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Try-catch ignoring the assignment of a variable when executing an SQL query

This piece of code uses an SQL query to return how many entries there are in a certain table.
public int countAmountOfEntries() {
int amount;
try (Connection conn = DriverManager.getConnection(Connection.JDBC_URL);
PreparedStatement query = conn.prepareStatement("SELECT COUNT(*) FROM Table")) {
try (ResultSet rs = query.executeQuery();) {
if (rs.next()) {
amount = rs.getInt("COUNT(*)");
}
}
} catch (SQLException ex) {
throw new RuntimeException(ex);
}
return amount;
}
This should return any int other than 0. Initialising the variable to 0 will result in a NullPointerException being thrown as I'm using the return value of this to set the length of an array. Using the same code in another class returns the int it should return. I've tried using an alias for the COUNT(*) but to no avail.
Running the query directly into MySQL returns the int as well. I've tried removing the nested try (it was pretty much obsolete since I know it won't throw an exception if no one messes with my DB).

Did you register the JDBC driver before using it?
Class.forName("com.mysql.jdbc.Driver");
Is it required to provide an username/password upon connecting?
DriverManager.getConnection(url, user, pass);
Did you create a Connection class yourself which overwrites the Connection class returned upon opening the connection. The reason I ask this is because you retrieve the URL to connect to using Connection.JDBC_URL which is (as far as I know) not in the Connection class.
Is there already a connection opened and your database only allows 1 open connection?
Note: do not forget to close the resultset, statement, and connection before returning:
rs.close();
query.close();
conn.close();
Besides that, restructure your function because a try without catch does not help at all.

This looks really weird:
amount = rs.getInt("COUNT(*)");
Try this instead
amount = rs.getInt(1);

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.

Prepared statement not returning results for string numbers

I am trying to retrieve records from oracle database for the SQL query - SELECT data FROM test where id = ?;
Here is my code to retrieve the data -
public static String retrieveData(String id) throws IOException {
String result = "";
Connection connection = null;
PreparedStatement ps = null;
ResultSet rs = null;
try {
String sql = "SELECT data FROM test where id = ?";
connection = getDBConnection(); //this method gets the db connection
ps = connection.prepareStatement(sql);
ps.setString(1, id);
rs = ps.executeQuery();
while(rs.next()) {
result = result + rs.getString("data");
}
} catch (SQLException e) {
e.printStackTrace();
} finally {
closeDBConnection(); //closes db connection
}
return result;
}
When I invoke the retrieveData in the main method, it returns null. When I debug, the resultSet is empty. I am guessing the reason is I am passing my id as String, but perhaps Oracle doesn't understand something like - SELECT data FROM test where id = "1234567890";
Can someone please guide me how do I fix this? I have tried way too many things, but I can't seem to fetch the data from the db for this id. I also tried converting the id to long using Long.valueOf(String s).longValue(); and setting this as the placeholder in ps.setLong, but it didn't help. Please advise.

Type issue of id column
This is probably due to spacing difference or another subtle difference that gives the impression that they are the same.
The strings ' 123 ' or '00123' (for example) are not equivalent to '123' in VARCHAR2 type, while they are equivalent in NUMERIC type, as Oracle converts all those strings to the numeral 123.
You are better off using a NUMERIC column as your id to avoid such problems. It also makes sense to limit your valid id values to whole numbers, rather than any arbitrary string.
With Oracle you can continue to use setString as it will automatically cast the string to an integer. But I recommend you to use setInt or setLong instead (after the conversion of the id column), as other database solutions like Postgres will not accept a string to match a NUMERIC column.
Exception handling issue
When you catch a Throwable or an Exception (or their subtypes), be sure to think about whether it can be ignored, or it must be thrown for further handling (for example, to stop the program, alert the user or correct the problem).
In your code, the catch block catches the SQLException and prints the stack trace. But it doesn't do anything else.
} catch (SQLException e) {
e.printStackTrace();
} ...
This means that you are ignoring the exception and continuing the program as though nothing happened. The execution will continue on to return result; after completing the finally block.
The caller of your method will never know that an SQLException occurred, and it will wrongly assume that there were no errors and the returned data is correct.
Unless that is what you really want, what you ought to do is as follows:
} catch (SQLException e) {
e.printStackTrace(); //it doesn't matter what you do here. the next line is what's important.
throw e; //throw the exception after printing stack trace, logging, etc.
} ...
In this case your method will throw the SQLException to the caller after completing the finally block. The execution will not continue to return result;.
This way the caller of your method will receive the exception and know that an error has occurred. The caller can then act accordingly; for example, by stopping the program, alerting the user or retrying the method with a different input.

may be the reason is DEBUG. if you have a breakpoint on the line "while(rs.next())" and you watch/inspect it then then rs increments one. Then the code executes rs.next for the second time where there is no record.
maybe...

