JNA 调用操作系统函数 和 系统调用

linux系统调用syscall 表：https://filippo.io/linux-syscall-table/

Linux Namespace 特性简要介绍

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

其他：

JNI的替代者—使用JNA访问Java外部功能接口

Invoke Syscalls from Java

 

java发起系统调用，本质还是用了JNA，调用OS提供的syacall函数：

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

1. pom 依赖

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

1. 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");
        }
    }

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
");
        for (int i=0;i < args.length;i++) {
            CLibrary.INSTANCE.printf("Argument %d: %s
", i, args[i]);
        }
    }
}

2. C 类型和 Java 类型映射。

1. 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。

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>
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

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