Ndk Development notes (III)-JNI Learning

1. This article mainly describes the use of the getfieldid and getmethodid methods.

 

 

Jclass clazz = (* env)-> getobjectclass (ENV, thiz); // clazz is the natvie class.

 

Public class native {<br/> Public string value = "hello "; <br/>/** <br/> * This will return from JNI <br/> * @ return <br/> */<br/> Public static int number = 0; <br/> Public static native string stringfromjni (); <br/>/** <br/> * pass the message to the JNI and <br/> * @ Param Meg <br/> * @ return JNI deal it message <br /> */<br/> Public static native string getmessage (string MEG ); <br/> Public native void getvalue (); </P> <p> Public void showmessage () {<br/> system. out. println ("Call the show message"); <br/>}< br/>} 

 

 

At first, when I was writing this program, I always reported getfileid... no such "value ".

After searching for a long time, I found that the getvalue () defined in native. Java also uses static

 

The defined value is not static.

An error is reported in FID = (* env)-> getfieldid (ENV, clazz, "value", "ljava/lang/string;"); because the static variable must be used in the static method.

So I removed the static method of the getvalue () method. below is the OK code.

Void Merge (jnienv * ENV, jobject thiz) {<br/> jfieldid FID; <br/> jfieldid fnumberid; <br/> jstring jstr; <br/> jmethodid mid; <br/> const char * sztemp; <br/> jclass clazz = (* env)-> getobjectclass (ENV, thiz); <br/> Print (clazz ); <br/> FID = (* env)-> getfieldid (ENV, clazz, "value", "ljava/lang/string ;"); <br/> If (FID = NULL) {<br/> return; <br/>}< br/> jstr = (* env)-> getobjectfield (ENV, thiz, FID); <br/> sztemp = (* env)-> getstringutfchars (ENV, jstr, null); <br/> Print (sztemp ); <br/> (* env)-> releasestringutfchars (ENV, jstr, sztemp); <br/> jstr = (* env)-> newstringutf (ENV, "Hello World"); <br/> If (jstr = NULL) <br/> return; <br/> (* env)-> setobjectfield (ENV, thiz, FID, jstr); <br/> // call the method showmessage <br/> fnumberid = (* env)-> getstaticfieldid (ENV, clazz, "Number ", "I"); <br/> If (fnumberid = NULL) <br/> return; <br/> jint num = (* env)-> getstaticintfield (ENV, clazz, fnumberid); <br/> // print (Num); <br/> (* env)-> setstaticintfield (ENV, clazz, fnumberid, 2 ); <br/> mid = (* env)-> getmethodid (ENV, clazz, "showmessage", "() V"); <br/> If (mid = NULL) <br/> return; <br/> (* env)-> callvoidmethod (ENV, thiz, mid); <br/>} 

 

 

I used static to solve all the problems.

FID = (* env)-> getstaticfieldid (ENV, clazz, "value", "ljava/lang/string ;");

 

I don't know why, or the above error occurs .. Later.

 

 

 

Try getmethodid () today ();

 

Mid = (* env)-> getmethodid (ENV, clazz, "showmessage", "() V"); // showmessage is a method in Java ,"() V "indicates that this method is the return void method.

(* Env)-> callvoidmethod (ENV, thiz, mid );

 

 

Corresponding to getmethodid is call ** method ().

 

The static attribute must be used.

 

 

Fnumberid = (* env)-> getstaticfieldid (ENV, clazz, "Number", "I ");

If (fnumberid = NULL)

Return;

Jint num = (* env)-> getstaticintfield (ENV, clazz, fnumberid );

// Print (Num );

(* Env)-> setstaticintfield (ENV, clazz, fnumberid, 2 );

 

 

 

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