Kristian Lunde

Archive for the ‘Java’ tag

My first introduction to java and databases

with 2 comments

The other day I did a complete reconfiguration of java on my system. Tonight I started to play around with java and postgresql. I installed PostgreSQL 8.2 (yes I know 8.3 is out there), and downloaded the jdbc driver for PosgreSQL.


After installing PostgreSQL, you have to add a user to the authorization file pg_hba.conf located in /etc/postgresql/8.x/main on a ubuntu system, remember to restart the postgreSQL server after editing pg_hba.conf. When that is done add the same user to the postgreSQL server either by using postgreSQL’s createuser function or by logging into the postgreSQL server and doing a SQL CREATE USER, I did the latter one.

  1. CREATE USER kristian WITH PASSWORD 'xxxxxxxx' CREATEDB;

This enables me to do a regular access of the database with:

  1. kristian@Saturn:~$ psql -U kristian crawler
  2. Welcome to psql 8.2.7, the PostgreSQL interactive terminal.</code>
  4. Type:  \copyright for distribution terms
  5. \h for help with SQL commands
  6. \? for help with psql commands
  7. \g or terminate with semicolon to execute query
  8. \q to quit
  10. crawler=#

My table:

  1. crawler=# \d document;
  2. TABLE "public.document"
  3. COLUMN     |            Type             |                       Modifiers
  4. —————+—————————–+————————
  5. id            | integer                     | NOT NULL DEFAULT NEXTVAL('document_id_seq'::regclass)
  6. url           | text                        | NOT NULL
  7. document      | text                        | NOT NULL
  8. ts_downloaded | timestamp without time zone | DEFAULT now()

Data in the table:

  1. crawler=# select * from document;
  2. id |          url          | document |       ts_downloaded
  3. —-+———————–+———-+—————————-
  4. 1 | | jalla    | 2008-04-19 21:11:11.805482


First thing to get the postgreSQL jdbc driver to work is to add it to the java classpath, and of course copy the actual jar file to its correct location, for instance /usr/lib/jvm/java-6-sun- is my java library path.
My classpath:

  1. /usr/lib/jvm/java-6-sun-

Now its about time to write some actual code:

  1. import java.sql.Connection;
  2. import java.sql.DriverManager;
  3. import java.sql.PreparedStatement;
  4. import java.sql.ResultSet;
  5. import java.sql.SQLException;
  6. import java.sql.Statement; </code>
  8. public class DatabaseTest
  9. {
  10. private Connection connection = null;
  12. private static final String USERNAME = "XXXXX";
  13. private static final String PASSWORD = "XXXXX";
  14. private static final String URL = "jdbc:postgresql:";
  15. private static final String DATABASE = "crawler";
  17. public DatabaseTest()
  18. {
  19. try
  20. {
  21. connection = DriverManager.getConnection(URL + DATABASE, USERNAME, PASSWORD);
  22. ResultSet rs = doQuery("SELECT * FROM document");
  25. System.out.println(rs.getString(2));
  26. }
  27. catch (SQLException e)
  28. {
  29. System.out.println("Database Exception!");
  30. System.out.println(e.toString());
  31. e.printStackTrace();
  32. }
  34. }
  36. public ResultSet doQuery(String sql)
  37. {
  38. ResultSet rs;
  39. try
  40. {
  41. Statement stmt = connection.createStatement();
  42. rs = stmt.executeQuery(sql);
  43. return rs;
  44. }
  45. catch (SQLException e) {
  46. e.printStackTrace();
  47. return null;
  48. }
  49. }
  50. }

Trying to execute my script:

  1. kristian@Saturn:~/workspace/crawler$  javac
  2. kristian@Saturn:~/workspace/crawler$  java -Djdbc.drivers=org.postgresql.Driver DatabaseTest

It works :D The installation and initial setup went pretty much by the book, but as a PHP developer I must say that I’m a little bit disappointed, JDBC cannot compare itself to the PHP PDO extension yet. But then again, maybe I just need to get to know the JDBC library better. Anyhow it was fun writing this little java app and refreshing some of my java skills.

Written by Kristian Lunde

May 5th, 2008 at 10:14 pm

Java on Ubuntu

without comments

The other day I decided that it was about time to refresh my java knowledge. I’ve done some java development at work lately, but besides that It has been about 3 years since I last wrote any decent java apps. I have a few ideas of some small cool apps I want to write, but first of all I had to check the java installation on my machine.

Im currently sitting on a Ubuntu 7.04 installation, so I started out by opening my console and typing

java -version

I was a bit surprised about the result, java version could not be determined, I found out that 4 different versions of java were installed and neither the classpath or java_home were set. The java version I wished to used were not among the installed versions. I used the java installation guide on to install my preferred java version, the java 6 version from Sun. The guide was straight forward and the java installation seemed to work as expected after completing the guide.

kristian@Saturn:/usr/lib$ java -version
java version "1.6.0_03"
Java(TM) SE Runtime Environment (build 1.6.0_03-b05)
Java HotSpot(TM) Client VM (build 1.6.0_03-b05, mixed mode, sharing)

Looking good. So I tried typing:

kristian@Saturn:/usr/lib$ javac -version
javac: error while loading shared libraries: cannot open shared object file: No such file or directory

Not looking that good, something was wrong :P I googled the result and found my mistake. Earlier while I was trying to get it all together with the 4 different java versions I changed the symlink in /usr/bin/ and I forgot to add the -s when I created the link. Removing the symblink and recreating it did the trick.

kristian@Saturn:/usr/bin$ sudo ln -s /etc/alternatives/javac javac

The javac in /etc/alternatives is a symblink to the current java version. And trying the all famous javac -version now result in the anticipated result:

kristian@Saturn:~$ javac -version
javac 1.6.0_03

Now I’m all set to go and try out my java skills :P

Written by Kristian Lunde

May 2nd, 2008 at 10:04 am

Posted in Java,Linux,Programming

Tagged with ,

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