android 调用jni

(用jni，jna速度差百倍)

1 、 android 里 Hello.java

package com.example.administrator.t1;

public class Hello {
    static {
        System.loadLibrary("sayhello");
    }
    public native void say();
    public native String getStringFromJNI();
}

2、用javah生成头文件到classes目录下

E:andprogramandproplayerJnitestRgbappuildintermediatesjavacdebugclasses>set classpath=E:/andprogram/program/newsdk/platforms/android-30/android.jar;.
后面的;.重要，不然把以前的覆盖了
E:andprogramandproplayerJnitestRgbappuildintermediatesjavacdebugclasses>javah com.example.administrator.t1.Hello

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_example_administrator_t1_Hello */
 
#ifndef _Included_com_example_administrator_t1_Hello
#define _Included_com_example_administrator_t1_Hello
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_example_administrator_t1_Hello
 * Method:    say
 * Signature: ()V
 */
JNIEXPORT void JNICALL Java_com_example_administrator_t1_Hello_say
  (JNIEnv *, jobject);
 
/*
 * Class:     com_example_administrator_t1_Hello
 * Method:    getStringFromJNI
 * Signature: ()Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_example_administrator_t1_Hello_getStringFromJNI
  (JNIEnv *, jobject);
 
#ifdef __cplusplus
}
#endif
#endif

3、下载ndk ,在ubuntu解压

vim /etc/profile,添加

export ANDROID_NDK=/home/wang/program/android-ndk-r21d
export PATH=$ANDROID_NDK:$PATH

source

ndk-build -v

安装好了

4、ubuntu上建立jnitest1目录

Application.mk

jni //目录

Android.mk

test.c

#include <stdio.h>
#include <jni.h>
JNIEXPORT void JNICALL Java_com_example_administrator_t1_Hello_say
  (JNIEnv * env, jobject obj){
  //  LOGI("Hello World~~~");
    return;
}

JNIEXPORT jstring JNICALL Java_com_example_administrator_t1_Hello_getStringFromJNI
  (JNIEnv * env, jobject obj){
    //LOGI("here will return a String 'Hello World'");
    jstring newstring = (*env)->NewStringUTF(env,"Hello World");
    const char* str= (*env)->GetStringUTFChars(env, newstring ,NULL);//使用
GetStringUTFChars方法将"Hello World"转成UTF－8格式的string的对象str
    (*env)->ReleaseStringUTFChars(env, newstring, str);//释放str对象的空间>，如果不显示的调用的话，JVM中会一直保存该对象，不会被垃圾回收器回收，因此就
会导致内存溢出
    return newstring;
}

Android.mk

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)
#LOCAL_MODULE表示生成的动态库名为sayhello
LOCAL_MODULE := sayhello
#LOCAL_SRC_FILES表示使用到的类
LOCAL_SRC_FILES := test.c
#LOCAL_SRC_FILES := sayhello.cpp
LOCAL_LDLIBS := -lm -llog -ljnigraphics

include $(BUILD_SHARED_LIBRARY)

Application.mk

# Uncomment this if you're using STL in your project
# See CPLUSPLUS-SUPPORT.html in the NDK documentation for more information
# APP_STL := stlport_static
#APP_BUILD_SCRIPT :=  Android.mk
#APP_ABI := armeabi-v7a arm64-v8a x86 x86_64

APP_ABI := all
# Min runtime API level
APP_PLATFORM=android-16

ndk-build 生成 libs，拷贝到android studio工程目录下

5、

、build.gradle(Module:app)里在buildTypes 后面添加 sourceSets 部分

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
    sourceSets {
        main {
            jniLibs.srcDirs = ['libs']
        }
    }

6、android studio调用

new Hello().getStringFromJNI()

注意test.c里函数名要和包路径对应