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Is there a way to Spring custom validator reject value without field having a getter method?

I'm currently studying building API's with Spring. I'm working with Spring Validator to validate my input. Here it is my custom validator:
public class NewHoldValidator implements Validator {
private EntityManager manager;
public NewHoldValidator(EntityManager manager) {
this.manager = manager;
}
#Override
public boolean supports(Class<?> clazz) {
return NewHoldRequest.class.isAssignableFrom(clazz);
}
#Override
public void validate(Object target, Errors errors) {
if (errors.hasErrors()) {
return;
}
NewHoldRequest request = (NewHoldRequest) target;
Patron patron = manager.find(Patron.class, request.patronId);
BookInstance bookInstance = manager.find(BookInstance.class, request.bookInstanceId);
Assert.state(patron != null, "Patron does not exists.");
Assert.state(bookInstance != null, "Book instance does not exists.");
if (!bookInstance.acceptToBeHoldTo(patron)) {
errors.reject(null, "This book instance cannot be hold to this patron");
}
if (!request.hasDaysHold()) {
if (!patron.researcher()) {
errors.rejectValue("daysHold", null, "You need to pass a daysHold attribute");
}
}
}
}
And here is my NewHoldRequest class:
public class NewHoldRequest {
#NotNull
public final Long patronId;
#NotNull
public final Long bookInstanceId;
#Positive
#Max(60)
public final Integer daysHold;
public NewHoldRequest(#NotNull Long patronId, #NotNull Long bookInstanceId, #Positive #Max(60) Integer daysHold) {
this.patronId = patronId;
this.bookInstanceId = bookInstanceId;
this.daysHold = daysHold;
}
#Override
public String toString() {
return "NewHoldRequest{" + "patronId=" + patronId + ", bookId=" + bookInstanceId + ", daysHold=" + daysHold + '}';
}
public boolean hasDaysHold() {
return this.daysHold != null;
}
Even if my field "daysHold" is public I still need to create a getter to it so Spring can show the rejected error properly, otherwise, it will throw a NotReadablePropertyException. Is there a way to define that Spring can reject public fields without getters or I will need to add accessor methods to all fields I want to reject?
Here is the message that shows up when my validation is triggered.
org.springframework.beans.NotReadablePropertyException: Invalid property 'daysHold' of bean class [com.api.library.usecases.newhold.NewHoldRequest]: Bean property 'daysHold' is not readable or has an invalid getter method: Does the return type of the getter match the parameter type of the setter?

I'll advise you to use the interface ConstraintValidator. It's a generic's interface, without casting and other problems.
You should create a custom constraint annotation for you it's #NewHoldValid after that implement CustomValidator interface:
#Component
public class NewHoldValidator implements ConstraintValidator<NewHoldValid, NewHoldRequest> {
#Override
public boolean isValid(NewHoldRequest value, ConstraintValidatorContext context) {
if(ANY_CONDITION){
return true;
}else {
return false;
}
}
After that use #NewHoldValid annotation above your NewHoldRequest.
If you'll want to set an error message text which different from default use that:
context.buildConstraintViolationWithTemplate("Your error message").addConstraintViolation();

Cleanest pattern for filtering objects of unknown type?

I have an application that takes json objects from a queue, deserializes them to a model, applies a list of filters, and sends the objects that pass all filters through to another queue.
The two complicating criteria are:
The set of filters is determined and injected via Spring profile at startup.
The type of object that the json is being deserialized to is also determined the by the Spring profile at startup.
The following solution is ugly because it involves casting:
public class MessageTypeOne {
public int someField;
}
public class MessageTypeTwo {
public int otherField;
}
public interface MessageFilter {
boolean doesFilterPass(Object object);
}
#Component
#Profile("ProfileOne")
public class OneOfMyMessageFilters implements MessageFilter {
public boolean doesFilterPass(Object object) {
MessageTypeOne message = (MessageTypeOne)object;
if (message.someField == something) {
return false;
} else return true;
}
}
#Component
#Profile("ProfileTwo")
public class AnotherOneOfMyMessageFilters implements MessageFilter {
public boolean doesFilterPass(Object object) {
MessageTypeTwo message = (MessageTypeTwo)object;
if (message.otherField == something) {
return false;
} else return true;
}
}
#Service
public class MessageFilterService {
// injected at runtime via Spring profile
private Set<MessageFilter> messageFilters
#AutoWired
public MessageFilterService(Set<MessageFilter> messageFilters) {
this.messageFilters = messageFilters;
}
public boolean passesAllFilters(Object object) throws IOException {
for (MessageFilter filter : messageFilters) {
if (!filter.doesFilterPass(object)) {
return false;
}
}
return true;
}
}
What's the cleanest pattern for cases like these? I've read about the visitor pattern but I'm not sure that's any better than casting like this.

