JDBC MYSQL连接问题

Question

I am finally able to create a simple java program through console and notepad which connects to MYSQL database. But when i access the database using

conn = DriverManager.getConnection(DB_URL,USER,PASS);
stmt = conn.createStatement();

my code simply keeps running for 1 or 2 minutes outputting a lot of information (I noticed some kind of trace) before it stops executing.

Here is my code:

public class Database1{

    static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";  
    static final String DB_URL = "jdbc:mysql://localhost:3306/emp?connectTimeout=3000";

    static final String USER = "root";
    static final String PASS = "admin";

    public static void main(String[] args) {

        Connection conn = null;
        Statement stmt = null;
        ResultSet rs=null;

        try{

            Class.forName("com.mysql.jdbc.Driver").newInstance();
        } catch(Exception e){

            System.out.println("Unable to connect!!");
        }

        System.out.println("Connecting to database...");

        try{

            conn = DriverManager.getConnection(DB_URL,USER,PASS);
        } catch(Exception e){
            System.out.println("Unable to access Database!!");
        }

        System.out.println("Creating statement...");

        try{
            stmt = conn.createStatement();
            rs = stmt.executeQuery("SELECT * FROM Employees");
        }catch(Exception e){
            System.out.println("Unable to access Database!!");
        }
        System.out.println("ID             NAME         ");
    }
}

Below is the output that i get(and i could copy only the details at the end of screen):

          ..............ql.jdbc.ByteArrayBuffer.getCapacity(..) returning 16384
Thu Jun 05 12:28:53 IST 2014 TRACE:       <--  MysqlIO.java:2357 com.mysql.jdbc.
MysqlIO.reclaimLargeReusablePacket(..) returning null
Thu Jun 05 12:28:53 IST 2014 TRACE:       --> ResultSet.java:6643 com.mysql.jdbc
.ResultSet.toString()
Thu Jun 05 12:28:53 IST 2014 TRACE:       <--  ResultSet.java:6643 com.mysql.jdb
c.ResultSet.toString(..) returning "com.mysql.jdbc.ResultSet@546ccad7"
Thu Jun 05 12:28:53 IST 2014 TRACE:      <--  MysqlIO.java:1411 com.mysql.jdbc.M
ysqlIO.readAllResults(..) returning com.mysql.jdbc.ResultSet@546ccad7
Thu Jun 05 12:28:53 IST 2014 TRACE:      --> ResultSet.java:6643 com.mysql.jdbc.
ResultSet.toString()
Thu Jun 05 12:28:53 IST 2014 TRACE:      <--  ResultSet.java:6643 com.mysql.jdbc
.ResultSet.toString(..) returning "com.mysql.jdbc.ResultSet@546ccad7"
Thu Jun 05 12:28:53 IST 2014 TRACE:     <--  MysqlIO.java:1634 com.mysql.jdbc.My
sqlIO.sqlQueryDirect(..) returning com.mysql.jdbc.ResultSet@546ccad7
Thu Jun 05 12:28:53 IST 2014 TRACE:     --> ConnectionProperties.java:1901 com.m
ysql.jdbc.ConnectionProperties.getMaintainTimeStats()
Thu Jun 05 12:28:53 IST 2014 TRACE:     <--  ConnectionProperties.java:1901 com.
mysql.jdbc.ConnectionProperties.getMaintainTimeStats(..) returning true
Thu Jun 05 12:28:53 IST 2014 TRACE:     --> ConnectionProperties.java:1809 com.m
ysql.jdbc.ConnectionProperties.getGatherPerformanceMetrics()
Thu Jun 05 12:28:53 IST 2014 TRACE:      --> ConnectionProperties.java:83 com.my
sql.jdbc.ConnectionProperties$BooleanConnectionProperty.getValueAsBoolean()
Thu Jun 05 12:28:53 IST 2014 TRACE:      <--  ConnectionProperties.java:83 com.m
ysql.jdbc.ConnectionProperties$BooleanConnectionProperty.getValueAsBoolean(..) r
eturning false
Thu Jun 05 12:28:53 IST 2014 TRACE:     <--  ConnectionProperties.java:1809 com.
mysql.jdbc.ConnectionProperties.getGatherPerformanceMetrics(..) returning false
Thu Jun 05 12:28:53 IST 2014 TRACE:     --> ResultSet.java:6643 com.mysql.jdbc.R
esultSet.toString()
Thu Jun 05 12:28:53 IST 2014 TRACE:     <--  ResultSet.java:6643 com.mysql.jdbc.
ResultSet.toString(..) returning "com.mysql.jdbc.ResultSet@546ccad7"
Thu Jun 05 12:28:53 IST 2014 TRACE:    <--  Connection.java:2964 com.mysql.jdbc.
Connection.execSQL(..) returning com.mysql.jdbc.ResultSet@546ccad7
Thu Jun 05 12:28:53 IST 2014 TRACE:    --> ResultSet.java:6643 com.mysql.jdbc.Re
sultSet.toString()
Thu Jun 05 12:28:53 IST 2014 TRACE:    <--  ResultSet.java:6643 com.mysql.jdbc.R
esultSet.toString(..) returning "com.mysql.jdbc.ResultSet@546ccad7"
Thu Jun 05 12:28:53 IST 2014 TRACE:   <--  Connection.java:2949 com.mysql.jdbc.C
onnection.execSQL(..) returning com.mysql.jdbc.ResultSet@546ccad7
Thu Jun 05 12:28:53 IST 2014 TRACE:   --> ResultSet.java:5634 com.mysql.jdbc.Res
ultSet.getUpdateID()
Thu Jun 05 12:28:53 IST 2014 TRACE:   <--  ResultSet.java:5634 com.mysql.jdbc.Re
sultSet.getUpdateID(..) returning -1
Thu Jun 05 12:28:53 IST 2014 TRACE:   --> ConnectionProperties.java:1638 com.mys
ql.jdbc.ConnectionProperties.getCacheResultSetMetadata()
Thu Jun 05 12:28:53 IST 2014 TRACE:   <--  ConnectionProperties.java:1638 com.my
sql.jdbc.ConnectionProperties.getCacheResultSetMetadata(..) returning false
Thu Jun 05 12:28:53 IST 2014 TRACE:   --> ResultSet.java:6643 com.mysql.jdbc.Res
ultSet.toString()
Thu Jun 05 12:28:53 IST 2014 TRACE:   <--  ResultSet.java:6643 com.mysql.jdbc.Re
sultSet.toString(..) returning "com.mysql.jdbc.ResultSet@546ccad7"
Thu Jun 05 12:28:53 IST 2014 TRACE:  <--  Statement.java:868 com.mysql.jdbc.Stat
ement.executeQuery(..) returning com.mysql.jdbc.ResultSet@546ccad7
ID             NAME

