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<title>JCommander</title>
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<h1>JCommander</h1>
<h2>Because life is too short to parse command line parameters</h2>
</td>
</tr>
<tr>
<td align="right">
Created: July 13th, 2010
</td>
</tr>
<tr><td align="right"><a href="mailto:cedric@beust.com">Cédric Beust</a></td></tr>
</table>
<h2>Table of contents</h2>
<div id="table-of-contents">
</div>
<h2><a class="section" name="Overview">Overview</a></h2>
JCommander is a very small Java framework that makes it trivial to parse command line parameters.
<p>
You annotate fields with descriptions of your options:
<pre class="brush: java">
import com.beust.jcommander.Parameter;
public class JCommanderTest {
@Parameter
public List<String> parameters = Lists.newArrayList();
@Parameter(names = { "-log", "-verbose" }, description = "Level of verbosity")
public Integer verbose = 1;
@Parameter(names = "-groups", description = "Comma-separated list of group names to be run")
public String groups;
@Parameter(names = "-debug", description = "Debug mode")
public boolean debug = false;
}
</pre>
and then you simply ask JCommander to parse:
<pre class="brush: java">
JCommanderTest jct = new JCommanderTest();
String[] argv = { "-log", "2", "-groups", "unit", "a", "b", "c" };
new JCommander(jct, argv);
Assert.assertEquals(jct.verbose.intValue(), 2);
</pre>
<h2><a class="section" name="Types_of_options">Types of options</a></h2>
The fields representing your parameters can be of any type. Basic types (<tt>Integer</tt>, <tt>Boolean</tt/>., etc...) are supported by default and you can write type converters to support any other type (<tt>File</tt>, etc...).
<h4>Boolean</h4>
When a <tt>Parameter</tt> annotation is found on a field of type <tt>boolean</tt> or <tt>Boolean</tt>, JCommander interprets it as an option with an <em>arity</em> of 0:
<pre class="brush: java">
@Parameter(names = "-debug", description = "Debug mode")
public boolean debug = false;
</pre>
Such a parameter does not require any additional parameter on the command line and if it's detected during parsing, the corresponding field will be set to <tt>true</tt>.
<h4>String, Integer, Long</h4>
When a <tt>Parameter</tt> annotation is found on a field of type <tt>String</tt>, <tt>Integer</tt>, <tt>int</tt>, <tt>Long</tt> or <tt>long</tt>, JCommander will parse the following parameter and it will attempt to cast it to the right type:
<pre class="brush: java">
@Parameter(names = "-log", description = "Level of verbosity")
public Integer verbose = 1;
</pre>
<pre class="brush: plain">
java Main -log 3
</pre>
will cause the field <tt>verbose</tt> to receive the value 3, however:
<pre class="brush: plain">
java Main -log test
</pre>
will cause an exception to be thrown.
<h4>Lists</h4>
When a <tt>Parameter</tt> annotation is found on a field of type <tt>List</tt>, JCommander will interpret it as an option that can occur multiple times:
<pre class="brush: java">
@Parameter(names = "-host", description = "The host")
public List<String> hosts = new ArrayList<String>();
</pre>
will allow you to parse the following command line:
<pre class="brush: plain">
java Main -host host1 -verbose -host host2
</pre>
When JCommander is done parsing the line above, the field <tt>hosts</tt> will contain the strings "host1" and "host2".
<h4>Password</h4>
If one of your parameters is a password or some other value that you do not wish to appear in your history or in clear, you can declare it of type <tt>password</tt> and JCommander will then ask you to enter it in the console:
<pre class="brush: java">
public class ArgsPassword {
@Parameter(names = "-password", description = "Connection password", password = true)
public String password;
}
</pre>
When you run your program, you will get the following prompt:
<pre class="brush: plain">
Value for -password (Connection password):
</pre>
You will need to type the value at this point before JCommander resumes.
<h2><a class="section" name="Custom_types">Custom types</a></h2>
<h3>By annotation</h3>
By default, JCommander parses the command line into basic types only (strings, booleans, integers and longs). Very often, your application actually needs more complex types, such as files, host names, lists, etc... To achieve this, you can write a type converter by implementing the following interface:
<pre class="brush: java">
public interface IStringConverter<T> {
T convert(String value);
}
</pre>
For example, here is a converter that turns a string into a <tt>File</tt>:
<pre class="brush: java">
public class FileConverter implements IStringConverter<File> {
@Override
public File convert(String value) {
return new File(value);
}
}
</pre>
Then, all you need to do is declare your field with the correct type and specify the converter as an attribute:
<pre class="brush: java">
@Parameter(names = "-file", converter = FileConverter.class)
File file;
</pre>
JCommander ships with a few common converters (e.g. one that turns a comma separated list into a <tt>List<String>)</tt>.
