CODEWITHSUNDEEP
javaannotations
Lipsyor
Lipsyor

Reputation: 448

How to get field of a class by its annotation

I created a simple annotation class:

@Retention(RUNTIME)
public @interface Column {
    public String name();
}

I use it in some classes like this:

public class FgnPzt extends Point {

    public static final String COLUMN_TYPE = "type";
    @Column(name=COLUMN_TYPE)
    protected String type;

}

I know that I can iterate over the declared fields and obtain the annotation like this:

for (Field field : current.getDeclaredFields()) {
    try {
        Column c = field.getAnnotation(Column.class);
        [...]
    } catch(Exception e) {
        [...]
    }
}

How can I obtain the field type directly by its annotated name without iterating over declared fields of the class?

Upvotes: 2

Views: 5470

Answers (1)

Dici
Dici

Reputation: 25980

If you need to make multiple accesses you can pre-process the annotations.

public class ColumnExtracter<T> {
    private final Map<String, Field> fieldsByColumn;

    public ColumnExtracter(Class<T> clazz) {
        this.fieldsByColumn = Stream.of(clazz.getDeclaredFields())
              .filter(field -> field.isAnnotationPresent(Column.class))
              .collect(Collectors.toMap(field -> field.getAnnotation(Column.class).name(), Function.identity()));
    }

    public Field getColumnField(String columnName) {
        return fieldsByColumn.get(columnName);
    }

    public <R> R extract(String columnName, T t, Class<R> clazz) throws IllegalAccessException {
        return clazz.cast(extract(columnName, t));
    }

    public Object extract(String columnName, T t) throws IllegalAccessException {
        return getColumnField(columnName).get(t);
    }
}

Upvotes: 6

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