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Should the connection be closed after each query in a long time transaction?

We are currently dealing with the function, that has to work partially with the database, and partially with a service, whice operations are time-consuming. So, generally speaking, here is a transactional method, that has a code like this:
Connection conn = null;
try {
conn = getConnection(); // This I get from the connection pool
Employee emp = queryDatabase(id);
// Point A - ??
Response resp = makeLongTimeServiceCall(emp);
// Point B - ??
anotherQueryDatabase(resp);
} catch (Exception e) {
throw e; // And this also rolls back the transaction
} finally {
// If conn is not closed, close it, so there would be no leaks
}
So the big question is - should I close the connection at point A and then get it again from the connection pool at point B, so that other servlets could use that connection while I interact with the service? My guess is that I should, but will this hurt my transaction?

In many circumstances: yes, closing and reopening the connection sounds good. However you need to understand the implication of doing them in two separate transactions (by closing and re-opening the connection you are inherently doing them in separate transaction).
If another user invokes an operation that changes the state of your data at Point B will the end result still be correct?

If you don't have a transaction you can close the connection and ask for a new connection when needed.
Remember that if you are using a connection pool closing a connection will not really close the connection, but only flag it as reusable by other threads.

Database connections shouldn't be left open. Open the connections just when you need to execute a query and close it as early as possible. So my solution is, YES.

if your application doesn't close connection properly may lead to some issues like the
connection pool maxing out.
Applications Not Properly Closing Connections:
When writing an application to use the WebSphere Application Server(WAS) datasource, the best way is fallowing get/use/close pattern.
1.get
-This is when the application makes a request to get a connection from the pool.
The application will first lookup the datasource name and then do a getConnection() call.
2.use
-This is when the application executes a query and waits for a response
3.close
-This is the final stage when the application has received the response from the database and is now done using that connection.
-The application should call close() in a finally block to close out the connection and return it to the free pool.
If your application does not follow this pattern, you may see connections staying open for long periods of time. This is especially seen when the application does not close the connection properly. After the response is received from the database, the application should be calling close in a finally block. If the application does not explicitly close the connection, that connection will stay open forever in WAS until that server is restarted. This is known as a connection leak. To resolve this type of issue, you will have to modify your application to close every connection
for further information: https://www.ibm.com/support/pages/common-reasons-why-connections-stay-open-long-period-time-websphere-application-server

Static "Connection" in JDBC: Can connections get mixed up? [duplicate]

I'm running a web application on Tomcat. I have a class that handles all DB queries.
This class contains the Connection object and methods that returns query results.
This is the connection object:
private static Connection conn = null;
It has only one instance (singleton).
In addition, I have methods that execute queries, such as search for a user in the db:
public static ResultSet searchUser(String user, String pass) throws SQLException
This method uses the static Connection object. My question is, is my use in static Connection object thread safe? Or can it cause problems when a lot of users will call the searchUser method?

is my use in static Connection object thread safe?
Absolutely not!
This way the connection going to be shared among all requests sent by all users and thus all queries will interfere with each other. But threadsafety is not your only problem, resource leaking is also your other problem. You're keeping a single connection open during the entire application's lifetime. The average database will reclaim the connection whenever it's been open for too long which is usually between 30 minutes and 8 hours, depending on DB's configuration. So if your web application runs longer than that, the connection is lost and you won't be able to execute queries anymore.
This problem also applies when those resources are held as a non-static instance variable of a class instance which is reused multiple times.
You should always acquire and close the connection, statement and resultset in the shortest possible scope, preferably inside the very same try-with-resources block as where you're executing the query according the following JDBC idiom:
public User find(String username, String password) throws SQLException {
User user = null;
try (
Connection connection = dataSource.getConnection();
PreparedStatement statement = connection.prepareStatement("SELECT id, username, email FROM user WHERE username=? AND password=md5(?)");
) {
statement.setString(1, username);
statement.setString(2, password);
try (ResultSet resultSet = statement.executeQuery()) {
if (resultSet.next()) {
user = new User();
user.setId(resultSet.getLong("id"));
user.setUsername(resultSet.getString("username"));
user.setEmail(resultSet.getString("email"));
}
}
}
return user;
}
Note that you should not return a ResultSet here. You should immediately read it and map it to a non-JDBC class and then return it, so that the ResultSet can safely be closed.
If you're not on Java 7 yet, then use a try-finally block wherein you manually close the closeable resources in the reverse order as you've acquired them. You can find an example here: How often should Connection, Statement and ResultSet be closed in JDBC?
If you worry about connecting performance, then you should be using connection pooling instead. This is built-in into many Java EE application servers and even barebones servletcontainers like Tomcat supports it. Just create a JNDI datasource in the server itself and let your webapp grab it as DataSource. It's transparently already a connection pool. You can find an example in the first link of the list below.
See also:
How should I connect to JDBC database / datasource in a servlet based application?
When my app loses connection, how should I recover it?
Am I Using JDBC Connection Pooling?
Show JDBC ResultSet in HTML in JSP page using MVC and DAO pattern
DAO tutorial with JDBC

