jni之hello world

最新推荐文章于 2019-04-06 15:36:05 发布

hb707934728

最新推荐文章于 2019-04-06 15:36:05 发布

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分类专栏： ndk

本文链接：https://blog.youkuaiyun.com/hb707934728/article/details/53762823

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这篇博客介绍了JNI开发的第一步，如何生成JNI头文件。通过创建包含native方法的Java文件，使用`javah`命令生成头文件，然后将头文件放入项目指定目录。接着配置项目的NDK路径，修改`local.properties`和`gradle.properties`文件，最后准备编写`android.mk`文件。

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第一步:生成jni头文件

建一个java文件,里面声明你的native

package test.com.myndk2;

/**
 * Created by hbin on 2016/12/20.
 */

public class NdkUtils {
    public native String getNativeString();
}

进入build\intermediates\classes\debug目录把里面的整个包都拷出来放桌面上（随便放哪儿都行）然后cmd中执行 javah -classpath . test.com.myndk2.NdkUtils

还有种方式直接cmd进入 build\intermediates\classes\debug目录执行 javah -classpath . test.com.myndk2.NdkUtils(注意中间有个 . 表示当前目录)

请确保上面的test目录和执行命令的目录在同一级别上,然后jni头文件就生成了(在main目录下面新建jni目录把这个头文件放入jni目录)。

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class test_com_myndk2_NdkUtils */

#ifndef _Included_test_com_myndk2_NdkUtils
#define _Included_test_com_myndk2_NdkUtils
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     test_com_myndk2_NdkUtils
 * Method:    getNativeString
 * Signature: ()Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_test_com_myndk2_NdkUtils_getNativeString
  (JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif

配置build.gradle

apply plugin: 'com.android.application'

def ndkDir = 'E:\\ndk\\android-ndk-r10e'
android {
    compileSdkVersion 23
    buildToolsVersion "24.0.3"
    defaultConfig {
        applicationId "test.com.myndk2"
        minSdkVersion 16
        targetSdkVersion 23
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        //ndk配置
        ndk{
            moduleName"myNativeLib"
            ldLibs "log", "z", "m"
            abiFilters "armeabi", "armeabi-v7a", "x86"
        }
    }

    //ndk配置
    sourceSets.main {
        jni.srcDirs = []
        jniLibs.srcDir 'src/main/libs'
    }

    //ndk配置
    task buildNative(type: Exec, description: 'Compile JNI source via NDK') {
        commandLine "$ndkDir\\ndk-build.cmd",
                '-C', file('src\\main\\jni').absolutePath, // Change src/main/jni the relative path to your jni source
                '-j', Runtime.runtime.availableProcessors(),
                'all',
                'NDK_DEBUG=1'
    }
    //ndk配置
    task cleanNative(type: Exec, description: 'Clean JNI object files') {
        commandLine "$ndkDir\\ndk-build.cmd",
                '-C', file('src\\main\\jni').absolutePath, // Change src/main/jni the relative path to your jni source
                'clean'
    }
    //ndk配置
    clean.dependsOn 'cleanNative'
    tasks.withType(JavaCompile) {
        compileTask -> compileTask.dependsOn buildNative
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'
    })
    compile 'com.android.support:appcompat-v7:23.4.0'
    testCompile 'junit:junit:4.12'
}

配置项目的ndk路径,这个就不说了，选中项目在file菜单下面的 project structure

配置项目根目录下面的local.properties 增加 ndk.dir=E\:\\ndk\\android-ndk-r10e(这个一般都自动生成了)

gradle.properties配置文件中新增：android.useDeprecatedNdk=true

最后编写android.mk

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := myNativeLib
LOCAL_SRC_FILES :=test_com_myndk2_NdkUtils.cpp
include $(BUILD_SHARED_LIBRARY)

编写cpp

#include "test_com_myndk2_NdkUtils.h"
 #include <stdio.h>
 #include <unistd.h>

 JNIEXPORT jstring JNICALL Java_test_com_myndk2_NdkUtils_getNativeString (JNIEnv *env, jobject obj)
 {
     return env->NewStringUTF("HelloFrom JNI!");
 }

验证执行结果

package test.com.myndk2;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;

public class MainActivity extends AppCompatActivity {

    static {
       System.loadLibrary("myNativeLib");
    }
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        NdkUtils ndkUtils=new NdkUtils();
        Log.i("hb-4","ndk返回的字符串："+ndkUtils.getNativeString());
    }
}

执行结果