JavaJNI调用C/C++代码

记于：2024-04-14
地点：浙江省·温州市·家里
天气：阴雨

背景#

因为一个项目需要Java JNI调用C++ SDK，只有编译好的Windows DLL，没有对应的源码。

测试#

简单demo#

JniTest.java

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package com.yeshimin.test.jni;

import java.lang.reflect.Field;
import java.util.Arrays;

public class JniTest {

    // 设置dll加载路径
    static {
        try {
            String path = "C:\\Users\\ysd\\IdeaProjects\\test\\dll";
            // 无效
//            System.setProperty("java.library.path", path);
            // 需要使用这种方式
            // 参考：https://fahdshariff.blogspot.com/2011/08/changing-java-library-path-at-runtime.html
            addLibraryPath(path);
        } catch (Exception e) {
            throw new RuntimeException(e);
        }

        System.loadLibrary("jnitest");
    }

    /**
     * Adds the specified path to the java library path
     *
     * @param pathToAdd the path to add
     * @throws Exception
     */
    public static void addLibraryPath(String pathToAdd) throws Exception {
        final Field usrPathsField = ClassLoader.class.getDeclaredField("usr_paths");
        usrPathsField.setAccessible(true);

        // get array of paths
        final String[] paths = (String[]) usrPathsField.get(null);

        // check if the path to add is already present
        for (String path : paths) {
            if (path.equals(pathToAdd)) {
                return;
            }
        }

        //add the new path
        final String[] newPaths = Arrays.copyOf(paths, paths.length + 1);
        newPaths[newPaths.length - 1] = pathToAdd;
        usrPathsField.set(null, newPaths);
    }

    // ================================================================================

    // 本地方法
    private static native String sayHello(String name);

    public static void main(String[] args) {
        String result = sayHello("ysm");
        System.out.println("result: " + result);
    }
}

生成本地方法对应的C++代码的头部文件；
可以使用javah或javac(jdk版本>=9)命令；
com_yeshimin_test_jni_JniTest.h

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/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_yeshimin_test_JniTest */

#ifndef _Included_com_yeshimin_test_jni_JniTest
#define _Included_com_yeshimin_test_jni_JniTest
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_yeshimin_test_jni_JniTest
 * Method:    sayHello
 * Signature: (Ljava/lang/String;)Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_yeshimin_test_jni_JniTest_sayHello
  (JNIEnv *, jclass, jstring);

#ifdef __cplusplus
}
#endif
#endif

接下需要根据头部文件中函数定义进行实现
JniTest.c

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#include "com_yeshimin_test_jni_JniTest.h"
// <jni.h>里已包含<stdio.h>

/*
 * Class:     com_yeshimin_test_jni_JniTest
 * Method:    sayHello
 * Signature: (Ljava/lang/String;)Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_yeshimin_test_jni_JniTest_sayHello
    (JNIEnv *env, jclass cls, jstring j_str) {

        const char *c_str = NULL;
        char buff[128] = {0};
        c_str = (*env)->GetStringUTFChars(env, j_str, NULL);
        if (c_str == NULL) {
            printf("out of memory\n");
            return NULL;
        }
        sprintf(buff, "Hello, %s\n", c_str);
        (*env)->ReleaseStringUTFChars(env, j_str, c_str);
        return (*env)->NewStringUTF(env, buff);
}

编译

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# 编译JniTest.c，输出jnitest.dll，并将其放到自定义dll目录下
# 我的自定义dll目录为C:\Users\ysd\IdeaProjects\test\dll
gcc -m64 -shared -o jnitest.dll -I"C:\Program Files\Eclipse Adoptium\jdk-8.0.402.6-hotspot\include" -I"C:\Program Files\Eclipse Adoptium\jdk-8.0.402.6-hotspot\include\win32" .jni\JniTest.c

# jni需要jni.h相关头部，这里需要指定头部所在目录；具体位置因系统和jdk版本而异
# -I"C:\Program Files\Eclipse Adoptium\jdk-8.0.402.6-hotspot\include" -I"C:\Program Files\Eclipse Adoptium\jdk-8.0.402.6-hotspot\include\win32"

运行

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result: Hello, ysm

不同包下#

场景为，调用点所在的包（和类）与目标本地代码的声明不一致时，调用会报错；
！！！重要前提，场景假设为，目标代码是编译好的dll；直接源码编译到一起的不算~

注意，JniTest.java和JniTestInternal.c是在不同包下，JniTestInternal是目标代码，JniTest是调用类

JniTest.java

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package com.yeshimin.test.jni;

import java.lang.reflect.Field;
import java.util.Arrays;

public class JniTest {

    // 设置dll加载路径
    static {
        // 略...