As far as design pattern is concerned, I think it is of type Strategy pattern. I am not talking about Spring way of implementation. You may have n number of filters, but you have to choose based upon the context. So strategy pattern is best fitted here. Others can provide other patterns. You can strategy pattern in the below link.
https://en.wikipedia.org/wiki/Strategy_pattern

What about visitor pattern with Java reflection? Here is an old article:
https://www.javaworld.com/article/2077602/java-tip-98--reflect-on-the-visitor-design-pattern.html

When you want to decouple messages from filters and relation is many to many you can always use Chain of Responsibility.
#Service
public class MessageFiltersAggregator {
private MessageFilter chainEntryNode;
#AutoWired
public MessageFilterService(Set<MessageFilter> messageFilters) {
this.chainEntryNode = buildChain(messageFilters);
}
public boolean passesAllFilters(Object object) throws IOException {
return chainEntryNode.doesFilterPass(object);
}
}
You need to implement buildChain method which creates chain from collection. Of course, each element in chain should have next property. In this case MessageFilter could look like below:
public abstract class MessageFilter {
private MessageFilter next;
//constructors, setters, etc
public boolean doesFilterPass(Object object) {
boolean res = true;
if (canHandle(object)) {
res = validate(object);
}
return res && next.doesFilterPass(object);
}
public abstract boolean validate(Object object);
public abstract boolean canHandle(Object object);
}
Abstract class contains chain logic you just need to implement two methods in each subclass. One of implementation could look like below:
public class AnotherOneOfMyMessageFilters extends MessageFilter {
public boolean canHandle(Object object) {
return object instanceof MessageTypeTwo;
}
public boolean validate(Object object) {
MessageTypeTwo message = (MessageTypeTwo)object;
return message.otherField == something;
}
}
All above classes are just example created without IDE so could have issues in syntax but should give you an idea how it should work.
See also:
Chain of Responsibility in Java
Chain of Responsibility Design Pattern in Java

If I understand your problem correctly, then it's possible to configure your Spring profile in a way that makes your filters throw ClassCastExceptions.
Assuming that you configuration options are the way you want, then it demonstrates the only real problem with your design -- your filters can be applied to any Object, and that's what the interface says -- doesFilterPass( Object ) -- but your filters only really work with certain types of objects.
That's what you need to fix. If the filter is applied to a strange type of object, does it pass or fail? You can decide this on a per-filter basis and then just fix it like this:
public boolean doesFilterPass(Object object) {
if (!(object instanceOf MessageTypeTwo)) {
return true;
}
MessageTypeTwo message = (MessageTypeTwo)object;
if (message.otherField == something) {
return false;
} else return true;
}
Easy peasy.
I know you don't like the cast, but it's a direct result of the configuration options you provide -- the profile can be configured to apply filters to any kind of object. You just need to support that, and that means there has to be casting somewhere.

