Develop Reference

Close context implicitly in Java EE

I'm trying to implement the following code in Java EE 8 (Java 11) to access a datasource in Wildfly 20. The objective is to close implicitly the JNDI context and the SQL connection:
try (InitialContext context = new InitialContext();
Connection conn = ((DataSource) context.lookup(pool)).getConnection()) {
// use the connection
}
catch (NamingException e) {
logger.error(e.getMessage());
}
catch (SQLException e) {
logger.error(e.getMessage());
}
Problem is that context is not closeable, as I get the compilation error:
The resource type InitialContext does not implement java.lang.AutoCloseable
I'm trying to avoid adding finally to close the context, is there a way to achieve this?

Try-with-resources works only with objects that are AutoCloseable. One way to do this without using finally is by wrapping the InitialContext in a wrapper that implements AutoCloseable that performs the necessary action in its close() method. Something like this:
public class AutoCloseableWrapper implements AutoCloseable {
private final InitialContext context;
public AutoCloseableWrapper(InitialContext context) {
this.context = context;
}
public InitialContext getContext() {
return this.context;
}
#Override
public void close() throws Exception {
this.context.close();
}
}
Use it like this:
try (AutoCloseableWrapper wrapper = new AutoCloseableWrapper(new InitialContext());
Connection conn = ((DataSource) wrapper.getContext().lookup(pool)).getConnection()) {
// ...
}

How to get same connection from a DataSource in multiple classes?

How to get same connection from a DataSource in multiple classes? Is it possible?
Suppose I have a DataSource which I pass to 2 classes. The DataSource is using a Connection Pooling.
Then I call a method in 1st Class which get a Connection from the DataSource, uses that Connection and then closes that Connection.
Then I call a method in 2nd Class which get a Connection from the DataSource, uses that Connection and then closes that Connection.
Is it possible to be sure that the method in 2nd Class will get the same Connection which was used by method in 1st Class?
This is the example code:
This is the 1st class whose method will be called by the unit of work:
public class Class1 {
private DataSource dataSource = null;
public Class1(DataSource dataSource) {
this.dataSource = dataSource;
}
public void class1Method1() throws Exception {
Connection conn = null;
try {
conn = dataSource.getConnection();
... // do your work
} finally {
try {
if (conn != null)
conn.close();
} catch (Exception e) {
} finally {
conn = null;
}
}
}
}
This is the 2nd class whose method will be called by the unit of work:
public class Class2 {
private DataSource dataSource = null;
public Class2(DataSource dataSource) {
this.dataSource = dataSource;
}
public void class2Method1() throws Exception {
Connection conn = null;
try {
conn = dataSource.getConnection();
... // do your work
} finally {
try {
if (conn != null)
conn.close();
} catch (Exception e) {
} finally {
conn = null;
}
}
}
}
And this is my unit of work:
InitialContext initialContext = null;
DataSource dataSource = null;
Class1 class1 = null;
Class2 class2 = null;
initialContext = new InitialContext();
dataSource = (DataSource) initialContext.lookup("java:comp/env/jdbc/MyDB");
class1 = new Class1(dataSource);
class2 = new Class2(dataSource);
class1.class1Method1();
class2.class2Method1();

Closing a connection releases the connection resources, see here and connections should always be closed. Depending on the DataSource implementation (e.g. connection pool) it could then for instance give that same connection back on the next getConnection call. But it does not have to do so, it can also terminate the first connection and return a new one each time.
For more info, see here and here.

As #EJP said in a comment, You would need to pass the Connection around into the other class before you close it. You'd also need to make sure the second class doesn't close it if you plan to use it later in the outer class. This is a low level solution and quite a headache.
You can use Spring or Java EE to solve this problem at a higher level. It lets you mark all your relevant methods to say, "these should be transactional". If you call one for the first time, a new transaction is started. When you leave that outer method, the transaction automatically ends. But, if that method calls another transactional method, it knows the transaction is still open so it reuses it. It removes a lot of headache.
I believe these two technologies use Aspect Oriented Programming under the hood.
Read more for JEE: http://docs.oracle.com/javaee/6/tutorial/doc/bncih.html
Read more for Spring: http://docs.spring.io/spring/docs/current/spring-framework-reference/html/transaction.html

still having You can't operate on a closed ResultSet error after using connection pool

