org.springframework.orm.jpa.support

## Class PersistenceAnnotationBeanPostProcessor

- java.lang.Object
- - org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor

- All Implemented Interfaces:
    Serializable, Aware, BeanFactoryAware, BeanPostProcessor, DestructionAwareBeanPostProcessor, InstantiationAwareBeanPostProcessor, MergedBeanDefinitionPostProcessor, Ordered, PriorityOrdered

      public class PersistenceAnnotationBeanPostProcessor
      extends Object
      implements InstantiationAwareBeanPostProcessor, DestructionAwareBeanPostProcessor, MergedBeanDefinitionPostProcessor, PriorityOrdered, BeanFactoryAware, Serializable

  BeanPostProcessor that processes `PersistenceUnit` and `PersistenceContext` annotations, for injection of the corresponding JPA resources `EntityManagerFactory` and `EntityManager`. Any such annotated fields or methods in any Spring-managed object will automatically be injected.

  This post-processor will inject sub-interfaces of `EntityManagerFactory` and `EntityManager` if the annotated fields or methods are declared as such. The actual type will be verified early, with the exception of a shared ("transactional") `EntityManager` reference, where type mismatches might be detected as late as on the first actual invocation.

  Note: In the present implementation, PersistenceAnnotationBeanPostProcessor only supports `@PersistenceUnit` and `@PersistenceContext` with the "unitName" attribute, or no attribute at all (for the default unit). If those annotations are present with the "name" attribute at the class level, they will simply be ignored, since those only serve as deployment hint (as per the Java EE 5 specification).

  This post-processor can either obtain EntityManagerFactory beans defined in the Spring application context (the default), or obtain EntityManagerFactory references from JNDI ("persistence unit references"). In the bean case, the persistence unit name will be matched against the actual deployed unit, with the bean name used as fallback unit name if no deployed name found. Typically, Spring's `LocalContainerEntityManagerFactoryBean` will be used for setting up such EntityManagerFactory beans. Alternatively, such beans may also be obtained from JNDI, e.g. using the `jee:jndi-lookup` XML configuration element (with the bean name matching the requested unit name). In both cases, the post-processor definition will look as simple as this:

       <bean class="org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor"/>

  In the JNDI case, specify the corresponding JNDI names in this post-processor's `"persistenceUnits" map`, typically with matching `persistence-unit-ref` entries in the Java EE deployment descriptor. By default, those names are considered as resource references (according to the Java EE resource-ref convention), located underneath the "java:comp/env/" namespace. For example:

       <bean class="org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor">
         <property name="persistenceUnits">
           <map/gt;
             <entry key="unit1" value="persistence/unit1"/>
             <entry key="unit2" value="persistence/unit2"/>
           </map/gt;
         </property>
       </bean>

  In this case, the specified persistence units will always be resolved in JNDI rather than as Spring-defined beans. The entire persistence unit deployment, including the weaving of persistent classes, is then up to the Java EE server. Persistence contexts (i.e. EntityManager references) will be built based on those server-provided EntityManagerFactory references, using Spring's own transaction synchronization facilities for transactional EntityManager handling (typically with Spring's `@Transactional` annotation for demarcation and `JtaTransactionManager` as backend).

  If you prefer the Java EE server's own EntityManager handling, specify entries in this post-processor's `"persistenceContexts" map` (or `"extendedPersistenceContexts" map`, typically with matching `persistence-context-ref` entries in the Java EE deployment descriptor. For example:

       <bean class="org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor">
         <property name="persistenceContexts">
           <map/gt;
             <entry key="unit1" value="persistence/context1"/>
             <entry key="unit2" value="persistence/context2"/>
           </map/gt;
         </property>
       </bean>

  If the application only obtains EntityManager references in the first place, this is all you need to specify. If you need EntityManagerFactory references as well, specify entries for both "persistenceUnits" and "persistenceContexts", pointing to matching JNDI locations.

  NOTE: In general, do not inject EXTENDED EntityManagers into STATELESS beans, i.e. do not use `@PersistenceContext` with type `EXTENDED` in Spring beans defined with scope 'singleton' (Spring's default scope). Extended EntityManagers are not thread-safe, hence they must not be used in concurrently accessed beans (which Spring-managed singletons usually are).

