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Should Java JDBC Select statements always be in a Try-Catch block?

Is it good practice to put all Java JDBC Select statements in a try-catch block ? Currently I write most of my code without it. However, I do try-catch for insert/update/delete.
Note: Currently using Sprint Boot.
String sqlQuery = "Select productId, productName, productStartDate from dbo.product where productId = 5"
public getProductData() {
....
List<Product> productList = namedJdbcTemplate.query(sqlQuery, new ProductMapper());

Since this question is tagged with spring-boot and you are using JdbcTemplate, I'm giving you a Spring-specific answer.
One of the points of Spring is to avoid boilerplate from developers. If you find yourself adding things repetitively, like putting try-catch blocks around code executing DML, that's cause for suspecting you're not doing something right. Adding your own try-catches in code using Spring isn't always wrong, but it usually is.
In the Spring reference doc https://docs.spring.io/spring-framework/docs/current/reference/html/data-access.html#jdbc there is a table showing what is the developer's responsibility and what is Spring's responsibility. Processing exceptions, handling the transactions, and closing jdbc resources are all shown as being Spring's responsibility.
SpringJdbc takes care of a lot of things for you. It handles closing JDBC resources and returning connections to their pool, and converts exceptions from SQLException to a hierarchy of unchecked DataAccessExceptions. In Spring unchecked exceptions thrown from a method wrapped in a transactional proxy cause the transaction to get rolled back. If you do your own try-catch logic you can prevent rollback from occurring when it needs to, if you catch the exception and the proxy never sees it. Adding try-catch logic can cause problems if you don't understand what Spring is doing.
Exceptions do need to be caught somewhere. In a Spring web application, you can set up an exception handler that catches anything thrown from the controller layer, so that you can log it. That way the action in progress gets broken off cleanly, the current transaction rolls back, and the exception gets handled in a consistent way. If you have other entry points, such as reading messages from a queue, those will need their own exception handler.
Exceptions are thrown in order to escape the current context, which isn't able to deal with the problem, and relocate control to somewhere safe. For most exceptions coming from JDBC, they aren't anything you can fix, you just want to let it be thrown, let the current transaction rollback, then let the central exception handler catch and log it.

First of all, if you're working with raw JDBC API, you should always use PreparedStatement.
Yes, you'll just have to wrap the code with try-catch block at some point, though it's a good practice to catch exceptions just right away or at the point where it's logically suits. In case of SQL queries, you actually should wrap all of them into some Service class that will give you an access to modify your database objects without running through JDBC API every time. For example:
public class UserService {
private static final String CREATE_USER_SQL = "...";
private final Connection jdbcConnection;
public #Nullable User createUser(final String name) {
try (final PreparedStatement stmt = jdbcConnection.prepareStatement(CREATE_USER_SQL)) {
jdbcConnection.setAutoCommit(false);
stmt.setString(1, name);
stmt.executeQuery();
jdbcConnection.commit();
return new User(name);
} catch (final SQLException createException) {
System.out.printf("User CREATE failed: %s\n", createException.getMessage());
try {
jdbcConnection.rollback();
} catch (final SQLException rollbackException) {
System.out.printf("Rollback failed: %s\n", rollbackException.getMessage());
}
return null;
}
}
}
This solves two problems right away:
You won't need to put boilerplate JDBC code everywhere;
It will log any JDBC errors right away, so you won't need to go through a complex debugging process.

Brief explanation:
First of all any resource involving I/O access (database access is I/O access) must always be closed or it will cause a memory leak.
Secondly, it is better to rely on try-with-resources to close any resource as having to call the .close() method manually is always exposed to the risk of not being effectively executed at runtime due to a potential Exception/RuntimeException/Error getting thrown beforehand; even closing the resource in a finally method is not preferable as such block executes at a different phase compared to the try-with-resources - auto closure of try-with-resources happens at the end of the try block, while finally executes at the end of all try/catch block - , in addition to the basic problem that it is not a secure solution as a throw might happen even inside the finally block, preventing it from completing correctly.
This said, you always need to close:
Statement/PreparedStatement/CallableStatement
any ResultSet
the whole Connection when you don't need DB access anymore

Try-catch for DB Layer code is important if you're querying with JDBC.
Think about, what if the connection broke? Or what if Database crashed ? Or some other unfortunate scenario comes up.
For these things, I will recommend you to always keep the DB layer code within try-catch.
It's also recommended for you to have some fallback mechanism in-case of the above events.

