Develop Reference

Proper way of obtaining the DBConnection for a Web Application

We have a Web Application which will serve more than 1000 concurrent users
Currently , the Utility class for obtaining the DB Connnection is
public static Connection getDBConnection()
{
Connection conn = null;
try
{
InitialContext ctx = new InitialContext();
DataSource ds = (DataSource) ctx.lookup("java:/comp/env/jdbc/MyLocalDB");
try
{
conn = ds.getConnection();
}
catch ( SQLException sqlEx )
{
System.out.println( "cannot get JDBC connection: " + sqlEx );
}
}
catch ( NamingException nEx )
{
nEx.printStackTrace();
}
return conn;
}
Option 2 :
public class DBConnection2 {
private static DataSource dataSource;
static {
try {
dataSource = (DataSource) new InitialContext().lookup("java:/comp/env/jdbc/MyLocalDB");
} catch (NamingException e) {
try {
throw new Exception("'jndifordbconc' not found in JNDI",e);
} catch (Exception e1) {
e1.printStackTrace();
}
}
}
public static Connection getConnection() {
try {
return dataSource.getConnection();
} catch (SQLException e) {
e.printStackTrace();
return null;
}
}
}
Please let me know whats the better option (I guess its second operation as lookup is a costly operation and i am doing it only for once in a application .)
please share your views .

You should adopt the second approach. As you say you don't need to keep looking up the datasource in JNDI.
I am assuming that you are using a pooled data source as provided by the tomcat-jdbc project, otherwise performance will be terrible.

On the second approach you should be sure to use connection pooling. Otherwise users will have to wait for others.
For example To Tomcat brings it own connection pool.
Or if you don't use Tomcat maybe you could have a look at C3p0: JDBC DataSources/Resource Pools.
see: Instantiating and Configuring a ComboPooledDataSource

The second option initialize dataSource only once when the class is loaded, then it is shared around all DBConnection2 instances. The first option initialize a new DataSource instance every time you call getDBConnection(). Thus the second option has better performance. But please be pay attention to connection release issue. Make sure close the connection if you choose the first option. I prefer use a framework to handle DB connection, like Spring JDBC: http://docs.spring.io/spring/docs/current/spring-framework-reference/html/jdbc.html

Singleton in JSP, how to properly tidy up on close?

I am just getting started with jsp and my question is this - when I have a singleton class, how do I tidy up after it?
In particular:
public class DBConnection {
private static Connection connection = null;
private static Statement statement = null;
public static ResultSet executeQuery(String query){
if (connection == null) { /*initConnection*/ }
if (statement == null) { /*initStatement*/ }
// do some stuff
}
}
Now, I use this class in several pages to get results from jdbc. However, I need to eventually call statement.close(); and connection.close(); - when should I call those?
I am using singleton, because it felt wrong to call for connection to a database over and over whenever I needed to make a query.