  Note: A default PersistenceAnnotationBeanPostProcessor will be registered by the "context:annotation-config" and "context:component-scan" XML tags. Remove or turn off the default annotation configuration there if you intend to specify a custom PersistenceAnnotationBeanPostProcessor bean definition.

  Since:
    2.0
  Author:
    Rod Johnson, Juergen Hoeller
  See Also:
    `PersistenceUnit`, `PersistenceContext`, Serialized Form

- - ### Field Summary

    - ### Fields inherited from interface org.springframework.core.Ordered

      `HIGHEST_PRECEDENCE, LOWEST_PRECEDENCE`

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `PersistenceAnnotationBeanPostProcessor()`

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `protected EntityManagerFactory` | `findDefaultEntityManagerFactory(String requestingBeanName)` Find a single default EntityManagerFactory in the Spring application context.
    `protected EntityManagerFactory` | `findEntityManagerFactory(String unitName, String requestingBeanName)` Find an EntityManagerFactory with the given name in the current Spring application context, falling back to a single default EntityManagerFactory (if any) in case of no unit name specified.
    `protected EntityManagerFactory` | `findNamedEntityManagerFactory(String unitName, String requestingBeanName)` Find an EntityManagerFactory with the given name in the current Spring application context.
    `int` | `getOrder()` Return the order value of this object, with a higher value meaning greater in terms of sorting.
    `protected EntityManager` | `getPersistenceContext(String unitName, boolean extended)` Return a specified persistence context for the given unit name, as defined through the "persistenceContexts" (or "extendedPersistenceContexts") map.
    `protected EntityManagerFactory` | `getPersistenceUnit(String unitName)` Return a specified persistence unit for the given unit name, as defined through the "persistenceUnits" map.
    `protected <T> T` | `lookup(String jndiName, Class<T> requiredType)` Perform a JNDI lookup for the given resource by name.
    `Object` | `postProcessAfterInitialization(Object bean, String beanName)` Apply this BeanPostProcessor to the given new bean instance after any bean initialization callbacks (like InitializingBean's `afterPropertiesSet` or a custom init-method).
    `boolean` | `postProcessAfterInstantiation(Object bean, String beanName)` Perform operations after the bean has been instantiated, via a constructor or factory method, but before Spring property population (from explicit properties or autowiring) occurs.
    `void` | `postProcessBeforeDestruction(Object bean, String beanName)` Apply this BeanPostProcessor to the given bean instance before its destruction.
    `Object` | `postProcessBeforeInitialization(Object bean, String beanName)` Apply this BeanPostProcessor to the given new bean instance before any bean initialization callbacks (like InitializingBean's `afterPropertiesSet` or a custom init-method).
    `Object` | `postProcessBeforeInstantiation(Class<?> beanClass, String beanName)` Apply this BeanPostProcessor before the target bean gets instantiated.
    `void` | `postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName)` Post-process the given merged bean definition for the specified bean.
    `PropertyValues` | `postProcessPropertyValues(PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName)` Post-process the given property values before the factory applies them to the given bean.
    `void` | `setBeanFactory(BeanFactory beanFactory)` Callback that supplies the owning factory to a bean instance.
    `void` | `setDefaultPersistenceUnitName(String unitName)` Specify the default persistence unit name, to be used in case of no unit name specified in an `@PersistenceUnit` / `@PersistenceContext` annotation.
    `void` | `setExtendedPersistenceContexts(Map<String,String> extendedPersistenceContexts)` Specify the extended persistence contexts for EntityManager lookups, as a Map from persistence unit name to persistence context JNDI name (which needs to resolve to an EntityManager instance).
    `void` | `setJndiEnvironment(Properties jndiEnvironment)` Set the JNDI environment to use for JNDI lookups.
    `void` | `setJndiTemplate(Object jndiTemplate)` Set the JNDI template to use for JNDI lookups.
    `void` | `setOrder(int order)`
    `void` | `setPersistenceContexts(Map<String,String> persistenceContexts)` Specify the transactional persistence contexts for EntityManager lookups, as a Map from persistence unit name to persistence context JNDI name (which needs to resolve to an EntityManager instance).
    `void` | `setPersistenceUnits(Map<String,String> persistenceUnits)` Specify the persistence units for EntityManagerFactory lookups, as a Map from persistence unit name to persistence unit JNDI name (which needs to resolve to an EntityManagerFactory instance).
    `void` | `setResourceRef(boolean resourceRef)` Set whether the lookup occurs in a J2EE container, i.e.

