gradle3.0以上進行jni呼叫的時候和之前的版本會略有不同

首先第一步:

首先新建一個工程, 我的工程目錄如下:

我本地使用的gradle版本:

app的build.gradle內容如下所示:

android {
apply plugin: 'com.android.application'

android {
    compileSdkVersion 27
    defaultConfig {
        applicationId "com.jni.jnidemo5"
        minSdkVersion 26
        targetSdkVersion 27
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:appcompat-v7:27.0.2'
    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'
}
 

以上就是新建的工程的基本設定, 接下來就是正式開始:

首先在包下新建一個JNINative.java的類用於寫native方法

package com.jni.jnidemo5;

public class JNINative {

    public native String getString();
}

很簡單隻有一個獲取字串的方法.

使用javah生成對應的.h檔案

具體方法如下:

開啟Android Studio的Terminal(控制檯)

輸入: cd app/src/main/java

輸入: javah com.jni.jnidemo5.JNINative

會在工程的 app/src/main/java/下生成對應的.h檔案

生成的.h檔案內容如下:

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_jni_jnidemo5_JNINative */

#ifndef _Included_com_jni_jnidemo5_JNINative
#define _Included_com_jni_jnidemo5_JNINative
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_jni_jnidemo5_JNINative
 * Method:    getString
 * Signature: ()Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_jni_jnidemo5_JNINative_getString
  (JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif
 

編寫對應的C檔案

在src上右鍵新建一個jni目錄, 目錄結構如下

在jni目錄下右鍵 新建一個hellotest.c檔案

檔案的內容如下(根據生成的.h檔案修改而成):

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_jni_jnidemo5_JNINative */

#ifndef _Included_com_jni_jnidemo5_JNINative
#define _Included_com_jni_jnidemo5_JNINative
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_jni_jnidemo5_JNINative
 * Method:    getString
 * Signature: ()Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_jni_jnidemo5_JNINative_getString
  (JNIEnv *env, jclass jobject){
 return (*env)->NewStringUTF(env,"hello world from c!!!");
  }
#ifdef __cplusplus
}
#endif
#endif

注意這裡修改的內容:

配置NDK環境

點選project structure 設定工程的ndk目錄

接下來是和低版本的gradle不一致的地方了, 因為gradle3.0以上useDeprecatedNdk這個東西不推薦使用了, 也就是我們不需要在gradle.properties裡新增android.useDeprecatedNdk=true這句話了

如果我們還使用這個東東, studio就會提示我們:

Error:Execution failed for task ':app:compileDebugNdk'. > 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.

推薦我們使用CMake, 這裡我就選擇CMake方式

CMake方式實現編譯so檔案

1. 開啟SDK Manager

勾選箭頭指向的兩個外掛, 安裝

2.在app的build.gradle裡的deaultConfig節點裡新增如下程式碼:

externalNativeBuild {
    cmake {
        cppFlags ""
        //生成多個版本的so檔案
        abiFilters 'arm64-v8a','armeabi-v7a','x86','x86_64'
    }
}

如下圖所示:

 3.在android節點裡新增如下程式碼

externalNativeBuild {
    cmake {
        path "CMakeLists.txt"  // 該檔案配置了c原始碼位置，以及編譯後so檔案的名字

    }
}

如下圖所示

4.在和app裡的build.gradle的同級目錄裡新建一個名為CMakeLists.txt的檔案, 並copy如下程式碼

# 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.
#CMakeLists.txt
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.
      # 設定so檔名稱.
       hello

       # Sets the library as a shared library.
       SHARED
       # 設定這個so檔案為共享.

       # Provides a relative path to your source file(s).
       # 設定我們的c/c++檔案路徑, 注意檔名要和我們的c檔案相同
       src/main/jni/hellotest.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.
            # 制定目標庫.注意要和上面的so檔名稱一致
            hello

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

注意自己要修改裡面帶註釋的地方, 主要是c檔名和編譯後的so的檔名

5.然後Build工程我們就能看到編譯成功的so庫檔案了, 如下所是:

6.Copy生成的so檔案到lib目錄下:

7.修改MainActivity裡的程式碼讓我們的程式跑起來

package com.jni.jnidemo5;

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

public class MainActivity extends AppCompatActivity {

    private TextView tv;

    static {
        System.loadLibrary("hello");
    }

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

        tv = findViewById(R.id.tv);

        tv.setText(new JNINative().getString());

    }
}

點選run執行結果如下:

demo下載地址:

https://github.com/githubStudy/jnidemo