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Unstable Oracle Database connection for Java-project

I'm a student and one of our assignments is creating a Java web project on a local GlassFish 5 webserver. The database used for this project is an OracleDB running locally in a Docker container.
I almost finished my project but some pages keep crashing (NullPointerException). I have to retrieve database records and save them in an ArrayList. But sometimes the SQLConnection doesn't return anything (but the records DO exist) and my code tries to preform actions on that empty ArrayList.
Now, as I said, the connection appears to be unstable, because at some seemingly random moments the database does respond with the appropriate records.
It's really frustrating and I cannot continue working on this project without a stable database connection. So I'd appreciate hearing from people with some more experience :-)
Thank you for your time.
Code for running a query:
protected ResultSet getRecords(String query) {
try {
Connection connection = DriverManager.getConnection(url, login, password);
Statement statement = connection.createStatement();
return (ResultSet) statement.executeQuery(query);
} catch (SQLException e) {
e.getStackTrace();
}
return null;
}
Code with the query:
List<Uitlening> uitleningen = new ArrayList<Uitlening>();
try {
ResultSet resultSet = getRecords("SELECT * FROM uitlening");
while(resultSet.next()) { //Here the code crashes because the ResultSet can sometimes be empty.
I think this is the actual error message: Listener refused the connection with the following error: ORA-12519, TNS:no appropriate service handler found
But I don't really understand what I should do now..
try {
ResultSet resultSet = getRecords("SELECT * FROM uitlening");
while(resultSet.next()) {
Uitlening uitlening = new Uitlening();
uitlening.setNr(resultSet.getInt("nr"));
uitleningen.add(uitlening);
}
} catch (SQLException e) {
e.addSuppressed(e);
}
return uitleningen;
It might be nothing, but it almost looks like the error only occurs when I run 2 queries almost immediately after each other. Is it possible that closing the connection takes a while?

Chances are that you run into the database connection problem because your code does not properly close the database connections as well as the statements and result sets.
A statement will also close its active result set. Most JDBC will also close the statement if the connection is closed.
So closing the connection is the most important part. It cannot be achieved with your current code structure because you create it in an inner method and do not return it.
It has also been mentioned that the exception handling is poor because you ignore exceptions and return null instead causing seemingly unrelated crashes later. In many cases it might be easier to declare that the method throws SQLException.
You might want to change your code like so:
List<Uitlening> retrieveData() {
final String query = "SELECT * FROM uitlening";
try (Connection connection = DriverManager.getConnection(url, login, password);
Statement statement = connection.createStatement();
ResultSet resultSet = statement.executeQuery(query)) {
return processResultSet(resultSet);
} catch (SQLException e) {
throw new RuntimeException(e);
}
}
List<Uitlening> processResultSet(ResultSet resultSet) throws SQLException {
List<Uitlening> uitleningen = new ArrayList<>();
while (resultSet.next()) {
Uitlening uitlening = new Uitlening();
uitlening.setNr(resultSet.getInt("nr"));
uitleningen.add(uitlening);
}
return uitleningen;
}
It closes the connection, the statement and the result set by using try/catch blocks that take advantage of AutoClosables (in this case: Connection, Statement, ResultSet).
The method processResultSet declares the SQLException so it doesn't need to handle it.
The code is rearrange so the data is fully processed before the code leaves the try/catch block that closes the connection.

The driver was unable to create a connection to mysql

I'm programming in java SE and I get an error when trying to access to create a connection to mysql. I can connect to mysql, in fact, the error shows up when running a bucle.
What I do in this program is to check for a String in the table Colors of my database and if it finds nothing it creates this String in the table with an autoincrementing id.
It works fine, but after having checked it for a while it gives me the error.
I attach the image of the error and the code where I create the connection.
public Integer codiColor(String col){
Integer codi=null;
if(col.equals(""))
return 1;
try {
Class.forName(driver).newInstance();
con = DriverManager.getConnection(url, usuari, password);//here is the error
try {
Statement st = con.createStatement();
String sql = "SELECT CODICOL FROM COLORES where COLOR ='"+col+"'";
ResultSet res = st.executeQuery(sql);
if(res.next()){
codi = res.getInt("CODICOL");
}
try { res.close(); } catch (Exception e) {}
try { st.close(); } catch (Exception e) {}
}
catch(SQLException s){
JOptionPane.showMessageDialog(null, "Error:\n"+s.getMessage(),
"ERROR.",JOptionPane.ERROR_MESSAGE);
}
finally{
try { con.close(); } catch (Exception e) {}
}
}
catch (Exception ex)
{
ex.printStackTrace();
}
return codi;
}
This code is the one that checks if the color already exists or not.
EDIT:
url = "jdbc:mysql://192.168.1.200:3306/mybbdd?zeroDateTimeBehavior=convertToNull";

