Android jni开发-0（用javah方式编译jni）

在Android studio2.2以前大部分用的是用javah的方式来编译jni，在编译之前首先确保有安装ndk，如果没下载，则下载ndk，下载到默认目录就好。如图所示：

下面具体介绍一下流程：

1、新建项目JniDemo,打开local.properties，在最下面加上（此处根据你的sdk和ndk目录而定）：

ndk.dir=E\:\\AndroidStudio\\sdk\\ndk-bundle
sdk.dir=E\:\\AndroidStudio\\sdk

如图：

2、新建类JniUtil.java，添加native方法和指定的lib库，如图：

public class JniUtil {    public native String getStringFromJni();    public native int getSum(int i,int j);    static {        System.loadLibrary("Jni_lib");    }}

3、打开app目录下的build.gradle文件，添加如下配置，注意moduleName后面的要和System.loadLibrary(“Jni_lib”)保持一致：

ndk {
            moduleName "Jni_lib" //编译生成so库的名字，注意不要lib，和.so加进来  ,而且要和上面loadLibrary里面的参数一致
            abiFilters "armeabi","armeabi-v7a","x86"//编译支持的平台
        }

4、在gradle.properties文件的最后一行添加一句：android.useDeprecatedNdk=true

5、此时点击build -> Make Project,然后在app->build->intermediates->classes->debug->com下面会看到编译的文件，JniUtil.class就是刚才编译的文件，如图：

6、利用cd命令进入到main目录下：

然后利用javah -d jni -classpath命令，javah -d jni -classpath E:\AndroidStudioProjects\ndk\app\build\intermediates\classes\debug com.sonic.jnidemo.JniUtil

如下图：

 注意：红框里面的是debug文件夹的绝对路径(项目的存放目录)，后面以包名的形式对应JniUtil这个类，此时会  在main目录下看到自动生成一个jni文件夹，里面有一个.h文件，如图：

 7、新建main.c文件，然后导入刚才生成的.h头文件，并且将头文件里面的方法复制过来实现即可：

#include "com_sonic_jnidemo_JniUtil.h"JNIEXPORT jstring JNICALL Java_com_sonic_jnidemo_JniUtil_getStringFromJni  (JNIEnv *env, jobject j){    return (*env)->NewStringUTF(env,"I am jni!");  }JNIEXPORT jint JNICALL Java_com_sonic_jnidemo_JniUtil_getSum        (JNIEnv *env, jobject j, jint m, jint n){    return m + n;}

8、在MainActivity里面调用：

至此，如果你的Android studio在2.3以下，现在运行就可以看到结果了，但是如果你升级到了3.0以上，会发现运行出错，提示如下：

Error: Flag android.useDeprecatedNdk is no longer supported and will be removed in the next version of Android Studio.  Please switch to a supported build system.    Consider using CMake or ndk-build integration. For more information, go to:     https://d.android.com/r/studio-ui/add-native-code.html#ndkCompile     To get started, you can use the sample ndk-build script the Android     plugin generated for you at:     E:\androidproject\JniDemo2\app\build\intermediates\ndk\debug\Android.mk    Alternatively, you can use the experimental plugin:     https://developer.android.com/r/tools/experimental-plugin.html    To continue using the deprecated NDK compile for another 60 days, set     android.deprecatedNdkCompileLease=1511489294355 in gradle.properties

是什么原因导致的呢？就是useDeprecatedNdk这个东西不再支持了，请切换到 CMake or ndk-build来编译Jni的C换C++代码，解决方式如下，打开app目录下的build.gradle，继续添加配置：

apply plugin: 'com.android.application'android {    compileSdkVersion 26    defaultConfig {        applicationId "com.sonic.jnidemo"        minSdkVersion 18        targetSdkVersion 26        versionCode 1        versionName "1.0"        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"        ndk {            moduleName "Jni_lib" //编译生成so库的名字，注意不要lib，和.so加进来  ,而且要和上面loadLibrary里面的参数一致            abiFilters "armeabi","armeabi-v7a","x86"//编译支持的平台        }    }    sourceSets.main {//            jniLibs.srcDir 'libs'        jni.srcDirs = [] //disable automatic ndk-build call    }    externalNativeBuild {        cmake {            path "CMakeLists.txt"        }    }    buildTypes {        release {            minifyEnabled false            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'        }    }}dependencies {    implementation fileTree(dir: 'libs', include: ['*.jar'])    implementation 'com.android.support:appcompat-v7:26.1.0'    implementation 'com.android.support.constraint:constraint-layout:1.0.2'    testImplementation 'junit:junit:4.12'    androidTestImplementation 'com.android.support.test:runner:1.0.1'    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.1'}

然后在main文件下面新建CMakeLists.txt文件，类容如下：

# For more information about using CMake with Android Studio, read the# documentation: https://d.android.com/studio/projects/add-native-code.html# Sets the minimum version of CMake required to build the native library.cmake_minimum_required(VERSION 3.4.1)# Creates and names a library, sets it as either STATIC# or SHARED, and provides the relative paths to its source code.# You can define multiple libraries, and CMake builds them for you.# Gradle automatically packages shared libraries with your APK.add_library( # Sets the name of the library.             Jni_lib             # Sets the library as a shared library.             SHARED             # Provides a relative path to your source file(s).             src/main/jni/main.c )# Searches for a specified prebuilt library and stores the path as a# variable. Because CMake includes system libraries in the search path by# default, you only need to specify the name of the public NDK library# you want to add. CMake verifies that the library exists before# completing its build.find_library( # Sets the name of the path variable.              log-lib              # Specifies the name of the NDK library that              # you want CMake to locate.              log )# Specifies libraries CMake should link to your target library. You# can link multiple libraries, such as libraries you define in this# build script, prebuilt third-party libraries, or system libraries.target_link_libraries( # Specifies the target library.                       Jni_lib                       # Links the target library to the log library                       # included in the NDK.                       ${log-lib} )

此时再去编译运行就没问题了，运行截图：

demo地址：点击打开链接