Managing multiple instances of services: OSGI service factories

And now the third post in my little series of OSGI related postings. I already showed how you can easily manage a number of services implementing the same interface using a service tracker or by using the right SCR annotations.

Sometimes you need to implement services, which just differ by configuration. A nice example for this is the logging, where you want to have the possibility to have multiple logging facilities being logged to log files at a different level. Somebody (normally the admin of the system) is then able to leverage this and configure the logging as she likes.

So more formally spoken you have zero to N instances of the same service, but just with different configuration. Just duplicating the code and create a logger1 with configurable values, a logger2, logger3 and so doesn’t make sense, as it’s just code duplication and inflexible (what happens, when you need logger100?).

OSGI offers for this kind of problem the concept of ServiceFactories. As the name already says, it’s a factory to create services, just by configuration.

As an example let’s assume, that you need to send out emails via a configurable number of SMTP servers, because for internal emails you need to use a different mailserver than for external users or partners. We will implement this as service, and on the service we will configure the details of the SMTP service we intend to use.

So let’s start with the service interface:

public interface MailService {
  public void sendMail (String from, String to, String body);

And a dummy implementation could look like this; we want to focus only on the properties, and not really on the details how to send emails 🙂

@Component(metatype=true,label="simple mailservice", description="simple mailservice")
public class MailServiceImpl implements MailService {

  private static final String DEFAULT_ADDRESS="localhost:25";
  @Property (description="adress of the SMTP server including port",value=DEFAULT_ADDRESS)
  private static final String ADDRESS = “mailservice.address”;
  private String address;
  private static final String DEFAULT_USERNAME="admin";
  @Property(description=“username to login to the SMTP server”,value=DEFAULT_USERNAME)
  private static final String USERNAME = “mailservice.username”;
  private String username;

  private static final String DEFAULT_PASSWORD="admin;
  @Property(description=“password to login to the SMTP server”,value=DEFAULT_PASSWORD)
  private static final String password = “mail service.password”;
  private String password;

  protected void activate (ComponentContext ctx) {
    address = PropertiesUtil.toString(ctx.getProperties().get(ADDRESS),DEFAULT_ADDRESS);
    username = PropertiesUtil.toString(ctx.getProperties().get(USERNAME),DEFAULT_USERNAME);
    password = PropertiesUtil.toString(ctx.getProperties().get(PASSWORD),DEFAULT_PASSWORD);

  public void sendMail(String from, String to, String body) {
    // login to the smtp server using address, username and password provided via OSGI 
    // properties and send the email

But how can you extend this and make sure, that you cannot just create a configuration for 1 mailserver, but for multiple ones? Easy, just add the property “configurationFactory=true” the @Component annotation.

@Component(metatype=true,label="simple mailservice", description="simple mailservice",

If you compile and deploy your service now, you can see in your Apache Felix Configuration Manager, that you have a “plus” sign in front of the service; and when you click it, you get a new instance of your service, which you can configure.

ConfigurationFactory in the Felix Console
ConfigurationFactory in the Felix Console

But when you have 3 mail services configured, which one do you get when you have something like this:

MailService mailService;

The answer: It’s not deterministic. You might get any of the configured mailservices. If you need a special one, the easiest way is to provide labels for them to make them unique. So add another property to your service:

@Property(description=“Label for this SMTP service”)
private static final String NAME = “mailservice.label”

And when you create then a configuration with the label “INTERNET”, you can reference exactly this service instance with this kind of reference:

MailService mailService;

This will resolve correctly when you have a mailservice service configured with the label “INTERNET”. As long as you don’t have such a service, any service containing such a reference won’t start (unless you create a dynamic reference …)

If you want to be more flexible and also implement some more logic in the lookup process (e.g. having a default mailservice or supporting a number of INTERNET mail services), you can use the whiteboard pattern to track all available MailService instances; based on their labels you can implement any logic you need.

As you see, OSGI is quite powerful when it comes to looking up and connecting to services. Combined with the power of SCR you can easily create lot of configurable services with very little effort. Managing these services and doing proper lookup is also just a few lines of code away.

Personally I really like the possibilities I have with the OSGI container inside of AEM, it gives me the flexibility to access lots of different parts of the system. And creating and injecting my own services is easier than ever.