CommandLinePropertySource (Spring Framework 3.2.1.RELEASE API)
Date: 2013-01-24

org.springframework.core.env

## Class CommandLinePropertySource<T>

- java.lang.Object
- - org.springframework.core.env.PropertySource<T>
  - - org.springframework.core.env.CommandLinePropertySource<T>

- Direct Known Subclasses:
    JOptCommandLinePropertySource, SimpleCommandLinePropertySource

      public abstract class CommandLinePropertySource<T>
      extends PropertySource<T>

  Abstract base class for `PropertySource` implementations backed by command line arguments. The parameterized type `T` represents the underlying source of command line options. This may be as simple as a String array in the case of `SimpleCommandLinePropertySource`, or specific to a particular API such as JOpt's `OptionSet` in the case of `JOptCommandLinePropertySource`.

  ### Purpose and General Usage

  For use in standalone Spring-based applications, i.e. those that are bootstrapped via a traditional `main` method accepting a `String[]` of arguments from the command line. In many cases, processing command-line arguments directly within the `main` method may be sufficient, but in other cases, it may be desirable to inject arguments as values into Spring beans. It is this latter set of cases in which a `CommandLinePropertySource` becomes useful. A `CommandLinePropertySource` will typically be added to the `Environment` of the Spring `ApplicationContext`, at which point all command line arguments become available through the `PropertyResolver.getProperty(String)` family of methods. For example:

       public static void main(String[] args) {
           CommandLinePropertySource clps = ...;
           AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
           ctx.getEnvironment().getPropertySources().addFirst(clps);
           ctx.register(AppConfig.class);
           ctx.refresh();
       }

  With the bootstrap logic above, the `AppConfig` class may `@Inject` the Spring `Environment` and query it directly for properties:

       @Configuration
       public class AppConfig {
           @Inject Environment env;

           @Bean
           public void DataSource dataSource() {
               MyVendorDataSource dataSource = new MyVendorDataSource();
               dataSource.setHostname(env.getProperty("db.hostname", "localhost"));
               dataSource.setUsername(env.getRequiredProperty("db.username"));
               dataSource.setPassword(env.getRequiredProperty("db.password"));
               // ...
               return dataSource;
           }
       }

  Because the `CommandLinePropertySource` was added to the `Environment`'s set of `MutablePropertySources` using the `#addFirst` method, it has highest search precedence, meaning that while "db.hostname" and other properties may exist in other property sources such as the system environment variables, it will be chosen from the command line property source first. This is a reasonable approach given that arguments specified on the command line are naturally more specific than those specified as environment variables.

  As an alternative to injecting the `Environment`, Spring's `@Value` annotation may be used to inject these properties, given that a `PropertySourcesPropertyResolver` bean has been registered, either directly or through using the `<context:property-placeholder>` element. For example:

       @Component
       public class MyComponent {
           @Value("my.property:defaultVal")
           private String myProperty;

           public void getMyProperty() {
               return this.myProperty;
           }

           // ...
       }

  ### Working with option arguments

  Individual command line arguments are represented as properties through the usual `PropertySource.getProperty(String)` and `PropertySource.containsProperty(String)` methods. For example, given the following command line:

       --o1=v1 --o2

  'o1' and 'o2' are treated as "option arguments", and the following assertions would evaluate true:

       CommandLinePropertySource ps = ...
       assert ps.containsProperty("o1") == true;
       assert ps.containsProperty("o2") == true;
       assert ps.containsProperty("o3") == false;
       assert ps.getProperty("o1").equals("v1");
       assert ps.getProperty("o2").equals("");
       assert ps.getProperty("o3") == null;

  Note that the 'o2' option has no argument, but `getProperty("o2")` resolves to empty string (`""`) as opposed to `null`, while `getProperty("o3")` resolves to `null` because it was not specified. This behavior is consistent with the general contract to be followed by all `PropertySource` implementations.

  Note also that while "--" was used in the examples above to denote an option argument, this syntax may vary across individual command line argument libraries. For example, a JOpt- or Commons CLI-based implementation may allow for single dash ("-") "short" option arguments, etc.

