ORACLE : Io exception: The Network Adapter could not establish the connection

Not connecting to correct Database in oracle :

jdbc:oracle:thin:@<server_host>:1521:<instance_name>

the following commands will help:

1. Oracle query command to check the SID (or instance name):

select sys_context('userenv','instance_name') from dual; 


2. Oracle query command to check database name (or server host):

select sys_context('userenv', 'server_host') from dual;