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Constructor Detail

    - #### PersistenceAnnotationBeanPostProcessor

          public PersistenceAnnotationBeanPostProcessor()

  - ### Method Detail

    - #### setJndiTemplate

          public void setJndiTemplate(Object jndiTemplate)

      Set the JNDI template to use for JNDI lookups.
      See Also:
        `JndiAccessor.setJndiTemplate(org.springframework.jndi.JndiTemplate)`

    - #### setJndiEnvironment

          public void setJndiEnvironment(Properties jndiEnvironment)

      Set the JNDI environment to use for JNDI lookups.
      See Also:
        `JndiAccessor.setJndiEnvironment(java.util.Properties)`

    - #### setResourceRef

          public void setResourceRef(boolean resourceRef)

      Set whether the lookup occurs in a J2EE container, i.e. if the prefix "java:comp/env/" needs to be added if the JNDI name doesn't already contain it. PersistenceAnnotationBeanPostProcessor's default is "true".
      See Also:
        `JndiLocatorSupport.setResourceRef(boolean)`

    - #### setPersistenceUnits

          public void setPersistenceUnits(Map<String,String> persistenceUnits)

      Specify the persistence units for EntityManagerFactory lookups, as a Map from persistence unit name to persistence unit JNDI name (which needs to resolve to an EntityManagerFactory instance).

      JNDI names specified here should refer to `persistence-unit-ref` entries in the Java EE deployment descriptor, matching the target persistence unit.

      In case of no unit name specified in the annotation, the specified value for the `default persistence unit` will be taken (by default, the value mapped to the empty String), or simply the single persistence unit if there is only one.

      This is mainly intended for use in a Java EE 5 environment, with all lookup driven by the standard JPA annotations, and all EntityManagerFactory references obtained from JNDI. No separate EntityManagerFactory bean definitions are necessary in such a scenario.

      If no corresponding "persistenceContexts"/"extendedPersistenceContexts" are specified, `@PersistenceContext` will be resolved to EntityManagers built on top of the EntityManagerFactory defined here. Note that those will be Spring-managed EntityManagers, which implement transaction synchronization based on Spring's facilities. If you prefer the Java EE 5 server's own EntityManager handling, specify corresponding "persistenceContexts"/"extendedPersistenceContexts".

    - #### setPersistenceContexts

          public void setPersistenceContexts(Map<String,String> persistenceContexts)

      Specify the transactional persistence contexts for EntityManager lookups, as a Map from persistence unit name to persistence context JNDI name (which needs to resolve to an EntityManager instance).

      JNDI names specified here should refer to `persistence-context-ref` entries in the Java EE deployment descriptors, matching the target persistence unit and being set up with persistence context type `Transaction`.

      In case of no unit name specified in the annotation, the specified value for the `default persistence unit` will be taken (by default, the value mapped to the empty String), or simply the single persistence unit if there is only one.

      This is mainly intended for use in a Java EE 5 environment, with all lookup driven by the standard JPA annotations, and all EntityManager references obtained from JNDI. No separate EntityManagerFactory bean definitions are necessary in such a scenario, and all EntityManager handling is done by the Java EE 5 server itself.

    - #### setExtendedPersistenceContexts

          public void setExtendedPersistenceContexts(Map<String,String> extendedPersistenceContexts)

      Specify the extended persistence contexts for EntityManager lookups, as a Map from persistence unit name to persistence context JNDI name (which needs to resolve to an EntityManager instance).

      JNDI names specified here should refer to `persistence-context-ref` entries in the Java EE deployment descriptors, matching the target persistence unit and being set up with persistence context type `Extended`.

      In case of no unit name specified in the annotation, the specified value for the `default persistence unit` will be taken (by default, the value mapped to the empty String), or simply the single persistence unit if there is only one.

