[Oracle AS, Datasource Pool]: java.lang.NullPointerException (Soap)

Question

I found below error on my applications that are on Oracle AS (data source pool). After I killed some processes on Oracle Database.
<SOAP-ENV:Fault>
<faultcode>SOAP-ENV:Server.Exception:(/faultcode>
<faultstring>java.lang.NullPointerException</faultstring>
<faultactor>/BBContent/BBWebService</faultactor>
</SOAP-ENV:Fault>

Answer 1

1 . I created Soap Application by use
/oracle/product/10.1.3/OracleAS/jdk/bin/java -jar $ORACLE_HOME/webservices/lib/WebServicesAssembler.jar -config config.xml
and deploy from ear file to AS
2. I edit data-sources.xml
<?xml version = '1.0' encoding = 'UTF-8'?>
<data-sources>
<managed-data-source login-timeout="1800" connection-pool-name="PoolVCMGR-COW2" jndi-name="jdbc/OracleDSw4SAP" name="DataSourcew4SAP"/>
<connection-pool name="PoolVCMGR-COW2">
<connection-factory factory-class="oracle.jdbc.pool.OracleDataSource" user="user" password="password" url="jdbc:oracle:thin:@//127.0.0.1:1521/DB"/>
</connection-pool>
</data-sources>
When I killed session on Database or restarted Database.
My application can't work. when we debug, we found
<SOAP-ENV:Fault>
<faultcode>SOAP-ENV:Server.Exception:(/faultcode>
<faultstring>java.lang.NullPointerException</faultstring>
<faultactor>/BBContent/BBWebService</faultactor>
</SOAP-ENV:Fault>
So We restarted this serverics, that worked.
I don't known why we kill session on Database or restart Database.
Our Application seem, that can't communicate Database.
Help me, How can we resolve this problem?