You should always handle it with try cactch.
Why: For example you started a connection to db then some exception happened, if you don't rollback your transaction it stay on db and performance will be decreased, and memory leak will be happen.
Imagine, if your connection limit is 100 and 100 exception throwed after transaction started and you didn't rollback it your system will be locked because of you can't create any new connection to your database.
But if you want an alternative for "try catch finally" you can use like this:
EmUtil.consEm(em -> {
System.out.println(em.createNativeQuery("select * from temp").getResultList().size());
});
Source code:
public final class EmUtil {
interface EmCons {
public void cons(EntityManager em);
}
public static void consEm(EmCons t) {
EntityManager em = null;
try {
em = getEmf().createEntityManager();
t.cons(em);
} finally {
if (em != null && em.getTransaction().isActive())
em.getTransaction().rollback();
if (em != null && em.isOpen())
em.close();
}
}
private static EntityManagerFactory getEmf() {
//TODO
}
}

Spring translates those exceptions as DataAccessException (for more detail link). It will be good to catch those exceptions and you can rollback with #Transactional.

JPA correct way to handle detached entity state in case of exceptions/rollback

I have this class and I tought three ways to handle detached entity state in case of persistence exceptions (which are handled elsewhere):
#ManagedBean
#ViewScoped
public class EntityBean implements Serializable
{
#EJB
private PersistenceService service;
private Document entity;
public void update()
{
// HANDLING 1. ignore errors
service.transact(em ->
{
entity = em.merge(entity);
// some other code that modifies [entity] properties:
// entity.setCode(...);
// entity.setResposible(...);
// entity.setSecurityLevel(...);
}); // an exception may be thrown on method return (rollback),
// but [entity] has already been reassigned with a "dirty" one.
//------------------------------------------------------------------
// HANDLING 2. ensure entity is untouched before flush is ok
service.transact(em ->
{
Document managed = em.merge(entity);
// some other code that modifies [managed] properties:
// managed.setCode(...);
// managed.setResposible(...);
// managed.setSecurityLevel(...);
em.flush(); // an exception may be thrown here (rollback)
// forcing method exit without [entity] being reassigned.
entity = managed;
}); // an exception may be thrown on method return (rollback),
// but [entity] has already been reassigned with a "dirty" one.
//------------------------------------------------------------------
// HANDLING 3. ensure entity is untouched before whole transaction is ok
AtomicReference<Document> reference = new AtomicReference<>();
service.transact(em ->
{
Document managed = em.merge(entity);
// some other code that modifies [managed] properties:
// managed.setCode(...);
// managed.setResposible(...);
// managed.setSecurityLevel(...);
reference.set(managed);
}); // an exception may be thrown on method return (rollback),
// and [entity] is safe, it's not been reassigned yet.
entity = reference.get();
}
...
}
PersistenceService#transact(Consumer<EntityManager> consumer) can throw unchecked exceptions.
The goal is to maintain the state of the entity aligned with the state of the database, even in case of exceptions (prevent entity to become "dirty" after transaction fail).
Method 1. is obviously naive and doesn't guarantee coherence.
Method 2. asserts that nothing can go wrong after flushing.
Method 3. prevents the new entity assigment if there's an exception in the whole transaction
Questions:
Is method 3. really safer than method 2.?
Are there cases where an exception is thrown between flush [excluded] and commit [included]?
Is there a standard way to handle this common problem?
Thank you
Note that I'm already able to rollback the transaction and close the EntityManager (PersistenceService#transact will do it gracefully), but I need to solve database state and the business objects do get out of sync. Usually this is not a problem. In my case this is the problem, because exceptions are usually generated by BeanValidator (those on JPA side, not on JSF side, for computed values that depends on user inputs) and I want the user to input correct values and try again, without losing the values he entered before.
Side note: I'm using Hibernate 5.2.1
this is the PersistenceService (CMT)
#Stateless
#Local
public class PersistenceService implements Serializable
{
#PersistenceContext
private EntityManager em;
#TransactionAttribute(TransactionAttributeType.REQUIRED)
public void transact(Consumer<EntityManager> consumer)
{
consumer.accept(em);
}
}
#DraganBozanovic
That's it! Great explanation for point 1. and 2.
I'd just love you to elaborate a little more on point 3. and give me some advice on real-world use case.
However, I would definitely not use AtomicReference or similar cumbersome constructs. Java EE, Spring and other frameworks and application containers support declaring transactional methods via annotations: Simply use the result returned from a transactional method.
When you have to modify a single entity, the transactional method would just take the detached entity as parameter and return the updated entity, easy.
public Document updateDocument(Document doc)
{
Document managed = em.merge(doc);
// managed.setXxx(...);
// managed.setYyy(...);
return managed;
}
But when you need to modify more than one in a single transaction, the method can become a real pain:
public LinkTicketResult linkTicket(Node node, Ticket ticket)
{
LinkTicketResult result = new LinkTicketResult();
Node managedNode = em.merge(node);
result.setNode(managedNode);
// modify managedNode
Ticket managedTicket = em.merge(ticket);
result.setTicket(managedTicket);
// modify managedTicket
Remark managedRemark = createRemark(...);
result.setRemark(managedemark);
return result;
}
In this case, my pain:
I have to create a dedicated transactional method (maybe a dedicated #EJB too)
That method will be called only once (will have just one caller) - is a "one-shot" non-reusable public method. Ugly.
I have to create the dummy class LinkTicketResult
That class will be instantiated only once, in that method - is "one-shot"
The method could have many parameters (or another dummy class LinkTicketParameters)
JSF controller actions, in most cases, will just call a EJB method, extract updated entities from returned container and reassign them to local fields
My code will be steadily polluted with "one-shotters", too many for my taste.
Probably I'm not seeing something big that's just in front of me, I'll be very grateful if you can point me in the right direction.

