guice and app configuration

[Stranger in the Q]

https://bitbucket.org/strangerintheq/guice-config

Usage example:

At first we have a properties file 'config.properties':

test.key=test value

And we want to inject this value like this:

@Inject
@Config( Property.TEST_KEY )
private String injectedValue;

We need to load contents of file 'config.properties' into java.util.Properties and pass it to Config module:

Properties props = new Properties();
props.load(...);
Module configModule = new ConfigModule( props, Property.values() );

... and injecting:

Injector injector = Guice.createInjector( configModule );
TestClass testClass = injector.getInstance( TestClass.class );
String injectedValue = testClass.getInjectedValue();

injected value will be 'test value'...

[Olivier Grégoire]

What is the added values when compared to https://github.com/google/guice/wiki/FrequentlyAskedQuestions#how-do-i-inject-configuration-parameters ?

It's not really clear to me.

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[Tim Boudreau]

FWIW, my Giulius library (heavily inspired by some of Eelco Hillenius' work) - https://github.com/timboudreau/giulius - which is for setting up property injection using @Named, has a little-used feature that sounds like what you're after.

In the general case, you feed it some properties files and default values.  It also has a @Namespace annotation you can use over a class or as a package-annotation, so to specify a specific file-name/collection of settings to use for injection into that class or package.  I added that feature in order to Guicify a legacy codebase that was littered with classes that all contained properties loading code for various file, so that first a Settings (like properties only read only) could be injected, and later the mechanism that located the properties files could be replaced.

So basically you do something like

Settings fooSettings = new SettingsBuilder("foo").add("someProp", "someValue").add(new File("/etc/foo.properties").build();

Settings barSettings = new SettingsBuilder("bar").add("someProp", "otherValue").add(new File("/etc/bar.properties").build();

// You can layer in default values, system properties, env vars, etc.

Injector inj = new DependenciesBuilder().add(fooSettings, "foo").add(barSettings, "bar").build().getInjector();

@Namespace ("foo")

public class A {

   @Inject A(@Named("someProp") String prop) {

      assert "someValue".equals(prop);

   }

}

@Namespace ("bar")

public class B {

   @Inject A(@Named("someProp") String prop) {

      assert "someValue".equals(prop);

   }

}

(you could also annotate A and B with @Defaults and not have to set up the default values when creating the settings)

A unit test may be worth a thousand words here, so:

https://github.com/timboudreau/giulius/blob/master/giulius/src/test/java/com/mastfrog/nstest/NamespaceTest.java

-Tim

[Nate Bauernfeind]

A unit test may be worth a thousand words here, so:

https://github.com/timboudreau/giulius/blob/master/giulius/src/test/java/com/mastfrog/nstest/NamespaceTest.java

Especially commented out unit tests! Though a large majority of the thousand words are expletives. ;)

Nate

[Cédric Beust ♔]

I also posted some work I did in this area a while ago:

http://beust.com/weblog/2013/07/12/flexible-configuration-with-guice/

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Cédric

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[Jens]

Whats wrong with Names.bindProperties(binder, myProps) provided by Guice? Isn't that effectively the same?

http://google.github.io/guice/api-docs/latest/javadoc/com/google/inject/name/Names.html#bindProperties-com.google.inject.Binder-java.util.Properties-

-- J.

[Tim Boudreau]

On Friday, April 10, 2015 at 2:13:16 PM UTC-4, Jens wrote:

Whats wrong with Names.bindProperties(binder, myProps) provided by Guice? Isn't that effectively the same?

Nothing, but there's a little more to it with Giulius:

 - Layering sets of properties on top of each other (e.g. layer $JAR/META-INF/settings/generated-foo.properties, /etc/foo.properties, ~/foo.properties, ./foo.properties) on top of each other so you can have defaults (annotation generated), system-wide, per-user and per-install properties, and parsing sendopts-style command-line argumenst (--foo bar) overlaid over that

 - Optional periodic refresh of file-based properties

 - Ability to replace properties files with loading config from a centralized config server such as etcd - https://github.com/timboudreau/giulius-settings-etcd - or zookeeper or similar, without major surgery

 - Ability to use @Namespace to choose the source of bound properties (i.e. when injecting @Named, giulius will check what class/package it's injecting into and select the right Settings to pull from) - useful in old codebases that read from heterogenous properties files

 - Having a test harness that lets test methods take injected arguments, which understands this stuff (i.e. you can override properties for a test using annotations or a properties file with the same name as the test) - https://github.com/timboudreau/giulius-tests

-Tim