Inject into collection

A tour of ElementLoader mechanism

Genie provides support on injecting Collection and Map type data. Unlike other type of Java bean injection, Collection/Map type injection require an additional step after the bean has been constructed: loading element into the container. Take an example below:

/**
 * Print out a Fibonacci number series injected
 */
public class FibonacciDemo {
    @Inject
    private List<Integer> series;

    public void print(PrintStream ps) {
        for (int n: series) {
            ps.println(n);
        }
    }
}

We want to create a type binding module:

public FibonacciModule extends Module {
    public void configure() {
        bind(List.class).to(ArrayList.class);
    }
}

Tips You don't have to do type binding for List and Map in your application because Genie has already done that for you.

Now if we load the bean with Genie:

FibonacciDemo demo = genie.get(FibonacciDemo.class);

We will get a FibonacciDemo with an empty ArrayList. One possible way is to use @Provides method:

public FibonacciModule {
    @Provides
    FibonacciDemo createDemo(ArrayList list) {
        // do logic to populate the list
    }
}

Now a problem is how many numbers should we populate into the list. We can either hard code a number, or anyway inject a number into the @Provides method. None of them satisfied our needs.

Genie comes up with an nice solution that allows application developer to define their data loading logic:

First, define an ElementLoader that loads the fibonacci series:

class FibonacciSeriesLoader extends ElementLoaderBase<Integer> {
    @Override
    public List<Integer> load(Map<String, Object> options, BeanSpec container, Genie genie) {
        int max = toInt(options.get("max")); // here "max" is provided by annotation class as shown below 
        int n1 = 1, n2 = 1, f;
        List<Integer> list = C.newList();
        list.add(n1);
        list.add(n2);
        for (;;) {
            f = n1 + n2;
            n1 = n2;
            n2 = f;
            if (f < max) {
                list.add(f);
            } else {
                break;
            }
        }
        return list;
    }
    ...
}

Note: the full code of the FibonacciSeriesLoader.java can be found at https://github.com/osglworks/java-di/blob/master/src/test/java/org/osgl/genie/FibonacciSeriesLoader.java

Now define the Loader annotation:

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Loader(FibonacciSeriesLoader.class)
public @interface FibonacciSeries {
    /**
     * The max value in the series
     * @return the max value
     */
    int max() default 100;
}

Finally modify our demo app by adding the @FibonacciSeries annotation:

/**
 * Print out a Fibonacci number series injected
 */
public class FibonacciDemo {
    @FibonacciSeries(max = 1000)
    private List<Integer> series;

    public void print(PrintStream ps) {
        for (int n: series) {
            ps.println(n);
        }
    }
}

Then you can use Genie to get the demo app and the series will get populated:

FibonacciDemo demo = genie.get(FibonacciDemo.class);
demo.print(System.out);

Filtering

Genie support filtering when loading container elements. Take the FibonacciDemo above as an example, suppose you need only the even number in the Fibonacci series. You need to define a ElementFilter class and a Filter annotation:

The ElementFilter logic

class EvenNumberFilter implements ElementFilter<Integer> {
    @Override
    public Osgl.Function<Integer, Boolean> filter(Map<String, Object> options, BeanSpec container) {
        return N.F.IS_EVEN;
    }
}

The Filter annotation that specify the ElementFilter implementation:

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Filter(EvenNumberFilter.class)
public @interface EvenNumber {
}

Now update your FibonacciDemo app by adding the EvenNumber annotation:

/**
 * Print out a Fibonacci number series injected
 */
public class FibonacciDemo {
    @FibonacciSeries(max = 1000)
    @EvenNumber
    private List<Integer> series;

    public void print(PrintStream ps) {
        for (int n: series) {
            ps.println(n);
        }
    }
}

Genie will ensure the data loaded in the series even Fibonacci numbers.

You can have many ElementFilters and only element pass all filters will be loaded. Actually every ElementLoader implementation is also ElementFilter, thus you can have multiple ElementLoaders and only the first one will be served as loader and the rest will be treated as filter.

Inject Map type bean

Things is a little bit different if we need to populate a Map type bean. In addition to ElementLoader and Loader annotation, we need another annotation MapKey to specify how to get the key from the element so we can put the element along with it's key into the map.

/**
 * Dispatch error to proper handlers
 */
class ErrorDispatcher {
    @TypeOf
    @MapKey("errorCode")
    Map<Integer, ErrorHandler> registry;

    String handle(int error) {
        ErrorHandler handler = registry.get(error);
        return null == handler ? "unknown" : handler.toString();
    }
}

In the above code the Map type bean registry has been tagged with @MapKey("errorCode") and @TypeOf annotation. Where @TypeOf is a built-in element Loader annotation which tells Genie to load element who close is sub class or implementation of ErrorHandler. And @MapKey("errorCode") is the annotation to tell Genie how to retrieve the key: get the key from bean's "errorCode" property. And here is the code of ErrorHandler:

/**
 * A test class: define a abstract class for error handlers
 */
public abstract class ErrorHandler {
    /**
     * Returns the error code the implementation is looking for
     * @return the interested error code
     */
    public abstract int getErrorCode();

    @Override
    public String toString() {
        return getClass().getSimpleName();
    }
}

Built-in ElementLoader(s)

Genie provides three built-in Element loader and their corresponding annotation

Loader	Annotation	Purpose
AnnotatedElementLoader	AnnotatedWith	Load element whose class has been annotated with specified annotation class
TypedElementLoader	TypeOf	Load element whose class is sub type or implementation of the specified class

Note It requires Genie to be setup property with certain IoC container to use the @AnnotatedWith and @TypeOf element loader.

Please refer the following section: integration