The problem is that con is evidently a member variable where it should be a local variable. If this piece of code and others like it are called from multiple threads, a con value will be overwritten and therefore lost, so a connection leak will result. You will almost certainly also have other problems due to concurrent use of the connection. Make it a local variable.
NB you haven't needed the Class.forName() line since 2007. The close of the connection, statement, and result set would be redundant if you used try-with-resources. And you should use a prepared statement.
String sql = "SELECT CODICOL FROM COLORES where COLOR =?";
try (con = DriverManager.getConnection(url, usuari, password);
PreparedStatement st = con.prepareStatement(sql);
) {
st.setObject(1, col);
ResultSet res = st.executeQuery();
if(res.next()){
codi = res.getInt("CODICOL");
}
}
catch(SQLException s){
JOptionPane.showMessageDialog(null, "Error:\n"+s.getMessage(),
"ERROR.",JOptionPane.ERROR_MESSAGE);
}
}
catch (Exception ex)
{
ex.printStackTrace();
}

I find the way out. I googled the number the java error gave to me and I found that adding a registry key to be able to do more connections should work.
I first did it on the server, to allow more connections from clients but it didn't still work so I tried to do it on my computer and for now it works.
This is the link from microsoft

In your code you giving a new connection on every call but you need to understand your OS allows you Only Limited Connection.
after cross Limit it will Not allow you to make another Connection.
i don't know what's your requirement but if you really need Connection
So better option is that you need to make Connection Pool. If user required a connection to do some task then user can Take Connection from connection Pool and after Work Finished user can return that connection to connection pool.
For Achieve this you need to Design Your Configuration File Or you need Design interface library interface and implement according to your Requirement.

showing thai language from sqlserver in java swing

I am facing some problem by getting thai word from sql server 2000 database in java netbeans 7.1
i've made the connection using net.sourceforge.jtds.jdbc.Driver and after making the connection when i want to read the table values (which are stored in thai language) say
เพชรสี่เหลี่ยม
is the desired value to get in a message box (for example) instead my program shows
ྪÃÊÕèàËÅÕèÂÁ
i am using jdk 1.7
here is my code snippet:
PreparedStatement pre = null;
try {
pre = con.prepareStatement("select * from setting");
} catch (SQLException ex) {
Logger.getLogger(NewFrm1.class.getName()).log(Level.SEVERE, null, ex);
}
ResultSet rs;
try {
rs = pre.executeQuery();
while(rs.next())
{
String strthai = rs.getString(3); \\ this is a varchar field in database
ShowMessage( strthai,"coding");
}
} catch (SQLException ex) {
Logger.getLogger(NewFrm1.class.getName()).log(Level.SEVERE, null, ex);
}
I've made this clear by making testing programs in both VB 6.0 and delphi 5 that those test programs are showing the correct thai words (ie เพชรสี่เหลี่ยม ) in their programs for the same table.
I am trying searching a lot and already spent 2 days to resolve this issue
please give me some solution regarding this issue.

Retain jdbc batch statements after exception

I am inserting data from java in to postgresql database. I am using jdbc postgresql driver to make connection. I am creating a batch of statements and sending to insert in one go. But if connection is lost then java tries to connect with database again using connection pooling. I tried to execute the batch again but no record is inserted.
PreparedStatement pstmt = connection.prepareStatement(INSERT_RECORD_TABLE_SQL);
while (iterator.hasNext()) {
pstmt.setLong(1, toLong(fields[0]));
pstmt.setLong(2, toLong(fields[1]));
....
pstmt.addBatch();
}
try{
pstmt.executeBatch();
} catch (Exception e) {
Thread.sleep(60000);
pstmt.executeBatch();
}
My question is that Can I retain the batch of statements that can be executed if exception occurs?
Thanks,
Saurabh Gupta

