我想使用Gsons库将MySQL结果集放入JsonArray中。我怎样才能做到最好。我读过这个： resultset to json using gson
但它出于某种原因使用了简单的Json库。如果可能，我不希望这样。有没有办法用gson库轻松实现这个目标？
非常感谢！
PlayerList.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 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...")
);
}
}
（香港专业教育学院评论了一些部分，因为它们不再相关）
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(); }
}
}
答案 0 :(得分：2)
使用gson库有没有办法轻松实现这个目标？
不是真的。 Gson和JDBC是两个不相关的东西，所以你必须实现一个自定义重映射功能来解码＆＃34; JDBC结果集行/字段和＆＃34;编码＆＃34;它们分别返回JSON数组/对象。从内存消耗的角度来看，累积JsonArray实例可能会很昂贵，甚至会使应用程序崩溃OutOfMemoryError以获得巨大的结果集。尽管如此，如果已知结果集较小或LIMIT，则它们很好。
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);
当然，别忘了关闭resultSet。
如果JsonArray应该写在别处，JsonWriter可以是一个更好的解决方案，能够处理逐行读取的大型结果集，并通过JSON元素编写JSON元素。
@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);
}
}
写入System.out的示例，但是，当然，只要提供适当的OutputStream实例，就可以将其写入：
final JsonWriter jsonWriter = new JsonWriter(new OutputStreamWriter(System.out))
resultSetToJsonArrayStream(resultSet, jsonWriter);
与ResultSet类似，JsonWriter也必须关闭。
我已经为SQLite编写了上面的代码，但它也适用于MySQL。例如，使用以下SQL语句创建和填充测试数据库：
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');
将导致
[{＆＃34; I＆＃34;：1，＆＃34; S＆＃34;：＆＃34;富＆＃34;}，{＆＃34; I＆＃34;：2＆＃34 ; S＆＃34;：＆＃34; BAR＆＃34;}，{＆＃34; I＆＃34;：3，＆＃34; S＆＃34;：＆＃34;巴兹＆＃34;}]
对象模型和流媒体方法。