Trim string field in JPA

The accepted answer (using JPA entity listeners / @Trim annotation) is a dangerous one. Calling the setter on the retrieved entity appears to mark the entity as dirty. When I tried this myself at a root entity level (using Spring3 / hibernate), it triggered tons of extraneous updates to related entities that were otherwise not modified during the transaction. It was a real mess in production, and tracking it down to this being the cause took time.

In the end I opted to go with the more straightforward approach of trimming each of the fields manually on-demand (in a custom entity-to-domain mapper, or in the entity getter) similar to Edwin's answer.

Or you can use lifecycle annotations:

@Entity
public class MyEntity {

    @PostLoad
    protected void repair(){
        if(myStringProperty!=null)myStringProperty=myStringProperty.trim();
    }

    private String myStringProperty;
    public String getMyStringProperty() {
        return myStringProperty;
    }
    public void setMyStringProperty(String myStringProperty) {
        this.myStringProperty = myStringProperty;
    }

}

If this occurs on multiple entities you can create a custom annotation and write a dedicated EntityListener.

Annotation

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface Trim {}

Listener

public class TrimListener {

    private final Map<Class<?>, Set<Field>> trimProperties = 
        new HashMap<Class<?>, Set<Field>>();

    @PostLoad
    public void repairAfterLoad(final Object entity) throws Exception {
        for (final Field fieldToTrim : getTrimProperties(entity.getClass())) {
            final String propertyValue = (String) fieldToTrim.get(entity);
            if (propertyValue != null)
                fieldToTrim.set(entity, propertyValue.trim());
        }
    }

    private Set<Field> getTrimProperties(Class<?> entityClass) throws Exception {
        if (Object.class.equals(entityClass))
            return Collections.emptySet();
        Set<Field> propertiesToTrim = trimProperties.get(entityClass);
        if (propertiesToTrim == null) {
            propertiesToTrim = new HashSet<Field>();
            for (final Field field : entityClass.getDeclaredFields()) {
                if (field.getType().equals(String.class)
                    && field.getAnnotation(Trim.class) != null) {
                    field.setAccessible(true);
                    propertiesToTrim.add(field);
                }
            }
            trimProperties.put(entityClass, propertiesToTrim);
        }
        return propertiesToTrim;
    }

}

Now annotate all relevant String fields with @Trim and register the Listener as default entity listener in your persistence.xml:

<persistence-unit ..>
    <!-- ... -->
    <default-entity-listeners>
      com.somepackage.TrimListener
      and.maybe.SomeOtherListener
    </default-entity-listeners>
</persistence-unit>