      This is mainly intended for use in a Java EE 5 environment, with all lookup driven by the standard JPA annotations, and all EntityManager references obtained from JNDI. No separate EntityManagerFactory bean definitions are necessary in such a scenario, and all EntityManager handling is done by the Java EE 5 server itself.

    - #### setDefaultPersistenceUnitName

          public void setDefaultPersistenceUnitName(String unitName)

      Specify the default persistence unit name, to be used in case of no unit name specified in an `@PersistenceUnit` / `@PersistenceContext` annotation.

      This is mainly intended for lookups in the application context, indicating the target persistence unit name (typically matching the bean name), but also applies to lookups in the `"persistenceUnits"` / `"persistenceContexts"` / `"extendedPersistenceContexts"` map, avoiding the need for duplicated mappings for the empty String there.

      Default is to check for a single EntityManagerFactory bean in the Spring application context, if any. If there are multiple such factories, either specify this default persistence unit name or explicitly refer to named persistence units in your annotations.

    - #### setOrder

          public void setOrder(int order)

    - #### getOrder

          public int getOrder()

      Description copied from interface: `Ordered`
      Return the order value of this object, with a higher value meaning greater in terms of sorting.

      Normally starting with 0, with `Integer.MAX_VALUE` indicating the greatest value. Same order values will result in arbitrary positions for the affected objects.

      Higher values can be interpreted as lower priority. As a consequence, the object with the lowest value has highest priority (somewhat analogous to Servlet "load-on-startup" values).

      Specified by:
        `getOrder` in interface `Ordered`
      Returns:
        the order value

    - #### setBeanFactory

          public void setBeanFactory(BeanFactory beanFactory)

      Description copied from interface: `BeanFactoryAware`
      Callback that supplies the owning factory to a bean instance.

      Invoked after the population of normal bean properties but before an initialization callback such as `InitializingBean.afterPropertiesSet()` or a custom init-method.

      Specified by:
        `setBeanFactory` in interface `BeanFactoryAware`
      Parameters:
        `beanFactory` - owning BeanFactory (never `null`). The bean can immediately call methods on the factory.
      See Also:
        `BeanInitializationException`

    - #### postProcessMergedBeanDefinition

          public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition,
                                             Class<?> beanType,
                                             String beanName)

      Description copied from interface: `MergedBeanDefinitionPostProcessor`
      Post-process the given merged bean definition for the specified bean.
      Specified by:
        `postProcessMergedBeanDefinition` in interface `MergedBeanDefinitionPostProcessor`
      Parameters:
        `beanDefinition` - the merged bean definition for the bean
        `beanType` - the actual type of the managed bean instance
        `beanName` - the name of the bean

    - #### postProcessBeforeInstantiation

          public Object postProcessBeforeInstantiation(Class<?> beanClass,
                                              String beanName)
                                                throws BeansException

      Description copied from interface: `InstantiationAwareBeanPostProcessor`
      Apply this BeanPostProcessor before the target bean gets instantiated. The returned bean object may be a proxy to use instead of the target bean, effectively suppressing default instantiation of the target bean.

      If a non-null object is returned by this method, the bean creation process will be short-circuited. The only further processing applied is the `BeanPostProcessor.postProcessAfterInitialization(java.lang.Object, java.lang.String)` callback from the configured `BeanPostProcessors`.

      This callback will only be applied to bean definitions with a bean class. In particular, it will not be applied to beans with a "factory-method".

      Post-processors may implement the extended `SmartInstantiationAwareBeanPostProcessor` interface in order to predict the type of the bean object that they are going to return here.

      Specified by:
        `postProcessBeforeInstantiation` in interface `InstantiationAwareBeanPostProcessor`
      Parameters:
        `beanClass` - the class of the bean to be instantiated
        `beanName` - the name of the bean
      Returns:
        the bean object to expose instead of a default instance of the target bean, or `null` to proceed with default instantiation
      Throws:
        `BeansException` - in case of errors
      See Also:
        `AbstractBeanDefinition.hasBeanClass()`, `AbstractBeanDefinition.getFactoryMethodName()`

    - #### postProcessAfterInstantiation

          public boolean postProcessAfterInstantiation(Object bean,
                                              String beanName)
                                                throws BeansException

      Description copied from interface: `InstantiationAwareBeanPostProcessor`
      Perform operations after the bean has been instantiated, via a constructor or factory method, but before Spring property population (from explicit properties or autowiring) occurs.

