Oracle8i JDBC Developer's Guide and Reference Release 3 (8.1.7) Part Number A83724-01 |
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## JDBC in the Server: the Server-Side Internal Driver

This section covers the following topics:

- Connecting to the Database with the Server-Side Internal Driver

- Exception-Handling Extensions for the Server-Side Internal Driver

- Session and Transaction Context for the Server-Side Internal Driver

- Testing JDBC on the Server

- Server-Side Character Set Conversion of oracle.sql.CHAR Data

Any Java program, Enterprise JavaBean (EJB), or Java stored procedure that runs inside the target database must use the server-side internal driver to access the local SQL engine.

This driver is intrinsically tied to the Oracle8i database and to the Java virtual machine (JVM). The driver runs as part of the same process as the database. It also runs within the default session--the same session in which the JVM was invoked.

The server-side internal driver is optimized to run within the database server and provide direct access to SQL data and PL/SQL subprograms on the local database. The entire JVM operates in the same address space as the database and the SQL engine. Access to the SQL engine is a function call; there is no network. This enhances the performance of your JDBC programs and is much faster than executing a remote Net8 call to access the SQL engine.

The server-side internal driver supports the same features, APIs, and Oracle extensions as the client-side drivers. This makes application partitioning very straightforward. For example, if you have a Java application that is data-intensive, you can easily move it into the database server for better performance, without having to modify the application-specific calls.

For general information about the Oracle Java platform server-side configuration or functionality, see the Oracle8i Java Developer's Guide.

### Connecting to the Database with the Server-Side Internal Driver

As described in the preceding section, the server-side internal driver runs within a default session. You are already "connected". There are two methods you can use to access the default connection:

- Use the static `DriverManager.getConnection()` method, with either `jdbc:oracle:kprb` or `jdbc:default:connection` as the URL string.

- Use the Oracle-specific `defaultConnection()` method of the `OracleDriver` class.

Using `defaultConnection()` is generally recommended.

The remainder of this section provides more information.

Note: With Oracle8i release 8.1.6 and higher, you are no longer required to register the `OracleDriver` class for connecting with the server-side internal driver, although there is no harm in doing so. This is true whether you are using `getConnection()` or `defaultConnection()` to make the connection.

#### Connecting with the OracleDriver Class defaultConnection() Method

The `oracle.jdbc.driver.OracleDriver` class `defaultConnection()` method is an Oracle extension and always returns the same connection object. Even if you invoke this method multiple times, assigning the resulting connection object to different variable names, just a single connection object is reused.

You do not need to include a connect string in the `defaultConnection()` call. For example:

    import java.sql.*;
    import oracle.jdbc.driver.*;

    class JDBCConnection
    {
      public static Connection connect() throws SQLException
      {
        Connection conn = null;
        try {
          // connect with the server-side internal driver
             OracleDriver ora = new OracleDriver();
             conn = ora.defaultConnection();
          }

        } catch (SQLException e) {...}
        return conn;
      }
    }

Note that there is no `conn.close()` call in the example. When JDBC code is running inside the target server, the connection is an implicit data channel, not an explicit connection instance as from a client. It should typically not be closed.

If you do call the `close()` method, be aware of the following:

- All connection instances obtained through the `defaultConnection()` method, which actually all reference the same connection object, will be closed and unavailable for further use, with state and resource cleanup as appropriate. Executing `defaultConnection()` afterward would result in a new connection object.

- Even though the connection object is closed, the implicit connection to the database will not be closed.

#### Connecting with the DriverManager.getConnection() Method

To connect to the internal server connection from code that is running within the target server, you can use the static `DriverManager.getConnection()` method with either of the following connect strings:

    DriverManager.getConnection("jdbc:oracle:kprb:");

or:

    DriverManager.getConnection("jdbc:default:connection:");

Any user name or password you include in the URL string is ignored in connecting to the server default connection.

The `DriverManager.getConnection()` method returns a new Java `Connection` object every time you call it. Note that although the method is not creating a new physical connection (only a single implicit connection is used), it is returning a new object.