If you are only running Select queries (searchUser sounds like only selecting data) there will be no issues, apart from thread contention.
As far as I know, a Connection can only handle one query at a time, so by using a single instance you will essentially serialize database access. But this does not necessarily mean, it is always safe to access a database like this in a multi threaded environment. There might still be issues if concurrent accesses are interleaving.

How do I safely use OracleDriver.defaultConnection() in a multithreaded Java Application?

I have a multithreaded application which uses OracleDriver.defaultConnection() to connect to the database.
This line of code is used in multiple code blocks which are accessed by different threads.
I am getting the error : Java Thread Deadlock detected
This is a sample piece of code :
private void function1() {
//something here
Connection conn = OracleDriver.defaultConnection();
// something here
conn.execute();
conn.close();
}
Second piece of code which is in a separate class :
private void function2() {
//something here
Connection conn = OracleDriver.defaultConnection();
// something here
conn.execute();
conn.close();
}
How do I make this connection "thread-safe"?
Also, I read that defaultConnection() returns a static object. So can this be caused due to the connection being closed by one thread while the other thread is using it?
I cannot use any connection method which requires me to input the database username and password.
Thanks in advance!

Java provides many ways for dealing with concurrency(synchronized, ReentrantLock, ReadWriteLock)
Considering the documentation which says:
The oracle.jdbc.OracleDriver class defaultConnection() method is an Oracle extension and always returns the same connection object.
Since it always returns the same connection object you can synchronize on it:
Connection conn = OracleDriver.defaultConnection();
synchronized (conn) {
// something here
conn.execute();
}
However you would need to remove all conn.close() calls due to:
If you do call the close() method, be aware of the following: All connection instances obtained through the defaultConnection() method, which actually all reference the same connection object, will be closed and unavailable for further use, with state and resource cleanup as appropriate. Executing defaultConnection() afterward would result in a new connection object.
Even though the connection object is closed, the implicit connection to the database will not be closed.

Should a database connection stay open all the time or only be opened when needed?

I have a bukkit plugin (minecraft) that requires a connection to the database.
Should a database connection stay open all the time, or be opened and closed when needed?

The database connection must be opened only when its needed and closed after doing all the necessary job with it. Code sample:
Prior to Java 7:
Connection con = null;
try {
con = ... //retrieve the database connection
//do your work...
} catch (SQLException e) {
//handle the exception
} finally {
try {
if (con != null) {
con.close();
}
} catch (SQLException shouldNotHandleMe) {
//...
}
}
Java 7:
try (Connection con = ...) {
} catch (SQLException e) {
}
//no need to call Connection#close since now Connection interface extends Autocloseable
But since manually opening a database connection is too expensive, it is highly recommended to use a database connection pool, represented in Java with DataSource interface. This will handle the physical database connections for you and when you close it (i.e. calling Connection#close), the physical database connection will just be in SLEEP mode and still be open.
Related Q/A:
Java Connection Pooling
Some tools to handle database connection pooling:
BoneCP
c3po
Apache Commons DBCP
HikariCP

Depends on what are your needs.
Creating a connection takes some time, so if you need to access database frequently it's better to keep the connection open. Also it's better to create a pool, so that many users can access database simultaneously(if it's needed).
If you need to use this connection only few times you may not keep it open, but you will have delay when you would like to access database. So i suggest you to make a timer that will keep connection open for some time(connection timeout).

You need to close your connections after each query executions.Sometimes you need to execute multiple queries at the same time because the queries are hanging from each other.Such as "first insert task then assign it to the employees".At this time execute your queries on the same transaction and commit it, if some errors occur then rollback.By default autocommit is disabled in JDBC. Example
Use connection pooling.If you are developing a webapplication then use App Server connection pooling.App server will use the same pooling for each of your applications so you can control the connection count from the one point.Highly recommend the Apache Tomcat Connection pooling.Example
As an additional info:
Connection, Statement and ResultSet.
1.If you close connection you don't need close statement or resultset.Both of them will be closed automatically
2.If you close Statement it will close ResultSet also
3.if you use try-with-resources like this:
try (Connection con = ...) {
} catch (SQLException e) {
}
it will close the connection automatically.Because try-with-resources require autoclosable objects and Connection is autocloseable.You can see the details about try-with-resources here

Actually, it's all matter on how you write your application! It's an art, but sadly everyone takes a tutorial for a good practice like Microsoft's tutorials.
If you know what you are coding, then you keep your connection open for the lifetime of the application. It's simple, not because you have to go at work in the morning that everyday we have to build a special route just for you! You take that single route or 2 or 4 like everyone does! You judge for the traffics and you build 2, 4 or 6 routes as needed. If there is traffic with these 4 or 6 routes, you wait!
Happy coding.

