AndroidStudio JNI development demo on Mac

AndroidStudio JNI development demo on Mac

Configuration

• Download NDK and CMake
• Configure the ndk path in local.properties
• Add the ndk path to the class path of the system-(export PATH = $PATH:$ ANDROID_NDK_HOME /)

Development

• Add in defaultConfig in app/build.gradle:

ndk {
            moduleName "hello"
            abiFilters "armeabi", "armeabi-v7a"
        }

• Create native methods in Java class

public class Md5Utils {
    
    
    static {
    
    
        System.loadLibrary("hello");//gradle中 ndk中的moduleName
    }

    public static native String sign(String source);
}

• Cd the location in the terminal to the java folder, execute

javah -jni 包名+类名（Md5Utils）

• The above command will produce the Md5Utils.h file under the Java file (the file naming rule is: replace. In the package name with _, and then add the class name .h)
• Create a jni folder under the main folder, copy the produced .h file to the jni folder, then create a .c/.cpp file under the jni folder, and then import the .h file in the .c/.cpp file ,code show as below:

#include <jni.h>
#include <stdlib.h>
#include<string.h>
#include "com_example_demos_Md5Utils.h"

char* Jstring2Cstr(JNIEnv* env, jstring strs){
    
    

char* rtn = NULL;
jclass classString = (*env)->FindClass(env, "java/lang/String");
jstring strenCode = (*env)->NewStringUTF(env,"UTF-8");
jmethodID mid = (*env)->GetMethodID(env,classString,"getBytes","(Ljava/lang/String;)[B");

jbyteArray barr = (jbyteArray) (*env)->CallObjectMethod(env,strs,mid,strenCode);
jsize alen = (*env)->GetArrayLength(env,barr);
jbyte* ba = (*env)->GetByteArrayElements(env,barr,JNI_FALSE);
if(alen > 0){
    
    
rtn = (char*)malloc(alen+1);
memcpy(rtn,ba,alen);
rtn[alen]=0;
}
(*env)->ReleaseByteArrayElements(env,barr,ba,0);
return rtn;
}

 jstring JNICALL Java_com_example_demos_Md5Utils_sign
          (JNIEnv* env, jclass cls, jstring strs){
    
    
    char* str = "::this hello is from jni";

    char* cstr = Jstring2Cstr(env,strs);

    strcat(cstr,str);
    jstring string = (*(*env)).NewStringUTF(env, cstr);
    return string;
}

• Create Android.mk in the jni folder, the content is as follows

# Copyright (C) 2009 The Android Open Source Project
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#      http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

LOCAL_PATH := $(call my-dir) 

include $(CLEAR_VARS)

LOCAL_MODULE    := hello 
LOCAL_SRC_FILES := Md5Utils.c

include $(BUILD_SHARED_LIBRARY)

• Create Application.mk in the root directory of the project with the following content:

# Build all machine code.
APP_ABI := ALL

• In the terminal, cd to the jni directory, and execute the ndk-build command (if it prompts command not found, splice the ndk path before ndk-build: ndkPath/ndk-build)

• After the command is executed successfully, the libs folder will be generated under the main file, and there will be the produced .so file inside.

• Create a jniLibs folder under the app folder, and add sourceSets.main.jniLibs.srcDirs = ['jniLibs'] to specify the lib path of the so file in the defaultConfig in app/build.gradle, and copy the so file generated by the above name to This directory.

• Then you can call the native method in the java code to implement the java call c code.