these two is that Flags.parse may throw an unchecked RuntimeException whereas Flags.parseWithExceptions forces you to deal with the checked exception. Here's a complete example:

import me.kennyyu.flags.Flag;
import me.kennyyu.flags.Flags;

import java.util.List;

public class MyApp {
  @FlagInfo(help = "maximum number of threads to use", altName = "n")
  private static final Flag<Integer> maxNumThreads = Flags.valueOf(3);

  @FlagInfo(help = "use real logger", altName = "r")
  private static final Flag<Boolean> useRealLogger = Flags.valueOf(false);

  @FlagInfo(help = "input list", altName = "l")
  private static final Flag<List<String>> inputList = Flags.valueOf(new ArrayList<String>());

  public static void main(String[] args) {

Providing Command Line Arguments

To pass in the value via command line, run the class with flags passed in the format:

$ java MyApp --maxNumThreads=5 --useRealLogger -l=foo,bar,baz

All classes referenced from the main class with flags will be available as options. If --help or -h is passed in, then a help menu will be printed with all available flag options, and the JVM will exit with a 0 exit status.

Loading Different Environments

Loading different environments is especially useful when we wish to separate testing flags from production flags: testing flags should never be allowed in a production environment.

For example, if we have a flag defined by:

@FlagInfo(help = "use mocking libraries", altName = "mock", environment = "test")
private static Flag<Boolean> useMockLibraries = Flags.valueOf(false);

Then we can choose to load the test environment in main with:

public static void main(String[] args) {
  Flags.parse(args, "test");

If no environment is provided in FlagInfo, then the empty string will be used as the default environment. If no environment is provided to Flags.parse or Flags.parseWithException, then the default environment will be loaded.

Supported Flag Types

Primitive Wrapper Classes

The currently supported types for flags include wrapper classes:

  • Integer
  • Long
  • Short
  • Boolean
  • Double
  • Float
  • Character
  • String
  • Byte

Boolean flags have short hand where --booleanFlag=true is the same as --boleanFlag.


Flags also support enumeration types. Example:

private enum Status {

@FlagInfo(help = "enum example")
private static final Flag<Status> status = Flags.valueOf(Status.RUNNING);

To run it:

$ java MyApp --status=TERMINATED


Flags also support java.util.Collection types.


To pass in a java.util.List:

@FlagInfo(help = "list example")
private static final Flag<List<Integer>> list = Flags.valueOf(new ArrayList<Integer>());

To run it:

$ java MyApp --list=3,4,5,6,6,7


To pass in a java.util.Set:

@FlagInfo(help = "set example")
private static final Flag<Set<String>> set = Flags.valueOf(new HashSet<String>());

To run it:

$ java MyApp --set=foo,cheese,bar


To pass in a java.util.Map:

@FlagInfo(help = "map example")
private static final Flag<Map<String, Integer>> map = Flags.valueOf(new HashMap<String, Integer>());

To run it:

$ java MyApp --map="foo:3 bar:4 cheese:5 bam:6"

The (key,value) pairs must be passed inside double quotes in the form key:value separated by spaces.


Java library for command line arguments







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