Why localhost\SQLEXPRESS Works in .NET

"Protocol Differences: .NET vs JDBC .NET (SqlClient / SSMS) Uses a protocol fallback order when you type localhost\SQLEXPRESS: Shared Memory Named Pipes TCP/IP That means even if TCP/IP is disabled or the SQL Browser service is off, SSMS can still connect locally via Shared Memory."

"JDBC (Microsoft JDBC Driver for SQL Server) Only supports TCP/IP. Named instances like SQLEXPRESS are resolved via the SQL Server Browser service on UDP port 1434. If the Browser is off, JDBC cannot figure out which port the instance is listening on. 👉 That's why the same connection string succeeds in .NET but times out in JDBC."

"Correct JDBC Connection Strings Instead of localhost\SQLEXPRESS, use one of these approaches: A) Use instanceName (requires SQL Server Browser) jdbc:sqlserver://localhost;instanceName=SQLEXPRESS;databaseName=myDb;encrypt=true;trustServerCertificate=true Pros: Looks familiar, works if Browser service is running. Cons: Breaks if Browser is stopped. B) Use a fixed port (recommended for backends) Enable TCP/IP for your instance (SQL Server Configuration Manager → Protocols for SQLEXPRESS). Either find the dynamic port under IPAll → TCP Dynamic Ports or assign a static port (e.g., 1433). Restart the SQL service. Use: jdbc:sqlserver://localhost:1433;databaseName=myDb;encrypt=true;trustServerCertificate=true Pros: No dependency on SQL Browser. Cons: You must manage the port configuration yourself."

JDBC drivers support only TCP/IP and rely on the SQL Server Browser to resolve named instances like SQLEXPRESS. .NET clients use a protocol fallback (Shared Memory, Named Pipes, TCP/IP) allowing local connections even when TCP/IP or the Browser service is disabled. Use jdbc:sqlserver://localhost;instanceName=SQLEXPRESS;... if the Browser runs, or prefer a fixed TCP port for backend services by enabling TCP/IP, assigning a static port, restarting the SQL service, and using jdbc:sqlserver://host:port;databaseName=...;encrypt=true;trustServerCertificate=true. A fixed port removes dependency on the Browser service.

Read at Medium