I have many threads accessing MYSQL database, at first I didn't use connection pool so I had this error "You can't operate on a closed ResultSet"
I searched on Google and found out that I should used connection pool so I tried c3p0 API for implementation, but I still have the same problem and nothing changed.
so should I Synchronize getAllcountries method or there's another better solution.
public class DataSource {
private static DataSource datasource;
private ComboPooledDataSource cpds ;
private DataSource() throws IOException, SQLException, PropertyVetoException {
cpds = new ComboPooledDataSource();
cpds.setDriverClass("com.mysql.jdbc.Driver"); //loads the jdbc driver
cpds.setJdbcUrl("jdbc:mysql://localhost/question_game");
cpds.setUser("root");
cpds.setPassword("");
cpds.setMaxPoolSize(500);
}
public static DataSource getInstance() throws IOException, SQLException, PropertyVetoException {
if (datasource == null) {
datasource = new DataSource();
return datasource;
} else {
return datasource;
}
}
public Connection getConnection() throws SQLException {
return this.cpds.getConnection();
}
public List<Country> getAllCountries() {
String query = "SELECT * FROM country order by name ";
List<Country> list = new ArrayList<Country>();
Country country = null;
ResultSet rs = null;
try {
try {
connection = DataSource.getInstance().getConnection();
} catch (IOException e) {
e.printStackTrace();
} catch (PropertyVetoException e) {
e.printStackTrace();
}
statement = connection.createStatement();
rs = statement.executeQuery(query);
while (rs.next()) {
//getting countries
}
} catch (SQLException ex) {
ex.printStackTrace();
} finally {
//DbUtil used to check if null
DbUtil.close(rs);
DbUtil.close(statement);
DbUtil.close(connection);
}
return list;
}

In addition to #stephen-c 's observation,
you basically have two options: either synchronize getAllCountries method, or make the database connection local to that method instead of having it as a class member.
As you have it now, 'connection' is a class member (available to all invocations of getAllCountries(), on all threads), so the connection is probably being overwritten by a second thread. Move it to a variable in the method, and then each invocation of the method will have its own connection.

I see a method called getConnection, I don't see where you are calling it. Instead, I see your getAllCountries method using a static DataSource object that is created like this:
new DataSource();
I searched on Google and found out that I should used connection pool so I tried c3p0 API for implementation, but I still have the same problem and nothing change
Yea ...
It looks like you just cut-and-pasted the code you found into your application without really thinking about it. Obviously ... you need to understand what the code is doing AND figure out how to use it in your application.

JDBC Connection pooling issue - Deadlock while getting connection

I am using JDBC connection pooling in Tomcat. To retrieve connections I have defined a connection factory as below:
public class ConnectionManager {
// reference to the ConnectionManager
private static ConnectionManager instance = null;
// Connection to MySQL database
private Connection connect = null;
private static DataSource ds = null;
// Logger
public static final Logger logger = Logger
.getLogger(ConnectionManager.class);
static {
try {
Context initCtx = new InitialContext();
Context envCtx = (Context) initCtx.lookup("java:comp/env");
ds = (DataSource) envCtx.lookup("jdbc/ConnectionManager");
} catch (NamingException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
/**
* Private Constructor .. since its a singleton
*
*/
private ConnectionManager() {
}
public static ConnectionManager getInstance() {
if (instance == null) {
instance = new ConnectionManager();
}
return instance;
}
public Connection getDbConnection() {
Connection conn = null;
try {
synchronized (DataSource.class) {
conn = ds.getConnection();
}
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return conn;
}
public void closeDbConnection() throws SQLException {
conn.close();
}
}
Now I see that my code always gets stuck at conn = ds.getConnection(); line. Please let me know what I am doing wrong. From my DAO methods I am using the following to get connection: conn = ds.getConnection();
Clearly its a multi-threading issue. What should I do?

Most of your class seems geared around retrieving the JNDI datasource and using it to create connections. Not necessarily a bad idea but in this case you have introduced some bugs into your program with the additional complexity.
First off, your singleton is not a singleton. Your are not synchronizing the getInstance method so its possible to multiple threads to invoke this method at the same time. The best method in Java (unfortunately) for implementing singletons is via an enum:
public enum ConnectionManager {
INSTANCE;
}
Your second significant issue is that you are synchronizing on a class that you don't explicitly control. There is nothing preventing third party JARs or even other classes in your own application from synchronizing on the DataSource class, making it a rife target for deadlocking issues. I would take out all the superfluous methods from the class and remove the synchronize block:
public enum ConnectionManager {
INSTANCE;
private DataSource ds = null;
ConnectionManager() {
try {
final Context initCtx = new InitialContext();
final Context envCtx = (Context) initCtx.lookup("java:comp/env");
ds = (DataSource) envCtx.lookup("jdbc/ConnectionManager");
} catch (NamingException e) {
e.printStackTrace();
}
}
public Connection getConnection() throws SQLException {
if(ds == null) return null;
return ds.getConnection();
}
}
Now, most datasource implementations are thread safe in my experience, so the above code should work most of the time. But, we shouldn't rely on implementations we cannot control, so lets add a safe synchronization to the code, like so:
public enum ConnectionManager {
INSTANCE;
private DataSource ds = null;
private Lock connectionLock = new ReentrantLock();
ConnectionManager() {
try {
final Context initCtx = new InitialContext();
final Context envCtx = (Context) initCtx.lookup("java:comp/env");
ds = (DataSource) envCtx.lookup("jdbc/ConnectionManager");
} catch (NamingException e) {
e.printStackTrace();
}
}
public Connection getConnection() throws SQLException {
if(ds == null) return null;
Connection conn = null;
connectionLock.lock();
try {
conn = ds.getConnection();
} finally {
connectionLock.unlock();
}
return conn;
}
}
You don't have to add wrapper methods to close the connection, that is the responsibility of the calling code. Good luck.

