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JDBC Multipoint Connection

I have the following Callable instance where SQLException is thrown here:

public long[] call() throws Exception {
    long[] stats = new long[6];
    try {
        executer.acquire();
        PreparedStatement statement =
            connection
                .prepareStatement("SELECT `War`.`EndTime` FROM `WarSim`.`War` WHERE `War`.`WarName` = ?");
        statement.setString(1, warName);
        ResultSet res = statement.executeQuery(); //<--------------SQLEXCEPTION HERE
        if (res.first()) {
        Timestamp ts = res.getTimestamp("EndTime");
        if (ts != null)
            stats[0] = 1;
        statement =
            connection
                .prepareStatement("SELECT COUNT(`ID`) FROM `Missile` WHERE `WarName` = ?");
        statement.setString(1, warName);
        res = statement.executeQuery();
        stats[1] = res.getInt(1);
        statement =
            connection
                .prepareStatement("SELECT COUNT(`ID`) FROM `Missile` WHERE `WarName` =  ?  AND `Intercepted` = '1'");
        statement.setString(1, warName);
        res = statement.executeQuery();
        stats[2] = res.getInt(1);
        stats[3] = stats[1] - stats[2];
        statement =
            connection
                .prepareStatement("SELECT COUNT(`ID`) FROM `EnemyLauncher` WHERE `WarName` = ? AND `Intercepted` = '1'");
        statement.setString(1, warName);
        res = statement.executeQuery();
        stats[4] = res.getInt(1);
        statement =
            connection
                .prepareStatement("SELECT SUM(`Damage`) FROM `Missile` WHERE `WarName` =  ? AND `Intercepted` = '0'");
        statement.setString(1, warName);
        res = statement.executeQuery();
        stats[5] = res.getInt(1);
        }
    } catch (SQLException e) {
        System.out.println(warName + " is problematic");
        while (e != null) {
        System.out.println("\tmsg: " + e.getMessage()+
                       "\n\tstate: " + e.getSQLState());
        e = e.getNextException();
        }
    } catch (InterruptedException e) {
        e.printStackTrace();
    } finally {
        executer.release();
    }
    return stats;
    }

executer

is a one-time, fair semaphore that I am using.

When I debug the code everything works fine (no exceptions), but when I run the program "normally" I get a "result set start" SQLException thrown along with SQLState S1000.

Why am I getting exceptions even though I used a semaphore to get the mutex for the request?

Help me please:)

EDIT: Here's a stack trace.

java.sql.SQLException: Before start of result set

    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1075)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:989)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:984)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:929)
    at com.mysql.jdbc.ResultSetImpl.checkRowPos(ResultSetImpl.java:841)
    at com.mysql.jdbc.ResultSetImpl.getInt(ResultSetImpl.java:2672)
    at db.jdbc.GetWarStatsTask.call(GetWarStatsTask.java:37)
    at db.jdbc.GetWarStatsTask.call(GetWarStatsTask.java:1)
    at java.util.concurrent.FutureTask.run(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

The class that handles the database connection:

public class JDBCConnection implements DBConnection {

    private Connection connection;
    private String dbUrl;
    private Semaphore executer;
    private ExecutorService es;
    private static JDBCConnection instance;

    public static JDBCConnection getInstance() {
    if (instance == null) {
        instance = new JDBCConnection();
    }
    return instance;
    }

    private JDBCConnection() {
    dbUrl = "jdbc:mysql://---------/WarSim";
    try {
        Class.forName("com.mysql.jdbc.Driver").newInstance();
        connection =
            DriverManager.getConnection(dbUrl, "------", "-------");
    } catch (InstantiationException e) {
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        e.printStackTrace();
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    } catch (SQLException e) {
        while (e != null) {
        System.out.println(e.getMessage());
        e = e.getNextException();
        }
    }
    es = Executors.newCachedThreadPool();
    executer = new Semaphore(1, true);
    }

    public Future<long[]> getWarStats(String warName) {
    return es.submit(new GetWarStatsTask(executer, connection, warName));
    }

    public void closeDB() {
    try {
     if (connection != null) {
        connection.close();
     }
    } catch (Exception e) {
        System.out.println("Could not close the current connection.");
        e.printStackTrace();
    }
}
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2 answers


Basically, you position the cursor before the first line and then query for data. You need to move the cursor to the first line.



So first call result.next();

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By using a semaphore, you understand the JDBC rule that a connection should only be used by one thread at a time. However, there are other topics that you need to be aware of, namely the garbage collector thread.

In this case, you need to close all statement objects created before the semaphore was freed. If you don't close the operator objects, they will be closed by the garbage collector thread indefinitely, resulting in unusual behavior on the connection.

So, before preparing a new statement object, you need to close the old one.



statement.close ()

statement = connection .prepareStatement ("SELECT COUNT ( ID

) FROM Missile

WHERE WarName

=?");



At the end, you need to close the instruction.



stats [5] = res.getInt (1);

statement.close ()



You might want to use the practice of running "findbugs" in your code. I think it might indicate a problem of not closing the Statement object.

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