The Connection must be closed always, and after you have executed all your database statements for the desired operations. Two examples:
Case 1: You must show a list of products to user filtered by criteria from database. Solution: get a connection, retrieve a list of products using the filter criteria, close the connection.
Case 2: The client selects some of these products and updates the minimum stock to get an alert and restock them. Solution: get a connection, update all the products, close the connection.
Based on these cases, we can learn lot of things:
You can execute more than a single statement while having/maintaining a single connection open.
The connection should live only in the block where it is used. It should not live before or after that.
Both cases can happen at the same time since they are in a multi threaded environment. So, a single database connection must not be available to be used by two threads at the same time, in order to avoid result problems. For example, user A searches the products that are in category Foo and user B searches the products that are in category Bar, you don't want to show the products in category Bar to user A.
From last sentence, each database operation ((or group of similar operations like Case 2) should be handled in an atomic operation. To assure this, the connection must not be stored in a singleton object, instead it must be live only in the method being used.
In consequence:
Do not declare the Connection nor the Statement nor the ResultSet nor other JDBC resource as static. It will simply fail. Instead, declare only the Connection as field of your DBConnection class. Let each method decide to handle each Statement (or PreparedStatement) and ResultSet and specific JDBC resources.
Since you must close the connection after its usage, then add two more methods: void open() and void close(). These methods will handle the database connection retrieval and closing that connection.
Additional, since the DBConnection looks like a wrapper class for Connection class and database connection operations, I would recommend to have at least three more methods: void setAutoCommit(boolean autoCommit), void commit() and void rollback(). These methods will be plain wrappers for Connection#setAutoCommit Connection#close and Connection#rollback respectively.
Then you can use the class in this way:
public List<Product> getProducts(String categoryName) {
String sql = "SELECT id, name FROM Product WHERE categoryName = ?";
List<Product> productList = new ArrayList<Product>();
DBConnection dbConnection = new DBConnection();
try {
dbConnection.open();
ResultSet resultSet = dbConnection.executeSelect(sql, categoryName); //execute select and apply parameters
//fill productList...
} catch (Exception e) {
//always handle your exceptions
...
} finally {
//don't forget to also close other resources here like ResultSet...
//always close the connection
dbConnection.close();
}
}
Note that in this example the PreparedStatement is not in the getProducts method, it will be a local variable of the executeSelect method.
Additional notes:
When working in an application server, you should not open connections naively e.g. using Class.forName("..."), instead use a database connection pool. You can roll on some database connection pooling libraries like C3P0 as explained here: How to establish a connection pool in JDBC?. Or configure one in your application server, as I explain here: Is it a good idea to put jdbc connection code in servlet class?
If this is for learning purposes, then roll on your own classes to handle the communication with your database. In real world applications, this is not recommended (doesn't mean you should not do it). Instead, use a database connectivity framework like ORMs e.g. JPA (Java official ORM framework) or Hibernate; there are no ORM frameworks that handles database communication like Spring JDBC and MyBatis. The choice is yours.
More info:
Should a database connection stay open all the time or only be opened when needed?
How do servlets work? Instantiation, sessions, shared variables and multithreading. Not directly related to your question, but it will help you understand why to not maintain state in resources that are used in multithreaded environments.

Define connection resource in mywebapp/META-INF/context.xml file
<Resource name="jdbc/mydb" auth="Container" type="javax.sql.DataSource"
maxActive="10" maxIdle="2" maxWait="20000"
driverClassName="com.mysql.jdbc.Driver"
username="myuser" password="mypwd"
url="jdbc:mysql://localhost:3306/mydb?useUnicode=true&characterEncoding=utf8"
validationQuery="SELECT 1" />
Create DB.java helper class to minimize code in other parts of app
import java.sql.*;
import javax.sql.DataSource;
import javax.naming.Context;
import javax.naming.InitialContext;
public class DB {
public static Connection createConnection() throws SQLException {
try {
Context ctx = new InitialContext();
DataSource ds = (DataSource)ctx.lookup("java:comp/env/jdbc/mydb");
return ds.getConnection();
} catch (SQLException ex) {
throw ex;
} catch (Exception ex) {
SQLException sqex = new SQLException(ex.getMessage());
sqex.initCause(ex);
throw sqex;
}
}
public static void close(ResultSet rs, Statement stmt, Connection conn) {
if (rs != null) try { rs.close(); } catch (Exception e) { }
if (stmt != null) try { stmt.close(); } catch (Exception e) { }
if (conn != null) try { conn.close(); } catch (Exception e) { }
}
public static void close(ResultSet rs, boolean closeStmtAndConn) {
if (rs==null) return;
try {
Statement stmt = rs.getStatement();
close(rs, stmt, stmt!=null ? stmt.getConnection() : null);
} catch (Exception ex) { }
}
}
And somewhere in your app DAO code use DB helper.
public List<MyBean> getBeans() throws SQLException {
List<MyBean> list = new ArrayList<MyBean>();
ResultSet rs=null;
try {
Connection con = DB.createConnection();
String sql = "Select * from beantable where typeid=?";
PreparedStatement stmt = con.prepareStatement(sql, Statement.NO_GENERATED_KEYS);
stmt.setInt(1, 101);
rs = stmt.executeQuery();
while(rs.next()
list.add( createBean(rs) );
} finally {
DB.close(rs, true); // or DB.close(rs, stmt, conn);
}
return list;
}
private MyBean createBean(ResultSet rs) throws SQLException {
MyBean bean = new MyBean();
bean.setId( rs.getLong("id") );
bean.setName( rs.getString("name" );
bean.setTypeId( rs.getInt("typeid") );
return bean;
}