      This is the ideal callback for performing field injection on the given bean instance. See Spring's own `AutowiredAnnotationBeanPostProcessor` for a typical example.

      Specified by:
        `postProcessAfterInstantiation` in interface `InstantiationAwareBeanPostProcessor`
      Parameters:
        `bean` - the bean instance created, with properties not having been set yet
        `beanName` - the name of the bean
      Returns:
        `true` if properties should be set on the bean; `false` if property population should be skipped. Normal implementations should return `true`. Returning `false` will also prevent any subsequent InstantiationAwareBeanPostProcessor instances being invoked on this bean instance.
      Throws:
        `BeansException` - in case of errors

    - #### postProcessPropertyValues

          public PropertyValues postProcessPropertyValues(PropertyValues pvs,
                                                 PropertyDescriptor[] pds,
                                                 Object bean,
                                                 String beanName)
                                                   throws BeansException

      Description copied from interface: `InstantiationAwareBeanPostProcessor`
      Post-process the given property values before the factory applies them to the given bean. Allows for checking whether all dependencies have been satisfied, for example based on a "Required" annotation on bean property setters.

      Also allows for replacing the property values to apply, typically through creating a new MutablePropertyValues instance based on the original PropertyValues, adding or removing specific values.

      Specified by:
        `postProcessPropertyValues` in interface `InstantiationAwareBeanPostProcessor`
      Parameters:
        `pvs` - the property values that the factory is about to apply (never `null`)
        `pds` - the relevant property descriptors for the target bean (with ignored dependency types - which the factory handles specifically - already filtered out)
        `bean` - the bean instance created, but whose properties have not yet been set
        `beanName` - the name of the bean
      Returns:
        the actual property values to apply to to the given bean (can be the passed-in PropertyValues instance), or `null` to skip property population
      Throws:
        `BeansException` - in case of errors
      See Also:
        `MutablePropertyValues`

    - #### postProcessBeforeInitialization

          public Object postProcessBeforeInitialization(Object bean,
                                               String beanName)
                                                 throws BeansException

      Description copied from interface: `BeanPostProcessor`
      Apply this BeanPostProcessor to the given new bean instance before any bean initialization callbacks (like InitializingBean's `afterPropertiesSet` or a custom init-method). The bean will already be populated with property values. The returned bean instance may be a wrapper around the original.
      Specified by:
        `postProcessBeforeInitialization` in interface `BeanPostProcessor`
      Parameters:
        `bean` - the new bean instance
        `beanName` - the name of the bean
      Returns:
        the bean instance to use, either the original or a wrapped one; if `null`, no subsequent BeanPostProcessors will be invoked
      Throws:
        `BeansException` - in case of errors
      See Also:
        `InitializingBean.afterPropertiesSet()`

    - #### postProcessAfterInitialization

          public Object postProcessAfterInitialization(Object bean,
                                              String beanName)
                                                throws BeansException

      Description copied from interface: `BeanPostProcessor`
      Apply this BeanPostProcessor to the given new bean instance after any bean initialization callbacks (like InitializingBean's `afterPropertiesSet` or a custom init-method). The bean will already be populated with property values. The returned bean instance may be a wrapper around the original.

      In case of a FactoryBean, this callback will be invoked for both the FactoryBean instance and the objects created by the FactoryBean (as of Spring 2.0). The post-processor can decide whether to apply to either the FactoryBean or created objects or both through corresponding `bean instanceof FactoryBean` checks.

      This callback will also be invoked after a short-circuiting triggered by a `InstantiationAwareBeanPostProcessor.postProcessBeforeInstantiation(java.lang.Class<?>, java.lang.String)` method, in contrast to all other BeanPostProcessor callbacks.

