如何使用gson在json文件中保存数据？

Question

在我的Web应用程序中,我成功地使用mybatis在html表中显示数据.现在我想在一个json文件中保存Mysql表的记录并创建一个用户数组,我用Gson,问题是只保存在文件中的一条记录.谢谢.
结果file.json如下:

{"data":
 [
 {"id":2,"Name":"Mike"}
 ]
}

servlet.java

SqlSession session = MyBatisSqlSessionFactory.getSession();
List<User> users = session.selectList("dao.UserDao.findAll");
for (User u : users) {
    Gson gson = new Gson();
    try {
        JsonWriter  writer = new JsonWriter(new FileWriter("C:\\file.json"));
        writer.beginObject();
        writer.name("data");
        writer.beginArray();
        writer.beginObject();
        writer.name("id").value(t.getId());
        writer.name("name").value(t.getNom());
        writer.endObject();
        writer.endArray();
        writer.endObject();
        writer.close();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

session.close();

Answer 1

您将所有用户写入同一文件中,C:\\file.json这样只保存了循环的最后一次迭代.

您可以将对象List<User>转换为json并将其写入一次(无需循环)

例:

try (Writer writer = new FileWriter("Output.json")) {
    Gson gson = new GsonBuilder().create();
    gson.toJson(users, writer);
}

Answer 2

我以前使用的是outputStream.writeObject和Serializable，带有默认的写入器/读取器，用于保存对象数据。由于代码可持续性方面的问题，我一直在追求别的东西。这就是结果。该BufferedWriter是强制性的，否则写入速度将下降8倍。注意，UTF-8声明是Json的默认编码。不知道是否不声明它是安全的。

例：

private void saveJson(Object object, Type type, String directory, String fileName) {

    File file = new File(getApplicationContext().getDir(directory, Context.MODE_PRIVATE),
            fileName);
    OutputStream outputStream = null;
    Gson gson = new GsonBuilder().enableComplexMapKeySerialization().setPrettyPrinting()
            .create();
    try {
        outputStream = new FileOutputStream(file);
        BufferedWriter bufferedWriter;
        if (android.os.Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
            bufferedWriter = new BufferedWriter(new OutputStreamWriter(outputStream,
                    StandardCharsets.UTF_8));
        } else {
            bufferedWriter = new BufferedWriter(new OutputStreamWriter(outputStream, "UTF-8"));
        }

        gson.toJson(object, type, bufferedWriter);
        bufferedWriter.close();

    } catch (FileNotFoundException e) {
        e.printStackTrace();
        if (DEBUG) Log.e(saveJson, "saveUserData, FileNotFoundException e: '" + e + "'");
    } catch (IOException e) {
        e.printStackTrace();
        if (DEBUG) Log.e(saveJson, "saveUserData, IOException e: '" + e + "'");
    } finally {
        if (outputStream != null) {
            try {
                outputStream.flush();
                outputStream.close();
            } catch (IOException e) {
                if (DEBUG) Log.e(saveJson, "saveUserData, finally, e: '" + e + "'");
            }
        }
    }

}


private Object loadJson(Type type,  String directory, String fileName) {
    Object jsonData = null;

    File file = new File(getApplicationContext().getDir(directory, Context.MODE_PRIVATE),
            fileName);
    InputStream inputStream = null;
    Gson gson = new GsonBuilder().enableComplexMapKeySerialization().setPrettyPrinting()
            .create();
    try {
        inputStream = new FileInputStream(file);
        InputStreamReader streamReader;
        if (android.os.Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
            streamReader = new InputStreamReader(inputStream,
                    StandardCharsets.UTF_8);
        } else {
            streamReader = new InputStreamReader(inputStream, "UTF-8");
        }

        jsonData = gson.fromJson(streamReader, type);
        streamReader.close();

    } catch (FileNotFoundException e) {
        e.printStackTrace();
        if (DEBUG) Log.e(TAG, "loadJson, FileNotFoundException e: '" + e + "'");
    } catch (IOException e) {
        e.printStackTrace();
        if (DEBUG) Log.e(TAG, "loadJson, IOException e: '" + e + "'");
    } finally {
        if (inputStream != null) {
            try {
                inputStream.close();
            } catch (IOException e) {
                if (DEBUG) Log.e(TAG, "loadJson, finally, e: '" + e + "'");
            }
        }
    }
    return jsonData;
}

以Type为例：

Type type = new TypeToken<Map<String, Object>>() { }.getType();