Solution to ORA-12520 errors

ORA-12520 listeners cannot find available handlers for the requested server type
Reference others' articles:

1) The database is a dedicated server, but the connection mode is set to shared in the tnsname. ora configuration. In this case, enable tnsname. ora,

Change (server = shared) to (server = dedicate)

2) It is caused by insufficient process.

Later, it was found that v $ process had risen to more than 140, and my database set 150. Based on this, we can conclude that process is not enough,

Use the following statement to modify the processes value of the database:

Alter system set processes = 500 scope = spfile (because processes is a static parameter, scope can only be spfile,

In the v $ sgastate condition of 'procesees', we can see that each process will be 4 bytes ),
Restart the database to make the new processes take effect. Then re-run eclipse, you can start it normally.
Note: If you choose c3p0 for hibernate. connection. provider_class in hibernate, this will happen, and others will not.

It is strange that c3p0 will open so many processs. In the shared connection mode, shared_server is the process value that has been connected to the database. Pay attention to this.

Current number of connections

select count(*) from v$process; 

select value from v$parameter where name = 'processes'; 

alter system set processes = 500 scope = spfile; 

As for the second check, the number of connections is indeed high. The maximum number of connections I set is 500, and the current number of connections has reached more than 490. So it must be this problem.

Instead of modifying the maximum number of connections, I restarted the current number of connections in oracle and suddenly returned to two digits. Problem Solving

Remember to restart oracle after modifying the maximum number of connections