The Connection should be opened only when required. If it is open before the actual need, it reduces one active connection from the connection pool..so it ultimately effects the users of the application.
So,it is always a better practice to open connection only when required and closing it after completion of process.
Always try puttting you connection close logic inside the finally block that will ensure that your connection will be closed,even if any exception occurs in the application
finally
{
connection.close()
}

Closing JDBC Connections in Pool

Our standard code section for using JDBC is...
Connection conn = getConnection(...);
Statement stmt = conn.conn.createStatement (ResultSet.TYPE_SCROLL_INSENSITIVE,
ResultSet.CONCUR_READ_ONLY);
ResultSet rset = stmt.executeQuery (sqlQuery);
// do stuff with rset
rset.close(); stmt.close(); conn.close();
Question 1: When using Connection Pool, should one close the Connection at the end? If so, isn't the purpose of pooling lost? And if not, how does the DataSource know when a particular instance of Connection is freed up and can be reused? I am a little confused on this one, any pointers appreciated.
Question 2: Is the following method anything close to standard? Looks like an attempt to get a connection from the pool, and if DataSource cannot be established, use the old fashioned DriverManager. We are not even sure which part is getting executed at runtime.
Repeating the question above, should one close the Connection coming out of such a method?
synchronized public Connection getConnection (boolean pooledConnection)
throws SQLException {
if (pooledConnection) {
if (ds == null) {
try {
Context envCtx = (Context)
new InitialContext().lookup("java:comp/env");
ds = (DataSource) envCtx.lookup("jdbc/NamedInTomcat");
return ds.getConnection();
} catch (NamingException e) {
e.printStackTrace();
}}
return (ds == null) ? getConnection (false) : ds.getConnection();
}
return DriverManager.getConnection(
"jdbc:mysql://"+ipaddy+":"+dbPort +"/" + dbName, uName, pWord);
}
Edit: I think we are getting the pooled connection since we do not see a stack trace.

When using Connection Pool, should one close the Connection at the end? If so, isn't the purpose of pooling lost? And if not, how does the DataSource know when a particular instance of Connection is freed up and can be reused? I am a little confused on this one, any pointers appreciated.
Yes, certainly you need to close the pooled connection as well. It's actually a wrapper around the actual connection. It wil under the covers release the actual connection back to the pool. It's further up to the pool to decide whether the actual connection will actually be closed or be reused for a new getConnection() call. So, regardless of whether you're using a connection pool or not, you should always close all the JDBC resources in reversed order in the finally block of the try block where you've acquired them. In Java 7 this can be further simplified by using try-with-resources statement.
Is the following method anything close to standard? Looks like an attempt to get a connection from the pool, and if DataSource cannot be established, use the old fashioned DriverManager. We are not even sure which part is getting executed at runtime. Repeating the question above, should one close the Connection coming out of such a method?
The example is pretty scary. You just need to lookup/initialize the DataSource only once during application's startup in some constructor / initialization of an applicationwide DB config class. Then just call getConnection() on the one and same datasource throughout the rest of application's lifetime. No need for synchronization nor nullchecks.
See also:
Is it safe to use a static java.sql.Connection instance in a multithreaded system?
Am I Using JDBC Connection Pooling?

The pools typically return you a wrapped Connection object, where the close() method is overridden, typically returning the Connection to the pool. Calling close() is OK and probably still required.
A close() method would probably look like this:
public void close() throws SQLException {
pool.returnConnection(this);
}
For your second question, you could add a logger to show whether the bottom block ever runs. I would imagine though you'd only want one way or the other for the configuration of your database connections. We solely use a pool for our database accesses. Either way, closing the connection would be pretty important to prevent leaks.

Actually, the best approach to connection management is to not farm them out to any code anywhere.
Create a SQLExecutor class that is the one and only location which opens and closes connections.
The entire rest of the application then pumps statements into the executor rather than getting connections from the pool and managing (or mismanaging them) all over the place.
You can have as many instances of the executor as you want, but no one should be writing code that opens and closes connections on its own behalf.
Conveniently, this also lets you log all your SQL from a single set of code.