MySQL ResultSet into Gson array - json

Question

I want to put a MySQL result set into a JsonArray using Gsons library. How can I best achieve this. I've read this:
resultset to json using gson
But it uses for some reason, the simple-Json library in addition. i dont want that if possible. Is there any way to achieve this easily with the gson library?
Thank you very much!
PlayerList.java:
/*
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* To change this template file, choose Tools | Templates
* and open the template in the editor.
*/
package de.freakyonline.ucone;
import de.freakyonline.ucone.Player;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.Socket;
import java.util.ArrayList;
import java.util.Iterator;
import javafx.application.Platform;
import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import javafx.scene.control.TextArea;
/**
*
* #author uwe
*/
public class PlayerList {
ObservableList<Player> playerList;
ObjectInputStream in;
ObjectOutputStream out;
Socket sock;
private Object obj = null;
private Object obj2 = null;
TextArea consoleOneTextArea;
public PlayerList(ObjectInputStream in, ObjectOutputStream out, Socket sock, TextArea consoleOneTextArea) {
this.in = in;
this.out = out;
this.sock = sock;
this.consoleOneTextArea = consoleOneTextArea;
getPlayersFromServer();
}
private void getPlayersFromServer() {
/* try {
out.writeObject("getplayers");
obj=in.readObject();
if(obj == null) {
System.out.println("ERROR! void getPlayersFromServer in PlayerList.java");
Platform.exit();
}
String command = obj.toString().toLowerCase();
String currentFromServer;
if(command.equalsIgnoreCase("getplayers")) {
while((obj2=in.readObject()) != null) {
currentFromServer = obj2.toString().toLowerCase();
for(String cell : currentFromServer.split(" ")) {
System.out.println(cell.toString());
}
if (currentFromServer.equalsIgnoreCase("done")) {
consoleOneTextArea.appendText("This is finished. Have fun!\n");
break;
}
consoleOneTextArea.appendText(currentFromServer + "\n");
}
} { System.out.println("ERROR"); }
} catch (Exception ex) { ex.printStackTrace(); }
*/
this.playerList = FXCollections.observableArrayList(
new Player("freakyy85","Owner","1810",31,"m", "missing..."),
new Player("Ky3ak","Owner","1920",34,"m", "missing...")
);
}
}
(ive commented out some parts, as they are not relevant anymore)
Player.java:
/*
* To change this license header, choose License Headers in Project Properties.
* To change this template file, choose Tools | Templates
* and open the template in the editor.
*/
package de.freakyonline.ucone;
import com.google.gson.stream.JsonReader;
import java.io.InputStreamReader;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.Socket;
import javafx.scene.control.TextArea;
/**
*
* #author uwe
*/
public class Remote implements Runnable {
private Object obj = null;
private Object obj2 = null;
private ObjectInputStream in;
private ObjectOutputStream out;
private Socket sock;
private TextArea consoleOneTextArea;
public Remote (ObjectInputStream in, ObjectOutputStream out, Socket sock) {
this.in = in;
this.out = out;
this.sock = sock;
}
public ObjectInputStream getIn() {
return in;
}
public ObjectOutputStream getOut() {
return out;
}
public Socket getSock() {
return sock;
}
public void setConsoleOneTextArea(TextArea consoleOneTextArea) {
this.consoleOneTextArea = consoleOneTextArea;
}
public void run() {
try {
while((obj=in.readObject()) != null && sock.isConnected()) {
String command = obj.toString().toLowerCase();
String currentFromServer;
switch(command) {
case "getplayers":
/* while((obj2=in.readObject()) != null) {
currentFromServer = obj2.toString().toLowerCase();
if (currentFromServer.equalsIgnoreCase("done")) {
consoleOneTextArea.appendText("This is finished. Have fun!\n");
break;
}
consoleOneTextArea.appendText(currentFromServer + "\n");
*/ }
JsonReader jsonReader = new JsonReader(new InputStreamReader(in, "UTF-8"));
jsonReader.close();
break;
}
} catch (Exception ex) { ex.printStackTrace(); }
}
}

Answer 1

Is there any way to achieve this easily with the gson library?
