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Tomcat doesn't sync with the mysql database

I'm currently working on a college project, and I'm creating a very simple e-commerce style website.
I'm using JDBC driver manager and connection pool for the connection to the db, while using Tomcat 9.0 as the container.
The problem is: when I modify some product through the website (let's say the amount available for example), the website doesn't always reflect the changes, while I can always see the data correctly in MySql Workbench.
It actually works one time out of two on the same query:
I run the query for the first time after the changes -> it shows the old value
I run the query for the second time after the changes -> it shows the new value
I run the query for the third time after the changes -> it shows the old value
And so on.
I've already tried to set caching off (from the query, using the SQL_NO_CACHE), but it didn't seem to solve the problem, I've tried to use Datasource instead, but it causes other problems that most likely I won't have the time to solve.
This is the connection pool file, which I think might be problem, I'm not that sure tho:
public class DriverManagerConnectionPool {
private static List<Connection> freeDbConnections;
static {
freeDbConnections = new LinkedList<Connection>();
try {
Class.forName("com.mysql.cj.jdbc.Driver");
} catch (ClassNotFoundException e) {
System.out.println("DB driver not found:"+ e.getMessage());
}
}
private static synchronized Connection createDBConnection() throws SQLException {
Connection newConnection = null;
String ip = "localhost";
String port = "3306";
String db = "storage";
String username = "root";
String password = "1234";
newConnection = DriverManager.getConnection("jdbc:mysql://"+ ip+":"+ port+"/"+db+"?useUnicode=true&useJDBCCompliantTimezoneShift=true&useLegacyDatetimeCode=false&serverTimezone=UTC", username, password);
newConnection.setAutoCommit(false);
return newConnection;
}
public static synchronized Connection getConnection() throws SQLException {
Connection connection;
if (!freeDbConnections.isEmpty()) {
connection = (Connection) freeDbConnections.get(0);
freeDbConnections.remove(0);
try {
if (connection.isClosed())
connection = getConnection();
} catch (SQLException e) {
connection.close();
connection = getConnection();
}
} else {
connection = createDBConnection();
}
return connection;
}
public static synchronized void releaseConnection(Connection connection) throws SQLException {
if(connection != null) freeDbConnections.add(connection);
}
}
I really hope you can help me, I haven't found any solution online!

I guess it is because of auto-commit is disabled. Please try using #Transactional or set auto-commit to true. You can also try to use db.commit after each statement.

As per your connection pool implementation, all connection in your pool seems to be auto committed false.
Please check you have properly committed the connection after executing the query or not.
So it might be the case that, when executing the query after changes with same connection it reflects those changes, done earlier and on other connections, old values are might get returned.

How to tell when there is a connection database issue?

I need to make a special treatment when a connection problem to the database is occurring like database server down and not an sql problem.
In the source code we can get various exceptions but which ones are belonging to the connection ones ?
We would like if this kind of problem occurs to make less logs.
EDITED
I have many methods that perform connection to the database but all get the session from the same method (initSession):
Here an example:
private Session initSession(HibernateUtil hibernateUtil) {
Session oSession = null;
try {
oSession = hibernateUtil.getSession();
} catch (Exception e) {
log.error("unable to log, Please check the details of your database");
}
return oSession;
}
public List findAlerts(int pFirstLine, int pNbElement) throws AnalyzerException {
List oAlerts = new ArrayList();
Session oSession = initSession(lHibernateUtil);
try {
oAlerts = AlertFinders.instance().findAlertByStatus(oSession, false, pFirstLine, pNbElement);
Iterator iterAlerts = oAlerts.iterator();
while (iterAlerts.hasNext()) {
...
}
} catch (UnableToLocateObjectException eU) {
throw new AnalyzerException(eU.getMessageSource(), eU.getClassNameSource(), eU.getMethodSource(), eU);
} finally {
oSession.close();
}
return oAlerts;
}

Multiple possible ways.
Use Java Connection isValid method.
Use connection pool - All major connection
pools support this functionality (including c3p0 and dbcp).They can
throw SQLException has getErrorCode() and getSQLState() methods
Write Java code & poll frequently - sample code below
Run arguments sample: jdbc:oracle:thin:#localhost:1521:XE system mypassword123 oracle.jdbc.driver.OracleDriver
public class DbConnCheck {
public static void main(String[] args) throws Exception {
String url = args[0];
String username = args[1];
String password = args[2];
String driver = args[3];
Class.forName(driver);
Connection conn = DriverManager.getConnection(url, username, password);
try {
Statement statement = conn.createStatement();
ResultSet rs = statement.executeQuery("SELECT SYSDATE FROM DUAL");
while(rs.next()) {
System.out.println(rs.getObject(1));
}
} finally {
conn.close();
}
}
}
Edit : Adding details on hibernate part
Not done in Hibernate but to be precise you can check in connection pool configuration.
If using c3p0 then check how you can best use setting like idle_test_period, preferredTestQuery and testConnectionOnCheckout;
If using dbcp then validationQuery can do the job.
If you want to use c3p0 with Hibernate and Spring check this link

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.