JNA 实际开发中若干问题解决方法
很早就已经听说过 JNI（Java Native Interface）Java 本地接口，奈何调用本地动态链接库太过于复杂，就一直没有再尝试。而最近因为工作需要调用 Windows DLL 动态链接库（对应 Linux 中 so 文件），而对 JNA 有了入坑体验，对实际工作中遇到的问题做出总结。
1. 调用 Windows 窗口打印Hello World
- pom 依赖
<dependency>
<groupId>net.java.dev.jna</groupId>
<artifactId>jna</artifactId>
<version>latest</version>
</dependency>- JNA 在加载驱动时提供两种加载方式 ， 直接映射 和 接口生成。
接口生成
public interface HelixcsDll extends StdCallLibrary {
// loadLibary 为动态链接库加载目录
HelixcsDll HELIXCS_DLL = Native.loadLibrary("helixcs.dll", HelixcsDll.class);
// 在 dll 中存在相同 SomeFunction 的函数
void SomeFunction(String content);
}
// 调用
HelixcsDll.HELIXCS_DLL.SomeFunction("Hello World");直接映射
class Helixcs{
static {
Native.register("helixcs.dll");
}
// 映射为本地方法
public static native void SomeFunction(String content);
public static void main(String[] args) {
SomeFunction("Hello World");
}
}2. C 类型和 Java 类型映射。
- JNA 官方提供的默认类型映射
Default Type Mappings 默认类型映射
Java primitive types (and their object equivalents) map directly to the native C type of the same size.
Java 原始类型以相同的大小映射 C 类型。
| char | 8-bit integer | byte | BYTE, TCHAR |
| short | 16-bit integer | short | WORD |
| wchar_t | 16/32-bit character | char | TCHAR |
| int | 32-bit integer | int | DWORD |
| int | boolean value | boolean | BOOL |
| long | 32/64-bit integer | NativeLong | LONG |
| long long | 64-bit integer | long | __int64 |
| float | 32-bit FP | float | |
| double | 64-bit FP | double | |
| char* | C string | String | LPTCSTR |
| void* | pointer | Pointer | LPVOID, HANDLE, LPXXX |
未签名类型作为签名类型来映射。 C 中枚举类型可替换为 “int”。
Unsigned types use the same mappings as signed types. C enums are usually interchangeable with "int".
3. 官方提供的详细的类型映射
Marshalling/Unmarshalling (Java/Native Type Conversions)
| C Type | Native Representation | Java Type |
| char | 8-bit integer | byte |
| wchar_t | platform-dependent | char |
| short | 16-bit integer | short |
| int | 32-bit integer | int |
| int | boolean flag | boolean |
| enum | enumeration type | int (usually) |
| long long, __int64 | 64-bit integer | long |
| float | 32-bit floating point | float |
| double | 64-bit floating point | double |
| pointer (e.g. void*) | platform-dependent (32- or 64-bit pointer to memory) | BufferPointer |
| pointer (e.g. void*), array |
32- or 64-bit pointer to memory (argument/return) contiguous memory (struct member) |
<P>[](array of primitive type) |
In addition to the above types, which are supported at the native layer, the JNA Java library automatically handles the following types. All but NativeMapped and NativeLong are converted to Pointer before being passed to the native layer. |
||
| long | platform-dependent (32- or 64-bit integer) | NativeLong |
| const char* | NUL-terminated array (native encoding or jna.encoding) |
String |
| const wchar_t* | NUL-terminated array (unicode) | WString |
| char** | NULL-terminated array of C strings | String[] |
| wchar_t** | NULL-terminated array of wide C strings | WString[] |
| void** | NULL-terminated array of pointers | Pointer[] |
| struct* struct |
pointer to struct (argument or return) (or explicitly)struct by value (member of struct) ( or explicitly) |
Structure |
| union | same as Structure |
Union |
| struct[] | array of structs, contiguous in memory | Structure[] |
| void (*FP)() | function pointer (Java or native) | Callback |
| pointer (<T> *) | same as Pointer |
PointerType |
| other | integer type | IntegerType |
| other | custom mapping, depends on definition | NativeMapped |
4. 经验总结默认映射关系
尽管 JNA 官方已经提供了详细的类型映射文档。但在实际中发现按照官方映射可能出现莫名问题。对此我们在实际开发中对于一些类型的映射报错，可以参考以下映射做出调整。
| C类型 | JNA类型 | 说明 |
|---|---|---|
| char * out | Pointer | Pointer room = new Memory(30); |
| uchar * out | Pointer | Pointer room = new Memory(30); |
| long * | long [ ] | |
| int * | init [ ] | |
| int | int | |
| char * | byte [ ] | |
| char * argv[] | String [] | |
| uchar | int | |
| long | NativeLong | 兼容 32和64位 |
5. 常见错误
1. UnsatisfiedLinkError 问题
存在多个动态链接库之间调用情况，可能缺少其中某一个动态链接库文件。
2. Error Memory Access 问题
较大情况下存在参数类型映射错误，参考 JNA 类型映射。
3. dll can not find in win32/86
无法加载到动态链接库文件路径，需要将动态链接库放到项目根目录下，推荐开启 JNA 驱动加载 debug 模式，
System.setProperty("jna.debug_load", "true"); 手动指定动态链接库文件路径
System.setProperty("jna.library.path",dllResourcePath);
System.setProperty("jna.platform.library.path",dllResourcePath);4. 程序在运行一段时间后崩溃
在JNA crash-protection 中，官方文档说明的崩溃的主要原因,( These are often caused by improper mappings or invalid arguments passed to the native library.) 未知的参数类型映射导致vm崩溃。在很大程度上使用 Pointer 来作为 Java 通用映射类型。
而 JNA 默认程序保护Native.setProtected(true)使得 java 错误来代替程序崩溃，可以设置 Native.setProtected(false) 来 dump 出崩溃日志。
问题可以反馈到 JNA Google Group : https://groups.google.com/forum/#!forum/jna-users
6. 参考
- JNA Google Group : https://groups.google.com/forum/#!forum/jna-users
- JNA API Documentation: https://java-native-access.github.io/jna/4.2.1/overview-summary.html
- JNA VM Crashed Protection :http://java-native-access.github.io/jna/4.5.1/javadoc/overview-summary.html#crash-protection
原文来自：https://iliangqunru.bitcron.com/post/2018/jna-shi-ji-kai-fa-zhong-ruo-gan-wen-ti-jie-jue-fang-fa
