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Extension for JUnit5 for managing set up and tear down of environments during testing

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/*
 * Copyright 2018 coldrye.eu, Carsten Klein
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package eu.coldrye.junit.env;

import eu.coldrye.junit.util.ReflectionUtils;
import org.junit.jupiter.api.extension.ExtensionContext;
import org.junit.platform.commons.util.Preconditions;

import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.List;
import java.util.Objects;

/**
 * The final class FieldInjector models an injector for fields for which it resolves
 * the instances for by querying the available {@link EnvProvider}S.
 *
 * @since 1.0.0
 */
class FieldInjector {

  /**
   * @param testInstance
   * @param context
   * @param providers
   * @throws Exception
   */
  public void inject(Object testInstance, ExtensionContext context, List providers) throws Exception {

    Preconditions.notNull(testInstance, "testInstance must not be null");
    Preconditions.notNull(providers, "providers must not be null");

    List fields = ReflectionUtils.getDeclaredFields(testInstance.getClass(), EnvProvided.class);
    for (Field field : fields) {
      for (EnvProvider provider : providers) {
        if (provider.canProvideInstance(field, field.getType())) {
          inject0(testInstance, field, provider);
          break;
        }
      }
    }
  }

  /**
   * @param testInstance
   * @param field
   * @param provider
   * @throws Exception
   */
  private void inject0(Object testInstance, Field field, EnvProvider provider) throws Exception {

    Object instance = provider.getOrCreateInstance(field, field.getType());
    Method setter = ReflectionUtils.findMethod(testInstance.getClass(), ReflectionUtils.setterName(field),
      field.getType());
    if (Objects.isNull(setter)) {
      field.setAccessible(true);
      field.set(testInstance, instance);
    } else {
      setter.setAccessible(true);
      setter.invoke(testInstance, instance);
    }
  }
}