JNI实战（一）：JNI HelloWorld

使用最新Android Studio的Cmake，创建一个Native C++项目后，我们就可以看到JNI的Hello World的项目及示例代码了。

JNI的项目代码，分为三层：Java层，C++层，CMake层。

其中Java层定义了调用C++的方法，C++层实现这个方法，CMake定义了如何将C++层的内容打包为一个动态库。

1. Java 层：

package com.renhui.jnihelloworld;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    // Used to load the 'native-lib' library on application startup.
    static {
        System.loadLibrary("native-lib");
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Example of a call to a native method
        TextView tv = findViewById(R.id.sample_text);
        tv.setText(stringFromJNI());
    }

    /**
     * A native method that is implemented by the 'native-lib' native library,
     * which is packaged with this application.
     */
    public native String stringFromJNI();
}

2. C++层：

#include <jni.h>
#include <string>

extern "C"   // 指示编译器这部分代码按C语言的进行编译
JNIEXPORT jstring JNICALL Java_com_renhui_jnihelloworld_MainActivity_stringFromJNI(JNIEnv *env, jobject /* this */) {
    std::string hello = "Hello from C++";
    return env->NewStringUTF(hello.c_str());
}

3. CMake编译脚本:

CMakeList.txt 

# 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.
        native-lib   // 生成的库的名字

        # Sets the library as a shared library.
        SHARED      // 动态库

        # Provides a relative path to your source file(s).
        native-lib.cpp)    // 使用那些文件生成这个库

# 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.    // 将哪些库链接到一起
        native-lib     // 目标库

        # Links the target library to the log library
        # included in the NDK.
        ${log-lib})    // 要链接的库