You shouldn't use datatype 'String' for id field, it can cause serious trouble. Try to covert it to numeric datatype is possible but why don't you use numeric at first. You should change to int or bigint. In additional try to catch exception like this:
try{
}catch(SQLException ex){
System.out.println("Error at:" +ex.getClass().getName() + ex.getMessage());
}
Try it again, if you can't debug it and find out why! Good luck!

Please try this. Remove all the spaces when defining field name with a ? and check
SELECT data FROM test where id=?

java.lang.NullPointerException ... Initializing the JDBC MYSQL connection

Please don't suggest me to use InternalFrame or Dialogs. I can't start the project from beginning.
Theme: I'm building a GUI program to display mark-sheet. I've taken 3 JFrames & 1 simple class...
Frame1.java
It's having 1 JTextField to enter roll_no. & 2 buttons to feedData in DB & showResult. feedData button calls Frame2 & showResult button calls Frame3.
Frame2.java
For feeding data have several JTextFields & Buttons that transfer content to mySQL DB.
Frame3.java
is a result window that fetches content from DB.
Support.java
Contains static variables & getter-setter methods for them
.....
.....//contains in Support.java
public boolean add() {
query = "Insert into table1 (enroll,Sname,Fname,sub1,sub2,sub3,sub4,sub5 )values(?,?,?,?,?)";
try {
PreparedStatement psmt = conn.prepareStatement(query);
psmt.setString(1, enroll);
psmt.setString(2, Sname);
psmt.setString(3, Fname);
psmt.setInt(4, sub1);
psmt.setInt(5, sub2);
psmt.setInt(6, sub3);
psmt.setInt(7, sub4);
psmt.setInt(8, sub5);
int y = 0;
y = psmt.executeUpdate();
if (y == 0) {
return false;
}
} catch (Exception e) {
e.printStackTrace();
return false;
}
return true;
}
add() is called on pressing save button in Frame2.java . . . If catch block is executing, why println(query) printing NULL

Based on some of your question tags and responses in the comments to other answers and on the question itself, I'm presuming that somewhere in your code, you intend to call
Class.forName("com.mysql.jdbc.Driver").newInstance();
conn = DriverManager.getConnection(url, username, password);
This is not happening before your add() method is called. In order to fix it, I'd recommend this (bulk of code borrowed from Vivek bhatnagar's answer):
try {
conn = DriverManager.getConnection(url, username, password);
PreparedStatement pstmt = conn.prepareStatement("INSERT INTO `table`
(pid,tid,rid,tspend,description) VALUE
(?,?,?,?,?)");
pstmt.setString(1, pid );
pstmt.setString(2, tid);
pstmt.setString(3, rid);
pstmt.setInt(4, tspent);
pstmt.setString(5,des );
pstmt.executeUpdate();
} catch (Exception e) {
// whatever you want to do to handle the exception
} finally {
// close your connection
}
If you're on Java 7, set up like this:
try (Connection conn = DriverManager.getConnection(url, username, password)) {
try (PreparedStatement pstmt = conn.prepareStatement(/*sql here*/)) {
// Your code here
} catch (SQLException sqle) {
// handle exceptions from the statement
}
} catch (SQLException outerSqlEx) {
// handle exceptions from connecting
}
How could I tell what your problem was (general help for NullPointerException)?
NullPointerException is only thrown when you try to call a method on a null variable (and at a few other specific times, as noted in the API documentation). The easy way to locate a NullPointerException is to look for the line the stack trace indicates, and then look for the dots on the line. There's only two lines in your try block that can throw a NullPointerException.
Statement stmt = conn.createStatement();
// could be here ----^
and
y = stmt.executeUpdate(query);
// or --^
So let's look at the dots. The first one will throw when conn is null. The second one will throw when stmt is null. In your original code, which you've now edited in response to the other answers, you set the value of query after you called conn.createStatement();. Since query was still null in your catch block, we know that it hadn't yet been set, and thus it must be the first one, so conn is null at that point in the program.
Furthermore, since the API Documentation for createStatement
implies that it will either return a valid Connection object or throw an SQLException, we can be pretty sure that stmt will never be null when executeUpdate is called.