  ### Working with non-option arguments

  Non-option arguments are also supported through this abstraction. Any arguments supplied without an option-style prefix such as "-" or "--" are considered "non-option arguments" and available through the special "nonOptionArgs" property. If multiple non-option arguments are specified, the value of this property will be a comma-delimited string containing all of the arguments. This approach ensures a simple and consistent return type (String) for all properties from a `CommandLinePropertySource` and at the same time lends itself to conversion when used in conjunction with the Spring `Environment` and its built-in `ConversionService`. Consider the following example:

       --o1=v1 --o2=v2 /path/to/file1 /path/to/file2

  In this example, "o1" and "o2" would be considered "option arguments", while the two filesystem paths qualify as "non-option arguments". As such, the following assertions will evaluate true:

       CommandLinePropertySource ps = ...
       assert ps.containsProperty("o1") == true;
       assert ps.containsProperty("o2") == true;
       assert ps.containsProperty("nonOptionArgs") == true;
       assert ps.getProperty("o1").equals("v1");
       assert ps.getProperty("o2").equals("v2");
       assert ps.getProperty("nonOptionArgs").equals("/path/to/file1,/path/to/file2");

  As mentioned above, when used in conjunction with the Spring `Environment` abstraction, this comma-delimited string may easily be converted to a String array or list:

       Environment env = applicationContext.getEnvironment();
       String[] nonOptionArgs = env.getProperty("nonOptionArgs", String[].class);
       assert nonOptionArgs[0].equals("/path/to/file1");
       assert nonOptionArgs[1].equals("/path/to/file2");

  The name of the special "non-option arguments" property may be customized through the `setNonOptionArgsPropertyName(String)` method. Doing so is recommended as it gives proper semantic value to non-option arguments. For example, if filesystem paths are being specified as non-option arguments, it is likely preferable to refer to these as something like "file.locations" than the default of "nonOptionArgs":

       public static void main(String[] args) {
           CommandLinePropertySource clps = ...;
           clps.setNonOptionArgsPropertyName("file.locations");

           AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
           ctx.getEnvironment().getPropertySources().addFirst(clps);
           ctx.register(AppConfig.class);
           ctx.refresh();
       }

  ### Limitations

  This abstraction is not intended to expose the full power of underlying command line parsing APIs such as JOpt or Commons CLI. It's intent is rather just the opposite: to provide the simplest possible abstraction for accessing command line arguments after they have been parsed. So the typical case will involve fully configuring the underlying command line parsing API, parsing the `String[]` of arguments coming into the main method, and then simply providing the parsing results to an implementation of `CommandLinePropertySource`. At that point, all arguments can be considered either 'option' or 'non-option' arguments and as described above can be accessed through the normal `PropertySource` and `Environment` APIs.
  Since:
    3.1
  Author:
    Chris Beams
  See Also:
    `PropertySource`, `SimpleCommandLinePropertySource`, `JOptCommandLinePropertySource`

- - ### Nested Class Summary

    - ### Nested classes/interfaces inherited from class org.springframework.core.env.PropertySource

      `PropertySource.StubPropertySource`

  - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `static String` | `COMMAND_LINE_PROPERTY_SOURCE_NAME` The default name given to `CommandLinePropertySource` instances: "commandLineArgs"
    `static String` | `DEFAULT_NON_OPTION_ARGS_PROPERTY_NAME` The default name of the property representing non-option arguments: "nonOptionArgs"

    - ### Fields inherited from class org.springframework.core.env.PropertySource

      `logger, name, source`

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `CommandLinePropertySource(String name, T source)` Create a new `CommandLinePropertySource` having the given name and backed by the given source object.
    `CommandLinePropertySource(T source)` Create a new `CommandLinePropertySource` having the default name "commandLineArgs" and backed by the given source object.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `protected abstract boolean` | `containsOption(String name)` Return whether the set of option arguments parsed from the command line contains an option with the given name.
    `boolean` | `containsProperty(String name)` Return whether this `PropertySource` contains a property with the given name.
    `protected abstract List<String>` | `getNonOptionArgs()` Return the collection of non-option arguments parsed from the command line.
    `protected abstract List<String>` | `getOptionValues(String name)` Return the collection of values associated with the command line option having the given name.
    `String` | `getProperty(String name)` Return the value associated with the given name, `null` if not found.
    `void` | `setNonOptionArgsPropertyName(String nonOptionArgsPropertyName)` Specify the name of the special "non-option arguments" property.

