如何在JNA结构中映射联合

Question

我需要使用JNA将一些C结构转换为java。我的结构是这样组成的：

struct s_xl_event {
XLeventTag tag;
unsigned char chanIndex;
unsigned short transId;
unsigned short portHandle;
unsigned char flags;
unsigned char reserved;
XLuint64 timeStamp;
union s_xl_tag_data tagData;
};

union s_xl_tag_data {
struct s_xl_can_msg msg;
struct s_xl_chip_state chipState;
union s_xl_lin_msg_api linMsgApi;
struct s_xl_sync_pulse syncPulse;
struct s_xl_daio_data daioData;
struct s_xl_transceiver transceiver;
};

struct s_xl_can_msg {
unsigned long id;
unsigned short flags;
unsigned short dlc;
XLuint64 res1;
unsigned char data[MAX_MSG_LEN];
XLuint64 res2;
};

我只添加了我需要的主结构，但也因为所有相关结构都类似于s_xl_can_msg。

在Java中，我使用JNA做到了这一点：

public static class s_xl_event extends Structure {
    public byte tag;
    public byte chanIndex;
    public short transId;
    public short portHandle;
    public byte flags;
    public byte reserved;
    public long timeStamp;
    public s_xl_tag_data tagData;

    @Override
    protected List<String> getFieldOrder() {
        return Arrays.asList("tag","chanIndex","transId","portHandle","flags","reserved","timeStamp","tagData");
    }
}

public static class s_xl_tag_data extends Union {
    public s_xl_can_msg msg;
    public s_xl_chip_state chipState;
    public s_xl_lin_msg_api linMsgApi;
    public s_xl_sync_pulse syncPulse;
    public s_xl_daio_data daioData;
    public s_xl_transceiver transceiver;
}

public static class s_xl_lin_msg extends Structure {        
    public byte id;
    public byte dlc;
    public short flags;
    public byte[] data = new byte[8];
    public byte crc;

    @Override
    protected List<String> getFieldOrder() {
        return Arrays.asList("id","dlc","flags","data","crc");
    }
}

当我使用该方法时

short xlReceive(long portHandle, IntByReference eventCount, s_xl_event eventList);

我获得了与s_xl_event中除tagData之外的所有字段相关的值，其中所有字段都填充为0。现在，我不确定我是否像预期的那样映射了所有的子结构和联合，或者我忘记了什么。有没有人跟这件事有关？

Answer 1

在读取像s_xl_tag_data这样的Union时，您必须通过使用带有类名或字段名的setType()来告诉Union正在读取它的哪个字段，然后在Union上调用read()。

第一次填充父结构时自动执行此操作的一种方法是覆盖父结构中的read()方法。

将此方法添加到s_xl_event结构中：

public void read() {
    // read from native memory, populate tag
    super.read(); 
    // set union type based on tag
    switch(tag) {
    case XL_RECEIVE_MSG:
    case XL_TRANSMIT_MSG: 
        tagData.setType(s_xl_can_msg.class);
        break;
    case XL_CHIP_STATE: 
        tagData.setType(s_xl_chip_state.class);
        break;
    case XL_LIN_MSG: 
        tagData.setType(s_xl_lin_msg_api.class);
        break;
    // ... add other cases here...
    default:
        // add default type or throw exception etc.  
        break;
    }
    // now read tagData from native memory
    tagData.read();
}

作为将类传递给联合元素的setType()的替代方法，您可以传递一个包含变量名称的String，例如"msg"、"chipState"或"linMsgApi"等。

还要注意的是，您在Structure中使用的getFieldOrder()方法在最新版本的Structure中已弃用。您应该改用@FieldOrder注释。