I have the following method in my database class that returns a resultset for a given SQL statement:
public static ResultSet sqlStatement(String query) throws SQLException{
ResultSet result = null;
Connection conn = connect();
Statement newStatement = conn.createStatement();
result = newStatement.executeQuery(query);
conn.close();
return result;
}
I want to close the connection to the database before i return the resultset but it throws the following exception:
java.sql.SQLException: out of memory
I'm a java noob and experimenting so any help is much appreciated.
In my past experience(No document or anything), I would understand the "ResultSet" like a pointer in C. I bet it would cache some rows from database when you execute your query..
Therefore, if you close connection and then try to use resultset, all those cached resultset does not have proper information and getting next cache etc. As a result, it would throw out of memory exception..
Anyway proper way to use those in java..
Connection con...
try{
create connection
execute query
use your resultset completely..
}catch(...){
}finally{
close connection;
}
Hope it would help
Put the conn.close() in a finally block so that it gets executed event if an exception is thrown.
always, always, always close your database resources in a finally block!
i doubt your logic will work as is. a ResultSet is only usable while the connection is open. you need to process the entire ResultSet before you close the connection.
Apart from closing resources in a finally block, you also have to close your PreparedStatement and your ResultSet. In fact, you shouldn't return the ResultSet and keep it open for too long. Instead, you could read it into some sort of intermediary data store, such as a List<Object[]>.
On the other hand, you're not getting a java.lang.OutOfMemoryError, so possibly it's your database itself that ran out of memory
Related
i created a kafka producer on java that works fine, now im trying to store the data produced in an mySQL database but i don't know how. i tried this code but it doesn't work
try {
String MyUrl = "jdbc:mysql://130.2.2.2/pfa";
Class.forName("com.mysql.jdbc.Driver");
Connection con = DriverManager.getConnection(MyUrl, "root", "");
Statement stm = con.createStatement();
ResultSet rs = stm.executeQuery("select * from tmp");
while(rs.next()) {
producer.send(new ProducerRecord<String, String>("test",rs.getString(1)));
con.close();
}
}catch(Exception e) {System.out.println(e);}
any help is much appreciated about sending data to the database
I'm not sure I understand what you're expecting to happen from that code.
You're reading from the database, never inserting into it
You're closing the database connection after every loop iteration, meaning you'll get an exception that the database is already closed on the second loop, and therefore the resultset might terminate as well (I'm a little fuzzy on if those are lazy objects or not)
Might I suggest that you follow the Deep Dive on the JDBC connector? https://dev.to/rmoff/kafka-connect-jdbc-sink-tips-tricks-video-walkthrough-2egf
i copy pasted it literally becoz the writer said it worked
This will never be a good way to learn anything. You can read the code and type it all back out, explaining it to yourself as you go... But never blindly copy something that you assume solves your problem
I am using apache derby and getting the above error here is my code, actually I'm just not getting all the children under the parents, it only give me one level of children, so kindly tell me how to make tree parent and child but this gives only one level of child with above error
getting connection
public static Connection getConnection(){
Connection connection = null;
try {
Class.forName("org.apache.derby.jdbc.ClientDriver").newInstance();//
//Get a connection
connection = DriverManager.getConnection(dbURL);
connection.setAutoCommit(false);
connection.getAutoCommit();
} catch (Exception except) {
System.out.println(except);
}
return connection;
}
and error in second loop at last reading rs1
Statement stmt;
stmt = MainUI.getConnection().createStatement();
Nesting the result sets is your problem, I believe.
To have two different result sets open, you must have two separate Statement instances, so each can have its own ResultSet. Otherwise, running the second query on the same Statement instance will close the first ResultSet.
See this answer for more information: https://stackoverflow.com/a/8579669/193453
Also, have a look at the Javadocs for the java.sql.Statement class:
By default, only one ResultSet object per Statement object can be open at the same time. Therefore, if the reading of one ResultSet
object is interleaved with the reading of another, each must have been
generated by different Statement objects. All execution methods in the
Statement interface implicitly close a statment's current ResultSet
object if an open one exists.
