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
CommandLinePropertySourcewill typically be added to the
Environment of the Spring
ApplicationContext, at which point all command line arguments become available
through the
Environment#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
@Valueannotation may be used to inject these properties, given that a
PropertySourcesPropertyResolver bean has been registered, either directly or through
using the
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
#DEFAULT_NON_OPTION_ARGS_PROPERTY_NAME 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
Environmentabstraction, 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.