Helper class for injecting objects when not in a Dependency Injection context - typically when unit testing.

PoorMansInjector.java

package dk.jlo.util;


import java.lang.reflect.Field;

/**
 * Injects an object into a (private) field in a bean; primarily for poor man's injection for unit testing when not running in a Dependency Injection environment.<p>
 * Travels through super classes if necessary.<p>
 * Usage:
 * <pre>
 *     import static dk.jlo.util.PoorMansInjector.inject;
 *     [...]
 *     inject(object).intoField("fieldName").inBean(bean);
 * </pre>
 */
@SuppressWarnings("unused")
public class PoorMansInjector {
    /**
     * Instantiates the injector.
     *
     * @param object the object to be injected.
     * @return The injector.
     */
    public static FieldInjector inject(Object object) {
        return new FieldInjector(object);
    }

    public static class FieldInjector {
        private Object object;

        FieldInjector(Object object) {
            this.object = object;
        }

        /**
         * Tells the injector where to inject the previously given object.
         *
         * @param privateFieldName the (usually private) field in question.
         * @return The injector.
         */
        public FinalInjector intoField(String privateFieldName) {
            FinalInjector finalInjector = new FinalInjector(object, privateFieldName);
            object = null;
            return finalInjector;
        }
    }

    public static class FinalInjector {
        private Object object;
        private String privateFieldName;

        FinalInjector(Object object, String privateFieldName) {
            this.object = object;
            this.privateFieldName = privateFieldName;
        }

        /**
         * Performs the injection.
         *
         * @param bean the bean/object containing the field.
         * @throws IllegalAccessException in case the injection is somehow not possible even after setting the field accessible.
         * @throws NoSuchFieldException   in case the field is not found in the class or superclass(es).
         */
        public void inBean(Object bean) throws IllegalAccessException, NoSuchFieldException {
            Field beanField = getFieldInClassHierarchy(bean, privateFieldName);
            beanField.setAccessible(true);
            beanField.set(bean, object);
            object = null;
            privateFieldName = null;
        }

        private Field getFieldInClassHierarchy(Object bean, String fieldName) throws NoSuchFieldException {
            Field beanField = null;
            Class<?> aClass = bean.getClass();
            do {
                try {
                    beanField = aClass.getDeclaredField(fieldName);
                } catch (NoSuchFieldException e) {
                    aClass = aClass.getSuperclass();
                    if (aClass == null) {
                        throw e;
                    }
                }
            } while (beanField == null);
            return beanField;
        }
    }
}