[SPARK-15031][EXAMPLE] Use SparkSession in Scala/Python/Java example.

## What changes were proposed in this pull request?

This PR aims to update Scala/Python/Java examples by replacing `SQLContext` with newly added `SparkSession`.

- Use **SparkSession Builder Pattern** in 154(Scala 55, Java 52, Python 47) files.
- Add `getConf` in Python SparkContext class: `python/pyspark/context.py`
- Replace **SQLContext Singleton Pattern** with **SparkSession Singleton Pattern**:
  - `SqlNetworkWordCount.scala`
  - `JavaSqlNetworkWordCount.java`
  - `sql_network_wordcount.py`

Now, `SQLContexts` are used only in R examples and the following two Python examples. The python examples are untouched in this PR since it already fails some unknown issue.
- `simple_params_example.py`
- `aft_survival_regression.py`

## How was this patch tested?

Manual.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #12809 from dongjoon-hyun/SPARK-15031.

1 files changed, 4 insertions, 6 deletions

diff --git a/examples/src/main/python/ml/count_vectorizer_example.py b/examples/src/main/python/ml/count_vectorizer_example.py
index e839f645f7..9dbf9959d1 100644
--- a/examples/src/main/python/ml/count_vectorizer_example.py
+++ b/examples/src/main/python/ml/count_vectorizer_example.py
@@ -17,19 +17,17 @@
 
 from __future__ import print_function
 
-from pyspark import SparkContext
-from pyspark.sql import SQLContext
+from pyspark.sql import SparkSession
 # $example on$
 from pyspark.ml.feature import CountVectorizer
 # $example off$
 
 if __name__ == "__main__":
-    sc = SparkContext(appName="CountVectorizerExample")
-    sqlContext = SQLContext(sc)
+    spark = SparkSession.builder.appName("CountVectorizerExample").getOrCreate()
 
     # $example on$
     # Input data: Each row is a bag of words with a ID.
-    df = sqlContext.createDataFrame([
+    df = spark.createDataFrame([
         (0, "a b c".split(" ")),
         (1, "a b b c a".split(" "))
     ], ["id", "words"])
@@ -41,4 +39,4 @@ if __name__ == "__main__":
     result.show()
     # $example off$
 
-    sc.stop()
+    spark.stop()