Not really. Gson and JDBC are too/two unrelated things so you have to implement a custom remapping function to "decode" JDBC result set rows/fields and "encode" them back to JSON array/object respectively. Accumulating a JsonArray instance may be expensive from the memory consumption point of view, or even crash the application with OutOfMemoryError for huge result sets. Nonetheless they are good if the result sets are known to be small or LIMITed.
Accumulating JsonArray
static JsonArray resultSetToJsonArray(final ResultSet resultSet)
throws SQLException {
final ResultSetMetaData metaData = resultSet.getMetaData();
// JsonArray is a Gson built-in class to hold JSON arrays
final JsonArray jsonArray = new JsonArray();
while ( resultSet.next() ) {
jsonArray.add(resultSetRowToJsonObject(resultSet, metaData));
}
return jsonArray;
}
private static JsonElement resultSetRowToJsonObject(final ResultSet resultSet, final ResultSetMetaData metaData)
throws SQLException {
final int columnCount = metaData.getColumnCount();
// Every result set row is a JsonObject equivalent
final JsonObject jsonObject = new JsonObject();
// JDBC uses 1-based loops
for ( int i = 1; i <= columnCount; i++ ) {
jsonObject.add(metaData.getColumnName(i), fieldToJsonElement(resultSet, metaData, i));
}
return jsonObject;
}
private static JsonElement fieldToJsonElement(final ResultSet resultSet, final ResultSetMetaData metaData, final int column)
throws SQLException {
final int columnType = metaData.getColumnType(column);
final Optional<JsonElement> jsonElement;
// Process each SQL type mapping a value to a JSON tree equivalent
switch ( columnType ) {
case Types.BIT:
case Types.TINYINT:
case Types.SMALLINT:
throw new UnsupportedOperationException("TODO: " + JDBCType.valueOf(columnType));
case Types.INTEGER:
// resultSet.getInt() returns 0 in case of null, so it must be extracted with getObject and cast, then converted to a JsonPrimitive
jsonElement = Optional.ofNullable((Integer) resultSet.getObject(column)).map(JsonPrimitive::new);
break;
case Types.BIGINT:
case Types.FLOAT:
case Types.REAL:
case Types.DOUBLE:
case Types.NUMERIC:
case Types.DECIMAL:
case Types.CHAR:
throw new UnsupportedOperationException("TODO: " + JDBCType.valueOf(columnType));
case Types.VARCHAR:
jsonElement = Optional.ofNullable(resultSet.getString(column)).map(JsonPrimitive::new);
break;
case Types.LONGVARCHAR:
case Types.DATE:
case Types.TIME:
case Types.TIMESTAMP:
case Types.BINARY:
case Types.VARBINARY:
case Types.LONGVARBINARY:
case Types.NULL:
case Types.OTHER:
case Types.JAVA_OBJECT:
case Types.DISTINCT:
case Types.STRUCT:
case Types.ARRAY:
case Types.BLOB:
case Types.CLOB:
case Types.REF:
case Types.DATALINK:
case Types.BOOLEAN:
case Types.ROWID:
case Types.NCHAR:
case Types.NVARCHAR:
case Types.LONGNVARCHAR:
case Types.NCLOB:
case Types.SQLXML:
case Types.REF_CURSOR:
case Types.TIME_WITH_TIMEZONE:
case Types.TIMESTAMP_WITH_TIMEZONE:
throw new UnsupportedOperationException("TODO: " + JDBCType.valueOf(columnType));
default:
throw new UnsupportedOperationException("Unknown type: " + columnType);
}
// If the optional value is missing, assume it's a null
return jsonElement.orElse(JsonNull.INSTANCE);
}
final JsonArray jsonArray = resultSetToJsonArray(resultSet);
System.out.println(jsonArray);
Don't forget to close the resultSet, of course.
JSON streaming
If the JsonArray is supposed to be written elsewhere, JsonWriter can be a better solution being able to process huge result sets reading row by row and writing JSON element by JSON element.