I have been making a program that is using HSQL to connect to a database that I created. For some reason some methods in my class can call on the database and perform commands, while other parts cannot. I keep getting this error,
java.sql.SQLFeatureNotSupportedException: feature not supported
and here is the method,
public List<CustomerInfo> DBgetInfo(String Customer)
throws ClassNotFoundException, SQLException {
Class.forName("org.hsqldb.jdbcDriver");
Connection con = DriverManager.getConnection(urlConnection, userId,
password);
Statement stmt= con.createStatement();
String query = "SELECT * FROM PUBLIC.CUSTOMER";
ResultSet rs = stmt.executeQuery(query);
rs.first(); //The error happens on this line
rs.close();
stmt.close();
con.close();
}
I have ran the debugger multiple times and there error is in this method on the rs.first line. I have tried remaking the DB, reimporting all the files, checking to make sure the command is correct, and ect... The weird thing is that earlier in this class I have a method very similar to this, but it has no issues. I really can't figure out what the problem is.
According to the documentation this error occurs:
Throws:
SQLException - if a database access error occurs, this method is called on a closed result set or the result set type is TYPE_FORWARD_ONLY
SQLFeatureNotSupportedException - if the JDBC driver does not support this method
Earlier on the same page, there is a section on HSQL specific details for result sets. To call first you need to modify your statement creation:
ResultSet object generated by HSQLDB is by default of ResultSet.TYPE_FORWARD_ONLY (as is standard JDBC behavior) and does not allow the use of absolute and relative positioning methods. If a statement is created with:
Statement stmt = conn.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_READ_ONLY);
then the ResultSet objects it produces support using all of the absolute and relative positioning methods of JDBC2 to set the position of the current row...
But you might want to think about why you need to call first.
In JDBC, can I use single Statement object to call executeQuery("") multiple times? Is it safe? Or should I close the statement object after each query, and create new object for executing another query.
E.G:
Connection con;
Statement s;
ResultSet rs;
ResultSet rs2;
try
{
con = getConnection();
// Initially I was creating the Statement object in an
// incorrect way. It was just intended to be a pseudocode.
// But too many answerers misinterpretted it wrongly. Sorry
// for that. I corrected the question now. Following is the
// wrong way, commented out now.
// s = con.prepareStatement();
// Following is the way which is correct and fits for my question.
s = con.createStatement();
try
{
rs = s.executeQuery(".......................");
// process the result set rs
}
finally
{
close(rs);
}
// I know what to do to rs here
// But I am asking, should I close the Statement s here? Or can I use it again for the next query?
try
{
rs2 = s.executeQuery(".......................");
// process the result set rs2
}
finally
{
close(rs2);
}
}
finally
{
close(s);
close(con);
}
Yes you can re-use a Statement(specifically a PreparedStatement) and should do so in general with JDBC. It would be inefficient & bad style if you didn't re-use your statement and immediately created another identical Statement object. As far as closing it, it would be appropriate to close it in a finally block, just as you are in this snippet.
For an example of what you're asking check out this link: jOOq Docs
I am not sure why you are asking. The API design and documentation show it is perfectly fine (and even intended) to reuse a Statement object for multiple execute, executeUpdate and executeQuery calls. If it wouldn't be allowed that would be explicitly documented in the Java doc (and likely the API would be different).
Furthermore the apidoc of Statement says:
All execution methods in the Statement interface implicitly close a statment's [sic] current ResultSet object if an open one exists.
This is an indication that you can use it multiple times.
TL;DR: Yes, you can call execute on single Statement object multiple times, as long as you realize that any previously opened ResultSet will be closed.
Your example incorrectly uses PreparedStatement, and you cannot (or: should not) be able to call any of the execute... methods accepting a String on a PreparedStatement:
SQLException - if [...] the method is called on a PreparedStatement or CallableStatement
But to answer for PreparedStatement as well: the whole purpose of a PreparedStatement is to precompile a statement with parameter placeholders and reuse it for multiple executions with different parameter values.