        System.loadLibrary("jnitestinternal");
    }

    // 本地方法
    private static native String sayHelloInternal(String name);

    public static void main(String[] args) {
        String result = sayHelloInternal("ysm");
        System.out.println("result: " + result);
    }
}

org_sample_JniTestInternal.h

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/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class org_sample_JniTest */

#ifndef _Included_org_sample_JniTestInternal
#define _Included_org_sample_JniTestInternal
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     org_sample_JniTestInternal
 * Method:    sayHelloInternal
 * Signature: (Ljava/lang/String;)Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_org_sample_JniTestInternal_sayHelloInternal
  (JNIEnv *, jclass, jstring);

#ifdef __cplusplus
}
#endif
#endif

JniTestInternal.c

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#include "org_sample_JniTestInternal.h"

/*
 * Class:     org_sample_JniTestInternal
 * Method:    sayHelloInternal
 * Signature: (Ljava/lang/String;)Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_org_sample_JniTestInternal_sayHelloInternal
    (JNIEnv *env, jclass cls, jstring j_str) {
    const char *c_str = NULL;
    char buff[128] = {0};
    c_str = (*env)->GetStringUTFChars(env, j_str, NULL);
    if (c_str == NULL) {
        printf("out of memory\n");
        return NULL;
    }
    sprintf(buff, "Hello, %s\n", c_str);
    (*env)->ReleaseStringUTFChars(env, j_str, c_str);
    return (*env)->NewStringUTF(env, buff);
}

编译

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# 编译JniTestInternal.c，输出jnitestinternal.dll，并将其放到自定义dll目录下
gcc -m64 -shared -o jnitestinternal.dll -I"C:\Program Files\Eclipse Adoptium\jdk-8.0.402.6-hotspot\include" -I"C:\Program Files\Eclipse Adoptium\jdk-8.0.402.6-hotspot\include\win32" .jni\JniTestInternal.c

运行

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result: Hello, ysm

如果将类名修改，使得调用点所在包（和类名）与目标定义不一致，比如将类名修改为”OtherName”，编译运行后，将会报错：

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Exception in thread "main" java.lang.UnsatisfiedLinkError: org.sample.OtherName.sayHelloInternal(Ljava/lang/String;)Ljava/lang/String;
	at org.sample.OtherName.sayHelloInternal(Native Method)
	at org.sample.OtherName.main(OtherName.java:60)

解决方案
使用“代理”的方式调用目标方法；即在同包名（和类名）对应的本地实现中包含目标头部，并且封装一个方法进行目标调用；具体见下面代码

JniTest.java

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package com.yeshimin.test.jni;

import java.lang.reflect.Field;
import java.util.Arrays;

public class JniTest {

    // 设置dll加载路径
    static {
        // 略...

        System.loadLibrary("jnitest");
        System.loadLibrary("jnitestinternal"); // 依赖的dll也要加载
    }

    private static native String sayHello(String name);

    private static native String sayHelloByProxy(String name);

    private static native String sayHelloInternal(String name);

    public static void main(String[] args) {

        // 调用同包名下的本地方法；执行成功
        String result = sayHello("ysm");
        System.out.println("result: " + result);

        // “代理”方式下，调用不同包名下的本地方法；执行成功
        String result2 = sayHelloByProxy("ysm");
        System.out.println("result2: " + result2);

        // 调用不同包名下的本地方法；执行报错
        String result3 = sayHelloInternal("ysm");
        System.out.println("result3: " + result3);
    }
}

com_yeshimin_test_jni_JniTest.h

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/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_yeshimin_test_JniTest */

#ifndef _Included_com_yeshimin_test_jni_JniTest
#define _Included_com_yeshimin_test_jni_JniTest
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_yeshimin_test_jni_JniTest
 * Method:    sayHello
 * Signature: (Ljava/lang/String;)Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_yeshimin_test_jni_JniTest_sayHello
  (JNIEnv *, jclass, jstring);

/*
* Class:     com_yeshimin_test_jni_JniTest
* Method:    sayHelloByProxy
* Signature: (Ljava/lang/String;)Ljava/lang/String;
*/
JNIEXPORT jstring JNICALL Java_com_yeshimin_test_jni_JniTest_sayHelloByProxy
  (JNIEnv *, jclass, jstring);

