Recently, there are more and more applications related to the Internet of Things, and for developers, the frequency of JNI calls and even NDK development is getting higher and higher, so I will write about my learning experience here, and I hope it will be helpful to those who start learning.
Ready to work:
1. Android Studio is above 2.3 and has support for C++ code, so we configure relevant content to help understand C++ code or methods in JNI in the future. First, open the sdk manager, download
these two tools, and then Check the support for C++ when you create a new project
2. NDK download, you can download it yourself, or you can download it directly in Android Sudio, click File -> OtherSettings -> Default Project Structure..., download in NDK, after finishing the project's local.properties will have ndk configuration information
Let's start to enter the topic, starting with a simple project that returns a string from C++
Create a new ndklibrary model as library, and then create a java class as the declaration of the native method called by jni,
public class StringJNI {
static {
System.loadLibrary("StringJNI");
}
public native String getHelloString();
}
The class is very simple, loads a local library, here this library is what we want to generate, and then defines a native method to get the string, and then build.gradle configures the compilation information
defaultConfig{
ndk{
moduleName "StringJNI" //so file name, may not be the same as the above class name
abiFilters 'armeabi-v7a', 'arm64-v8a' // 'x86', 'x86_64' 这里如果是写‘armeabi’，在新的NDK里
//是不支持导致编译不过
}
}
然后能过javah命令开生成头文件，AS下打开terminal,输入以下命令
这时，如果提示javah不是内部命令，就先去配置环境变量，在有些类里，不加-encoding会报
编码GBK的不可映射字符。
然后就会在当前目录下生成一个头文件com_example_qjl_ndklibrary_StringJNI.h，这个文件的命名是固定的，包名加类名用_连接组成。然后new一个jni folder，把头文件放进去，并创建一个.cpp的C++类，引入头文件，由于前面加了C++的支持，这时我们打#in时，就会有代码补全的提示
有了这些提示，对我们写C++的代码也很有帮助，然后写完C++的方法
#include "com_example_qjl_ndklibrary_StringJni.h"
JNIEXPORT jstring JNICALL Java_com_example_qjl_ndklibrary_StringJni_getHelloString(JNIEnv *env, jobject jobject1) {
return (env) -> NewStringUTF("Hello JNI !");
}
这里先不考究代码的详细信息，只要知道返回了一个String,这个String是方法NewStringUTF转化的java的String。
这时代码的编写就完成了，然后编译代码，成功后会在
build的ndk目录下有相应的so库
如果项目是依赖于整个ndklibrary工程的话，so库不用管，如果是生成jar包用的话，就要把so库放到当前的项目的libs文件夹下。
最后在要使用的地方
new StringJni().getHelloString()就可以了，由于后来类重命名过，所以有些是StringJni有些是StringJNI，这个大家不用理会，知道就好。
好了，整个JNI调用的步骤大概就这样子，希望对大家有所帮助，忘后再详细说说怎样配置，编译so库和一些jni里常用的函数等内容。