This became much cleaner with generics. Since I know what type of Object each filter can handle I can just do this, eliminating the casting:
public class MessageTypeOne {
public int someField;
}
public class MessageTypeTwo {
public int otherField;
}
public interface MessageFilter<T> {
boolean doesFilterPass(T message);
}
#Component
#Profile("ProfileOne")
public class OneOfMyMessageFilters<T extends MessageTypeOne> implements MessageFilter<T> {
public boolean doesFilterPass(MessageTypeOne message) {
if (message.someField == something) {
return false;
} else return true;
}
}
#Component
#Profile("ProfileTwo")
public class AnotherOneOfMyMessageFilters<T extends MessageTypeTwo> implements MessageFilter<T> {
public boolean doesFilterPass(MessageTypeTwo message) {
if (message.otherField == something) {
return false;
} else return true;
}
}
#Service
public class MessageFilterServiceImpl<T> implements MessageFilterService<T> {
// injected at runtime via Spring profile
private Set<MessageFilter<T>> messageFilters
#AutoWired
public MessageFilterService(Set<MessageFilter<T>> messageFilters) {
this.messageFilters = messageFilters;
}
public boolean passesAllFilters(T message) throws IOException {
for (MessageFilter filter : messageFilters) {
if (!filter.doesFilterPass(message)) {
return false;
}
}
return true;
}
}
public interface MessageFilterService<T> {
boolean passesAllFilters(T rawEvent) throws IllegalArgumentException;
}

For a large validation task is chain of responsibility pattern a good bet?

I need to build a process which will validate a record against ~200 validation rules. A record can be one of ~10 types. There is some segmentation from validation rules to record types but there exists a lot of overlap which prevents me from cleanly binning the validation rules.
During my design I'm considering a chain of responsibility pattern for all of the validation rules. Is this a good idea or is there a better design pattern?

Validation is frequently a Composite pattern. When you break it down, you want to seperate the what you want to from the how you want to do it, you get:
If foo is valid
then do something.
Here we have the abstraction is valid -- Caveat: This code was lifted from currrent, similar examples so you may find missing symbology and such. But this is so you get the picture. In addition, the
Result
Object contains messaging about the failure as well as a simple status (true/false).
This allow you the option of just asking "did it pass?" vs. "If it failed, tell me why"
QuickCollection
and
QuickMap
Are convenience classes for taking any class and quickly turning them into those respected types by merely assigning to a delegate. For this example it means your composite validator is already a collection and can be iterated, for example.
You had a secondary problem in your question: "cleanly binding" as in, "Type A" -> rules{a,b,c}" and "Type B" -> rules{c,e,z}"
This is easily managed with a Map. Not entirely a Command pattern but close
Map<Type,Validator> typeValidators = new HashMap<>();
Setup the validator for each type then create a mapping between types. This is really best done as bean config if you're using Java but Definitely use dependency injection
public interface Validator<T>{
public Result validate(T value);
public static interface Result {
public static final Result OK = new Result() {
#Override
public String getMessage() {
return "OK";
}
#Override
public String toString() {
return "OK";
}
#Override
public boolean isOk() {
return true;
}
};
public boolean isOk();
public String getMessage();
}
}
Now some simple implementations to show the point:
public class MinLengthValidator implements Validator<String> {
private final SimpleResult FAILED;
private Integer minLength;
public MinLengthValidator() {
this(8);
}
public MinLengthValidator(Integer minLength) {
this.minLength = minLength;
FAILED = new SimpleResult("Password must be at least "+minLength+" characters",false);
}
#Override
public Result validate(String newPassword) {
return newPassword.length() >= minLength ? Result.OK : FAILED;
}
#Override
public String toString() {
return this.getClass().getSimpleName();
}
}
Here is another we will combine with
public class NotCurrentValidator implements Validator<String> {
#Autowired
#Qualifier("userPasswordEncoder")
private PasswordEncoder encoder;
private static final SimpleResult FAILED = new SimpleResult("Password cannot be your current password",false);
#Override
public Result validate(String newPassword) {
boolean passed = !encoder.matches(newPassword,user.getPassword());
return (passed ? Result.OK : FAILED);
}
#Override
public String toString() {
return this.getClass().getSimpleName();
}
}
Now here is a composite:
public class CompositePasswordRule extends QuickCollection<Validator> implements Validator<String> {
public CompositeValidator(Collection<Validator> rules) {
super.delegate = rules;
}
public CompositeValidator(Validator<?>... rules) {
super.delegate = Arrays.asList(rules);
}
#Override
public CompositeResult validate(String newPassword) {
CompositeResult result = new CompositeResult(super.delegate.size());
for(Validator rule : super.delegate){
Result temp = rule.validate(newPassword);
if(!temp.isOk())
result.put(rule,temp);
}
return result;
}
public static class CompositeResult extends QuickMap<Validator,Result> implements Result {
private Integer appliedCount;
private CompositeResult(Integer appliedCount) {
super.delegate = VdcCollections.delimitedMap(new HashMap<PasswordRule, Result>(), "-->",", ");
this.appliedCount = appliedCount;
}
#Override
public String getMessage() {
return super.delegate.toString();
}
#Override
public String toString() {
return super.delegate.toString();
}
#Override
public boolean isOk() {
boolean isOk = true;
for (Result r : delegate.values()) {
isOk = r.isOk();
if(!isOk)
break;
}
return isOk;
}
public Integer failCount() {
return this.size();
}
public Integer passCount() {
return appliedCount - this.size();
}
}
}
and now a snippet of use:
private Validator<String> pwRule = new CompositeValidator<String>(new MinLengthValidator(),new NotCurrentValidator());
Validator.Result result = pwRule.validate(newPassword);
if(!result.isOk())
throw new PasswordConstraintException("%s", result.getMessage());
user.obsoleteCurrentPassword();
user.setPassword(passwordEncoder.encode(newPassword));
user.setPwExpDate(DateTime.now().plusDays(passwordDaysToLive).toDate());
userDao.updateUser(user);