      Specified by:
        `postProcessAfterInitialization` in interface `BeanPostProcessor`
      Parameters:
        `bean` - the new bean instance
        `beanName` - the name of the bean
      Returns:
        the bean instance to use, either the original or a wrapped one; if `null`, no subsequent BeanPostProcessors will be invoked
      Throws:
        `BeansException` - in case of errors
      See Also:
        `InitializingBean.afterPropertiesSet()`, `FactoryBean`

    - #### postProcessBeforeDestruction

          public void postProcessBeforeDestruction(Object bean,
                                          String beanName)
                                            throws BeansException

      Description copied from interface: `DestructionAwareBeanPostProcessor`
      Apply this BeanPostProcessor to the given bean instance before its destruction. Can invoke custom destruction callbacks.

      Like DisposableBean's `destroy` and a custom destroy method, this callback just applies to singleton beans in the factory (including inner beans).

      Specified by:
        `postProcessBeforeDestruction` in interface `DestructionAwareBeanPostProcessor`
      Parameters:
        `bean` - the bean instance to be destroyed
        `beanName` - the name of the bean
      Throws:
        `BeansException` - in case of errors
      See Also:
        `DisposableBean`, `AbstractBeanDefinition.setDestroyMethodName(java.lang.String)`

    - #### getPersistenceUnit

          protected EntityManagerFactory getPersistenceUnit(String unitName)

      Return a specified persistence unit for the given unit name, as defined through the "persistenceUnits" map.
      Parameters:
        `unitName` - the name of the persistence unit
      Returns:
        the corresponding EntityManagerFactory, or `null` if none found
      See Also:
        `setPersistenceUnits(java.util.Map<java.lang.String, java.lang.String>)`

    - #### getPersistenceContext

          protected EntityManager getPersistenceContext(String unitName,
                                            boolean extended)

      Return a specified persistence context for the given unit name, as defined through the "persistenceContexts" (or "extendedPersistenceContexts") map.
      Parameters:
        `unitName` - the name of the persistence unit
        `extended` - whether to obtain an extended persistence context
      Returns:
        the corresponding EntityManager, or `null` if none found
      See Also:
        `setPersistenceContexts(java.util.Map<java.lang.String, java.lang.String>)`, `setExtendedPersistenceContexts(java.util.Map<java.lang.String, java.lang.String>)`

    - #### findEntityManagerFactory

          protected EntityManagerFactory findEntityManagerFactory(String unitName,
                                                      String requestingBeanName)
                                                           throws NoSuchBeanDefinitionException

      Find an EntityManagerFactory with the given name in the current Spring application context, falling back to a single default EntityManagerFactory (if any) in case of no unit name specified.
      Parameters:
        `unitName` - the name of the persistence unit (may be `null` or empty)
        `requestingBeanName` - the name of the requesting bean
      Returns:
        the EntityManagerFactory
      Throws:
        `NoSuchBeanDefinitionException` - if there is no such EntityManagerFactory in the context

    - #### findNamedEntityManagerFactory

          protected EntityManagerFactory findNamedEntityManagerFactory(String unitName,
                                                           String requestingBeanName)
                                                                throws NoSuchBeanDefinitionException

      Find an EntityManagerFactory with the given name in the current Spring application context.
      Parameters:
        `unitName` - the name of the persistence unit (never empty)
        `requestingBeanName` - the name of the requesting bean
      Returns:
        the EntityManagerFactory
      Throws:
        `NoSuchBeanDefinitionException` - if there is no such EntityManagerFactory in the context

    - #### findDefaultEntityManagerFactory

          protected EntityManagerFactory findDefaultEntityManagerFactory(String requestingBeanName)
                                                                  throws NoSuchBeanDefinitionException

      Find a single default EntityManagerFactory in the Spring application context.
      Returns:
        the default EntityManagerFactory
      Throws:
        `NoSuchBeanDefinitionException` - if there is no single EntityManagerFactory in the context

    - #### lookup

          protected <T> T lookup(String jndiName,
                     Class<T> requiredType)
                      throws Exception

      Perform a JNDI lookup for the given resource by name.

      Called for EntityManagerFactory and EntityManager lookup when JNDI names are mapped for specific persistence units.

      Parameters:
        `jndiName` - the JNDI name to look up
        `requiredType` - the required type of the object
      Returns:
        the obtained object
      Throws:
        `Exception` - if the JNDI lookup failed