I would add two methods to the class:
public static void open() throws SomeException;
public static void close() throws SomeException;
then your calling code looks something like this{
try {
DBConnection.open();
... code to use the connection one or more times ...
} finally {
DBConnection.close();
}
Wrap all your database calls inside that and it will take care of closing whether there is an exception thrown or not.
Of course, this isn't much different than having a regular class, which I might recommend:
try {
DBConnection conn = new DBConnection();
conn.open();
... all the code to use the database (but you pass 'conn' around) ...
} finally {
conn.close();
}
And you might want to look at the java.lang.AutoCloseable and java.io.Closeable to see if that helps you.
2
If you are keeping it open across page loads, there isn't any place to put the try ... finally stuff so you can open it and close it when the servlet closes or the server closes or something like that.
If you are going to leave it open, you need to make sure and add code to verify it doesn't close when you aren't looking. A short network glitch, for example, could close it down. In that case, you need to reopen it when it gets closed. Otherwise, all database access from that point will fail.
You might want to look into the concept of a DataBase Pool. Apache has one -- DBCP. Tomcat has its own that's quite good. Other containers, like JBOSS, WebSphere, WebLogic all have them. There's a couple that can be used with the Spring Framework. What it does is manage one or more database connections. Your code asks it for one and it returns an open one, unless none is available and then it opens one and returns it. You call close when your code gets through with it but it doesn't really close the connection, it just returns it to the pool.
You can usually configure the pool to check for shut down connections and reopen if needed.

Separate database connection method for jsp and servlets

I was trying to write a separate database connection method in .java file which can be called upon by any servlet or jsp file needing the database connection. My code is
import java.sql.*;
import java.lang.*;
public class ConnectionClass {
private String username="root";
private String password="passwd";
/* Adjust the above two as per the username
* password combination of your MySql databse */
public Connection connect()
{
try
{
Class.forName("com.mysql.jdbc.Driver");
String url="jdbc:mysql://localhost/schooldatabase";
Connection con = DriverManager.getConnection(url,username,password);
return con;
}
catch(Exception e)
{
response.sendRedirect("studentserr.html");
out.println(e);
}
}
}
Now, the problem is that i'll be returning a Connection type so that all the servlets (which require database connection) may use it to execute various statements. However, in my code what should i return in the catch block ( which means that the connection to the database could not be established) ? Also, in case of a connection failure, I'm redirecting the user to the following page:
"studentserr.html"
This works fine if i use it in a servlet but not in .java class. What should i do for this ??