<h3>By factory</h3>
If the custom types you use appear multiple times in your application, having to specify the converter in each annotation can become tedious. To address this, you can use an <tt>IStringConverterFactory</tt>:
<pre class="brush: java">
public interface IStringConverterFactory {
<T> Class<? extends IStringConverter<T>> getConverter(Class<T> forType);
}
</pre>
For example, suppose you need to parse a string representing a host and a port:
<pre class="brush: plain">
java App -target example.com:8080
</pre>
You define the holder class :
<pre class="brush: java">
public class HostPort {
public String host;
public Integer port;
}
</pre>
and the string converter to create instances of this class:
<pre class="brush: java">
class HostPortConverter implements IStringConverter<HostPort> {
@Override
public HostPort convert(String value) {
HostPort result = new HostPort();
String[] s = value.split(":");
result.host = s[0];
result.port = Integer.parseInt(s[1]);
return result;
}
}
</pre>
The factory is straightforward:
<pre class="brush: java">
public class Factory implements IStringConverterFactory {
public Class<? extends IStringConverter<?>> getConverter(Class forType) {
if (forType.equals(HostPort.class)) return HostPortConverter.class;
else return null;
}
</pre>
You can now use the type <tt>HostPort</tt> as a parameter without any <tt>converterClass</tt> attribute:
<pre class="brush: java">
public class ArgsConverterFactory {
@Parameter(names = "-hostport")
public HostPort hostPort;
}
</pre>
All you need to do is add the factory to your JCommander object:
<pre class="brush: java">
ArgsConverterFactory a = new ArgsConverterFactory();
JCommander jc = new JCommander(a);
jc.addConverterFactory(new Factory());
jc.parse("-hostport", "example.com:8080");
Assert.assertEquals(a.hostPort.host, "example.com");
Assert.assertEquals(a.hostPort.port.intValue(), 8080);
</pre>
Another advantage of using string converter factories is that your factories can come from a dependency injection framework.
<h2><a class="section" name="Main_parameter">Main parameter</a></h2>
So far, all the <tt>@Parameter</tt> annotations we have seen had defined an attribute called <tt>names</tt>. You can define one (and at most one) parameter without any such attribute. This parameter needs to be a <tt>List<String></tt> and it will contain all the parameters that are not options:
<pre class="brush: java">
@Parameter(description = "Files")
public List<String> files = new ArrayList<String>();
@Parameter(names = "-debug", description = "Debugging level")
public Integer debug = 1;
</pre>
will allow you to parse:
<pre class="brush: plain">
java Main -debug file1 file2
</pre>
and the field <tt>files</tt> will receive the strings "file1" and "file2".
<h2><a class="section" name="Private_parameters">Private parameters</a></h2>
Parameters can be private:
<pre class="brush: java">
public class ArgsPrivate {
@Parameter(names = "-verbose")
private Integer verbose = 1;
public Integer getVerbose() {
return verbose;
}
}
</pre>
<pre class="brush: java">
ArgsPrivate args = new ArgsPrivate();
new JCommander(args, "-verbose", "3");
Assert.assertEquals(args.getVerbose().intValue(), 3);
</pre>
<h2><a class="section" name="Separators">Parameter separators</a></h2>
By default, parameters are separated by spaces, but you can change this setting to allow different separators:
<pre class="brush: plain">
java Main -log:3
</pre>
or
<pre class="brush: plain">
java Main -level=42
</pre>
You define the separator with the <tt>@Parameters</tt> annotation:
<pre class="brush: java">
@Parameters(separators = "=")
public class SeparatorEqual {
@Parameter(names = "-level")
public Integer level = 2;
}
</pre>
<h2><a class="section" name="Multiple_descriptions">Multiple descriptions</a></h2>
You can spread the description of your parameters on more than one
class. For example, you can define the following two classes:
<p>
<h3 class="sourcetitle">ArgsMaster.java</h3>
<pre class="brush: java">
public class ArgsMaster {
@Parameter(names = "-master")
public String master;
}
</pre>
<h3 class="sourcetitle">ArgsSlave.java</h3>
<pre class="brush: java">
public class ArgsSlave {
@Parameter(names = "-slave")
public String slave;
}
</pre>
and pass these two objects to JCommander:
<pre class="brush: java">
ArgsMaster m = new ArgsMaster();
ArgsSlave s = new ArgsSlave();
String[] argv = { "-master", "master", "-slave", "slave" };
new JCommander(new Object[] { m , s }, argv);
Assert.assertEquals(m.master, "master");
Assert.assertEquals(s.slave, "slave");
</pre>
<h2><a class="section" name="Syntax">@ syntax</a></h2>
JCommander supports the @ syntax, which allows you to put all your options into a file and pass this file as parameter:
<p>
<div class="sourcetitle">/tmp/parameters</div>
<pre class="brush: plain">
-verbose
file1
file2
file3
</pre>
<pre class="brush: plain">
java Main @/tmp/parameters
</pre>
<h2><a class="section" name="Arities">Arities (multiple values for parameters)</a></h2>
If some of your parameters require more than one value, such as the
following example where two values are expected after <tt>-pairs</tt>:
<pre class="brush: plain">
java Main -pairs slave master foo.xml
</pre>
then you need to define your parameter with the <tt>arity</tt>
attribute and make that parameter a <tt>List<String></tt>:
<pre class="brush: java">
@Parameter(names = "-pairs", arity = 2, description = "Pairs")
public List<String> pairs;
</pre>
You don't need to specify an arity for parameters of type
<tt>boolean</tt> or <tt>Boolean</tt> (which have a default arity of 0)
and of types <tt>String</tt>, <tt>Integer</tt>, <tt>int</tt>,
<tt>Long</tt> and <tt>long</tt> (which have a default arity of 1).
