Using JNI in Android

Recently, I want to adjust a project, which uses JNI, and make a note. The environment is: Windows7+eclipse.

Reference sticker:

http://blog.csdn.net/shulianghan/article/details/18964835

 

Process:

1.Down the simplest project using JNI

https://github.com/han1202012/NDKHelloworld.git

 

2. Add a batch file to the above project

 

 

common problem:

1. The include <string> is missing and the prompt cannot be found

Solution:

http://stackoverflow.com/questions/10373788/c-unresolved-inclusion-iostream
 

2. Down a project from the company server, there are C++ related parts in the right-click properties, but down a project from the Internet, but can't see the shadow of the CDT plug-in? Why is this?

Later, I copied a .project file from the company's project, and that's it! Thinking about it, it seems that my computer has not installed the CDT plug-in. It is estimated that there is something in this .project file.

 

3. The program crashes inexplicably, and after searching for a long time, I found out that a function is declared in the java code

native public static boolean sendMessageToOneMobile(String mobile,String msg);

When defining a function in c++, the first two parameters (JNIEnv *env, jobject thiz) must be taken with

Because when calling, the actual parameters will be passed! ! !

JNIEXPORT bool JNICALL Java_shuliang_han_ndkhelloworld_MainActivity_sendMessageToOneMobile

(JNIEnv *env,jobject thiz,jstring mobile,jstring msg ){}

 

4.native reads the String[] passed by java and stores it as char**

Reference sticker:

http://liuxp0827.blog.51cto.com/5013343/1378277 

 

JNIEXPORT void JNICALL Java_shuliang_han_ndkhelloworld_MainActivity_nativeSendMessageToMobiles(
JNIEnv *env,jobject thiz,jobjectArray mobiles,jstring msg ){
	jstring jstr;
	jsize arraysize = env->GetArrayLength(mobiles);
	const char **pstr = (const char **) malloc(arraysize*sizeof(char *));
	for (int i=0 ; i<arraysize; i++) {
		jstr = (jstring)env->GetObjectArrayElement(mobiles, i);
		pstr[i] = (char *)env->GetStringUTFChars(jstr, 0);
	}

	const char *nativemsg = env->GetStringUTFChars(msg, 0);
	DataTransDemo::sendMessageToMobiles(pstr,arraysize,nativemsg);

	env->ReleaseStringUTFChars(msg,nativemsg);
	for(int i=0;i<arraysize;i++){
		jstr = (jstring)env->GetObjectArrayElement(mobiles, i);
		env->ReleaseStringUTFChars(jstr,pstr[i]);
		env->DeleteLocalRef(jstr);
	}
	free(pstr);
}

 

 

5.native replaces char** with String[] and passes it to java

Reference sticker:

https://coderanch.com/t/326467/java/Returning-String-array-program-Java

http://zhiweiofli.iteye.com/blog/1830319 

 

bool DataTransDemo::sendMessageToMobiles(const char* mobiles[],const char* msg){
	SGJniMethodInfo t;
	bool bRlt = true;

	if (SGSDKJniHelper::getStaticMethodInfo(t, CLASS_NAME, "sendMessageToMobiles","([Ljava/lang/Object;Ljava/lang/String;)Z")) {
		SGLOGD("DataTransDemo::sendMessageToMobiles begin");

		jstring strMessage = t.env->NewStringUTF(msg);
		jobjectArray mobilearray;
		mobilearray= (jobjectArray)t.env->NewObjectArray(size,t.env->FindClass("java/lang/String"),NULL);
		for(int i=0;i<size;i++){
			jstring mobilestring = t.env->NewStringUTF(mobiles[i]);
			t.env->SetObjectArrayElement(mobilearray,i,mobilestring);
			t.env->DeleteLocalRef(mobilestring);
		}

		bRlt = t.env->CallStaticBooleanMethod(t.classID, t.methodID, mobilearray, strMessage);
		t.env->DeleteLocalRef(t.classID);
		t.env->DeleteLocalRef(strMessage);
		t.env->DeleteLocalRef(mobilearray);

		SGLOGD("DataTransDemo::sendMessageToMobiles ok");
	}else{
		SGLOGD("DataTransDemo::sendMessageToMobiles getStaticMethodInfo error");
		bRlt = false;
	}

	return bRlt;
}

 

 6.关于jni里释放资源

参考贴：http://iaiai.iteye.com/blog/2245785