You should only be catching exceptions at exactly that moment where you can sensibly deal with them. You can't sensibly deal with them in the getConnection() method, so you should instead throw it so that the caller itself needs to deal with it.
Displaying an error page in case of a specific exception is however the responsibility of the servlet container itself. You normally configure the error page in web.xml as follows:
<error-page>
<exception-type>java.sql.SQLException</exception-type>
<location>/WEB-INF/errorpages/database.jsp</location>
</error-page>
You only need to change your code accordingly that you never catch the exception, or at least rethrow as ServletException where necessary.
Here's a minor rewrite:
public class Database {
private String url = "jdbc:mysql://localhost/schooldatabase";
private String username = "root";
private String password = "passwd";
static {
try {
Class.forName("com.mysql.jdbc.Driver"); // You don't need to load it on every single opened connection.
} catch (ClassNotFoundException) {
throw new ExceptionInInitializerError("MySQL JDBC driver missing in classpath", e);
}
}
public static Connection getConnection() throws SQLException {
return DriverManager.getConnection(url, username, password);
}
}
And here's how you should use it in your DAO classes:
public List<Student> list() throws SQLException {
List<Student> students = new ArrayList<Student>();
Connection connection = null;
// ...
try {
connection = Database.getConnection();
// ...
} finally { // Note: no catch block!
// ...
if (connection != null) try { connection.close(); } catch (SQLException ignore) {}
}
return students;
}
And here is how you should use the DAO class in your servlet's doGet() or doPost().
try {
List<Student> students = studentDAO.list();
request.setAttribute("students", students);
request.getRequestDispatcher("/WEB-INF/students.jsp").forward(request, response);
} catch (SQLException e) {
throw new ServletException(e);
}
It has to be rethrown as ServletException simply because you can't add SQLException to the throws clause of any HttpServlet method. The servletcontainer will unwrap the SQLException while locating the error page.

Return a null value in case an exception is thrown. In case of getting a null value from your method, handle the exception and don't perform any database operation.
Also, remember to separate the database logic (connection, statement execution) from the business logic and the presentation. In your actual method, this code
response.sendRedirect("studentserr.html");
should never be in the database logic, because is presentation logic.
More info:
Java 101: Layered Architecture
How to avoid Java Code in JSP-Files?

Try moving class "ConnectionClass" into some package. and then call using this package in your java class where you require this (seems to be classpath issue in java file) or in jsp or servlet page.
Example
You need this in Demo.java as
pkg1.ConnectionClass obj = new pkg1.ConnectionClass ();
It is recommended to have database connection class as singleton. So that through out the application only a single instance of connection will be created and shared.

Custom java.sql.Driver Implementation Connection Handling

Currently, I load the below custom driver (TestDriver.java), get a connection, create a Statement, execute a query, gets the results and close the connection. I open and close a connection for each query. Is this common practice or is there an standard way to share the open connections?
public static void main(String[] args) {
Class.forName("com.sql.TestDriver");
java.sql.Connection conn = DriverManager.getConnection("jdbc:test://8888/connectme", props);
Statement stmt = conn.createStatement;
ResultSet rs = stmt.executeQuery("select * from table");
//loop through rs and pull out needed data
conn.close();
}
public class TestDriver implements java.sql.Driver{
private final TestSchema schema;
private Properties props = null;
static {
try {
DriverManager.registerDriver(new TestDriver());
} catch (SQLException e) {
e.printStackTrace();
}
protected TestDriver() throws SQLException {
schema = TestSchemaFactory.getInstance().getDbSchemaFromFile(SCHEMA_FILE);
//loads in and parses a file containing tables, columns used for business logic
}
public Connection connect(String url, Properties info)
throws SQLException {
TestSqlConnection conn=null;
//connect logic here
return conn; //will return an instance of TestSqlConnection
}
#Override
public boolean jdbcCompliant() {
return false;
}
}

Yes, it's more common to use a database connection pool. This will allow connections to be reused without the overhead or closing/re-opening. Here's a link to DBCP which is one implementation of a database connection pool: http://commons.apache.org/dbcp/

Ideally you should write a separate factory class (can be static)
say ConnectionFactory which returns a connection object.
Also I see that you are not using try/catch/finally block while creating
connection.I strongly suggest to close the connection in finally
clause otherwise you program may suffer from connection leak if any
exception is raised and causes abrupt behavior.
Ideally you should close the connection after your operation is complete in finally
clause.In web based application if you are using connections pool
then closing connection will return the connection back to pool and
will be available for use.

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.