SPARK-5239 [CORE] JdbcRDD throws "java.lang.AbstractMethodError:...

SPARK-5239 [CORE] JdbcRDD throws "java.lang.AbstractMethodError: oracle.jdbc.driver.xxxxxx.isClosed()Z"

This is a completion of https://github.com/apache/spark/pull/4033 which was withdrawn for some reason.

Author: Sean Owen <sowen@cloudera.com>

Closes #4470 from srowen/SPARK-5239.2 and squashes the following commits:

2398bde [Sean Owen] Avoid use of JDBC4-only isClosed()

core/src/main/scala/org/apache/spark/rdd/JdbcRDD.scala

@@ -99,21 +99,21 @@ class JdbcRDD[T: ClassTag](
 
     override def close() {
       try {
-        if (null != rs && ! rs.isClosed()) {
+        if (null != rs) {
           rs.close()
         }
       } catch {
         case e: Exception => logWarning("Exception closing resultset", e)
       }
       try {
-        if (null != stmt && ! stmt.isClosed()) {
+        if (null != stmt) {
           stmt.close()
         }
       } catch {
         case e: Exception => logWarning("Exception closing statement", e)
       }
       try {
-        if (null != conn && ! conn.isClosed()) {
+        if (null != conn) {
           conn.close()
         }
         logInfo("closed connection")

sql/core/src/main/scala/org/apache/spark/sql/jdbc/JDBCRDD.scala

@@ -370,21 +370,21 @@ private[sql] class JDBCRDD(
     def close() {
       if (closed) return
       try {
-        if (null != rs && ! rs.isClosed()) {
+        if (null != rs) {
           rs.close()
         }
       } catch {
         case e: Exception => logWarning("Exception closing resultset", e)
       }
       try {
-        if (null != stmt && ! stmt.isClosed()) {
+        if (null != stmt) {
           stmt.close()
         }
       } catch {
         case e: Exception => logWarning("Exception closing statement", e)
       }
       try {
-        if (null != conn && ! conn.isClosed()) {
+        if (null != conn) {
           conn.close()
         }
         logInfo("closed connection")