Is method 3. really safer than method 2.?
Yes. Not only is it safer (see point 2), but it is conceptually more correct, as you change transaction-dependent state only when you proved that the related transaction has succeeded.
Are there cases where an exception is thrown between flush [excluded] and commit [included]?
Yes. For example:
LockMode.OPTIMISTIC:
Optimistically assume that transaction will not experience contention
for entities. The entity version will be verified near the transaction
end.
It would be neither performant nor practically useful to check optimistick lock violation during each flush operation within a single transaction.
Deferred integrity constraints (enforced at commit time in db). Not used often, but are an illustrative example for this case.
Later maintenance and refactoring. You or somebody else may later introduce additional changes after the last explicit call to flush.
Is there a standard way to handle this common problem?
Yes, I would say that your third approach is the standard one: Use the results of a complete and successful transaction.
However, I would definitely not use AtomicReference or similar cumbersome constructs. Java EE, Spring and other frameworks and application containers support declaring transactional methods via annotations: Simply use the result returned from a transactional method.

Not sure if this is entirely to the point, but there is only one way to recover after exceptions: rollback and close the EM. From https://docs.jboss.org/hibernate/entitymanager/3.6/reference/en/html/transactions.html#transactions-basics-issues
An exception thrown by the Entity Manager means you have to rollback
your database transaction and close the EntityManager immediately
(discussed later in more detail). If your EntityManager is bound to
the application, you have to stop the application. Rolling back the
database transaction doesn't put your business objects back into the
state they were at the start of the transaction. This means the
database state and the business objects do get out of sync. Usually
this is not a problem, because exceptions are not recoverable and you
have to start over your unit of work after rollback anyway.
-- EDIT--
Also see http://piotrnowicki.com/2013/03/jpa-and-cmt-why-catching-persistence-exception-is-not-enough/
ps: downvote is not mine.

Java thread safe database connections

I'm writing a servlet that handles each request by accessing and modifying some table(s) in the database. I want the connections to the database to be thread safe. I don't want to use already existing libraries/frameworks for this (spring, hibernate, etc.).
I know I can use java's ThreadLocal for this in the following way :
public class DatabaseRegistry { //assume it's a singleton
private Properties prop = new Properties();
public static final ThreadLocal<Connection> threadConnection = new ThreadLocal<Connection>();
private Connection connect() throws SQLException {
try {
// This will load the MySQL driver, each DB has its own driver
Class.forName("com.mysql.jdbc.Driver");
// Setup the connection with the DB
Connection connection = DriverManager
.getConnection("jdbc:mysql://" + prop.getProperty("hostname") + "/" + prop.getProperty("database") + "?"
+ "user=" + prop.getProperty("username") + "&password=" + prop.getProperty("password"));
return connection;
} catch (SQLException e) {
throw e;
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
return null;
}
public Connection getConnection() throws SQLException {
if (threadConnection.get() == null) {
Connection connection = connect();
threadConnection.set(connection);
return threadConnection.get();
} else {
return threadConnection.get();
}
}
private void freeConnection(Connection connection) throws SQLException {
connection.close();
threadConnection.remove();
}
}
Each time you call getConnection(), the new connection is added to the ThreadLocal object and then removed when you free the connection.
Is this the proper way of doing this or should the DatabaseRegistry itself extend the ThreadLocal<Connection> class? Or is there an even better way to do this to make all connections thread safe?

I don't think that making database connections thread-safe is a common practice. Usually what you want is either:
Serialize the access to some part of your servlet, so that there is no more than one servlet executing code at a time (e.g. implementing the SingleThreadModel interface).
Locking a particular table / table page / row so you can operate on some particular tuple (by changing the database isolation level).
Using optimistic locking to detect modified rows in a table (using some reference attribute of the table to check if the current version is the same that the one in the table).
AFAIK, the typical use of ThreadLocal<Connection> is to store a unique database connection per thread, so that the same connection can be used in different methods in your business logic without the need of passing it as a parameter each time. Because the common servlet container implementation uses a thread to fulfill an HTTP request, then two different requests are guaranteed to use two different database connections.