It is a bad thing to catch the general Exception.
It is a bad thing to sleep for a minute, or any other "human" time value.
It is a bad thing to re-execute the same code in the catch block just like if nothing had occurred, but you're handling an exception there! And you should catch the new possible exception in the catch block.
Better to:
try
{
int[] updateCounts = pstmt.executeBatch();
}
catch (BatchUpdateException be)
{
// if one of the commands sent to the database fails to execute properly
// or attempts to return a result set
handleException(be);
return;
}
catch (SQLException se)
{
//if a database access error occurs, this method is called on a closed Statement
//or the driver does not support batch statements
handleException(se);
return;
}
Do you need a transaction? That is, if an error occurs should you rollback to the state the db was before you started, or is it ok to retry?

How to catch constraint violation inside the resultset loop?

I was working on a servlet that will generate a unique code and update that in a mySQL database.
Now, in that, I want to catch any exception thrown in case that unique code already exists in the mySQL table and generate a new code and try updating the database. The problem is I want to do this WITHIN the for loop itself. The code is as follows:
try
{
connection = datasource.getConnection();
SQLUpdate = "INSERT INTO Voucher_dump VALUES( '"+unique_code+"','08-10-2011 04:48:48','0')";
PreparedStatement ps1 = connection.prepareStatement(SQLUpdate);
ps1.executeUpdate();
ResultSet r = ps1.getResultSet(); // this is where I'm checking if it's a duplicate
if(r==null)
out.println("This is a duplicate");
else out.println("Updated");
trial12= "08-10-2011 04:48:480.03999855056924717a";
SQLUpdate = "INSERT INTO Voucher_dump VALUES( '"+trial12+"','08-10-2011 04:48:48','0')";
ps1 = connection.prepareStatement(SQLUpdate);
ps1.executeUpdate();
r = ps1.getResultSet();
if(r==null)
out.println("This is a duplicate");
else out.println("Updated");
}
catch (SQLException sqle)
{
sqle.printStackTrace();
}
I don't want to wait till the end of the entire loop to catch the SQLException (I have already defined this key in mySQL as primary). The moment, the result comes back as a duplicate entry, I want to re-generate this key and attempt the update again.My output for this particular code is coming blank on my output page (all other parameters are showing correctly). Neither is "This is a duplicate" displayed nor is "Updated". Maybe, ResultSet is not the best way to do it. Could you guys give me some advice on what would be the best way forward ?

Some advice in no particular order:
Close the connection in a finally block.
Close statements individually if you'll be creating many of them before closing the connection. ("Many" is defined by your DBAs.)
Format your code.
Don't use stdout and/or stderr from real code. Pick a logging framework.
Consider using some helper classes to simplify (and correct) your database access, like Spring's JdbcTemplate.
Make sure to include relevant context when you post example code.
Due to #6, I don't know what out is, but I suspect the reason you're not seeing anything is that you're inserting a duplicate value with the first statement, which will cause a SQLException from that line, not at getResultSet(), where you seem to expect it. Since the error is written to stdout, it'll show up in your server logs somewhere, but nothing will be written to out. I'm not sure why you think getResultSet() will return null or not null depending on whether there was a constraint violation. Take a look at the javadoc for that method.
Update: 7. As BalusC points out, never, ever concatenate a string directly into a JDBC Statment. Use PreparedStatment's placeholders and set* methods. For info on SQL injection, see Wikipedia and XKCD.

How about this code?
try {
Class.forName(driver).newInstance();
conn = DriverManager.getConnection(url + dbName);
System.out.println("Connected to the database");
int i = 1; //get the unique code
boolean isInserted = false;
while (!isInserted) {
try {
PreparedStatement preparedStatement = conn.prepareStatement("INSERT INTO test values (?)");
preparedStatement.setInt(1, i);
preparedStatement.executeUpdate();
isInserted = true;
} catch (com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException e) { //Catch the particular exception which throws error on unique constraint. This may depend on Java/MySQL your version
i++; //get the next unique code
}
}
System.out.println("Disconnected from database");
} catch (Exception e) {
e.printStackTrace();
} finally {
try {
conn.close();
} catch (Exception e) {
}
}