Java 使用 ByteBuffer 解码来自 GPS 设备的字节

Question

我在解码从 GPS 设备发送到我的 java UDP 监听器程序的字节时遇到问题。我可以提取一些信息，但对于其他信息，我无法理解如何执行此操作。设备手册中的数据格式如下:

Name       | Type          | Length
length:    | short         | 2
cmd:       | short         | 2
enabled:   | unsigned char | 1
alarm:     | unsigned char | 1
speed:     | char          | 1
direction: | short         | 2
longitude: | double        | 4
latitude:  | double        | 4
datetime:  | long          | 4
userid:    | char          | 11
iostate:   | char          | 1
oilstate:  | char          | 1

在我的程序中，我可以解码长度、命令、警报、速度、方向和设备。我陷入了纬度、经度和日期时间的困境，需要一些帮助。我得到的值与正确的值完全相去甚远。使用 struct.unpack 函数在 python 上运行没有任何问题。

函数中代码如下:

private void SetupUDP() throws IOException {
        String message;
        // Setup UDP Stuff and listen on all interfaces
        DatagramSocket serverSocket = new DatagramSocket(9876, InetAddress.getByName("0.0.0.0"));
        byte[] receiveData = new byte[1024];
        while(true) {
            // Start receiving UDP packets from devices
            DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
            serverSocket.receive(receivePacket);

            ByteBuffer data = ByteBuffer.wrap(receivePacket.getData());
            data.order(ByteOrder.LITTLE_ENDIAN);
            if (receivePacket.getLength() == 15) {
                short len = data.getShort(0);
                short cmd = data.getShort(2);
                String device = "";
                for (int x=4; x < 14; x++) {
                    byte b = data.get(x);
                    device += (char) b;
                }
                System.out.println(len + " " + cmd + " -- " + device);
            }
            if (receivePacket.getLength() == 34) {
                short len = data.getShort(0);
                short cmd = data.getShort(2);
                byte benabled = data.get(4);
                int enabled = (int) benabled;
                byte balarm = data.get(5);
                // Alarm (OK)
                int alarm =  Math.abs((int) balarm);
                // Speed (OK)
                byte bspeed = data.get(6);
                int speed = (int) bspeed;
                // Direction (OK)
                short direction = data.getShort(7);
                // Coordinates
                double longitude = data.getDouble(9);
                double latitude = data.getDouble(13);
                // Device ID (OK)
                String device = "";
                for (int x=21; x < 31; x++) {
                    byte bdevice = data.get(x);
                    device += (char) bdevice;
                }

                // Datetime (testing)
                int ddatetime = data.get(17);
                Date datetime = new Date(ddatetime * 1000);
                //System.out.println("-->" + datetime + ", " + longitude);
                System.out.println(len + " " + cmd + " " + enabled + " " + alarm + " " + speed + " " + direction + " " + longitude + " " + latitude + " " + datetime + " " + device);
            }
        }
    }

Answer 1

您正在读取的纬度和经度各为 8 个字节，您的规范显示为 4。java中的double和ByteBuffer中的getDouble都是8个字节，你需要getFloat。

当您读得太远时，这也会损坏数据。