Chain of responsibility implies that there is an order in which the validations must take place. I would probably use something similar to the Strategy pattern where you have a Set of validation strategies that are applied to a specific type of record. You could then use a factory to examine the record and apply the correct set of validations.

Is it possible to set multiple messages using oval AbstractAnnotationCheck?

I am using the Oval validation framework to validate fields that HTML fields cannot hold malicious javascript code. For the malicious code detection, I am using an external framework that returns me a list of errors that I would like to use as error messages on the field. The problem I am running into is that I can only setMessage in the check implementation, while I would rather do something like setMessages(List). So while I am currently just joining the errors with a comma, I would rather pass them back up as a list.
Annotation
#Target({ ElementType.METHOD, ElementType.FIELD})
#Retention( RetentionPolicy.RUNTIME)
#Constraint(checkWith = HtmlFieldValidator.class)
public #interface HtmlField {
String message() default "HTML could not be validated";
}
Check
public class HtmlFieldValidator extends AbstractAnnotationCheck<HtmlDefaultValue> {
public boolean isSatisfied( Object o, Object o1, OValContext oValContext, Validator validator ) throws OValException {
if (o1 == null) {
return true;
} else {
CleanResults cleanResults = UIowaAntiSamy.cleanHtml((String) o1);
if (cleanResults.getErrorMessages().size() > 0) {
String errors = StringUtils.join(cleanResults.getErrorMessages(), ", ");
this.setMessage(errors);
return false;
} else {
return true;
}
}
}
}
Model class
class Foo {
#HtmlField
public String bar;
}
Controller code
Validator validator = new Validator(); // use the OVal validator
Foo foo = new Foo();
foo.bar = "<script>hack()</script>";
List<ConstraintViolation> violations = validator.validate(bo);
if (violations.size() > 0) {
// inform the user that I cannot accept the string because
// it contains invalid html, using error messages from OVal
}

