This is most common exception when connecting with database through Spring boot application. It’s happen because of connection failed to the database.

com.microsoft.sqlserver.jdbc.SQLServerException: The TCP/IP connection to the host ABCKXYZ356, port 1345 has failed. Error: "ABCKXYZ356. 
Verify the connection properties. Make sure that an instance of SQL Server is running on the host and accepting TCP/IP connections at the port. 
Make sure that TCP connections to the port are not blocked by a firewall.".    
at com.microsoft.sqlserver.jdbc.SQLServerException.makeFromDriverError(SQLServerException.java:234) ~[mssql-jdbc-8.2.1.jre8.jar:na]    
at com.microsoft.sqlserver.jdbc.SQLServerException.ConvertConnectExceptionToSQLServerException(SQLServerException.java:285) ~[mssql-jdbc-8.2.1.jre8.jar:na]    
at com.microsoft.sqlserver.jdbc.SocketFinder.findSocket(IOBuffer.java:2434) ~[mssql-jdbc-8.2.1.jre8.jar:na]    
at com.microsoft.sqlserver.jdbc.TDSChannel.open(IOBuffer.java:659) ~[mssql-jdbc-8.2.1.jre8.jar:na]    
at com.microsoft.sqlserver.jdbc.SQLServerConnection.connectHelper(SQLServerConnection.java:2546) ~[mssql-jdbc-8.2.1.jre8.jar:na]    
at com.microsoft.sqlserver.jdbc.SQLServerConnection.login(SQLServerConnection.java:2216) ~[mssql-jdbc-8.2.1.jre8.jar:na]    
at com.microsoft.sqlserver.jdbc.SQLServerConnection.connectInternal(SQLServerConnection.java:2067) ~[mssql-jdbc-8.2.1.jre8.jar:na]    
at com.microsoft.sqlserver.jdbc.SQLServerConnection.connect(SQLServerConnection.java:1204) ~[mssql-jdbc-8.2.1.jre8.jar:na]    
at com.microsoft.sqlserver.jdbc.SQLServerDriver.connect(SQLServerDriver.java:825) ~[mssql-jdbc-8.2.1.jre8.jar:na]    
at com.zaxxer.hikari.util.DriverDataSource.getConnection(DriverDataSource.java:138) ~[HikariCP-4.0.3.jar:na]    
at com.zaxxer.hikari.pool.PoolBase.newConnection(PoolBase.java:364) ~[HikariCP-4.0.3.jar:na]    
at com.zaxxer.hikari.pool.PoolBase.newPoolEntry(PoolBase.java:206) ~[HikariCP-4.0.3.jar:na]    
at com.zaxxer.hikari.pool.HikariPool.createPoolEntry(HikariPool.java:476) ~[HikariCP-4.0.3.jar:na]    
at com.zaxxer.hikari.pool.HikariPool.checkFailFast(HikariPool.java:561) ~[HikariCP-4.0.3.jar:na]    
at com.zaxxer.hikari.pool.HikariPool.<init>(HikariPool.java:115) ~[HikariCP-4.0.3.jar:na]

Reason of Exception

These are main reason of this Exception:

• This issue can occurred because of wrong properties configured for database connection.
• This issue can also occurred if the database is not running.
• This issue can also occurred if database only allow to access through vpn.

Solutions

These are some most common solution to resolve this issue:

• Verify the connection properties.
• Make sure that an instance of SQL Server is running on the host and accepting TCP/IP connections at the port.
• Make sure that TCP connections to the port are not blocked by a firewall.
• Check the vpn connection if DB access allow over the vpn.