When working with Oracle databases in Java applications, selecting the correct JDBC driver version is crucial for compatibility and performance. The version numbering follows Java's JDBC API versions:

// Legacy drivers
classes12.jar - JDBC 2.0 (Java 1.2/1.3)
ojdbc14.jar - JDBC 3.0 (Java 1.4/1.5)

// Modern drivers  
ojdbc5.jar - JDBC 3.0 + Java 5 features
ojdbc6.jar - JDBC 4.0 (Java 6+)

When upgrading from Java 1.4 to Java 1.6 (as in JRun migration), note these technical specifics:

• ojdbc14.jar: While technically works in Java 6 due to backward compatibility, lacks:
• JDBC 4.0 features (e.g., automatic driver loading)
• Java 5+ generics support
• ojdbc6.jar advantages:
• Native support for JDBC 4.0 API
• Optimized for Java 6 JVM
• Additional Oracle-specific features

Here's how connection handling differs between versions:

// ojdbc14.jar (Java 1.4 style)
Class.forName("oracle.jdbc.driver.OracleDriver");
Connection conn = DriverManager.getConnection(
    "jdbc:oracle:thin:@localhost:1521:ORCL", 
    "user", "password");

// ojdbc6.jar (Java 6+ with auto-loading)
Connection conn = DriverManager.getConnection(
    "jdbc:oracle:thin:@localhost:1521/ORCL", 
    "user", "password"); // No Class.forName needed

Benchmark tests show significant improvements:

For your JRun/Java 6 migration:

1. Test ojdbc6.jar first - it's the officially supported version
2. If encountering compatibility issues with legacy code:
• Try ojdbc5.jar as interim solution
• Gradually refactor to ojdbc6
3. Never use classes12.jar in Java 6 environment

When upgrading Java environments, the Oracle JDBC driver versions often cause confusion. Here's the technical breakdown:

// Historical Oracle JDBC drivers timeline:
classes12.jar - JDBC 2.0 compliant (Java 1.2-1.3)
ojdbc14.jar - JDBC 3.0 compliant (Java 1.4-1.5)
ojdbc5.jar - JDBC 3.0/4.0 features (Java 5)
ojdbc6.jar - Full JDBC 4.0 support (Java 6+)

The version numbering indicates the minimum Java version required, not maximum compatibility. For example:

• ojdbc14.jar works with Java 6 but lacks JDBC 4.0 features
• ojdbc6.jar won't work with Java 1.4 as it requires Java 6 features
• classes12.jar should be avoided as it's deprecated

When moving from Java 1.4 to 1.6:

// Sample connection code with ojdbc6.jar
import java.sql.*;
import oracle.jdbc.OracleDriver;

public class OracleTest {
    public static void main(String[] args) {
        try {
            // Java 6 auto-loading driver feature
            Connection conn = DriverManager.getConnection(
                "jdbc:oracle:thin:@localhost:1521:ORCL",
                "username",
                "password");
            
            // JDBC 4.0 specific features
            DatabaseMetaData meta = conn.getMetaData();
            System.out.println("JDBC Driver Version: " + meta.getDriverVersion());
            
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

Newer drivers offer significant improvements:

When mixing versions:

// Error you might see with wrong driver:
java.lang.UnsupportedClassVersionError: 
oracle/jdbc/OracleDriver : Unsupported major.minor version 50.0
    at java.lang.ClassLoader.defineClass1(Native Method)
    
// Solution: Ensure your Java version matches the driver requirements

For production environments, always use the newest driver version that your Java version supports. The ojdbc6.jar is recommended for Java 6 environments as it provides full compatibility and access to modern JDBC features.