如何在 JNA 结构中映射联合

Question

我需要使用 JNA 将几个 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要结构，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完成了此操作：

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中的所有字段相关的值，除了所有字段都填充为 0 的tagData。现在，我不确定我是否按预期映射了所有子结构和联合，或者我忘记了什么。有人和这样的事情有什么关系吗？

Answer 1

当读取像s_xl_tag_data这样的Union时，您必须通过使用带有类或字段名称的setType()来告诉联合正在读取它的哪些字段，然后在联合上调用read()。

首次填充父结构时自动执行此操作的一种方法是重写父结构中的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，您可以改为传递包含变量名的String，例如"msg"、"chipState"或"linMsgApi"等。

同样值得注意的是，您在Structure中使用的getFieldOrder()方法在最新版本的 JNA 中已弃用。 应改用@FieldOrder批注。