Exception Handling in Scala

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 :: ")
}

MODIFIED CODE
try {
var sqlSelect = """select * from emp"""
while (rs.next){
}
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())
}
}
IMPROVISED CODE
try {

var sqlSelect = """select * from emp1"""
while (rs.next){
}
oraStmt.close()
}
catch {
case ex: Exception => throw new CandAllExceptions(ex)
}
finally {
// Nothing
}
}
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

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)
}
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)

@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 {
}
}
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

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: " +
}
else if (exception.isInstanceOf[org.apache.thrift.transport.TTransportException]){
throw new org.apache.thrift.transport.TTransportException("ThriftTransportException: " +
}
else if (exception.isInstanceOf[java.lang.NullPointerException]){
throw new java.lang.NullPointerException("NullPointerException: " +
}
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: " +
}
else{
throw new Exception("Exception: " + exception.printStackTrace)
}
}
logger.info(" :: End Object :: CandAllExceptions :: ")
}

FINAL NOTE
Enhancedexceptionshandlingcode isinjectedtothe existingScalacode. There isno change inthe
existingfunctionality.

Exception Handling in Scala

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