I know you said you don't want to use libraries to do this, but you're going to be way better off if you do. Pick a standard connection pool (C3P0, DBCP, or something) and you'll be way happier than if you bake your own. Why can't you use a library to do this?

I am not sure why you want your DB connections to be thread safe. Most of the time establishing connection to the database is the longest part of the transaction. Typically connections are reused between requests and pools of open connections are managed (via frameworks or more typically application servers).
If you are worried about concurrent modifications to the same tables you might want to look at synchronized methods: http://docs.oracle.com/javase/tutorial/essential/concurrency/syncmeth.html

Should I manually commit this Play! transaction?

After finding out how to manually commit a Play! JPA transaction ... I'm not sure I really need to.
I have two controller actions: one that adds a website, and immediately redirects to the next one ... which shows its edit form.
public static void added(String title){
Task task= new Task();
website.title = title;
task.save();
// Do I really need to commit this transaction here?
// Note that task.id is already filled here, somehow
// https://stackoverflow.com/questions/8169640/how-does-an-entity-get-an-id-before-a-transaction-is-committed-in-jpa-play
JPA.em().getTransaction().commit();
edit(task.id);
}
public static void edit(long taskId) {
Task task = Task.find("byId", taskId).first();
render(task);
}
Is there a need to commit the transaction before redirecting to edit()?

No. :) (Though there may be situations in the future where you may need to take control of the transaction handling, this does not appear to be one of them; as you also discovered, Play! flushes the session immediately after saving, so you have access to the auto-generated PK ID. As this appears to be the only reason you were attempting this, I would let Play! do its thing and only hijack control if/when you really need to.)

How to do transactional without lose encapsulation?

I have a code that saves a bean, and updates another bean in a DB via Hibernate. It must be do in the same transaction, because if something wrong occurs (f.ex launches a Exception) rollback must be executed for the two operations.
public class BeanDao extends ManagedSession {
public Integer save(Bean bean) {
Session session = null;
try {
session = createNewSessionAndTransaction();
Integer idValoracio = (Integer) session.save(bean); // SAVE
doOtherAction(bean); // UPDATE
commitTransaction(session);
return idBean;
} catch (RuntimeException re) {
log.error("get failed", re);
if (session != null) {
rollbackTransaction(session);
}
throw re;
}
}
private void doOtherAction(Bean bean) {
Integer idOtherBean = bean.getIdOtherBean();
OtherBeanDao otherBeanDao = new OtherBeanDao();
OtherBean otherBean = otherBeanDao.findById(idOtherBean);
.
. (doing operations)
.
otherBeanDao.attachDirty(otherBean)
}
}
The problem is:
In case that
session.save(bean)
launches an error, then I get AssertionFailure, because the function doOtherAction (that is used in other parts of the project) uses session after a Exception is thrown.
The first thing I thought were extract the code of the function doOtherAction, but then I have the same code duplicate, and not seems the best practice to do it.
What is the best way to refactor this?

It's a common practice to manage transactions at one level above DAOs, in services or other business logic classes. That way you can, based on the business/service logic, in one case do two DAO operations in one transaction and, in another case, do them in separate transactions.

I'm a huge fan of Declarative Transaction Management. If you can spare the time to get it working (piece of cake with an Application Server such as GlassFish or JBoss, and easy with Spring). If you annotate your business method with #TransactionAttribute(REQUIRED) (it can even be set to be done as default) and it calls the two DAO methods you will get exactly what you want: everything gets committed at once or rolled back over an Exception.
This solution is about as loosely coupled as it gets.

The others are correct in that they take in to account what are common practice currently.
But that doesn't really help you with your current practice.
What you should do is create two new DAO methods. Such as CreateGlobalSession and CommitGlobalSession.
What these do is the same thing as your current create and commit routines.
The difference is that they set a "global" session variable (most likely best done with a ThreadLocal). Then you change the current routines so that they check if this global session already exists. If your create detects the global session, then simply return it. If your commit detects the global session, then it does nothing.
Now when you want to use it you do this:
try {
dao.createGlobalSession();
beanA.save();
beanb.save();
Dao.commitGlobalSession();
} finally {
dao.rollbackGlobalSession();
}
Make sure you wrap the process in a try block so that you can reset your global session if there's an error.
While the other techniques are considered best practice and ideally you could one day evolve to something like that, this will get you over the hump with little more than 3 new methods and changing two existing methods. After that the rest of your code stays the same.