Question

我正在开发项目，需要从表中获取记录并将其写入平面文件，如txt。所以我使用Java IO api中的BufferedWriter来实现这一点。但不幸的是，文件似乎没有被写入。

以下是我的尝试：

public class PrintConnectionStatistics {

    public static void main(String[] args) {
         String userName = "root";
         String pass = "root";
         String url = "jdbc:mysql://localhost:3306/monitoring";
         String driver ="com.mysql.jdbc.Driver";

         Connection con = null;
         Statement stm = null;
         ResultSet rs = null;


            try{
                //registering the driver.
                Class.forName(driver);
                con = DriverManager.getConnection(url,userName,pass);
                stm = con.createStatement();
                String sql = "SELECT  id, start_time, end_time, layer_name, client, value, created_on, parameter FROM monitoring.status ORDER BY start_time DESC, value DESC";
                rs = stm.executeQuery(sql);
                StringBuilder statRow = null;
                StringBuilder cellVal = null;
                String columnSeperator = ",";
                int rowCounter = 0;

                if(rs != null){
                    long startTimeTracker = 0;

                    File file = new File("E:/Files/User Tracking/ConnectionTrackingReport.txt");

                    // if file doesnt exists, then create it
                    if (!file.exists()) {
                        file.createNewFile();
                    }

                    //true = append file. This is for open file in append mode
                    FileWriter fileWritter = new FileWriter(file.getName(),true);
                    BufferedWriter bw = new BufferedWriter(fileWritter);

                     while (rs.next()){

                     rowCounter++;

                     long startTime = rs.getLong("start_time");
                     long end_time = rs.getLong("end_time");
                     String layer = rs.getString("layer_name");
                     String client = rs.getString("client");
                     int val = rs.getInt("value");

                     if(startTime != startTimeTracker){
                      //reset start time tracker
                      startTimeTracker = startTime;     
                      //next column started
                      if(statRow != null){
                       //append to output file
//                     System.out.println(statRow.toString());
                        bw.write(statRow.toString());
                       statRow = null;
                      }
                      statRow = new StringBuilder();
                      String strStartTime = converMillisecondsToString(startTime);
                      String strEndTime = converMillisecondsToString(end_time);
                      statRow.append(strStartTime).append("-").append(strEndTime);

                     }

                     cellVal = new StringBuilder();

                     if(client != null && !("".equals(client.trim()))){
                      cellVal.append(client.trim());
                     }
                     else{
                      cellVal.append(layer);
                     }

                     cellVal.append("-").append(val);
                     statRow.append(columnSeperator).append(cellVal.toString());
                     cellVal = null;

                    }
                    //this is for last row
                    if(statRow != null){
                     //append to output file
//                   System.out.println(statRow.toString());
                        bw.write(statRow.toString());// I am trying output here to the file which fails to write. But System.out.println print it to the console properly.
                     statRow = null;
                    }
                   }
            } catch (Exception e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            finally{
                try{
                    con.close();
                    con = null;
                   }
                   catch(Exception e){
                   }
           } 

    }

}

输出如下：

12:39:35-12:39:35,MYSQL-62,demo-1
12:39:35-12:39:35,MYSQL-57,demo-1

我不明白代码有什么问题。但是，它会在给定位置创建File，但不会将数据写入其中。请纠正我错在哪里。

Answer 1

尝试使用

BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(path + "\\" + fileName),"UTF-8"))

或者，如果您使用的是java 1.7或更高版本，则可以尝试使用资源选项。

try (BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(path + "\\" + fileName),"UTF-8"))) {
            writer.write(outPut.toString());
        }

Answer 2

您需要在bw.close()块中执行finally。否则，您写入bw的所有内容只会进入内存缓冲区，并且永远不会被写入底层文件。另外@mattymanme建议尝试资源也很好。无论哪种方式，你必须在程序退出之前关闭你的BufferedWriter，否则你写的最后一位数据将会丢失。

Answer 3

您需要关闭BufferedWriter实例，否则会出现资源泄漏。

bw.write(statRow.toString());
// I am trying output here to the file which fails to write. But System.out.println print it to the console properly.
 statRow = null;
 bw.close()

将输出写入通过结果集获取的文件

3 个答案: