ORA-12514, TNS:listener does not currently know of service requested in connect descriptor

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Applies to:
Weblogic: 10.3.6
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Problem Summary:
Data Source connection test is failing with ORA-12514, TNS:listener does not currently know of service requested in connect descriptor

Error:
Connection test failed with the following exception: weblogic.common.resourcepool.ResourceDeadException: 0:weblogic.common.ResourceException: Listener refused the connection with the following error: ORA-12514, TNS:listener does not currently know of service requested in connect descriptor

<BEA-001156> <Stack trace associated with message 001129 follows:
java.sql.SQLException: Listener refused the connection with the following error:
ORA-12514, TNS:listener does not currently know of service requested in connect descriptor
        at oracle.jdbc.driver.T4CConnection.logon(T4CConnection.java:419)
        at oracle.jdbc.driver.PhysicalConnection.<init>(PhysicalConnection.java:538)
        at oracle.jdbc.driver.T4CConnection.<init>(T4CConnection.java:228)
        at oracle.jdbc.driver.T4CDriverExtension.getConnection(T4CDriverExtension.java:32)
        at oracle.jdbc.driver.OracleDriver.__AW_connect(OracleDriver.java:521)
        at oracle.jdbc.driver.OracleDriver.connect(OracleDriver.java)
......................
Caused By: oracle.net.ns.NetException: Listener refused the connection with the following error:
ORA-12514, TNS:listener does not currently know of service requested in connect descriptor
        at oracle.net.ns.NSProtocol.__AW_connect(NSProtocol.java:394)
        at oracle.net.ns.NSProtocol.connect(NSProtocol.java)
        at oracle.jdbc.driver.T4CConnection.connect(T4CConnection.java:1056)
        at oracle.jdbc.driver.T4CConnection.logon(T4CConnection.java:308)
        at oracle.jdbc.driver.PhysicalConnection.<init>(PhysicalConnection.java:538)

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Root Cause:
Database is under maintenance or in inconsistent state. Exception clearly shows that connection requested by weblogic failed to make connectivity with DB.

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Resolution:
1. Make sure Database is up and running and able to accept connections. listener should be listening.
2. Use below command on DB to verify listener status,
    SQL>lsnrctl status
3. We can use SQL developer to verify DB state. Create a new connection and see if you are able to connect to it.
4. Once DB is fine you won't face this issue anymore.