#ifdef __cplusplus
}
#endif
#endif

JniTest.c

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#include "com_yeshimin_test_jni_JniTest.h"
#include "org_sample_JniTestInternal.h"

/*
 * Class:     com_yeshimin_test_jni_JniTest
 * Method:    sayHello
 * Signature: (Ljava/lang/String;)Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_yeshimin_test_jni_JniTest_sayHello
    (JNIEnv *env, jclass cls, jstring j_str) {

        const char *c_str = NULL;
        char buff[128] = {0};
        c_str = (*env)->GetStringUTFChars(env, j_str, NULL);
        if (c_str == NULL) {
            printf("out of memory\n");
            return NULL;
        }
        sprintf(buff, "Hello, %s\n", c_str);
        (*env)->ReleaseStringUTFChars(env, j_str, c_str);
        return (*env)->NewStringUTF(env, buff);
}

/*
 * Class:     com_yeshimin_test_jni_JniTest
 * Method:    sayHelloByProxy
 * Signature: (Ljava/lang/String;)Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_yeshimin_test_jni_JniTest_sayHelloByProxy
    (JNIEnv *env, jclass cls, jstring j_str) {

    return Java_org_sample_JniTestInternal_sayHelloInternal(env, cls, j_str);
}

org_sample_JniTestInternal.h

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/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class org_sample_JniTest */

#ifndef _Included_org_sample_JniTestInternal
#define _Included_org_sample_JniTestInternal
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     org_sample_JniTestInternal
 * Method:    sayHelloInternal
 * Signature: (Ljava/lang/String;)Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_org_sample_JniTestInternal_sayHelloInternal
  (JNIEnv *, jclass, jstring);

#ifdef __cplusplus
}
#endif
#endif

JniTestInternal.c

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#include "org_sample_JniTestInternal.h"

/*
 * Class:     org_sample_JniTestInternal
 * Method:    sayHelloInternal
 * Signature: (Ljava/lang/String;)Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_org_sample_JniTestInternal_sayHelloInternal
    (JNIEnv *env, jclass cls, jstring j_str) {
    const char *c_str = NULL;
    char buff[128] = {0};
    c_str = (*env)->GetStringUTFChars(env, j_str, NULL);
    if (c_str == NULL) {
        printf("out of memory\n");
        return NULL;
    }
    sprintf(buff, "Hello, %s\n", c_str);
    (*env)->ReleaseStringUTFChars(env, j_str, c_str);
    return (*env)->NewStringUTF(env, buff);
}

编译

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# 编译JniTestInternal.c，输出jnitestinternal.dll，并将其放到自定义dll目录下
gcc -m64 -shared -o jnitestinternal.dll -I"C:\Program Files\Eclipse Adoptium\jdk-8.0.402.6-hotspot\include" -I"C:\Program Files\Eclipse Adoptium\jdk-8.0.402.6-hotspot\include\win32" .jni\JniTestInternal.c

# 编译JniTest.c，输出jnitest.dll，并将其放到自定义dll目录下
gcc -m64 -shared -o jnitest.dll -I"C:\Program Files\Eclipse Adoptium\jdk-8.0.402.6-hotspot\include" -I"C:\Program Files\Eclipse Adoptium\jdk-8.0.402.6-hotspot\include\win32" -L"C:\Users\ysd\IdeaProjects\test\dll" -l"jnitestinternal" .jni\JniTest.c

# -L"C:\Users\ysd\IdeaProjects\test\dll" -l"jnitestinternal" // 用于指定依赖的dll

运行

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result: Hello, ysm

result2: Hello, ysm

Exception in thread "main" java.lang.UnsatisfiedLinkError: com.yeshimin.test.jni.JniTest.sayHelloInternal(Ljava/lang/String;)Ljava/lang/String;
	at com.yeshimin.test.jni.JniTest.sayHelloInternal(Native Method)
	at com.yeshimin.test.jni.JniTest.main(JniTest.java:69)

调用dll#

场景跟上面【不同包下】类似，区别为目标是第三方sdk中的dll文件，且没有头部文件

JniTest.java

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package com.yeshimin.test.jni;

import java.lang.reflect.Field;
import java.util.Arrays;

public class JniTest {

    // 设置dll加载路径
    static {
        // 略...

