How to bind String to variable in Guice?

Question

I'm new to Guice and here is a naive question. I learned that we could bind String to a particular value through:

bind(String.class)
        .annotatedWith(Names.named("JDBC URL"))
        .toInstance("jdbc:mysql://localhost/pizza");

But what if I want to bind String to any possible charactors?

Or I think it could be desribed this way:

How can I replace "new SomeClass(String strParameter)" with Guice?

Answer 1

I find a solution in the FAQ of Guice:

http://code.google.com/docreader/#p=google-guice&s=google-guice&t=FrequentlyAskedQuestions

In addition to define an annotation and a String attribute in MyModule, I need to write below line to get a instance of SomeClass:

SomeClass instance = Guice.createInjector(new MyModule("any string i like to use")).getInstance(SomeClass.class);

But I remembered that Injector.getInstance() should not be used except for the root object, so is there any better way to do this?

Answer 2

You first need to annotate the constructor for SomeClass:

class SomeClass {
  @Inject
  SomeClass(@Named("JDBC URL") String jdbcUrl);
    this.jdbcUrl = jdbcUrl;
  }
}

I prefer to use custom annotations, like this:

class SomeClass {
  @Inject
  SomeClass(@JdbcUrl String jdbcUrl);
    this.jdbcUrl = jdbcUrl;
  }

  @Retention(RetentionPolicy.RUNTIME)
  @Target({ElementType.FIELD, ElementType.PARAMETER})
  @BindingAnnotation
  public @interface JdbcUrl {}
}

Then you need to provide a binding in your Module:

public class SomeModule extends AbstractModule {
  private final String jdbcUrl; // set in constructor

  protected void configure() {
    bindConstant().annotatedWith(SomeClass.JdbcUrl.class).to(jdbcUrl);
  }
}

Then an time Guice creates SomeClass, it will inject the parameter. For instance, if SomeOtherClass depends on SomeClass:

class SomeOtherClass {
  @Inject
  SomeOtherClass(SomeClass someClass);
    this.someClass = someClass;
  }

Often, when you think you want to inject a String, you want to inject an object. For instance, if the String is a URL, I often inject a URI with a binding annotation.

This all assumes there is some constant value you can define at module creation time for the String. If the value isn't available at module creation time, you can use AssistedInject.