#arya, seems like you are having the problem of connection leak, and because of that the pool is getting exhausted and the code just waits till it gets a new connection, To analyze the problem , use any of the database monitoring tools, or manually try to trace the leak (The point in code where you have consumed a connnection but forgot to return it to the pool after use).

Well I would say first try out your dataSource is working or not with a test source.
I suggest look at Apache Tomcat JNDI Data Resource How To, for Apache Tomcat 6.0 and for Apache Tomcat 7.0.
Look at the instructions carefully and analyse what's going wrong in your code, then update your question with specific problem.

That code is virtually guaranteed to cause connection leaks in a multithreaded system. closeDbConnection() closes only the last connection borrowed from pool - so if 10 threads have called getDbConnection(), and after that closeDbConnection(), only 1 connection is closed and 9 still alive. Repeat that several times and pool is exhausted (unless connection is cleaned up in finalize(), but that's probably not the case). I would get rid of the whole class, or reworked it to act only as a datasource locator.

Java connecting to multiple databases

I am creating a java application that connects to multiple databases. A user will be able to select the database they want to connect to from a drop down box.
The program then connects to the database by passing the name to a method that creates an initial context so it can talk with an oracle web logic data source.
public class dbMainConnection {
private static dbMainConnection conn = null;
private static java.sql.Connection dbConn = null;
private static javax.sql.DataSource ds = null;
private static Logger log = LoggerUtil.getLogger();
private dbMainConnection(String database) {
try {
Context ctx = new InitialContext();
if (ctx == null) {
log.info("JDNI Problem, cannot get InitialContext");
}
database = "jdbc/" + database;
log.info("This is the database string in DBMainConnection" + database);
ds = (javax.sql.DataSource) ctx.lookup (database);
} catch (Exception ex) {
log.error("eMTSLogin: Error in dbMainConnection while connecting to the database : " + database, ex);
}
}
public Connection getConnection() {
try {
return ds.getConnection();
} catch (Exception ex) {
log.error("Error in main getConnection while connecting to the database : ", ex);
return null;
}
}
public static dbMainConnection getInstance(String database) {
if (dbConn == null) {
conn = new dbMainConnection(database);
}
return conn;
}
public void freeConnection(Connection c) {
try {
c.close();
log.info(c + " is now closed");
} catch (SQLException sqle) {
log.error("Error in main freeConnection : ", sqle);
}
}
}
My problem is what happens if say someone forgets to create the data source for the database but they still add it to the drop down box? Right now what happens is if I try and connect to a database that doesn't have a data source it errors saying it cannot get a connection. Which is what I want but if I connect to a database that does have a data source first, which works, then try and connect to the database that doesn't have a data source, again it errors with
javax.naming.NameNotFoundException: Unable to resolve 'jdbc.peterson'. Resolved 'jdbc'; remaining name 'peterson'.
Which again I would expect but what is confusing me is it then grabs the last good connection which is for a different database and process everything as if nothing happened.
Anyone know why that is? Is weblogic caching the connection or something as a fail safe? Is it a bad idea to create connections this way?

You're storing a unique datasource (and connection, and dbMainConnection) in a static variable of your class. Each time someone asks for a datasource, you replace the previous one by the new one. If an exception occurs while getting a datasource from JNDI, the static datasource stays as it is. You should not store anything in a static variable. Since your dbMainConnection class is constructed with the name of a database, and there are several database names, it makes no sense to make it a singleton.
Just use the following code to access the datasource:
public final class DataSourceUtil {
/**
* Private constructor to prevent unnecessary instantiations
*/
private DataSourceUtil() {
}
public static DataSource getDataSource(String name) {
try {
Context ctx = new InitialContext();
String database = "jdbc/" + name;
return (javax.sql.DataSource) ctx.lookup (database);
}
catch (NamingException e) {
throw new IllegalStateException("Error accessing JNDI and getting the database named " + name);
}
}
}
And let the callers get a connection from the datasource and close it when they have finished using it.

You're catching JNDI exception upon lookup of the nonexistent datasource but your singleton still keeps the reference to previously looked up datasource. As A.B. Cade says, null reference to ds upon exception, or even before that.
On a more general note, perhaps using Singleton is not the best idea.