<p>
Also, note that only <tt>List<String></tt> is allowed for
parameters that define an arity. You will have to convert these values
yourself if the parameters you need are of type <tt>Integer</tt> or
other (this limitation is due to Java's erasure).
<h2><a class="section" name="Multiple_option_names">Multiple option names</a></h2>
You can specify more than one option name:
<pre class="brush: java">
@Parameter(names = { "-d", "--outputDirectory" }, description = "Directory")
public String outputDirectory;
</pre>
will allow both following syntaxes:
<pre class="brush: plain">
java Main -d /tmp
java Main --outputDirectory /tmp
</pre>
<h2><a class="section" name="Required_and_optional">Required and optional parameters</a></h2>
If some of your parameters are mandatory, you can use the
<tt>required</tt> attribute (which default to <tt>false</tt>):
<pre class="brush: java">
@Parameter(names = "-host", required = true)
public String host;
</pre>
If this parameter is not specified, JCommander will throw an exception
telling you which options are missing.
<h2><a class="section" name="Default_values">Default values</a></h2>
The most common way to specify a default value for your parameters is to initialize the field at declaration time:
<pre class="brush: java">
private Integer logLevel = 3;
</pre>
For more complicated cases, you might want to be able to reuse identical default values across several main classes or be able to specify these default values in a centralized location such as a .properties or an XML fie. In this case, you can use an <tt>IDefaultProvider</tt>
<pre class="brush: java">
public interface IDefaultProvider {
/**
* @param optionName The name of the option as specified in the names() attribute
* of the @Parameter option (e.g. "-file").
*
* @return the default value for this option.
*/
String getDefaultValueFor(String optionName);
}
</pre>
By passing an implementation of this interface to your <tt>JCommander</tt> object, you can now control which default value will be used for your options. Note that the value returned by this method will then be passed to a string converter, if any is applicable, thereby allowing you to specify default values for any types you need.
<p>
For example, here is a default provider that will assign a default value of 42 for all your parameters except "-debug":
<pre class="brush: java">
private static final IDefaultProvider DEFAULT_PROVIDER = new IDefaultProvider() {
@Override
public String getDefaultValueFor(String optionName) {
return "-debug".equals(optionName) ? "false" : "42";
}
};
// ...
JCommander jc = new JCommander(new Args());
jc.setDefaultProvider(DEFAULT_PROVIDER);
</pre>
<h2><a class="section" name="Complex">More complex syntaxes (commands)</a></h2>
Complex tools such as <tt>git</tt> or <tt>svn</tt> understand a whole set of commands, each of which with their own specific syntax:
<pre class="brush: plain">
git commit --amend -m "Bug fix"
</pre>
Words such as "commit" above are called "commands" in JCommander, and you can specify them by creating one arg object per command:
<pre class="brush: java">
@Parameters(separators = "=", commandDescription = "Record changes to the repository")
public class CommandCommit {
@Parameter(description = "The list of files to commit")
public List<String> files;
@Parameter(names = "--amend", description = "Amend")
public Boolean amend = false;
@Parameter(names = "--author")
public String author;
}
</pre>
<pre class="brush: java">
@Parameters(commandDescription = "Add file contents to the index")
public class CommandAdd {
@Parameter(description = "File patterns to add to the index")
public List<String> patterns;
@Parameter(names = "-i")
public Boolean interactive = false;
}
</pre>
Then you register these commands with your JCommander object. After the parsing phase, you call <tt>getParsedCommand()</tt> on your JCommander object, and based on the command that is returned, you know which arg object to inspect (you can still use a main arg object if you want to support options before the first command appears on the command line):
<pre class="brush: java">
CommandMain cm = new CommandMain();
JCommander jc = new JCommander(cm);
CommandAdd add = new CommandAdd();
jc.addCommand("add", add);
CommandCommit commit = new CommandCommit();
jc.addCommand("commit", commit);
jc.parse("-v", "commit", "--amend", "--author=cbeust", "A.java", "B.java");
Assert.assertTrue(cm.verbose);
Assert.assertEquals(jc.getParsedCommand(), "commit");
Assert.assertTrue(commit.amend);
Assert.assertEquals(commit.author, "cbeust");
Assert.assertEquals(commit.files, Arrays.asList("A.java", "B.java"));
</pre>
<h2><a class="section" name="Exceptions">Exception</a></h2>
Whenever JCommander detects an error, it will throw a
<tt>ParameterException</tt>. Note that this is a Runtime Exception,
since your application is probably not initialized correctly at this
point.
