Getting Started with JNI

JNIData types and ANDJava数据类型的映射关系

javaThe correspondence between data types and signature types

Static registration function

根据函数名来建立 java 方法与 JNI 函数的一一对应关系;
命名规则如下：Java+方法的全路径（must be used in the path“_”代替Javain the full path"“.”）

在MainActivity里面定义了一个native方法:

 public native String stringFromJNI();

Follow the naming rules,JNIThe layer's function is registered as ：

extern "C" JNIEXPORT jstring JNICALL
Java_com_anniljing_jnidemo_MainActivity_stringFromJNI(
        JNIEnv *env,
        jobject /* this */) {
    std::string hello = "Hello from C++";
    LOGD("Get native message success");
    return env->NewStringUTF(hello.c_str());
}

动态注册函数

原理：利用 RegisterNatives 方法来注册 java 方法与 JNI 函数的一一对应关系;
实现流程：
1、实现 JNI_OnLoad 方法,该方法在jni.h的头文件中定义,which also definesJNI_OnUnload方法,在加载动态库后,执行动态注册;

JNIEXPORT jint JNI_OnLoad(JavaVM* vm, void* reserved);
JNIEXPORT void JNI_OnUnload(JavaVM* vm, void* reserved);

JNIEXPORT int JNICALL JNI_OnLoad(JavaVM *vm, void *reserved) {
}

2、调用 FindClass 方法,获取 java 对象;

 jclass jcls;
    jcls = env->FindClass(name);
    if (jcls == nullptr) {
        return JNI_FALSE;
    }

3、利用结构体 JNINativeMethod 数组记录 java 方法与 JNI 函数的对应关系;

typedef struct {
    
    const char* name;
    const char* signature;
    void*       fnPtr;
} JNINativeMethod;

name为java层的方法名,signature为javaThe type signature of the layer's parameters and return value,fnPtr为jni层方法名

static JNINativeMethod gMethods[] = {
        {"sum",             "(II)I",                (void *) sum},
        {"getNativeString", "()Ljava/lang/String;", (void *) getMessage}
};

4、调用 RegisterNatives 方法,传入 java 对象,以及 JNINativeMethod 数组,以及注册数目完成注册;

 if (env->RegisterNatives(jcls, methods, nMethods) < 0) {
        return JNI_FALSE;
    }

JNI层访问JavaLayer class properties

JNI层设置JavaLayer class attribute value

第一步、获取到Java类

jclass jc = env->GetObjectClass(people);

第二步、GetFieldID方法获取到JavaLayer class properties

jfieldID fidAge = env->GetFieldID(jc, "age", "I");

第三步、调用SetXXXField()方法,为javaThe layer's class attribute sets the value

env->SetIntField(people, fidAge, 18);

JNI层访问JavaLayer class member methods

第一步、获取Java类

jclass jc = env->GetObjectClass(people);

第二步、调用GetMethodID（）方法,获取jMethodID

jmethodID jMethod = env->GetMethodID(jc, "doIntroduce", "()V");

第三步、调用CallVoidMethod（）方法,调用Java类成员方法

 env->CallVoidMethod(people, jMethod);

问题1：新增加的AndroidLog.hThe header files have been unable to be precompiled<android/log.h>和<jni.h>头文件

解决方法：

先创建空的AndroidLog.h文件,然后在其他的cppSource files are added to their precompiled header files,重新编译一下,AndroidLogThe header files are then re-added to the relevant header files,编译即可通过

问题2：新增加jni_dynamic_load.cpp,实现动态注册jni,CMakeList.txt的编写如下：

新增了如下代码：

add_library(
        dynamicLoad-lib
        SHARED
        jni_dynamic_load.cpp
)

但是编译的时候,throw beforeAndroidLog.hFile undefined exception

解决方法：

1、把jni_dynamic_load.cpp放到 native-lib里面,CMakeList.txt如下：

cmake_minimum_required(VERSION 3.10.2)

project("jnidemo")


add_library(
        native-lib
        SHARED
        native-lib.cpp
        jni_dynamic_load.cpp
)
find_library(
        log-lib
        log
)
include_directories(${CMAKE_HOME_DIRECTORY}/base)
target_link_libraries(
        native-lib
        ${log-lib})

这样的话,in a library.

2、But this is not the effect I wanted at first,I am trying to generate two libraries.The reason must be that there is a problem where the library is linked,于是CMakeList.txt改为如下内容：

cmake_minimum_required(VERSION 3.10.2)

project("jnidemo")


add_library(
        native-lib
        SHARED
        native-lib.cpp
)
add_library(
        dynamicLoad-lib
        SHARED
        jni_dynamic_load.cpp
)
find_library(
        log-lib
        log
)
include_directories(${CMAKE_HOME_DIRECTORY}/base)
target_link_libraries(
        native-lib
        ${log-lib})
target_link_libraries(
        dynamicLoad-lib
        ${log-lib}
)

The core code is the second one at the bottomtarget_link_libraries,就是做了dynamicLoad-lib库与Log库的连接.

target_link_libraries(
        dynamicLoad-lib
        ${log-lib}

问题3：illegal class name

cppThe file is not recognized“.”的路径,要把“.”换成“/”即可

#define JAVA_CLASS "com/anniljing/jnidemo/JniDynamicLoad"

GitHub源码