Write access, principles

Connecting an application to a database by establishing a connection between client and database server:

Figure 822. Networking between clients and database server Slide presentation
Networking between clients and database server

Figure 823. JDBC features Slide presentation
  • Protocol connecting database client and server.

  • Vendor dependent implementations.


So JDBC™ is just one among a whole bunch of protocol implementations connecting database servers and applications. Consequently JDBC™ is expected to appear in the lower layer of multi-tier applications. We take a three-tier application as a starting point:

Figure 824. JDBC™ in a three-tier application Slide presentation
JDBC™ in a three-tier application

We may add an additional layer. Web applications are typically being build on top of an application server (WebSphere, Glassfish, Jboss,...) providing additional services:

Figure 825. JDBC™ connecting application server and database. Slide presentation
JDBC™ connecting application server and database.

Opening a connection to a database server requires:

Figure 826. JDBC connection parameter Slide presentation
  1. Database server type i.e. Oracle, DB2, Informix, Postgresql, Mysql etc. due to vendor specific JDBC™ protocol implementations.

  2. Server DNS name or IP number.

  3. Server service's port number.

  4. The database name within the given server.

  5. Optional: A database user's account name and password.


Items 1 - 4 will be encapsulated into a so called JDBCURL. We consider a typical example corresponding to the previous parameter list:

Figure 827. Components of a JDBC™ URL Slide presentation
Components of a JDBC™ URL

Figure 828. IETF Uniform Resource Identifier Slide presentation

https://www.ietf.org/rfc/rfc2396.txt:

absoluteURI   = scheme ":" ( hier_part | opaque_part )

hier_part     = ( net_path | abs_path ) [ "?" query ]

net_path      = "//" authority [ abs_path ]

abs_path      = "/"  path_segments
...

Figure 829. URL examples Slide presentation
  • http://www.hdm-stuttgart.de/aaa

  • http://someserver.com:8080/someResource

    Non-standard port 8080

  • ftp://mirror.mi.hdm-stuttgart.de/Firmen


Figure 830. Sub protocol examples Slide presentation
Database JDBC URI
PostgreSQL jdbc:postgresql://<HOST>:<PORT>/[database]
MySQL jdbc:mysql://[host][:port]/[database][?p1=v1]...
Oracle jdbc:oracle:thin:[user/password]@[host][:port]:SID
DB2 jdbc:db2://<HOST>:<PORT>/[database]
Derby jdbc:derby://[host][:port]/[database]
MS. SQL S. jdbc:sqlserver://host[:port];user=xxx;password=xyz
Sybase jdbc:sybase:Tds:<HOST>:<PORT>/[database]

Figure 831. No standard port assignments ... Slide presentation
Postgresql:

5432

IBM DB2:

50000

Oracle:

1521

  • No official IETF standard port assignments

  • Vendor specific defaults

  • Explicit port specification required


Figure 832. ... but Postgresql made it into Linux Slide presentation
>grep postgresql /etc/services 
postgresql      5432/tcp       postgres      # PostgreSQL Database
postgresql      5432/udp       postgres

Writing JDBC™ based applications follows a simple scheme:

Figure 833. JDBC architecture Slide presentation
JDBC™ architecture

From a programmer's point of view the java.sql.DriverManager is a bootstrapping object: Other objects like Connection instances are being created from this central and unique object.

Figure 834. DriverManager: Bootstrapping connections Slide presentation

Figure 835. Example: Mysql connection implementation Slide presentation

Figure 836. Driver libraries Slide presentation
  • postgresql-42.1.4.jar

  • mysql-connector-java-x.y.z.jar

  • ojdbc6.jar


Figure 837. Driver libraries by Maven Slide presentation
  • <groupId>postgresql</groupId>
    <artifactId>postgresql</artifactId>
    <version>9.1-901-1.jdbc4</version>
  • <groupId>com.oracle</groupId>  <!-- requires access credentials -->
    <artifactId>ojdbc7</artifactId>
    <version>12.1.0</version>

Figure 838. Driver unavailable Slide presentation
  • conn = DriverManager.getConnection(
     "jdbc:postgresqll://localhost/hdm", "hdmuser", "XYZ");
  • java.sql.SQLException: No suitable driver found for
     jdbc:postgresqll://localhost/hdm
    
      at java.sql.DriverManager.getConnection(DriverManager.java:689)
      at java.sql.DriverManager.getConnection(DriverManager.java:247)
      at de.hdm_stuttgart.mi.sda1.DatabaseTest.initDatabase(DatabaseTest.java:34)
     ...

Figure 833, “JDBC™ architecture ” does not show details about the relations between java.sql.Connection, java.sql.Statement and java.sql.ResultSet objects. We start by giving a rough description of these three interfaces' tasks and responsibilities:

Figure 839. Connection interface Slide presentation
java.sql.Connection

Figure 840. Statement interface Slide presentation
java.sql.Statement

Two distinct operation classes:

executeUpdate()

INSERT, UPDATE, DELETE: Integer return code

executeQuery()

SELECT: Returning java.sql.ResultSet, see the section called “Read Access”.


Figure 841. JDBC™ instances and relationships. Slide presentation
JDBC™ instances and relationships.


Figure 843. Important Statement methods Slide presentation

Figure 844. JDBC and threading. Slide presentation

From JDBC and Multithreading:

Because all Oracle JDBC API methods are synchronized, if two threads try to use the connection object simultaneously, then one will be forced to wait until the other one finishes its use.

Consequence:


Figure 845. JDBC connection pooling Slide presentation
try (final Connection conn =
       C3P0DataSource.getInstance().getConnection()) {

  final PreparedStatement pstmt = conn.create...;
    ...
  pstmt.executeUpdate();
  // Auto close connection, back to pool.
} catch (SQLException e) {
  e.printStackTrace();
}