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A utility library to handle Java reflection with ease
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package refutils;

import java.lang.reflect.InvocationTargetException;
import refutils.util.ConstructorHelper;
import refutils.util.FieldHelper;

/** This class is used to set protected fields in classes and access private constructors. */
public final class ReflectionHelper {
  /** The object that contains the field. */
  private final Object instance;

  private final Class fieldDefinitions;

  /**
   * Instantiates a new reflection helper with the instance where the reflection operations are
   * performed.
   *
   * @param instance the instance
   */
  public ReflectionHelper(final Object instance) {
    this(instance, instance == null ? null : instance.getClass());
  }

  /**
   * Advanced instantiation of a new reflection helper where the ClassType
   * reflection operations are performed on the instance, but the field definitions will be taken
   * from a superclass. Example: SuperClass has a private field String name SubClass inherits from
   * SuperClass new ReflectionHelper(subClass).setField("value"); does not work since the "name"
   * field is not visible in SubClass new ReflectionHelper(subClass,
   * SuperClass.class).setField("value"); works since SuperClass is scanned for fields
   *
   * @param instance the instance to manipulate
   * @param fieldDefinitions where the fields are defined
   */
  public ReflectionHelper(final Object instance, final Class fieldDefinitions) {
    if (instance == null) {
      throw new NullPointerException("The instance in the ReflectionHelper cannot be null");
    }
    this.instance = instance;
    this.fieldDefinitions = fieldDefinitions;
  }

  /**
   * Instantiate private empty constructor.
   *
   * @param clazz the class to instantiate
   * @param  type of the class to instantiate
   * @return the new instance
   */
  public static  T instantiatePrivateConstructor(final Class clazz) {
    try {
      return new ConstructorHelper<>(clazz).instantiatePrivate();
    } catch (NoSuchMethodException | InstantiationException | InvocationTargetException ex) {
      throw ReflectionHelperException.createCannotInstantiateClass(clazz, ex);
    } catch (IllegalAccessException ex) {
      throw new IllegalStateException(
          "This should never happen, since the constructor is always made accessible", ex);
    }
  }

  /**
   * Sets a value for a field in the instance object. The method uses type-matching to set the
   * field. This method can only be used if a class has one field of the specified type.
   *
   * @param fieldValue The value that the field should be set to.
   */
  public void setField(final Object fieldValue) {
    try {
      FieldHelper fieldHelper = new FieldHelper(instance, fieldDefinitions);
      fieldHelper.setValueByType(fieldValue);
    } catch (IllegalAccessException | NoSuchFieldException ex) {
      throw new ReflectionHelperException(ex);
    }
  }

  /**
   * Sets a value for a field in the instance object. This method can be used if a class has more
   * than one field of the specified type or if the field is private and inherited.
   *
   * @param fieldName The name of the field
   * @param fieldValue The value that the field should be set to.
   */
  public void setField(final String fieldName, final Object fieldValue) {
    try {
      FieldHelper fieldHelper = new FieldHelper(instance, fieldDefinitions);
      fieldHelper.setValueByName(fieldName, fieldValue);
    } catch (IllegalAccessException | NoSuchFieldException ex) {
      throw new ReflectionHelperException(ex);
    }
  }

  /**
   * Gets the value of a field in the instance object. This method can be used if a class has more
   * than one field of the specified type or if the field is private and inherited. The returned
   * value must be cast to the field class.
   *
   * @param fieldName The name of the field
   * @return the value of the field
   */
  public Object getField(String fieldName) {
    try {
      FieldHelper fieldHelper = new FieldHelper(instance, fieldDefinitions);
      return fieldHelper.getValueByName(fieldName);
    } catch (IllegalAccessException ex) {
      throw new IllegalStateException(
          "This should never happen, since the field is always made accessible", ex);
    } catch (NoSuchFieldException ex) {
      throw new ReflectionHelperException(ex);
    }
  }

  /**
   * Gets the value of a field in the instance object. The method uses type-matching to set the
   * field. This method can only be used if a class has one field of the specified type.
   *
   * @param fieldClass the class of the field
   * @param  field class
   * @return the value of the field with the right type
   */
  public  T getField(Class fieldClass) {
    try {
      FieldHelper fieldHelper = new FieldHelper(instance, fieldDefinitions);
      return fieldHelper.getValueByType(fieldClass);
    } catch (IllegalAccessException ex) {
      throw new IllegalStateException(
          "This should never happen, since the field is always made accessible", ex);
    } catch (NoSuchFieldException ex) {
      throw new ReflectionHelperException(ex);
    }
  }
}