The document discusses exception handling in Scala code. It provides examples of the original code with try/catch blocks, a modified version that handles specific exception types in catch, and an improved approach that centralizes exception handling. The improved approach throws a custom CandAllExceptions for any exception in the catch block, allowing exceptions to be handled in one place rather than throughout the code. It also discusses best practices like using ProcessLogger instead of Process to catch process exceptions, creating custom exceptions with messages and causes, and annotating methods with @throws only if calling from Java.

1. CAND NAG ARVIND GUDISEVA 1
EXCEPTION HANDLING IN SCALA
JAVA WAY OF EXCEPTION HANDLING IN SCALA
TRY / CATCH / FINALLY
Onlyone catch blockis neededandcasesare usedto handle individual exceptions
PRESENT CODE
def oracleSample(oraConnWeb: Connection): Unit = {
logger.info(" :: Start Def :: oracleSample :: ")
try {
val oraStmt = oraConnWeb.createStatement()
var sqlSelect = """select * from emp"""
logger.info("SQL: " + sqlSelect)
val rs = oraStmt.executeQuery(sqlSelect)
while (rs.next){
println(rs.getInt(1) + " " + rs.getString(2) + " " + rs.getString(3))
}
oraStmt.close()
}
catch {
case e: Throwable => e.printStackTrace()
}
logger.info(" :: End Def :: oracleSample :: ")
}

2. CAND NAG ARVIND GUDISEVA 2
MODIFIED CODE
def oracleSample(oraConnWeb: Connection): Unit = {
logger.info(" :: Start Def :: oracleSample :: ")
try {
val oraStmt = oraConnWeb.createStatement()
var sqlSelect = """select * from emp"""
logger.info("SQL: " + sqlSelect)
val rs = oraStmt.executeQuery(sqlSelect)
while (rs.next){
println(rs.getInt(1) + " " + rs.getString(2) + " " + rs.getString(3))
}
oraStmt.close()
}
catch {
case ex: java.sql.SQLException => throw new java.sql.SQLException("SQLException: " +
ex.printStackTrace())
case ex: oracle.net.ns.NetException => throw new
oracle.net.ns.NetException(1,ex.printStackTrace().toString)
case ex: java.net.UnknownHostException => throw new
java.net.UnknownHostException("UnknownHostException: " + ex.printStackTrace())
case ex: java.sql.SQLSyntaxErrorException => throw new
java.sql.SQLSyntaxErrorException("SQLSyntaxErrorException: " + ex.printStackTrace())
case ex: Exception => throw new Exception("Exception: " + ex.printStackTrace())
}
logger.info(" :: End Def :: oracleSample :: ")
}
IMPROVISED CODE
def oracleSample(oraConnWeb: Connection): Unit = {
logger.info(" :: Start Def :: oracleSample :: ")
try {

3. CAND NAG ARVIND GUDISEVA 3
val oraStmt = oraConnWeb.createStatement()
var sqlSelect = """select * from emp1"""
logger.info("SQL: " + sqlSelect)
val rs = oraStmt.executeQuery(sqlSelect)
while (rs.next){
println(rs.getInt(1) + " " + rs.getString(2) + " " + rs.getString(3))
}
oraStmt.close()
}
catch {
case ex: Exception => throw new CandAllExceptions(ex)
}
finally {
// Nothing
}
logger.info(" :: End Def :: oracleSample :: ")
}
WHAT IS THE PROBLEM?
Scala hasomittedCheckedExceptions. These exceptionsviolate SOLID –Open/ Close principle,
because we can’tthrow a newexceptioninasubclasswithoutpropagatingitaroundtoall the
existingclassesinthe hierarchy,aswell as methodscallingthose methods.
Scala’ssupportfor CheckedExceptionsisforcompatibilitywithJavacode. If we don’tintendtocall
a Scala code from Javacode,we shouldnotbe usingCheckedExceptions.
THROW NEW EXCEPTION
Throwingthe exceptionback tothe callingfunctionandhandlingthe exceptionsstackina
centralizedfashion.
PRESENT CODE
private def executeShell(shellStr: String) = {
logger.info(" :: Start Def :: executeShell :: ")
val shellCommand: String = shellStr
logger.info("Shell Command: " + shellCommand)
val sqoopProcess = Process(shellCommand).!!
logger.info("Shell Process: " + sqoopProcess)
logger.info(" :: End Def :: executeShell :: ")
}
IMPROVISED CODE

