Add support for writing Json object per line

json/src/main/java/tech/tablesaw/io/json/JsonWriteOptions.java

@@ -3,6 +3,7 @@
 import java.io.Writer;
 import tech.tablesaw.io.Destination;
 import tech.tablesaw.io.WriteOptions;
+import tech.tablesaw.io.jsonl.JsonlWriteOptions;
 
 public class JsonWriteOptions extends WriteOptions {
 

json/src/main/java/tech/tablesaw/io/jsonl/JsonlWriteOptions.java

@@ -0,0 +1,31 @@
+package tech.tablesaw.io.jsonl;
+
+import java.io.Writer;
+import tech.tablesaw.io.Destination;
+import tech.tablesaw.io.WriteOptions;
+
+public class JsonlWriteOptions extends WriteOptions {
+
+  private JsonlWriteOptions(Builder builder) {
+    super(builder);
+  }
+
+  public static Builder builder(Writer writer) {
+    return new Builder(new Destination(writer));
+  }
+
+  public static Builder builder(Destination destination) {
+    return new Builder(destination);
+  }
+
+  public static class Builder extends WriteOptions.Builder {
+
+    protected Builder(Destination destination) {
+      super(destination);
+    }
+
+    public JsonlWriteOptions build() {
+      return new JsonlWriteOptions(this);
+    }
+  }
+}

json/src/main/java/tech/tablesaw/io/jsonl/JsonlWriter.java

@@ -0,0 +1,67 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package tech.tablesaw.io.jsonl;
+
+import java.io.IOException;
+import java.io.Writer;
+
+import com.fasterxml.jackson.databind.JsonNode;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.node.ObjectNode;
+import com.fasterxml.jackson.datatype.jsr310.JavaTimeModule;
+
+import tech.tablesaw.api.Table;
+import tech.tablesaw.io.DataWriter;
+import tech.tablesaw.io.Destination;
+import tech.tablesaw.io.RuntimeIOException;
+import tech.tablesaw.io.WriterRegistry;
+
+public class JsonlWriter implements DataWriter<JsonlWriteOptions> {
+
+    private static final JsonlWriter INSTANCE = new JsonlWriter();
+    private static final ObjectMapper mapper = new ObjectMapper().registerModule(new JavaTimeModule());
+
+    static {
+        register(Table.defaultWriterRegistry);
+    }
+
+    public static void register(WriterRegistry registry) {
+        registry.registerExtension("jsonl", INSTANCE);
+        registry.registerOptions(JsonlWriteOptions.class, INSTANCE);
+    }
+
+    public void write(Table table, JsonlWriteOptions options) {
+        try (Writer writer = options.destination().createWriter()) {
+            for (int r = 0; r < table.rowCount(); r++) {
+                ObjectNode row = mapper.createObjectNode();
+                for (int c = 0; c < table.columnCount(); c++) {
+                    row.set(table.column(c).name(), mapper.convertValue(table.get(r, c), JsonNode.class));
+                }
+                String str = mapper.writeValueAsString(row);
+                writer.write(str);
+                if (r < table.rowCount() - 1) {
+                    writer.write("\n");
+                }
+            }
+        } catch (IOException e) {
+            throw new RuntimeIOException(e);
+        }
+    }
+
+    @Override
+    public void write(Table table, Destination dest) {
+        write(table, JsonlWriteOptions.builder(dest).build());
+    }
+}

json/src/test/java/tech/tablesaw/io/jsonl/JsonlWriterTest.java

@@ -0,0 +1,32 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package tech.tablesaw.io.jsonl;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+import tech.tablesaw.api.Table;
+
+public class JsonlWriterTest {
+
+    @Test
+    public void arrayOfObjects() {
+        String json = "{\"a\":1453438800000,\"b\":-2.144}\n{\"a\":1454043600000,\"b\":-2.976}\n{\"a\":1454648400000,\"b\":-2.954}";
+        Table table = Table.read().string(json, "jsonl");
+        String output = table.write().toString("jsonl");
+        assertEquals(json, output);
+    }
+}