有句話說得好：逆向的高度取決于正向開發的高度，

前言：

1.Java 的 native 方法可以通過靜態和動態的方式注冊JNI來鏈接 C/C++中的函式，

2.產生so的方式有兩種：1.cmake編譯 2.通過撰寫android.mk編譯

本篇你將學到：

1.通過靜態注冊來訪問 C/C++中的函式

2.在as中，通過cmake編譯cpp，產生so

3.jni日志列印

開始準備作業

1.在as中新創建一個native c++

2.完成相關配置后，build時會報NDK not configured. Download it with SDK manager.，此時需要配置ndk：

如果你沒有下載過ndk，則點擊提示的 Install NDK '21.0.6113669' and sync project

如果你下載過：點擊File->project structure->sdk location 會看到android ndk location 然后選擇你的ndk安裝的目錄

點擊確認后，會自動進行重新build，

3.build 成功后，此時的結構目錄：

（如果想直接看多so撰寫的話，直接到階段三）

階段一：

1.修改結構目錄如下

2.修改 MainActivity.java代碼如下：

?
package com.test.so.myapplication111111111111;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.widget.TextView;

import com.myso.test1;

public class MainActivity extends AppCompatActivity {

    @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(test1.stringFromJNI());
    }
}

?

新增 test1.java 代碼如下：

package com.myso;

public class test1 {
    // Used to load the 'native-lib' library on application startup.
    static {
        System.loadLibrary("native-lib");
    }


    public  static native String stringFromJNI();

}

此時我們的 stringFromJNI 方法顯示為紅色，不急，將滑鼠移到該方法上會出現下圖紅框中的，點擊它

點擊后會自動跳到 native-lib.cpp中，顯示如下代碼：

直接修改它，如下：（關于cpp中的對應java中的native方法的方法名的命名規則可以去了解下）

我們點擊綠色運行，將app安裝到測驗機中，如不出意外，應該會看到上述的字串：

Hello from C++6666666666666666666

階段二：

1.修改test1.java檔案為：

package com.myso;

public class test1 {
    // Used to load the 'native-lib' library on application startup.
    static {
        System.loadLibrary("native-lib");
    }


    public  static native String stringFromJNI();
    public  static native String stringFromJNI(String str);

}

2.修改 native-lib.cpp中的 Java_com_myso_test1_stringFromJNI 為 Java_com_myso_test1_stringFromJNI__

（多載函式命名需要在最后加上 __）

3.回傳 test1.java 可以看到 public static native String stringFromJNI(String str); 變紅了，將滑鼠移到紅色上進行創建，

4.此時 native-lib.cpp 代碼如下：

5.可以將 Java_com_myso_test1_stringFromJNI__Ljava_lang_String_2 修改如下

extern "C"
JNIEXPORT jstring JNICALL
Java_com_myso_test1_stringFromJNI__Ljava_lang_String_2(JNIEnv *env, jclass clazz, jstring str) {
    const char *c_str = NULL;
    char buff[128] = {0};
    c_str = env->GetStringUTFChars(str, NULL);
    if (c_str == NULL) {
        return NULL;
    }
    sprintf(buff, "hello come from c++: %s", c_str);
    env->ReleaseStringUTFChars(str, c_str);
    return env->NewStringUTF(buff);
}

6.接下來的測驗該多載方法省略，請自行測驗，不出意外的話，你能看到你輸入的字串，

階段三：

1，添加并修改結構目錄如下：

cpp中的 CMakeLists.txt 代碼如下：

#指定需要CMAKE的最小版本
cmake_minimum_required(VERSION 3.4.1)


#C 的編譯選項是 CMAKE_C_FLAGS
# 指定編譯引數，可選
#SET(CMAKE_CXX_FLAGS "-Wno-error=format-security -Wno-error=pointer-sign")

#設定生成的so動態庫最后輸出的路徑
set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${PROJECT_SOURCE_DIR}/../jniLibs/${ANDROID_ABI})

#設定頭檔案搜索路徑（和此txt同個路徑的頭檔案無需設定），可選
#INCLUDE_DIRECTORIES(${CMAKE_CURRENT_SOURCE_DIR}/common)

#指定用到的系統庫或者NDK庫或者第三方庫的搜索路徑，可選，
#LINK_DIRECTORIES(/usr/local/lib)


#為了撰寫多個so,添加子目錄,將會呼叫子目錄中的CMakeLists.txt
ADD_SUBDIRECTORY(one)
ADD_SUBDIRECTORY(two)

jni日志列印，mylog.h 代碼如下：

#include <android/log.h>

#ifndef MYLOG_H
#define MYLOG_H
/*
 * 添加日志列印
 */

#define TAG "testApp" // 這個是自定義的標識
#define LOGD(...) __android_log_print(ANDROID_LOG_DEBUG,TAG ,__VA_ARGS__)
#define LOGI(...) __android_log_print(ANDROID_LOG_INFO,TAG ,__VA_ARGS__)
#define LOGW(...) __android_log_print(ANDROID_LOG_WARN,TAG ,__VA_ARGS__)
#define LOGE(...) __android_log_print(ANDROID_LOG_ERROR,TAG ,__VA_ARGS__)
#define LOGF(...) __android_log_print(ANDROID_LOG_FATAL,TAG ,__VA_ARGS__)