The fact that `DriverManager.getConnection()` returns a new connection object every time you call it is significant if you are working with object maps (or "type maps"). A type map is associated with a specific `Connection` object and with any state that is part of the object. If you want to use multiple type maps as part of your program, then you can call `getConnection()` to create a new `Connection` object for each type map.

### Exception-Handling Extensions for the Server-Side Internal Driver

The server-side internal driver, in addition to having standard exception-handling capabilities such as `getMessage()`, `getErrorCode()`, and `getSQLState()` (as described in "Processing SQL Exceptions"), offers extended features through the `oracle.jdbc.driver.OracleSQLException` class. This class is a subclass of the standard `java.sql.SQLException` class and is not available to the client-side JDBC drivers or the server-side Thin driver.

When an error condition occurs in the server, it often results in a series of related errors being placed in an internal error stack. The JDBC server-side internal driver retrieves errors from the stack and places them in a chain of `OracleSQLException` objects.

You can use the following methods in processing these exceptions:

- `SQLException getNextException()` (standard method)

  This method returns the next exception in the chain (or `null` if no further exceptions). You can start with the first exception you receive and work through the chain.

- `int getNumParameters()` (Oracle extension)

  Errors from the server usually include parameters, or variables, that are part of the error message. These may indicate what type of error occurred, what kind of operation was being attempted, or the invalid or affected values.

  This method returns the number of parameters included with this error.

- `Object[] getParameters()` (Oracle extension)

  This method returns a Java `Object[]` array containing the parameters included with this error.

##### Example

Following is an example of server-side error processing:

    try
    {
       // should get "ORA-942: table or view does not exist"
       stmt.execute("drop table no_such_table");
    }
    catch (OracleSQLException e)
    {
       System.out.println(e.getMessage());
       // prints "ORA-942: table or view does not exist"

       System.out.println(e.getNumParameters());
       // prints "1"

       Object[] params = e.getParameters();
       System.out.println(params[0]);
       // prints "NO_SUCH_TABLE"
    }

### Session and Transaction Context for the Server-Side Internal Driver

The server-side driver operates within a default session and default transaction context. The default session is the session in which the JVM was invoked. In effect, you are already connected to the database on the server. This is different from the client side where there is no default session: you must explicitly connect to the database.

Auto-commit mode is disabled in the server. You must manage transaction `COMMIT` and `ROLLBACK` operations explicitly by using the appropriate methods on the connection object:

    conn.commit();

or:

    conn.rollback();

### Testing JDBC on the Server

Almost any JDBC program that can run on a client can also run on the server. All the programs in the `samples` directory can be run on the server with only minor modifications. Usually, these modifications concern only the connection statement.

For example, consider the test program `JdbcCheckup.java` described in "Testing JDBC and the Database Connection: JdbcCheckup". If you want to run this program on the server and connect with the `DriverManager.getConnection()` method, then open the file in your favorite text editor and change the driver name in the connection string from "`oci8`" to "`kprb`". For example:

    Connection conn = DriverManager.getConnection
                      ("jdbc:oracle:kprb:@" + database, user, password);

The advantage of using this method is that must change only a short string in your original program. The disadvantage is that you still must provide the user, password, and database information, even though the driver will discard it. In addition, if you issue the `getConnection()` method again, the driver will create another new (and unnecessary) connection object.

However, if you connect with `defaultConnection()`, the preferred method of connecting to the database from the server-side internal driver, you do not have to enter any user, password, or database information. You can delete these statements from your program.

For the connection statement, use:

    Connection conn =  new oracle.jdbc.driver.OracleDriver().defaultConnection();

The following example is a rewrite of the `JdbcCheckup.java` program which uses the `defaultConnection()` connection statement. The connection statement is printed in bold. The unnecessary user, password, and database information statements, along with the utility function to read from standard input, have been deleted.