    - ### Methods inherited from class org.springframework.core.env.PropertySource

      `equals, getName, getSource, hashCode, named, toString`

    - ### Methods inherited from class java.lang.Object

      `clone, finalize, getClass, notify, notifyAll, wait, wait, wait`

- - ### Field Detail

    - #### COMMAND_LINE_PROPERTY_SOURCE_NAME

          public static final String COMMAND_LINE_PROPERTY_SOURCE_NAME

      The default name given to `CommandLinePropertySource` instances: "commandLineArgs"
      See Also:
        Constant Field Values

    - #### DEFAULT_NON_OPTION_ARGS_PROPERTY_NAME

          public static final String DEFAULT_NON_OPTION_ARGS_PROPERTY_NAME

      The default name of the property representing non-option arguments: "nonOptionArgs"
      See Also:
        Constant Field Values

  - ### Constructor Detail

    - #### CommandLinePropertySource

          public CommandLinePropertySource(T source)

      Create a new `CommandLinePropertySource` having the default name "commandLineArgs" and backed by the given source object.

    - #### CommandLinePropertySource

          public CommandLinePropertySource(String name,
                                   T source)

      Create a new `CommandLinePropertySource` having the given name and backed by the given source object.

  - ### Method Detail

    - #### setNonOptionArgsPropertyName

          public void setNonOptionArgsPropertyName(String nonOptionArgsPropertyName)

      Specify the name of the special "non-option arguments" property. The default is "nonOptionArgs".

    - #### containsProperty

          public final boolean containsProperty(String name)

      Return whether this `PropertySource` contains a property with the given name.

      This implementation first checks to see if the name specified is the special "non-option arguments" property, and if so delegates to the abstract `getNonOptionArgs()` method checking to see whether it returns an empty collection. Otherwise delegates to and returns the value of the abstract `containsOption(String)` method.

      Overrides:
        `containsProperty` in class `PropertySource<T>`
      Parameters:
        `name` - the property name to find

    - #### getProperty

          public final String getProperty(String name)

      Return the value associated with the given name, `null` if not found.

      This implementation first checks to see if the name specified is the special "non-option arguments" property, and if so delegates to the abstract `getNonOptionArgs()` method. If so and the collection of non-option arguments is empty, this method returns `null`. If not empty, it returns a comma-separated String of all non-option arguments. Otherwise delegates to and returns the result of the abstract `getOptionValues(String)` method.

      Specified by:
        `getProperty` in class `PropertySource<T>`
      Parameters:
        `name` - the property to find
      See Also:
        `PropertyResolver.getRequiredProperty(String)`

    - #### containsOption

          protected abstract boolean containsOption(String name)

      Return whether the set of option arguments parsed from the command line contains an option with the given name.

    - #### getOptionValues

          protected abstract List<String> getOptionValues(String name)

      Return the collection of values associated with the command line option having the given name.
      - if the option is present and has no argument (e.g.: "--foo"), return an empty collection (`[]`)
      - if the option is present and has a single value (e.g. "--foo=bar"), return a collection having one element (`["bar"]`)
      - if the option is present and the underlying command line parsing library supports multiple arguments (e.g. "--foo=bar --foo=baz"), return a collection having elements for each value (`["bar", "baz"]`)
      - if the option is not present, return `null`

    - #### getNonOptionArgs

          protected abstract List<String> getNonOptionArgs()

      Return the collection of non-option arguments parsed from the command line. Never `null`.