C:\JavaP>   

Can anyone here help me to understand what the output is all about and how do i control it please...

Answer 1

Your logs

As I can see in your code in the Dropbox, you are using a PostgreSQL database - not a MySQL database a mentioned in your question! That makes a difference.

Take a look at http://jdbc.postgresql.org/development/privateapi/org/postgresql/Driver.html

This is the documentation of the driver you use. This driver has a logger and a certain log level (in your case trace ). You can set the log level or even the logger.

org.postgresql.Driver.setLogLevel(java.util.logging.Level.INFO);

Some general things

Your code and output looks fine. Well, you do not close your connection and statements, but beside of that, it seems to work.

You make a connection, you create a statement and as fare as I can see there was no exception. In the end you make a query, but you do nothing with the result. That is the reason why you do not see anything. Then your program ends and stops (because the code finished).

However, I recommend to change your code a little bit. You simple log away all possible exception

catch(Exception e){
    System.out.println("Unable to access Database!!");
}

If you do this you will never know what went wrong. Instead, print the exception, it is the only piece of information which could tell you what happens:

catch(Exception e){
    System.out.println("Unable to access Database!!");
    e.printStackTrace();
}

Now you should only think about, if your program could continue if such an exception occurs. Eg if you cannot connect to your database, it is useless to continue. It will not work! In this case, it is better to leave (or handle the exception):

catch(Exception e){
    System.out.println("Unable to access Database!!");
    e.printStackTrace();
    return;
}

And last but not least, you need to use the result of your query to see something:

rs = stmt.executeQuery("SELECT * FROM Employees");
while (rs.next()) {
    // get your stuff
}

Answer 2

public class DBConnect {

Statement st;
ResultSet rs;

public DBConnect() {
    try {
        Class.forName("com.mysql.jdbc.Driver");
        Connection con = DriverManager.getConnection("jdbc:mysql://localhost/db_name", "root", "root");
        st = con.createStatement();
    } catch (Exception e) {
    }
}

public ResultSet readData(String s) {
    try {
        System.out.println("SQL : : : " + s);
        rs = st.executeQuery(s);
    } catch (Exception e) {
    }
    return rs;
}

public int putData(String s) {
    int i = 0;
    try {
        System.out.println("SQL : : : " + s);
        i = st.executeUpdate(s);
    } catch (Exception e) {}
    return i;
}
}