    /*
     * This sample can be used to check the JDBC installation.
     * Just run it and provide the connect information.  It will select
     * "Hello World" from the database.
     */
    // You need to import the java.sql package to use JDBC
    import java.sql.*;
    // We import java.io to be able to read from the command line
    import java.io.*;

    class JdbcCheckup
    {
       public static void main (String args []) throws SQLException, IOException
       {
          // Load the Oracle JDBC driver
          DriverManager.registerDriver(new oracle.jdbc.driver.OracleDriver());

          Connection conn =
               new oracle.jdbc.driver.OracleDriver ().defaultConnection ();

           // Create a statement
           Statement stmt = conn.createStatement ();

           // Do the SQL "Hello World" thing
           ResultSet rset = stmt.executeQuery ("SELECT 'Hello World' FROM dual");

           while (rset.next ())
              System.out.println (rset.getString (1));
           System.out.println ("Your JDBC installation is correct.");
       }
    }

### Loading an Application into the Server

When loading an application into the server, you can load `.class` files that you have already compiled on the client, or you can load `.java` source files and have them compiled automatically in the server.

In either case, use the Oracle `loadjava` client-side utility to load your files. You can either specify source file names on the command line (note that the command line understands wildcards), or put the files into a JAR file and specify the JAR file name on the command line. The `loadjava` utility is discussed in detail in the Oracle8i Java Developer's Guide.

The `loadjava` script, which runs the actual utility, is in the `bin` subdirectory under your `[Oracle Home]` directory. This directory should already be in your path once Oracle has been installed.

Note: As of Oracle8i release 8.1.6, the `loadjava` utility does support compressed files.

#### Loading Class Files into the Server

Consider a case where you have three class files in your application: `Foo1.class`, `Foo2.class`, and `Foo3.class`. The following three examples demonstrate: 1) specifying the individual class file names; 2) specifying the class file names using a wildcard; and 3) specifying a JAR file that contains the class files.

Each class is written into its own class schema object in the server.

These three examples use the default OCI8 driver in loading the files:

    loadjava -user scott/tiger Foo1.class Foo2.class Foo3.class

or:

    loadjava -user scott/tiger Foo*.class

or:

    loadjava -user scott/tiger Foo.jar

Or use the following command to load with the Thin driver (specifying the `-thin` option and an appropriate URL):

    loadjava -thin -user scott/tiger@localhost:1521:ORCL Foo.jar

(Whether to use an OCI driver or the Thin driver to load classes depends on your particular environment and which performs better for you.)

Note: Because the server-side embedded JVM uses JDK 1.2.x, it is advisable to compile classes under JDK 1.2.x if they will be loaded into the server. This will catch incompatibilities during compilation, instead of at runtime (for example, JDK 1.1.x artifacts such as leftover use of the `oracle.jdbc2` package).

#### Loading Source Files into the Server

If you enable the `loadjava -resolve` option in loading a `.java` source file, then the server-side compiler will compile your application as it is loaded, resulting in both a source schema object for the original source code, and one or more class schema objects for the compiled output.

If you do not specify `-resolve`, then the source is loaded into a source schema object without any compilation. In this case, however, the source is implicitly compiled the first time an attempt is made to use a class defined in the source.

For example, run `loadjava` as follows to load and compile `Foo.java`, using the default OCI driver:

    loadjava -user scott/tiger -resolve Foo.java

Or use the following command to load with the Thin driver (specifying the `-thin` option and an appropriate URL):

    loadjava -thin -user scott/tiger@localhost:1521:ORCL -resolve Foo.java

Either of these will result in appropriate class schema objects being created in addition to the source schema object.

Note: Oracle generally recommends compiling source on the client whenever possible, and loading the `.class` files instead of the source files into the server.

### Server-Side Character Set Conversion of oracle.sql.CHAR Data

The server-side internal driver performs character set conversions for `oracle.sql.CHAR` in C. This is a different implementation than for the client-side drivers, which perform character set conversions for `oracle.sql.CHAR` in Java, and offers better performance. For more information on the `oracle.sql.CHAR` class, see "Class oracle.sql.CHAR".

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