If setMessage(String message) is a method created by a superclass, you can override it and once it receives the data, simply split the string into a list and call a second function in which you would actually place your code. On a side note, I would also recommend changing the separating string to something more unique as the error message itself could include a comma.
Your question doesn't really make much sense though. If you are "passing them back up" to a method implemented in a superclass, then this voids the entire point of your question as the superclass will be handling the data.
I am going to assume the setError methods is a simple setter that sets a String variable to store an error message that you plan to access after checking the data. Since you want to have the data in your preferred type, just create a new array of strings in your class and ignore the superclass. You can even use both if you so desire.
public class HtmlFieldValidator extends AbstractAnnotationCheck<HtmlDefaultValue> {
public String[] errorMessages = null;
public void setErrorMessages(String[] s) {
this.errorMessages = s;
}
public boolean isSatisfied( Object o, Object o1, OValContext oValContext, Validator validator ) throws OValException {
if (o1 == null) {
return true;
} else {
CleanResults cleanResults = UIowaAntiSamy.cleanHtml((String) o1);
if (cleanResults.getErrorMessages().size() > 0) {
//String errors = StringUtils.join(cleanResults.getErrorMessages(), ", ");
//this.setMessage(errors);
this.setErrorMessages(cleanResults.getErrorMessages());
return false;
} else {
return true;
}
}
}
}
Elsewhere:
HtmlFieldValidator<DefaultValue> hfv = new HtmlFieldValidator<DefaultValue>();
boolean satisfied = hfv.isSatisfied(params);
if (!satisfied) {
String[] errorMessages = hfv.errorMessages;
//instead of using their error message
satisfy(errorMessages);//or whatever you want to do
}
EDIT:
After you updated your code I see what you mean. While I think this is sort of overdoing it and it would be much easier to just convert the string into an array later, you might be able to do it by creating a new class that extends Validator its setMessage method. In the method, you would call super.setMethod as well as splitting and storing the string as an array in its class.
class ValidatorWithArray extends Validator {
public String[] errors;
public final static String SPLIT_REGEX = ";&spLit;";// Something unique so you wont accidentally have it in the error
public void setMessage(String error) {
super.setMessage(error);
this.errors = String.split(error, SPLIT_REGEX);
}
}
In HtmlFieldValidator:
public boolean isSatisfied( Object o, Object o1, OValContext oValContext, Validator validator ) throws OValException {
if (o1 == null) {
return true;
} else {
CleanResults cleanResults = UIowaAntiSamy.cleanHtml((String) o1);
if (cleanResults.getErrorMessages().size() > 0) {
String errors = StringUtils.join(cleanResults.getErrorMessages(), ValidatorWithArray.SPLIT_REGEX);
this.setMessage(errors);
return false;
} else {
return true;
}
}
}
And now just use ValidatorWithArray instead of Validator

The situation in which I want to achieve this was different from yours, however what I found was best in my case was to create an annotation for each error (rather than having one that would return multiple errors). I guess it depends on how many errors you are likely to be producing in my case it was only two or three.
This method makes also makes your code really easy to reuse as you can just add the annotations wherenever you need them and combine them at will.

Trim string field in JPA

I have a db table with column of datatype char(20). I'm not allowed to change it to a varchar.
I'm writing a JPA entity mapped to this table. I would like the string field representing this column in my entity class to always contain the trimmed value, not the 20-character value padded with spaces that exists in the db.
I can't see any easy way to do this. (an annotation would rock!). At the moment I'm just returning a trimmed value from my getter(), but this feels like a kludge.
A google search is offering no help on this. Any ideas?

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>

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.

It's an old question but it was very useful for me to get to my answer. In my case, the easiest way was to create a simple "javax.persistence.Converter", like this:
#Converter
public class StringTrimConverter implements AttributeConverter<String, String> {
#Override
public String convertToDatabaseColumn(String attribute) {
return attribute;
}
#Override
public String convertToEntityAttribute(String dbData) {
return dbData.trim();
}
}
And you can use it like this:
#Entity
#Table(name = "ViewAddress")
public class PostalAddress extends DbObject {
#Convert(converter = StringTrimConverter.class)
private String street;
#Convert(converter = StringTrimConverter.class)
private String number;
(...)
It works just fine.

Put the annotation on the getter method, set the #Acesss to AccessType.Property and trim the field there using String.trim() method.
Or simply put the trim in the getter method and always access the field through it. It is not going to get any simpler than that.
If you do not mind using pure Hibernate and deviating from the JPA standard you can use Hibernate #ColumnTransformer provided that you have a database function to do the work of
You can find how to do it in the Hibernate reference:
http://docs.jboss.org/hibernate/core/3.6/reference/en-US/html/mapping.html#mapping-column-read-and-write
I hope that helps!