        System.loadLibrary("jnitest");
        System.loadLibrary("jnitestinternal"); // 依赖的dll也要加载
    }

    private static native int dynamicInvoke();

    public static void main(String[] args) {
        int intResult = dynamicInvoke();
        System.out.println("intResult: " + intResult);
    }
}

com_yeshimin_test_jni_JniTest.h

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/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_yeshimin_test_JniTest */

#ifndef _Included_com_yeshimin_test_jni_JniTest
#define _Included_com_yeshimin_test_jni_JniTest
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_yeshimin_test_jni_JniTest
 * Method:    dynamicInvoke
 * Signature: (Ljava/lang/Integer;)Ljava/lang/Integer;
 */
JNIEXPORT jint JNICALL Java_com_yeshimin_test_jni_JniTest_dynamicInvoke
  (JNIEnv *, jclass);

#ifdef __cplusplus
}
#endif
#endif

JniTest.c

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#include <windows.h>
#include <stdbool.h>
#include "com_yeshimin_test_jni_JniTest.h"
#include "org_sample_JniTestInternal.h"

/*
 * Class:     com_yeshimin_test_jni_JniTest
 * Method:    dynamicInvoke
 * Signature: (Ljava/lang/Integer;)Ljava/lang/Integer;
 */
JNIEXPORT jint JNICALL Java_com_yeshimin_test_jni_JniTest_dynamicInvoke
    (JNIEnv *env, jclass cls) {

    typedef int (__stdcall *TargetMethodPtr)(); // 声明 TargetMethodPtr 类型，名称自定义
    TargetMethodPtr targetMethod;

    HMODULE hModule = LoadLibrary(TEXT("Log_MD_VC120_v3_1.dll")); // 加载DLL
    if (hModule != NULL) {
        targetMethod = (TargetMethodPtr)GetProcAddress(hModule, "?ConfigureFromEnvironment@CLog@GenICam_3_1@@SA_NXZ"); // 获取函数地址
        if (targetMethod != NULL) {
            printf("function found");
            bool result = targetMethod(); // 调用DLL函数
            FreeLibrary(hModule); // 释放DLL模块
            return result ? 1 : 0;
        } else {
            printf("function not found");
        }
        FreeLibrary(hModule); // 释放DLL模块
    } else {
        // getLastError
        DWORD dwError = GetLastError();
        printf("Error code: %lu\n", dwError);
    }
    return -1; // 返回 NULL 表示失败
}

编译

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# 编译JniTestInternal.c，输出jnitestinternal.dll，并将其放到自定义dll目录下
gcc -m64 -shared -o jnitestinternal.dll -I"C:\Program Files\Eclipse Adoptium\jdk-8.0.402.6-hotspot\include" -I"C:\Program Files\Eclipse Adoptium\jdk-8.0.402.6-hotspot\include\win32" .jni\JniTestInternal.c

# 编译JniTest.c，输出jnitest.dll，并将其放到自定义dll目录下
gcc -m64 -shared -o jnitest.dll -I"C:\Program Files\Eclipse Adoptium\jdk-8.0.402.6-hotspot\include" -I"C:\Program Files\Eclipse Adoptium\jdk-8.0.402.6-hotspot\include\win32" -L"C:\Users\ysd\IdeaProjects\test\dll" -l"jnitestinternal" .jni\JniTest.c

# -L"C:\Users\ysd\IdeaProjects\test\dll" -l"jnitestinternal" // 用于指定依赖的dll

运行
将上面两个dll以及用到的第三方dll及其依赖dll放到项目根目录下才行正常运行，原因未知，见【遇到的问题】第3点

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intResult: 1

遇到的问题#

1.提示32位代码在64位架构下跑不通；是下载错了mingw架构，应该下载64位的
2.指定自定义dll目录无效；解决方法见上面【简单demo】代码
3.在【调用dll】测试过程中，dll文件放到自定义dll目录下，链接不到，放到项目根目录下就可以，暂时搞不清楚
4.在【调用dll】测试过程中，一直获取不到目标函数；
开始是通过dllexp工具查看的函数函数名称为 public: static bool __cdecl GenICam_3_1::CLog::ConfigureFromEnvironment(void)，
也试过 GenICam_3_1::CLog::ConfigureFromEnvironment(void) 和 GenICam_3_1::CLog::ConfigureFromEnvironment，都不行
后来是通过visual studio的dumpbin工具查看到dll的正确函数名 ?ConfigureFromEnvironment@CLog@GenICam_3_1@@SA_NXZ

参考资料#