diff --git a/flink-formats/flink-protobuf/src/test/java/org/apache/flink/formats/protobuf/ProtobufSQLITCase.java b/flink-formats/flink-protobuf/src/test/java/org/apache/flink/formats/protobuf/ProtobufSQLITCase.java
index 1c9306fdde977..f1e0d49cfd388 100644
--- a/flink-formats/flink-protobuf/src/test/java/org/apache/flink/formats/protobuf/ProtobufSQLITCase.java
+++ b/flink-formats/flink-protobuf/src/test/java/org/apache/flink/formats/protobuf/ProtobufSQLITCase.java
@@ -59,10 +59,11 @@ public void testSource() {
     }
 
     @Test
-    public void testSink() throws Exception {
+    public void testSourceWithDefaultValue() {
+        MapTest mapTest = MapTest.newBuilder().build();
+
         TestProtobufSourceFunction.messages.clear();
-        TestProtobufSourceFunction.messages.add(getProtoTestObject());
-        TestProtobufSinkFunction.results.clear();
+        TestProtobufSourceFunction.messages.add(mapTest);
 
         env().setParallelism(1);
         String sql =
@@ -73,10 +74,23 @@ public void testSink() throws Exception {
                         + ")  with ("
                         + "	'connector' = 'protobuf-test-connector', "
                         + "	'format' = 'protobuf', "
-                        + " 'protobuf.message-class-name' = 'org.apache.flink.formats.protobuf.testproto.MapTest'"
+                        + " 'protobuf.message-class-name' = 'org.apache.flink.formats.protobuf.testproto.MapTest', "
+                        + " 'protobuf.read-default-values' = 'true' "
                         + ")";
         tEnv().executeSql(sql);
-        sql =
+        TableResult result = tEnv().executeSql("select * from bigdata_source");
+        Row row = result.collect().next();
+        assertEquals(0, (int) row.getField(0));
+    }
+
+    @Test
+    public void testSink() throws Exception {
+        TestProtobufSourceFunction.messages.clear();
+        TestProtobufSourceFunction.messages.add(getProtoTestObject());
+        TestProtobufSinkFunction.results.clear();
+
+        env().setParallelism(1);
+        String sql =
                 "create table bigdata_sink ( "
                         + "	a int, "
                         + "	map1 map<string,string>,"
@@ -88,7 +102,8 @@ public void testSink() throws Exception {
                         + ")";
         tEnv().executeSql(sql);
         TableResult tableResult =
-                tEnv().executeSql("insert into bigdata_sink select * from bigdata_source");
+                tEnv().executeSql(
+                                "insert into bigdata_sink select 1, map['a', 'b', 'c', 'd'], map['f', row(1,cast(2 as bigint))] ");
         tableResult.await();
 
         byte[] bytes = TestProtobufSinkFunction.results.get(0);
@@ -100,4 +115,36 @@ public void testSink() throws Exception {
         assertEquals(1, innerMessageTest.getA());
         assertEquals(2L, innerMessageTest.getB());
     }
+
+    @Test
+    public void testSinkWithNullLiteral() throws Exception {
+        TestProtobufSourceFunction.messages.clear();
+        TestProtobufSourceFunction.messages.add(getProtoTestObject());
+        TestProtobufSinkFunction.results.clear();
+
+        env().setParallelism(1);
+        String sql =
+                "create table bigdata_sink ( "
+                        + "	a int, "
+                        + "	map1 map<string,string>,"
+                        + " map2 map<string, row<a int, b bigint>>"
+                        + ")  with ("
+                        + "	'connector' = 'protobuf-test-connector', "
+                        + "	'format' = 'protobuf', "
+                        + " 'protobuf.message-class-name' = 'org.apache.flink.formats.protobuf.testproto.MapTest', "
+                        + " 'protobuf.write-null-string-literal' = 'NULL' "
+                        + ")";
+        tEnv().executeSql(sql);
+        TableResult tableResult =
+                tEnv().executeSql(
+                                "insert into bigdata_sink select 1, map['a', null], map['b', cast(null as row<a int, b bigint>)]");
+        tableResult.await();
+
+        byte[] bytes = TestProtobufSinkFunction.results.get(0);
+        MapTest mapTest = MapTest.parseFrom(bytes);
+        assertEquals(1, mapTest.getA());
+        assertEquals("NULL", mapTest.getMap1Map().get("a"));
+        MapTest.InnerMessageTest innerMessageTest = mapTest.getMap2Map().get("b");
+        assertEquals(MapTest.InnerMessageTest.getDefaultInstance(), innerMessageTest);
+    }
 }
diff --git a/flink-formats/flink-protobuf/src/test/java/org/apache/flink/formats/protobuf/table/TestProtobufTableFactory.java b/flink-formats/flink-protobuf/src/test/java/org/apache/flink/formats/protobuf/table/TestProtobufTableFactory.java
index 71a4ac72630a1..d3ee7980b589d 100644
--- a/flink-formats/flink-protobuf/src/test/java/org/apache/flink/formats/protobuf/table/TestProtobufTableFactory.java
+++ b/flink-formats/flink-protobuf/src/test/java/org/apache/flink/formats/protobuf/table/TestProtobufTableFactory.java
@@ -18,7 +18,7 @@
 import java.util.HashSet;
 import java.util.Set;
 
-/** */
+/** Test protobuf table factory. Only used in flink ptotobuf test module. */
 public class TestProtobufTableFactory
         implements DynamicTableSourceFactory, DynamicTableSinkFactory {
 
diff --git a/flink-formats/flink-protobuf/src/test/java/org/apache/flink/formats/protobuf/table/TestProtobufTableSink.java b/flink-formats/flink-protobuf/src/test/java/org/apache/flink/formats/protobuf/table/TestProtobufTableSink.java
index f32a684041a1d..435e96850922c 100644
--- a/flink-formats/flink-protobuf/src/test/java/org/apache/flink/formats/protobuf/table/TestProtobufTableSink.java
+++ b/flink-formats/flink-protobuf/src/test/java/org/apache/flink/formats/protobuf/table/TestProtobufTableSink.java
@@ -8,7 +8,7 @@
 import org.apache.flink.table.data.RowData;
 import org.apache.flink.table.types.DataType;
 
-/** */
+/** Table sink for protobuf table factory test. */
 public class TestProtobufTableSink implements DynamicTableSink {
     private final EncodingFormat<SerializationSchema<RowData>> encodingFormat;
     private final DataType dataType;
diff --git a/flink-formats/flink-protobuf/src/test/java/org/apache/flink/formats/protobuf/table/TestProtobufTableSource.java b/flink-formats/flink-protobuf/src/test/java/org/apache/flink/formats/protobuf/table/TestProtobufTableSource.java
index 150e8d65d1906..610a38ddbeea5 100644
--- a/flink-formats/flink-protobuf/src/test/java/org/apache/flink/formats/protobuf/table/TestProtobufTableSource.java
+++ b/flink-formats/flink-protobuf/src/test/java/org/apache/flink/formats/protobuf/table/TestProtobufTableSource.java
@@ -9,7 +9,7 @@
 import org.apache.flink.table.data.RowData;
 import org.apache.flink.table.types.DataType;
 
-/** */
+/** Table source for protobuf table factory test. */
 public class TestProtobufTableSource implements ScanTableSource {
     private final DecodingFormat<DeserializationSchema<RowData>> decodingFormat;
     private final DataType producedDataType;