#endif //MYLOG_H

one中 CMakeLists.txt代碼如下：

# 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.
        native-one

        # Sets the library as a shared library.
        SHARED

        # Provides a relative path to your source file(s).
        native-lib1.cpp)

# 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.
        native-one

        # Links the target library to the log library
        # included in the NDK.
        ${log-lib})

native-lib1.cpp代碼如下：

#include <jni.h>
#include <string>
#include "../mylog.h"

extern "C"
JNIEXPORT jstring JNICALL Java_com_myso_test1_stringFromJNI__(JNIEnv *env, jclass clazz) {
    std::string hello = "Hello from C++ 6666666666666666666677";
    return env->NewStringUTF(hello.c_str());
}

extern "C"
JNIEXPORT jstring JNICALL
Java_com_myso_test1_stringFromJNI__Ljava_lang_String_2 (JNIEnv *env, jclass clazz, jstring str) {
    const char *c_str = NULL;
    char buff[128] = {0};
    c_str = env->GetStringUTFChars(str, NULL);
    if (c_str == NULL) {
        LOGI("out of memory.\n");
        return NULL;
    }
    LOGI("lib1 java str：%s", c_str);
    sprintf(buff, "hello come from c++: %s", c_str);
    env->ReleaseStringUTFChars(str, c_str);
    return env->NewStringUTF(buff);

}

two中 CMakeLists.txt代碼如下：

# 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.
        native-two

        # Sets the library as a shared library.
        SHARED

        # Provides a relative path to your source file(s).
        native-lib2.cpp)

# 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.
        native-two

        # Links the target library to the log library
        # included in the NDK.
        ${log-lib})

native-lib2.cpp代碼如下:

#include <jni.h>
#include <string>
#include "../mylog.h"


extern "C"
JNIEXPORT jstring JNICALL
Java_com_myso_test1_stringFromJNI__Ljava_lang_String_2Ljava_lang_String_2(JNIEnv *env, jclass clazz, jstring str, jstring str1) {
    LOGI("我來自：222222222222222222");
    return env->NewStringUTF("2 str2222");
}

extern "C"
JNIEXPORT jstring JNICALL
Java_com_myso_test1_stringFromJNI__Ljava_lang_String_2Ljava_lang_String_2Ljava_lang_String_2(JNIEnv *env, jclass clazz, jstring str, jstring str1, jstring str2) {
    LOGI("我來自：33333333333333333333");
    return env->NewStringUTF("3 str3333");
}

build.gradle(:app)代碼如下：

apply plugin: 'com.android.application'

android {
    compileSdkVersion 30
    buildToolsVersion "30.0.2"

    defaultConfig {
        applicationId "com.test.so.myapplication111111111111"
        minSdkVersion 21
        targetSdkVersion 30
        versionCode 1
        versionName "1.0"

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
        externalNativeBuild {
            cmake {
                cppFlags ""
            }
        }
        ndk {
            ldLibs "log"            //實作__android_log_print
            // 設定支持的so構架
            abiFilters 'armeabi-v7a', 'arm64-v8a'// , 'x86', 'x86_64'
        }
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
    externalNativeBuild {
        cmake {
            path "src/main/cpp/CMakeLists.txt"
            version "3.10.2"
        }
    }
    //解決：在增加多個cpp后,編譯時出現More than one file was found with OS independent path 'lib/arm64-v8a/libnative-two.so'問題，添加如下代碼
    packagingOptions {
        pickFirst 'lib/armeabi-v7a/libnative-one.so'
        pickFirst 'lib/armeabi-v7a/libnative-two.so'
        pickFirst 'lib/arm64-v8a/libnative-one.so'
        pickFirst 'lib/arm64-v8a/libnative-two.so'
    }
}

dependencies {
    implementation fileTree(dir: "libs", include: ["*.jar"])
    implementation 'androidx.appcompat:appcompat:1.2.0'
    implementation 'androidx.constraintlayout:constraintlayout:2.0.4'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test.ext:junit:1.1.2'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.3.0'

}

然后sync now下，發現cpp中的紅色都消失了，

2.點擊 build -> make project 會看到有so產生

3.修改 test1.java 代碼如下：

package com.myso;

public class test1 {
    static {
        System.loadLibrary("native-one");
        System.loadLibrary("native-two");
    }

    public static native String stringFromJNI();
    public static native String stringFromJNI(String str);
    public static native String stringFromJNI(String str,String str1);
    public static native String stringFromJNI(String str,String str1,String str2);

}

3.修改MainActivity.java代碼如下：

package com.test.so.myapplication111111111111;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.widget.TextView;

import com.myso.test1;

public class MainActivity extends AppCompatActivity {

    @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(test1.stringFromJNI());
//        tv.setText(test1.stringFromJNI("1111111199999999999999"));
        tv.setText(test1.stringFromJNI("",""));
    }
}

5.點擊綠色運行安裝app，不出意外，你將在app中看到 2 str2222，在logcat中看到：我來自：222222222222222222