<h2><a class="section" name="Usage">Usage</a></h2>
You can invoke <tt>usage()</tt> on the <tt>JCommander</tt> instance that you used to parse your command line in order to generate a summary of all the options that your program understands:
<pre class="brush: plain">
Usage: <main class> [options]
Options:
-debug Debug mode (default: false)
-groups Comma-separated list of group names to be run
* -log, -verbose Level of verbosity (default: 1)
-long A long number (default: 0)
</pre>
Options preceded by an asterisk are required.
<h2><a class="section" name="Hiding">Hiding parameters</a></h2>
If you don't want certain parameters to appear in the usage, you can mark them as "hidden":
<pre class="brush: java">
@Parameter(names = "-debug", description = "Debug mode", hidden = true)
public boolean debug = false;
</pre>
<h2><a class="section" name="Internationalization">Internationalization</a></h2>
You can internationalize the descriptions of your parameters.
<p>
First you use the <tt>@Parameters</tt> annotation at the top of your class to define the name of your message bundle, and then you use the <tt>descriptionKey</tt> attribute instead of <tt>description</tt> on all the <tt>@Parameters</tt> that require translations. This <tt>descriptionKey</tt> is the key to the string into your message bundle:
<h3 class="sourcetitle">I18N.java</h3>
<pre class="brush:java">
@Parameters(resourceBundle = "MessageBundle")
public class ArgsI18N2 {
@Parameter(names = "-host", description = "Host", descriptionKey = "host")
String hostName;
}
</pre>
Your bundle needs to define this key:
<br>
<h3 class="sourcetitle">MessageBundle_fr_FR.properties</h3>
<pre class="brush: plain">
host: Hôte
</pre>
JCommander will then use the default locale to resolve your descriptions.
<h2><a class="section" name="Scala">JCommander in Scala</a></h2>
Here is a quick example of how to use JCommander in Scala (courtesy of Patrick Linskey):
<pre class="brush: java">
import java.io.File
import com.beust.jcommander.{JCommander, Parameter}
import collection.JavaConversions._
object Main {
object Args {
// Declared as var because JCommander assigns a new collection; declared
// as java.util.List because that's what JCommander will replace with.
// It'd be nice if JCommander would just use the provided List so this
// could be a val and a Scala LinkedList.
@Parameter(
names = Array("-f", "--file"),
description = "File to load. Can be specified multiple times.")
var file: java.util.List[String] = null
}
def main(args: Array[String]): Unit = {
new JCommander(Args, args.toArray: _*)
for (filename <- Args.file) {
val f = new File(filename)
printf("file: %s\n", f.getName)
}
}
}
</pre>
<h2><a class="section" name="More_examples">More examples</a></h2>
TestNG uses JCommander to parse its command line, here is <a href="http://github.com/cbeust/testng/blob/master/src/main/java/org/testng/CommandLineArgs.java">its definition file</a>.
<h2><a class="section" name="Javadocs">Javadocs</a></h2>
The Javadocs for JCommander can be found <a href="http://beust.com/jcommander/apidocs/">here</a>.
<h2><a class="section" name="Download">Download</a></h2>
You can download JCommander from the following locations:
<ul>
<li><a href="http://github.com/cbeust/jcommander">Source on github</a></li>
<li><a href="http://beust.com/jcommander/jcommander-1.7.jar">Jar file</a></li>
<li>Or if you are using Maven, add the following dependency to your <tt>pom.xml</tt>:
<pre class="brush: xml">
<dependency>
<groupId>com.beust</groupId>
<artifactId>jcommander</artifactId>
<version>1.7</version>
</dependency>
</pre>
</ul>
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