In your try block, you are calling a method that is possible to throw an exception before setting the variable in question:
Statement stmt = conn.createStatement();
query = "Insert into table1 (enroll,Sname,Fname,sub1,sub2,sub3,sub4,sub5 )values('" + getEnroll() + "','" + getSname() + "','"+getFname()+"',"+getSub1()+","+getSub2()+","+getSub3()+","+getSub4()+","+getSub5()+")";
Therefore, if your code fails on the conn.createStatement() line, it will enter the catch block without the query variable being initialized.
You can fix this simply by switching the order of the statements, or by putting the query line outside and before the try/catch blocks.

Adding to what #Southpaw answered :
you can use something like this also :
PreparedStatement pstmt = con.prepareStatement("INSERT INTO `table`
(pid,tid,rid,tspend,description) VALUE
(?,?,?,?,?)");
pstmt.setString(1, pid );
pstmt.setString(2, tid);
pstmt.setString(3, rid);
pstmt.setInt(4, tspent);
pstmt.setString(5,des );
pstmt.executeUpdate();
Kindly Note its benefits:
1."Query is rewritten and compiled by the database server"
If you don't use a prepared statement, the database server will have to parse, and compute an execution plan for the statement each time you run it. If you find that you'll run the same statement multiple times (with different parameters) then its worth preparing the statement once and reusing that prepared statement. If you are querying the database adhoc then there is probably little benefit to this.
2."Protected against SQL injection"
This is an advantage you almost always want hence a good reason to use a PreparedStatement everytime. Its a consequence of having to parameterize the query but it does make running it a lot safer. The only time I can think of that this would not be useful is if you were allowing adhoc database queries; You might simply use the Statement object if you were prototyping the application and its quicker for you, or if the query contains no parameters.

Best Practice for handling duplicate INSERT statements

I am currently using Java to try to INSERT a customer in my Database:
public void saveCustomer(Customer c) {
getConnection();
try {
CallableStatement pstmt = con.prepareCall("{call sp_saveCustomer(?,?)}");
pstmt.setString(1, c.getName());
pstmt.setString(2, a.getLastName());
pstmt.executeUpdate();
closeConnection();
} catch (SQLException e) {
// Handle Exception
}
}
When a user tries to INSERT a duplicate record, the catch block will trigger. What I'm worried about is that I don't know if this means a performance hit. Isn't a better way to check if a customer already exists? Something like:
public void saveCustomer(Customer c) {
getConnection();
boolean exists = false;
CallableStatement pstmt = con.prepareCall("{call sp_checkCustomerExistence(?,?)}");
pstmt.setString(1, c.getName());
pstmt.setString(2, a.getLastName());
ResultSet rs = pstmt.executeQuery();
if (rs.next()) exists = true;
if ( exists ){
closeConnection();
return;
}
CallableStatement pstmt = con.prepareCall("{call sp_saveCustomer(?,?)}");
pstmt.setString(1, c.getName());
pstmt.setString(2, a.getLastName());
pstmt.executeUpdate();
closeConnection();
}
What would be the best solution based on the best performance? Should I check myself if the customer exists or should I let the Exception be thrown?

I think checking the duplicate is preferable than catching the exceptions. Only suggestion I have it to perform the check in your procedure itself if you can i.e. create a procedure as sp_check_and_save_customer which will check for duplicate and save if it doesn't already exist. This way there will be very less time difference between the check and insert, which becomes more crucial in heavy loaded applications.
Still, please have your exception handling as well as it is still possible that another process inserts the data in between the check and insert.

Checking before hand = two database calls or two IO interactions. Attempting to just insert with the possibility of an exception occurring exception is only one. As long as the exceptions are handled, they aren't necessarily a bad thing.
Although probably not preferred, it would be the fastest of the two methods you've listed.

} catch (SQLException e) {
The SQLException that you are catching could mean anything. It doesn't necessarily mean that it occured because of a duplicate row in the database table. So, simply it is not reliable enough to be used for that use-case.
You just have to make a call before trying to create one, to handle it in a proper manner.