Android JNI Programming 1 (Operation on a basic type string)

Recently, I have been learning about the JNI of Android, the main reference is the Black Horse Programmer's video tutorial, really good.

Then summarize it yourself, as a review of the study.

This blog is also the main reference to the blogger: http://www.cnblogs.com/activity-life/p/3643047.html, here thank you

Through his blog and video tutorials, I combined the two methods, the function realized, after all, this is the application of the main, their own not fine, here accumulate under:

Summary of the first blog all Java fetch JNI is about string operations, including the following three types:

//access to a basic data type string//0. No input, with output as String    Public nativeString HELLOFROMC (); //1. Have input, have output access string   Private nativestring getnativestring (String str); //2. Input string, with length, output string   Public nativestring encode (string text,intlength);  Public nativeString decode (string text,intlength);

So, according to the tutorial, when an input string is passed as a parameter, it is converted to a C-language string function pointer using the following function:

//string that converts a Java string into CChar* JSTRING2CSTR (jnienv*env, jstring jstr) {     Char* Rtn =NULL; Jclass clsstring= (*env)->findclass (env, "java/lang/string")); Jstring Strencode= (*env)->newstringutf (env, "GB2312")); Jmethodid Mid= (*env)->getmethodid (env,clsstring, "GetBytes", "(ljava/lang/string;) [B]); Jbytearray Barr= (Jbytearray) (*env)->callobjectmethod (Env,jstr,mid,strencode);//String. GetByte ("GB2312");Jsize alen = (*env)getarraylength (Env,barr); Jbyte* ba = (*env)getbytearrayelements (Env,barr,jni_false); if(Alen > 0) {Rtn=   (Char*) malloc (alen+1);//" the"memcpy (Rtn,ba,alen); Rtn[alen]=0; }     (*env)->releasebytearrayelements (env,barr,ba,0);//     returnRtn;}

After that, I'll show the use of JNI in the above three types, respectively:

0. No input, there is output as a string:

Jstring JAVA_COM_SWUST_STRING_MAINACTIVITY_HELLOFROMC (jnienv* env,jobject obj) {   //  C language String    Char* cstr= "Hello Paopao from C";    // converting a C string to a Java string     // jstring     (*NEWSTRINGUTF) (jnienv*, const char*);     // jstring jstr= (* (*env)). Newstringutf (ENV,CSTR)    jstring jstr = (*env), Newstringutf (env, CStr);     return jstr;}

1. There is an input, there is an output access string:

//string that converts a Java string into CChar* JSTRING2CSTR (jnienv*env, jstring jstr) {     Char* Rtn =NULL; Jclass clsstring= (*env)->findclass (env, "java/lang/string")); Jstring Strencode= (*env)->newstringutf (env, "GB2312")); Jmethodid Mid= (*env)->getmethodid (env,clsstring, "GetBytes", "(ljava/lang/string;) [B]); Jbytearray Barr= (Jbytearray) (*env)->callobjectmethod (Env,jstr,mid,strencode);//String. GetByte ("GB2312");Jsize alen = (*env)getarraylength (Env,barr); Jbyte* ba = (*env)getbytearrayelements (Env,barr,jni_false); if(Alen > 0) {Rtn=   (Char*) malloc (alen+1);//" the"memcpy (Rtn,ba,alen); Rtn[alen]=0; }     (*env)->releasebytearrayelements (env,barr,ba,0);//     returnRtn;} Jniexport jstring jnicall java_com_swust_string_mainactivity_getnativestring (jnienv*env, Jobject obj, jstring jstr) {    //gets the input string    Char* cstr=jstring2cstr (ENV,JSTR); Char* Putf8str = "from Jni String"; strcat (CStr,"/");//Add a delimiterstrcat (CStr, PUTF8STR);//joins a local string//C-language string conversion to jstring    return(*env)Newstringutf (ENV,CSTR);}

The basic function of your own implementation is to merge and return the string of Java passed in and C.

2. Input string, with length, output string:
This type is based on the tutorial two functions (encoding and decoding), which can be considered the same type:

//string that converts a Java string into CChar* JSTRING2CSTR (jnienv*env, jstring jstr) {     Char* Rtn =NULL; Jclass clsstring= (*env)->findclass (env, "java/lang/string")); Jstring Strencode= (*env)->newstringutf (env, "GB2312")); Jmethodid Mid= (*env)->getmethodid (env,clsstring, "GetBytes", "(ljava/lang/string;) [B]); Jbytearray Barr= (Jbytearray) (*env)->callobjectmethod (Env,jstr,mid,strencode);//String. GetByte ("GB2312");Jsize alen = (*env)getarraylength (Env,barr); Jbyte* ba = (*env)getbytearrayelements (Env,barr,jni_false); if(Alen > 0) {Rtn=   (Char*) malloc (alen+1);//" the"memcpy (Rtn,ba,alen); Rtn[alen]=0; }     (*env)->releasebytearrayelements (env,barr,ba,0);//     returnRtn;} Jniexport jstring jnicall java_com_swust_string_mainactivity_getnativestring (jnienv*env, Jobject obj, jstring jstr) {    //gets the input string    Char* cstr=jstring2cstr (ENV,JSTR); Char* Putf8str = "from Jni String"; strcat (CStr,"/");//Add a delimiterstrcat (CStr, PUTF8STR);//joins a local string//C-language string conversion to jstring    return(*env)Newstringutf (ENV,CSTR);} Jniexport jstring jnicall java_com_swust_string_mainactivity_encode (jnienv*env, Jobject obj, jstring jstr, jint length) {  //generates a C-language character pointer based on the function given by the teacher//Arg1:java Environment pointer Arg2:java string    Char* cstr=jstring2cstr (ENV,JSTR); intI=0; //The C language is the same as int and jint, so use length directly     for(i=0;i<length;i++){        * (cstr+i) +=1; }    //C-language string conversion to jstring    return(*env)Newstringutf (ENV,CSTR);}

For the code to look clear, the project code is not posted here, only show:

0. No input, with output as String: Display "Hello Paopao from C"

1. Have input, have output access string: Display "Hello from Java/hello from JNI"

2. Input string, with length, output string: Display encoding and decoding capabilities:

Android JNI Programming 1 (Operation on a basic type string)