I can't find anything in the API docs that would state, that you shouldn't call executeQuery() on a given PreparedStatement instance more than once.
However your code does not close the PreparedStatement - a call to executeQuery() would throw a SQLException in that case - but the ResultSet that is returned by executeQuery(). A ResultSet is automatically closed, when you reexecute a PreparedStatement. Depending on your circumstances you should close it, when you don't need it anymore. I would close it, because i think it's bad style not to do so.
UPDATE Upps, I missed your comment between the two try blocks. If you close your PreparedStatement at this point, you shouldn't be able to call executeQuery() again without getting a SQLException.
A Prepared Statement tells the database to remember your query and to be prepared to accept parameterized variables to execute in that query. It's a lot like a stored procedure.
Prepared Statement accomplishes two main things:
It automatically escapes your query variables to help guard against SQL Injection.
It tells the database to remember the query and be ready to take variables.
Number 2 is important because it means the database only has to interpret your query once, and then it has the procedure ready to go. So it improves performance.
You should not close a prepared statement and/or the database connection in between execute calls. Doing so is incredibly in-efficient and it will cause more overhead than using a plain old Statement since you instruct the database each time to create a procedure and remember it. Even if the database is configured for "hot spots" and remembers your query anyways even if you close the PreparedStatement, you still incur network overhead as well as small processing time.
In short, keep the Connection and PreparedStatement open until you are done with them.
Edit: To comment on not returning a ResultSet from the execution, this is fine. executeQuery will return the ResultSet for whatever query just executed.
Firstly I am confused about your code
s = con.prepareStatement();
Is it work well?I can't find such function in JAVA API,at least one parameter is needed.Maybe you want to invoke this function
s = con.createStatement();
I just ran my code to access DB2 for twice with one single Statement instance without close it between two operation.It's work well.I think you can try it yourself too.
String sql = "";
String sql2 = "";
String driver = "com.ibm.db2.jcc.DB2Driver";
String url = "jdbc:db2://ip:port/DBNAME";
String user = "user";
String password = "password";
Class.forName(driver).newInstance();
Connection conn = DriverManager.getConnection(url, user, password);
Statement statement = conn.createStatement();
ResultSet resultSet = statement.executeQuery(sql);
int count = 0;
while (resultSet.next()) {
count++;
}
System.out.println("Result row count of query number one is: " + count);
count = 0;
resultSet = statement.executeQuery(sql2);
while (resultSet.next()) {
count++;
}
System.out.println("Result row count of query number two is: " + count);
We are trying to fetch data from Oracle DB using a PreparedStatement. It keeps fetching zero records while the same runs and fetches data when run from PL/SQL developer.
We found the root cause while trying to debug. While debugging the code fetched the two records properly.
We did a temporary fix by placing this piece of code.
ResultSet rs = ps.executeQuery();
while(!rs.hasNext()){
ps.executeQuery();}
This works. But this is not the best solution since it results in an unwanted DB hit.It clearly looks like a time issue. We also explicitly committed earlier transactions since they can affect the result of this query.
What could be causing this. What's the best way to solve this?
The method is quite big: I'll just post some parts here:
private static boolean loadCommission(Member member){
Connection conn = getConnection("schema1"); //obtained through connection pool
//insertion into table
conn.close();
Conn conn2 = getConnection("schema2"); //obtained through connection pool
PreparedStatement ps = conn2.prepareStatement(sql);
//this sql combines data from schema1
// and 2 with DB links
ResultSet rs = ps.executeQuery();
//business logic
conn2.close();
return true;
}
Thanks
We tried a few more things yesterday. We replaced the second connection code with direct jdbc connection like so
Connection conn = DriverManager.getConnection(URL, USER, PASS);
This too works. Now we are not sure if the delay is in getting connection from pool or in completing previous transaction like we thought earlier.
If your query selects from a materialized view, then there may be some elapsed time before it will yield results (as materialized views do not necessarily refresh instantly after a commit, depending upon how they've been created).
If this is the case, then you can resolve the problem by either selecting directly from the base table (or equivalent non-materialized views), or forcing the materialized view to refresh.