#SuppressWarnings("resource")
static void resultSetToJsonArrayStream(final ResultSet resultSet, final JsonWriter jsonWriter)
throws SQLException, IOException {
// Write the [ token
jsonWriter.beginArray();
final ResultSetMetaData metaData = resultSet.getMetaData();
while ( resultSet.next() ) {
// Write row by row
writeRow(resultSet, jsonWriter, metaData);
}
// Finish the array with the ] token
jsonWriter.endArray();
}
#SuppressWarnings("resource")
private static void writeRow(final ResultSet resultSet, final JsonWriter jsonWriter, final ResultSetMetaData metaData)
throws SQLException, IOException {
final int columnCount = metaData.getColumnCount();
// Similarly to the outer array: the { token starts a new object representing a row
jsonWriter.beginObject();
for ( int i = 1; i <= columnCount; i++ ) {
// Write the column name and try to resolve a JSON literal to be written
jsonWriter.name(metaData.getColumnName(i));
writeField(resultSet, jsonWriter, metaData, i);
}
// Terminate the object with }
jsonWriter.endObject();
}
#SuppressWarnings("resource")
private static void writeField(final ResultSet resultSet, final JsonWriter jsonWriter, final ResultSetMetaData metaData, final int column)
throws SQLException, IOException {
final int columnType = metaData.getColumnType(column);
switch ( columnType ) {
case Types.BIT:
case Types.TINYINT:
case Types.SMALLINT:
throw new UnsupportedOperationException("TODO: " + JDBCType.valueOf(columnType));
case Types.INTEGER:
jsonWriter.value((Integer) resultSet.getObject(column));
break;
case Types.BIGINT:
case Types.FLOAT:
case Types.REAL:
case Types.DOUBLE:
case Types.NUMERIC:
case Types.DECIMAL:
case Types.CHAR:
throw new UnsupportedOperationException("TODO: " + JDBCType.valueOf(columnType));
case Types.VARCHAR:
jsonWriter.value((String) resultSet.getObject(column));
break;
case Types.LONGVARCHAR:
case Types.DATE:
case Types.TIME:
case Types.TIMESTAMP:
case Types.BINARY:
case Types.VARBINARY:
case Types.LONGVARBINARY:
case Types.NULL:
case Types.OTHER:
case Types.JAVA_OBJECT:
case Types.DISTINCT:
case Types.STRUCT:
case Types.ARRAY:
case Types.BLOB:
case Types.CLOB:
case Types.REF:
case Types.DATALINK:
case Types.BOOLEAN:
case Types.ROWID:
case Types.NCHAR:
case Types.NVARCHAR:
case Types.LONGNVARCHAR:
case Types.NCLOB:
case Types.SQLXML:
case Types.REF_CURSOR:
case Types.TIME_WITH_TIMEZONE:
case Types.TIMESTAMP_WITH_TIMEZONE:
throw new UnsupportedOperationException("TODO: " + JDBCType.valueOf(columnType));
default:
throw new UnsupportedOperationException("Unknown type: " + columnType);
}
}
Example of writing to System.out, but, of course, it can be written anywhere just supplying an appropriate OutputStream instance:
final JsonWriter jsonWriter = new JsonWriter(new OutputStreamWriter(System.out))
resultSetToJsonArrayStream(resultSet, jsonWriter);
Similarly to ResultSet, JsonWriter must be closed as well.
I've written the above code for SQLite, but it should work for MySQL too. For example, the test database created and populated with the following SQL statements:
CREATE TABLE `table` (i NUMBER NOT NULL, s TEXT NOT NULL);
INSERT INTO `table` (i, s) VALUES (1, 'foo');
INSERT INTO `table` (i, s) VALUES (2, 'bar');
INSERT INTO `table` (i, s) VALUES (3, 'baz');
will result in
[{"i":1,"s":"foo"},{"i":2,"s":"bar"},{"i":3,"s":"baz"}]
for both object model and streaming approaches.