[SPARK-19980][SQL] Add NULL checks in Bean serializer

## What changes were proposed in this pull request?
A Bean serializer in `ExpressionEncoder`  could change values when Beans having NULL. A concrete example is as follows;
```
scala> :paste
class Outer extends Serializable {
  private var cls: Inner = _
  def setCls(c: Inner): Unit = cls = c
  def getCls(): Inner = cls
}

class Inner extends Serializable {
  private var str: String = _
  def setStr(s: String): Unit = str = str
  def getStr(): String = str
}

scala> Seq("""{"cls":null}""", """{"cls": {"str":null}}""").toDF().write.text("data")
scala> val encoder = Encoders.bean(classOf[Outer])
scala> val schema = encoder.schema
scala> val df = spark.read.schema(schema).json("data").as[Outer](encoder)
scala> df.show
+------+
|   cls|
+------+
|[null]|
|  null|
+------+

scala> df.map(x => x)(encoder).show()
+------+
|   cls|
+------+
|[null]|
|[null]|     // <-- Value changed
+------+
```

This is because the Bean serializer does not have the NULL-check expressions that the serializer of Scala's product types has. Actually, this value change does not happen in Scala's product types;

```
scala> :paste
case class Outer(cls: Inner)
case class Inner(str: String)

scala> val encoder = Encoders.product[Outer]
scala> val schema = encoder.schema
scala> val df = spark.read.schema(schema).json("data").as[Outer](encoder)
scala> df.show
+------+
|   cls|
+------+
|[null]|
|  null|
+------+

scala> df.map(x => x)(encoder).show()
+------+
|   cls|
+------+
|[null]|
|  null|
+------+
```

This pr added the NULL-check expressions in Bean serializer along with the serializer of Scala's product types.

## How was this patch tested?
Added tests in `JavaDatasetSuite`.

Author: Takeshi Yamamuro <yamamuro@apache.org>

Closes #17347 from maropu/SPARK-19980.

1 files changed, 19 insertions, 0 deletions

diff --git a/sql/core/src/test/java/test/org/apache/spark/sql/JavaDatasetSuite.java b/sql/core/src/test/java/test/org/apache/spark/sql/JavaDatasetSuite.java
index ca9e5ad2ea..ffb4c6273f 100644
--- a/sql/core/src/test/java/test/org/apache/spark/sql/JavaDatasetSuite.java
+++ b/sql/core/src/test/java/test/org/apache/spark/sql/JavaDatasetSuite.java
@@ -1380,4 +1380,23 @@ public class JavaDatasetSuite implements Serializable {
     CircularReference4Bean bean = new CircularReference4Bean();
     spark.createDataset(Arrays.asList(bean), Encoders.bean(CircularReference4Bean.class));
   }
+
+  @Test(expected = RuntimeException.class)
+  public void testNullInTopLevelBean() {
+    NestedSmallBean bean = new NestedSmallBean();
+    // We cannot set null in top-level bean
+    spark.createDataset(Arrays.asList(bean, null), Encoders.bean(NestedSmallBean.class));
+  }
+
+  @Test
+  public void testSerializeNull() {
+    NestedSmallBean bean = new NestedSmallBean();
+    Encoder<NestedSmallBean> encoder = Encoders.bean(NestedSmallBean.class);
+    List<NestedSmallBean> beans = Arrays.asList(bean);
+    Dataset<NestedSmallBean> ds1 = spark.createDataset(beans, encoder);
+    Assert.assertEquals(beans, ds1.collectAsList());
+    Dataset<NestedSmallBean> ds2 =
+      ds1.map((MapFunction<NestedSmallBean, NestedSmallBean>) b -> b, encoder);
+    Assert.assertEquals(beans, ds2.collectAsList());
+  }
 }