Quiz yourself: Service types and service providers in Java modules

The ServiceLoader class is key to using the Java Platform Module System.

September 3, 2021 | Download a PDF of this article
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Which statements are correct about the Java Platform Module System (JPMS)? Choose two.

A. The service type must be a Java interface.

B. The service provider type must be a concrete public Java class.

C. The programmer has an option to inspect the service provider type before obtaining an instance.

D. The service provider type may be a Java interface.

Answer. Option A is incorrect. The service type is commonly an interface, but it may also be a class (either abstract or concrete).

Option B is incorrect, because the service provider type may also be an interface or a class, either abstract or concrete. The reason that abstract types can be used here is that a public static provider() method can be provided to serve as a factory. If this method is present, it will be used by the ServiceLoader.

There are two possibilities concerning the relationship between the service type and the service provider type.

  • If the service provider type has a public static provider() method, the service provider type need not have any relationship to the service type, but the return type of the provider() method (which will be the actual service implementation, in this case) must be assignment-compatible to the service type.
  • If no provider() method exists, the service provider type must have a public zero-argument constructor, and that service provider type itself must be assignment-compatible to the service type.

Option C is correct. A stream drawn from the ServiceLoader provides a means to find and inspect implementation classes without having instantiated those services. This allows lazy loading such that there is no need to load service implementations that do not appear to be appropriate.

For example, assume a project’s code uses @com.lang.Online and @com.lang.Offline annotations to describe the operating mode of a particular translator factory provider. Therefore, you might have a translator factory class that looks like the following:


@Online
public class GoogleTranslatorFactory implements TranslatorFactory {
    ... // code here
}

The following code will instantiate only those translator factories that carry the @Online annotation:


ServiceLoader<TranslatorFactory> loader = ServiceLoader.load(TranslatorFactory.class);
Set<TranslatorFactory> onlineTF = loader
    .stream()                                            
    .filter(p -> p.type().isAnnotationPresent(Online.class))
    .map(Provider::get)                                    
    .collect(Collectors.toSet());

Notice that the stream contains ServiceLoader.Provider objects; these are a kind of wrapper around the actual service. The Provider has two methods: type(), which gives access to the java.lang.Class object that describes the service provider, and get(), which loads and instantiates that service provider and returns the object.

Option D is also correct. As mentioned above, the provider type can be an interface; in that case, the public static provider() method must be present and must return an object that’s assignable to the service type.

Conclusion. The correct answers are options C and D.

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Mikalai Zaikin

Mikalai Zaikin is a lead Java developer at IBA IT Park in Minsk, Belarus. During his career, he has helped Oracle with development of Java certification exams, and he has been a technical reviewer of several Java certification books, including three editions of the famous Sun Certified Programmer for Java study guides by Kathy Sierra and Bert Bates.

Simon Roberts

Simon Roberts joined Sun Microsystems in time to teach Sun’s first Java classes in the UK. He created the Sun Certified Java Programmer and Sun Certified Java Developer exams. He wrote several Java certification guides and is currently a freelance educator who publishes recorded and live video training through Pearson InformIT (available direct and through the O’Reilly Safari Books Online service). He remains involved with Oracle’s Java certification projects.

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