Using Google Guice to inject java properties

Question

I want to use google guice to make properties available in all classes of my application. I defined a Module which loads and binds the properties file Test.properties.

Property1=TEST
Property2=25

package com.test;

import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.util.Properties;

import com.google.inject.AbstractModule;
import com.google.inject.name.Names;

public class TestConfiguration extends AbstractModule {

    @Override
    protected void configure() {
    Properties properties = new Properties();
    try {
        properties.load(new FileReader("Test.properties"));
        Names.bindProperties(binder(), properties);
    } catch (FileNotFoundException e) {
        System.out.println("The configuration file Test.properties can not be found");
    } catch (IOException e) {
        System.out.println("I/O Exception during loading configuration");
    }

    }
}

I'm using a main class where I create a injector to inject the properties.

package com.test;

import com.google.inject.Guice;
import com.google.inject.Injector;

public class Test {

    public static void main(String[] args) {
    TestConfiguration config = new TestConfiguration();
    Injector injector = Guice.createInjector(config);
    TestImpl test = injector.getInstance(TestImpl.class);
    }
}

package com.test;

import com.google.inject.Inject;
import com.google.inject.name.Named;

public class TestImpl {
    private final String property1;
    private final Integer property2;

        @Inject
        public TestImpl(@Named("Property1") String property1, @Named("Property2") Integer property2) {

        System.out.println("Hello World");
        this.property1 = property1;
        this.property2 = property2;

        System.out.println(property1);
        System.out.println(property2);

        }
     }

Now my question. If my TestImpl creates other classes where I also need to inject properties, and those classes also need to inject properties what is the correct way to do this?

1. Pass the injector to all subclasses and then use injector.getInstance(...) to create the subclasses?

2. Instanciate a new injector like

   TestConfiguration config = new TestConfiguration();
   Injector injector = Guice.createInjector(config);
   TestImpl test = injector.getInstance(TestImpl.class);
   

in all nested classes?

1. Is there an other approach to make the properties available in all classes?

Answer 1

Pass the injector to all subclasses and then use injector.getInstance(...) to create the subclasses?

no, by doing this you are defeating the purpose of the dependency injection pattern and also coupling all your implementation to Guice. Your implementations should not interact at all with guice, except through the (now standardized) annotations.

Instanciate a new injector like

TestConfiguration config = new TestConfiguration(); 
Injector injector = Guice.createInjector(config); 
TestImpl test = injector.getInstance(TestImpl.class); 

in all nested classes?

no, and this is even worse cause you will end up with multiple injectors, hence multiple contexts which will prevent a proper usage of the scopes.

Ideally, you should only use the injector during the bootstrapping of your application. Of course the way to bootstrap it will largely depend of the application.

Is there an other approach to make the properties available in all classes?

The properties could be injected the same way you did for TestImpl. If you want TestImpl to use let say a service which also needs some properties (or other services), just let Guice inject it to TestImpl. Guice is taking care of all the instantiation/wiring. You should only tell Guice "how to proceed", by using the binder, when Guice cannot figure this out itself :

public class TestImpl {
    private final String property1;
    private final Integer property2;
    private final IService service;


        @Inject
        public TestImpl(@Named("Property1") String property1, @Named("Property2") Integer property2, IService service) {
           this.property1 = property1;
           this.property2 = property2;
           this.service= service;
        }
    }
}