NDK二

在升级Android Studio 到3.0后，因为项目的需求需要进行NDK开发，因为以前做过NDK开发，感觉so easy.

呵呵，在按照以前总结的经验，然后一栋操作猛如虎，然后......，各种报错。心里一万头*飞奔。

但是大体的思路还是和以前一样。参照NDK一开发博客。

在Android studio3.0后ndk开发必须要用cmake.

在这里需要注意的是cmake配置必须在ndk12r+下支持。

首先建立CMakeLists.txt文件在app下，必须是txt文件。不然会出现：

Error:Gradle project cmake.path specifies CMakelists.txt but it must be CMakeLists.txt   的错误。

CMakeLists.txt文件

cmake_minimum_required(VERSION 3.4.1)

include_directories(
    ${CMAKE_SOURCE_DIR}/src/main/cpp/include #h文件目录
)
add_library( # Sets the name of the library.
             jni-lib                        #c/cpp代码将要编译成为so库的名称，java代码加载库文件要用这个名称
             SHARED
             #src/main/cpp/hello-cjni.c      #c代码文件路径
             src/main/jni/test.cpp  #cpp代码文件路径 这里可以随意添加c、c++文件
              )
target_link_libraries( # Specifies the target library.
                       jni-lib
                       )

然后配置CmakeLists.txt,

 externalNativeBuild {
        cmake {
            path "CMakeLists.txt"//默认路径是在app/
        }
    }

然后配置cpu类型

externalNativeBuild {
    cmake {
        // Sets a flag to enable format macro constants for the C++ compiler.

        cppFlags "-std=c++11 -D__CORRECT_ISO_CPP_WCHAR_H_PROTO"
    }
    ndk {
        abiFilters "armeabi", "armeabi-v7a","x86"
    }
}

在grade下的整体结构如下

apply plugin: 'com.android.application'

android {
    compileSdkVersion 26
    defaultConfig {
        applicationId "com.wzxl.nativedemo"
        minSdkVersion 19
        targetSdkVersion 26
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        externalNativeBuild {
            cmake {
                // Sets a flag to enable format macro constants for the C++ compiler.

                cppFlags "-std=c++11 -D__CORRECT_ISO_CPP_WCHAR_H_PROTO"
            }
            ndk {
                abiFilters "armeabi", "armeabi-v7a","x86"
            }
        }
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
            externalNativeBuild {
                cmake {
                    // Sets a flag to enable format macro constants for the C++ compiler.

                    cppFlags "-std=c++11 -D__CORRECT_ISO_CPP_WCHAR_H_PROTO"
                }
                ndk {
                    abiFilters "armeabi", "armeabi-v7a","x86"
                }
            }
        }
    }
    externalNativeBuild {
        cmake {
            path "CMakeLists.txt"
        }
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:appcompat-v7:26.1.0'
    implementation 'com.android.support.constraint:constraint-layout:1.1.0'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.2'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
}

然后就是上代码

public class TestNative {
    static {
        System.loadLibrary("jni-lib");
    }
    public static native String getShow();
    public static native int getNum(int a,int b);

}

然后进入Terminal编译器 进入Java 目录下，输入命令：

cd app/src/main/java
javah com.wzxl.nativedemo.TestNative

在Java目录下生成.h文件

在main目录下创建jni文件夹 

把.h文件放到jni目录下

创建test.cpp文件导入.h 

#include"com_wzxl_nativedemo_TestNative.h"

复制.h文件里面的方法 书写返回值

JNIEXPORT jstring JNICALL Java_com_wzxl_nativedemo_TestNative_getShow
  (JNIEnv *env, jobject obj){
    return env->NewStringUTF("hello");
}
JNIEXPORT jint JNICALL Java_com_wzxl_nativedemo_TestNative_getNum
        (JNIEnv *env, jclass, jint a, jint b){
    return a+b;
}

然后rebuild 在build/intermediates/cmake下生成so动态库

然后调用代码：

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        TextView textView = findViewById(R.id.text);
        TestNative nat = new TestNative();
        Log.d("MainActivity",nat.getShow());
        Log.d("MainActivity","a+b的和等于"+nat.getNum(333,333));
        textView.setText(nat.getShow()+"\n"+"a+b的和等于"+nat.getNum(333,333));
    }
}

运行程序吧！！！！！

可能出现

java.lang.UnsatisfiedLinkError: No implementation found for java.lang.String com.wzxl.nativedemo.TestNative.getShow() (tried Java_com_wzxl_nativedemo_TestNative_getShow and Java_com_wzxl_nativedemo_TestNative_getShow__)

这是因为你的.h文件中参数的类型和.cpp文件中的不同。

修改为相同就可以了

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

#ifndef _Included_com_wzxl_nativedemo_TestNative
#define _Included_com_wzxl_nativedemo_TestNative
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_wzxl_nativedemo_TestNative
 * Method:    getShow
 * Signature: ()Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_wzxl_nativedemo_TestNative_getShow
  (JNIEnv *, jobject);

/*
 * Class:     com_wzxl_nativedemo_TestNative
 * Method:    getNum
 * Signature: (II)I
 */
JNIEXPORT jint JNICALL Java_com_wzxl_nativedemo_TestNative_getNum
  (JNIEnv *, jclass, jint, jint);

#ifdef __cplusplus
}
#endif
#endif