I am using this method, which makes the trimming transparent without having to use annotations in every string field.
In the same package that you have your session factory class (the one you use to get Sessions, e.g org.blablabla.yourpackage.etc.SessionGetter.getSession(), you must create a file named package-info.java and put this content inside it:
#TypeDefs({
#TypeDef(name = "trimmedStringType",
defaultForType = String.class,
typeClass = StringUserType.class)
})
package org.blablabla.yourpackage.etc;
import org.hibernate.annotations.TypeDef;
import org.hibernate.annotations.TypeDefs;
Then you create the class StringUserType in this same package:
import org.hibernate.HibernateException;
import org.hibernate.engine.spi.SessionImplementor;
import org.hibernate.type.StandardBasicTypes;
import org.hibernate.usertype.EnhancedUserType;
import java.io.Serializable;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Types;
public class StringUserType implements EnhancedUserType, Serializable {
private static final int[] SQL_TYPES = new int[]{Types.VARCHAR};
#Override
public int[] sqlTypes() {
return SQL_TYPES;
}
#Override
public Class returnedClass() {
return String.class;
}
#Override
public boolean equals(Object x, Object y) throws HibernateException {
if (x == y) {
return true;
}
if (x == null || y == null) {
return false;
}
String dtx = (String) x;
String dty = (String) y;
return dtx.equals(dty);
}
#Override
public int hashCode(Object object) throws HibernateException {
return object.hashCode();
}
#Override
public Object nullSafeGet(ResultSet resultSet, String[] names, SessionImplementor session, Object owner)
throws HibernateException, SQLException {
Object s = StandardBasicTypes.STRING.nullSafeGet(resultSet, names, session, owner);
if (s == null) {
return null;
}
return s.toString().trim();
}
#Override
public void nullSafeSet(PreparedStatement preparedStatement, Object value, int index, SessionImplementor session)
throws HibernateException, SQLException {
if (value == null) {
StandardBasicTypes.STRING.nullSafeSet(preparedStatement, null, index, session);
} else {
StandardBasicTypes.STRING.nullSafeSet(preparedStatement, value.toString().trim(), index, session);
}
}
#Override
public Object deepCopy(Object value) throws HibernateException {
return value;
}
#Override
public boolean isMutable() {
return false;
}
#Override
public Serializable disassemble(Object value) throws HibernateException {
return (Serializable) value;
}
#Override
public Object assemble(Serializable cached, Object value) throws HibernateException {
return cached;
}
#Override
public Object replace(Object original, Object target, Object owner) throws HibernateException {
return original;
}
#Override
public String objectToSQLString(Object object) {
throw new UnsupportedOperationException();
}
#Override
public String toXMLString(Object object) {
return object.toString();
}
#Override
public Object fromXMLString(String string) {
return string;
}
}
And that's it, no need to create custom annotations in your beans, it will "magically" trim the strings whenever you get the objects from the database.

What JPA provider are you using?
If you are using EclipseLink CHAR fields are trimmed by default. You can disable this through the session trimStrings property (ensure you have not set this).

Accepted answer works except registering the listener in persistence.xml. I did it with orm.xml.
<?xml version="1.0" encoding="UTF-8"?>
<entity-mappings xmlns="http://java.sun.com/xml/ns/persistence/orm"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence/orm
http://xmlns.jcp.org/xml/ns/persistence/orm_2_1.xsd"
version="2.1">
<persistence-unit-metadata>
<persistence-unit-defaults>
<entity-listeners>
<entity-listener class="org.example.TrimListener">
</entity-listener>
</entity-listeners>
</persistence-unit-defaults>
</persistence-unit-metadata>
</entity-mappings>

All you have to do is put this on your controller and it works as expected you dont need a listener or anything of that
#InitBinder
public void initBinder(WebDataBinder binder) {
binder.registerCustomEditor(String.class, new StringTrimmerEditor(true));
}

If your domain requirement states it needs trimmed information, you need to store the data in trimmed value. I see nothing wrong about it.
Domain Model
An object model of the
domain that incorporates both behavior
and data. (PEAA - Martin Fowler)
If you explicitly have to enforce the business rule at the database level, one option is that you have a choice of writing a trigger, you can use built-in SQL trim method. But it will be like using a rocket to crack an egg.