实例详解Android中JNI的使用方法

前言

做Android开发的程序员应该都知道，Android的开发语言我们都是在使用JAVA(Kotlin和Flutter我们暂时不考虑)。但是，有时候我们也需要使用到C语言进行一些功能的开发。这个时候我们就需要用到JNI了。

1.导入C语言的类

首先我们需要把C语言写的功能类放入我们的项目中。这里我直接从资料中找了一个，毕竟我不会写。路径在src/main/jni中

find_name.cpp

#include <jni.h>
#include <string.h>
 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <netdb.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/select.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
 
#define send_MAXSIZE 50
#define recv_MAXSIZE 1024
 
struct NETBIOSNS {
	unsigned short int tid; //unsigned short int 占2字节
	unsigned short int flags;
	unsigned short int questions;
	unsigned short int answerRRS;
	unsigned short int authorityRRS;
	unsigned short int additionalRRS;
	unsigned char name[34];
	unsigned short int type;
	unsigned short int classe;
};
 
char *getNameFromIp(const char *ip);
 
extern "C"
 
jstring Java_com_hao_cmake_MainActivity_cpuFromJNI(JNIEnv* env, jobject thiz, jstring ip) {
	const char* str_ip;
	str_ip = env->GetStringUTFChars(ip, 0);
	return env->NewStringUTF(getNameFromIp(str_ip));
}
 
char *getNameFromIp(const char *ip) {
	char str_info[1024] = { 0 };
	struct sockaddr_in toAddr; //sendto中使用的对方地址
	struct sockaddr_in fromAddr; //在recvfrom中使用的对方主机地址
	char send_buff[send_MAXSIZE];
	char recv_buff[recv_MAXSIZE];
	memset(send_buff, 0, sizeof(send_buff));
	memset(recv_buff, 0, sizeof(recv_buff));
	int sockfd; //socket
	unsigned int udp_port = 137;
	int inetat;
	if ((inetat = inet_aton(ip, &toAddr.sin_addr)) == 0) {
		sprintf(str_info, "[%s] is not a valid IP address\n", ip);
		return str_info;
	}
	if ((sockfd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0) {
		sprintf(str_info, "%s socket error sockfd=%d, inetat=%d\n", ip, sockfd, inetat);
		return str_info;
	}
	bzero((char*) &toAddr, sizeof(toAddr));
	toAddr.sin_family = AF_INET;
	toAddr.sin_addr.s_addr = inet_addr(ip);
	toAddr.sin_port = htons(udp_port);
 
	//构造netbios结构包
	struct NETBIOSNS nbns;
	nbns.tid = 0x0000;
	nbns.flags = 0x0000;
	nbns.questions = 0x0100;
	nbns.answerRRS = 0x0000;
	nbns.authorityRRS = 0x0000;
	nbns.additionalRRS = 0x0000;
	nbns.name[0] = 0x20;
	nbns.name[1] = 0x43;
	nbns.name[2] = 0x4b;
	int j = 0;
	for (j = 3; j < 34; j++) {
		nbns.name[j] = 0x41;
	}
	nbns.name[33] = 0x00;
	nbns.type = 0x2100;
	nbns.classe = 0x0100;
	memcpy(send_buff, &nbns, sizeof(nbns));
	int send_num = 0;
	send_num = sendto(sockfd, send_buff, sizeof(send_buff), 0,
			(struct sockaddr *) &toAddr, sizeof(toAddr));
	if (send_num != sizeof(send_buff)) {
		sprintf(str_info,
				"%s sendto() error sockfd=%d, send_num=%d, sizeof(send_buff)=%d\n",
				ip, sockfd, send_num, sizeof(send_buff));
		shutdown(sockfd, 2);
		return str_info;
	}
	int recv_num = recvfrom(sockfd, recv_buff, sizeof(recv_buff), 0,
			(struct sockaddr *) NULL, (socklen_t*) NULL);
	if (recv_num < 56) {
		sprintf(str_info, "%s recvfrom() error sockfd=%d, recv_num=%d\n", ip,
				sockfd, recv_num);
		shutdown(sockfd, 2);
		return str_info;
	}
	//这里要初始化。因为发现linux和模拟器都没问题，真机上该变量若不初始化，其值就不可预知
	unsigned short int NumberOfNames = 0;
	memcpy(&NumberOfNames, recv_buff + 56, 1);
	char str_name[1024] = { 0 };
	unsigned short int mac[6] = { 0 };
	int i = 0;
	for (i = 0; i < NumberOfNames; i++) {
		char NetbiosName[16];
		memcpy(NetbiosName, recv_buff + 57 + i * 18, 16);
		//依次读取netbios name
		if (i == 0) {
			sprintf(str_name, "%s", NetbiosName);
		}
	}
	sprintf(str_info, "%s|%s|", ip, str_name);
	for (i = 0; i < 6; i++) {
		memcpy(&mac[i], recv_buff + 57 + NumberOfNames * 18 + i, 1);
		sprintf(str_info, "%s%02X", str_info, mac[i]);
		if (i != 5) {
			sprintf(str_info, "%s-", str_info);
		}
	}
	return str_info;
}

这里要注意一点，jstring Java_com_hao_cmake_MainActivity_cpuFromJNI方法中，com_hao_cmake是我们的包名，MainActivity是调用JNI的Activity名称，cpuFromJNI是对应方法的名字。

2.接着导入Android.mk文件

这个文件也是放在jni文件夹中

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
 
# 指定so库文件的名称
LOCAL_MODULE    := jni_mix
# 指定需要编译的源文件列表
LOCAL_SRC_FILES := find_name.cpp
# 指定C++的编译标志
LOCAL_CPPFLAGS += -fexceptions
# 指定要加载的静态库
#LOCAL_WHOLE_STATIC_LIBRARIES += android_support
# 指定需要链接的库
LOCAL_LDLIBS    := -llog
 
include $(BUILD_SHARED_LIBRARY)
$(call import-module, android/support)

3.我们配置一下build.gradle文件

android -> defaultConfig 下添加

externalNativeBuild{
    ndkBuild{
        abiFilters "arm64-v8a","armeabi-v7a"
    }
}

android 下添加

externalNativeBuild {
    ndkBuild {
        path file('src/main/jni/Android.mk')
    }
}
packagingOptions{
    pickFirst 'lib/arm64-v8a/libjni_mix.so'
    pickFirst 'lib/armeabi-v7a/libjni_mix.so'
}

4.好了，此时可以编译一下项目了

5.此时我们可以找一下我们生成的so包了

在build → intermediates → ndkBuild → debug → obj → local下，我们可以找到我们生成的相关配置平台的so文件

6.将生成的so文件拷入src/main/jniLibs中

这个样子的

7.调用C语言方法的Activity如下

public class MainActivity extends AppCompatActivity {
 
    public native String cpuFromJNI(String ip);
 
    static {
        System.loadLibrary("jni_mix");
    }
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        String str = cpuFromJNI("192.168.0.163");
        Toast.makeText(this,str,Toast.LENGTH_SHORT).show();
    }
}

这样我们就完成了用C语言类生成so包，并使用JNI进行调用的全流程。

注意：在使用JNI进行调用的时候，我们的环境一定要有NDK，这个我这里就不说了，大家如果没有搭建需要上网找找搭建一下。

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