linux系統調用syscall 表：https://filippo.io/linux-syscall-table/
原文：https://iliangqunru.bitcron.com/post/2018/jna-shi-ji-kai-fa-zhong-ruo-gan-wen-ti-jie-jue-fang-fa
其他：
java發起系統調用，本質還是用了JNA，調用OS提供的syacall函數：
import com.sun.jna.Library; import com.sun.jna.Native; public class Test { public interface CStdLib extends Library { int syscall(int number, Object... args); } public static void main(String[] args) { CStdLib c = (CStdLib)Native.loadLibrary("c", CStdLib.class); // WARNING: These syscall numbers are for x86 only System.out.println("PID: " + c.syscall(20)); System.out.println("UID: " + c.syscall(24)); System.out.println("GID: " + c.syscall(47)); c.syscall(39, "/tmp/create-new-directory-here"); } }
很早就已經聽說過 JNI（Java Native Interface）Java 本地接口，奈何調用本地動態鏈接庫太過於復雜，就一直沒有再嘗試。而最近因為工作需要調用 Windows DLL 動態鏈接庫（對應 Linux 中 so 文件），而對 JNA 有了入坑體驗，對實際工作中遇到的問題做出總結。
1. 調用 Windows 窗口打印Hello World
- pom 依賴
<dependency>
<groupId>net.java.dev.jna</groupId> <artifactId>jna</artifactId> <version>latest</version> </dependency>
- JNA 在加載驅動時提供兩種加載方式 ， 直接映射 和 接口生成。
接口生成
public interface HelixcsDll extends StdCallLibrary { // loadLibary 為動態鏈接庫加載目錄 HelixcsDll HELIXCS_DLL = Native.loadLibrary("helixcs.dll", HelixcsDll.class); // 在 dll 中存在相同 SomeFunction 的函數 void SomeFunction(String content); } // 調用 HelixcsDll.HELIXCS_DLL.SomeFunction("Hello World");
直接映射
class Helixcs{ static { Native.register("helixcs.dll"); } // 映射為本地方法 public static native void SomeFunction(String content); public static void main(String[] args) { SomeFunction("Hello World"); } }
官方例子：
package com.sun.jna.examples; import com.sun.jna.Library; import com.sun.jna.Native; import com.sun.jna.Platform; /** Simple example of JNA interface mapping and usage. */ public class HelloWorld { // This is the standard, stable way of mapping, which supports extensive // customization and mapping of Java to native types. public interface CLibrary extends Library { CLibrary INSTANCE = (CLibrary) Native.load((Platform.isWindows() ? "msvcrt" : "c"), CLibrary.class); void printf(String format, Object... args); } public static void main(String[] args) { CLibrary.INSTANCE.printf("Hello, World\n"); for (int i=0;i < args.length;i++) { CLibrary.INSTANCE.printf("Argument %d: %s\n", i, args[i]); } } }
2. C 類型和 Java 類型映射。
- JNA 官方提供的默認類型映射
Java primitive types (and their object equivalents) map directly to the native C type of the same size.
Java 原始類型以相同的大小映射 C 類型。
| Native Type | Size | Java Type | Common Windows Types |
| 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 |
未簽名類型作為簽名類型來映射。 C 中枚舉類型可替換為 “int”。
Unsigned types use the same mappings as signed types. C enums are usually interchangeable with "int".
3. 官方提供的詳細的類型映射
| 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) | Buffer |
| pointer (e.g. void*),array | 32- or 64-bit pointer to memory (argument/return)contiguous memory (struct member) | <P> |
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. 經驗總結默認映射關系
盡管 JNA 官方已經提供了詳細的類型映射文檔。但在實際中發現按照官方映射可能出現莫名問題。對此我們在實際開發中對於一些類型的映射報錯，可以參考以下映射做出調整。
| C類型 | JNA類型 | 說明 |
|---|---|---|
| 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. 常見錯誤
存在多個動態鏈接庫之間調用情況，可能缺少其中某一個動態鏈接庫文件。
較大情況下存在參數類型映射錯誤，參考 JNA 類型映射。
無法加載到動態鏈接庫文件路徑，需要將動態鏈接庫放到項目根目錄下，推薦開啟 JNA 驅動加載 debug 模式，
System.setProperty("jna.debug_load", "true");
手動指定動態鏈接庫文件路徑
System.setProperty("jna.library.path",dllResourcePath); System.setProperty("jna.platform.library.path",dllResourcePath);
在JNA crash-protection 中，官方文檔說明的崩潰的主要原因,( These are often caused by improper mappings or invalid arguments passed to the native library.) 未知的參數類型映射導致vm崩潰。在很大程度上使用 Pointer 來作為 Java 通用映射類型。而 JNA 默認程序保護Native.setProtected(true)使得 java 錯誤來代替程序崩潰，可以設置 Native.setProtected(false) 來 dump 出崩潰日志。
問題可以反饋到 JNA Google Group : https://groups.google.com/forum/#!forum/jna-users
6. 參考
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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