4. CAND NAG ARVIND GUDISEVA 4
private def executeShell(shellStr: String) = {
logger.info(" :: Start Def :: executeShell :: ")
try {
val shellCommand: String = shellStr
logger.info("Shell Command: " + shellCommand)
val stdOutput = new StringBuilder
val stdError = new StringBuilder
val processStatus: Int = shellCommand ! ProcessLogger(stdOutput append _, stdError append
_)
logger.info("Status Code: " + processStatus)
logger.info("Standard Output: " + stdOutput)
logger.info("Standard Error: " + stdError)
if(processStatus != 0){
val customCause = new RuntimeException("Non-zero Exit Code: (" + processStatus + ")")
val customException = new RuntimeException(customCause)
throw new CandCustomException(customException.getMessage, customException.getCause)
}
}
catch {
case ex: CandCustomException => throw new CandCustomException("CandCustomException: " +
ex.getMessage)
case ex: Exception => throw new CandAllExceptions(ex)
}
finally {
// Nothing
}
logger.info(" :: End Def :: executeShell :: ")
}
WHAT IS THE PROBLEM?
Shell / Processexceptionswere nevercaughtbyScalacode. Itwas handledbyJVM,whichwas
runningthe process. Enhance the code to use ProcessLoggerinsteadof Process.
PROCESS SHELL EXCEPTIONS:
Caused by: java.lang.RuntimeException: Unable to instantiate
org.apache.hadoop.hive.ql.metadata.SessionHiveMetaStoreClient
java.io.IOException: Hive exited with status 1
USE PROCESSLOGGER INSTEAD OF PROCESS
INFO contrct.GenCustContract$: Status Code: 1
INFO contrct.GenCustContract$: Standard Output: Warning: Please set $ZOOKEEPER_HOME to the root of
your Zookeeper installation.
INFO contrct.GenCustContract$: Standard Error:
ERROR util.CandCustomException: Message: java.lang.RuntimeException: Non-zero Exit Code: (1)

5. CAND NAG ARVIND GUDISEVA 5
@THROWS ANNOTATION
Onlyuseful if Scalacode iscalledfromJavaProgram. Is notof muchvalue as we needto
encapsulate the callingfunctionwithatryand catch.
@throws(classOf[CandAllExceptions])
def testThrows (oraConnWeb: Connection) = {
logger.info(" :: Start Def :: testThrows :: ")
val i: Int = 1
val j: Int = 0
println("Result: " + i/j)
logger.info(" :: End Def :: testThrows :: ")
}
def callTest = {
try{
testThrows(OraDBCon.getNaiApplCon())
}
catch {
case ex: Exception => throw new CandAllExceptions(ex)
}
}
CUSTOM EXCEPTIONS
Everyexceptionhasamessage andcause. CustomExceptionare extendedfromRuntimeException.
Hence,message andcause needstobe populatedwhengeneratingaCustomException.
package com.cisco.cand.util
import grizzled.slf4j.Logger
class CandCustomException(message: String = null, cause: Throwable = null) extends
RuntimeException(CandCustomException.customMessage(message, cause), cause)
/**
* Created by Nag Arvind Gudiseva on 08-July-2016.
*/
object CandCustomException {
val logger = Logger(classOf[CandCustomException])
logger.info(" :: Start Object :: CandCustomException :: ")
def customMessage(message: String, cause: Throwable) = {
if (message != null) {
logger.error("Message: " + message)
message
}
else if (cause != null){
logger.error("Cause: " + cause.toString())
cause.toString()
}
else

6. CAND NAG ARVIND GUDISEVA 6
null
}
logger.info(" :: End Object :: CandCustomException :: ")
}
EXCEPTIONS STACK / CENTRALIZED HANDLING OF ALL EXCEPTIONS
The purpose of thisCentralizedExceptionhandlingistokeepthe catchclause of code cleanand
simple. Exceptiontype isrecognizedasperthe exceptioninstance andthe stacktrace isprinted.
package com.cisco.cand.util
import grizzled.slf4j.Logger
/**
* Created by Nag Arvind Gudiseva on 11-July-2016.
*/
object CandAllExceptions {
val logger = Logger(classOf[CandAllExceptions])
logger.info(" :: Start Object :: CandAllExceptions :: ")
class CandAllExceptions(exception: Exception) extends Exception {
if (exception.isInstanceOf[java.sql.SQLSyntaxErrorException]){
throw new java.sql.SQLSyntaxErrorException("SQLSyntaxErrorException: " +
exception.printStackTrace)
}
else if(exception.isInstanceOf[java.sql.SQLException]){
throw new java.sql.SQLException("SQLException: " + exception.printStackTrace)
}
else if (exception.isInstanceOf[oracle.net.ns.NetException]){
throw new oracle.net.ns.NetException(1,exception.printStackTrace.toString)
}
else if (exception.isInstanceOf[java.net.UnknownHostException]){
throw new java.net.UnknownHostException("UnknownHostException: " +
exception.printStackTrace)
}
else if (exception.isInstanceOf[org.apache.thrift.transport.TTransportException]){
throw new org.apache.thrift.transport.TTransportException("ThriftTransportException: " +
exception.printStackTrace)
}
else if (exception.isInstanceOf[java.lang.NullPointerException]){
throw new java.lang.NullPointerException("NullPointerException: " +
exception.printStackTrace)
}
else if (exception.isInstanceOf[java.io.IOException]){
throw new java.io.IOException("IOException: " + exception.printStackTrace)
}
else if (exception.isInstanceOf[java.lang.RuntimeException]){
throw new java.lang.RuntimeException("RuntimeException: " + exception.printStackTrace)
}
else if (exception.isInstanceOf[java.lang.ArithmeticException]){
throw new java.lang.ArithmeticException("ArithmeticException: " +
exception.printStackTrace)
}
else{
throw new Exception("Exception: " + exception.printStackTrace)
}
}
logger.info(" :: End Object :: CandAllExceptions :: ")
}

7. CAND NAG ARVIND GUDISEVA 7
FINAL NOTE
Enhancedexceptionshandlingcode isinjectedtothe existingScalacode. There isno change inthe
existingfunctionality.