JNA 調用操做系統函數 和 系統調用

linux系統調用syscall 表：https://filippo.io/linux-syscall-table/

Linux Namespace 特性簡要介紹

原文：https://iliangqunru.bitcron.com/post/2018/jna-shi-ji-kai-fa-zhong-ruo-gan-wen-ti-jie-jue-fang-fa

其餘：

JNI的替代者—使用JNA訪問Java外部功能接口

Invoke Syscalls from Java

 

java發起系統調用，本質仍是用了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");
    }
}

 

 

 

很早就已經據說過 JNI（Java Native Interface）Java 本地接口，奈何調用本地動態連接庫太過於複雜，就一直沒有再嘗試。而最近由於工做須要調用 Windows DLL 動態連接庫（對應 Linux 中 so 文件），而對 JNA 有了入坑體驗，對實際工做中遇到的問題作出總結。

1. 調用 Windows 窗口打印Hello World

1. pom 依賴

<dependency>
            <groupId>net.java.dev.jna</groupId> <artifactId>jna</artifactId> <version>latest</version> </dependency> 

1. 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 爲動態連接庫加載目錄 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"); } // 映射爲本地方法 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。

1. JNA 官方提供的默認類型映射

Default Type Mappings 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

未簽名類型做爲簽名類型來映射。 C 中枚舉類型可替換爲 「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

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>
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係

儘管 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。對此咱們在實際開發中對於一些類型的映射報錯，能夠參考如下映射作出調整。

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

1. UnsatisfiedLinkError 問題

存在多個動態連接庫之間調用狀況，可能缺乏其中某一個動態連接庫文件。

2. Error Memory Access 問題

較大狀況下存在參數類型映射錯誤，參考 JNA 類型映射。

3. dll can not find in win32/86

沒法加載到動態連接庫文件路徑，須要將動態連接庫放到項目根目錄下，推薦開啓 JNA 驅動加載 debug 模式，

System.setProperty("jna.debug_load", "true"); 

手動指定動態連接庫文件路徑

System.setProperty("jna.library.path",dllResourcePath); System.setProperty("jna.platform.library.path",dllResourcePath); 

4. 程序在運行一段時間後崩潰

在JNA crash-protection 中，官方文檔說明的崩潰的主要緣由,( These are often caused by improper mappings or invalid arguments passed to the native library.) 未知的參數類型映射致使vm崩潰。在很大程度上使用 Pointer 來做爲 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，能夠設置 Native.setProtected(false) 來 dump 出崩潰日誌。

問題能夠反饋到 JNA Google Group : https://groups.google.com/forum/#!forum/jna-users

6. 參考

1. JNA Google Group : https://groups.google.com/forum/#!forum/jna-users

2. JNA API Documentation: https://java-native-access.github.io/jna/4.2.1/overview-summary.html

3. JNA VM Crashed Protection :http://java-native-access.github.io/jna/4.5.1/javadoc/overview-summary.html#crash-protection