10:java simple data transfer between Android technology and C language

Because of the many types of inconsistencies between the C and Java languages, data type conversions are required when using native. The following illustrates passing data by passing integers, strings, integer arrays, and Java static methods, respectively.

1. Create a native method

We create a separate Nativeclass class to hold the native method

1  Packagecom.forsta.ndk;2 3  Public classNativeclass {4      Public native intAddintXinty);5      Public nativestring showstring (String str);6      Public native int[] Array (int[] a);7      Public Static native intSubintXinty);8}

2. Get the corresponding header file

By using the Javah com.forsta.ndk.Native in the appropriate directory, get the Com_forsta_ndk_nativeclass.h

1 /*Do not EDIT this file-it are machine generated*/2#include <jni.h>3 /*Header for Class Com_forsta_ndk_nativeclass*/4 5 #ifndef _included_com_forsta_ndk_nativeclass6 #define_included_com_forsta_ndk_nativeclass7 #ifdef __cplusplus8 extern "C" {9 #endifTen /* One * Class:com_forsta_ndk_nativeclass A * Method:add - * Signature: (II) I -  */ the jniexport jint jnicall java_com_forsta_ndk_nativeclass_add -(JNIENV *, Jobject, Jint, jint); -  - /* + * Class:com_forsta_ndk_nativeclass - * method:showstring + * Signature: (ljava/lang/string;) ljava/lang/string; A  */ at jniexport jstring jnicall java_com_forsta_ndk_nativeclass_showstring -(JNIENV *, Jobject, jstring); -  - /* - * Class:com_forsta_ndk_nativeclass - * Method:array in * Signature: ([i) [I -  */ to jniexport jintarray jnicall java_com_forsta_ndk_nativeclass_array +(JNIENV *, Jobject, Jintarray); -  the /* * * Class:com_forsta_ndk_nativeclass $ * Method:subPanax Notoginseng * Signature: (II) I -  */ the jniexport jint jnicall java_com_forsta_ndk_nativeclass_sub +(JNIENV *, Jclass, Jint, jint); A  the #ifdef __cplusplus + } - #endif $ #endif

3. Implement the method in the header file

1#include <stdio.h>2#include <jni.h>3#include <malloc.h>4#include"Com_forsta_ndk_nativeclass.h"5 6 7 /**8 * Return value char* This represents the first address of a char array9 * JSTRING2CSTR Converts the type of jstring in Java into a char string in the C languageTen  */ One Char* JSTRING2CSTR (jnienv*env, jstring jstr) A { -      Char* Rtn =NULL; -Jclass clsstring = (*env)->findclass (env,"java/lang/string");//String theJstring Strencode = (*env)->newstringutf (env,"GB2312");//get a Java string "GB2312" -Jmethodid mid = (*env)->getmethodid (env,clsstring,"getBytes","(ljava/lang/string;) [B");//[String.getbytes ("gb2312"); -Jbytearray barr= (Jbytearray) (*env)->callobjectmethod (Env,jstr,mid,strencode);//String. GetByte ("GB2312"); -Jsize alen = (*env)->getarraylength (Env,barr);//the length of a byte array +jbyte* ba = (*env)getbytearrayelements (env,barr,jni_false); -      if(Alen >0) +      { ARTN = (Char*) malloc (alen+1);//" the" at memcpy (Rtn,ba,alen); -rtn[alen]=0; -      } -(*env)->releasebytearrayelements (Env,barr,ba,0);// -      returnRtn; - } in  - jniexport jint jnicall java_com_forsta_ndk_nativeclass_add to(JNIENV *env, Jobject obj, Jint x, Jint y) { +     returnx+y; - } the  * jniexport jstring jnicall java_com_forsta_ndk_nativeclass_showstring $(JNIENV *env, Jobject obj, jstring str) {Panax Notoginseng     //no string type in C language -     Char* s=jstring2cstr (ENV,STR); the     //the strings in the C language End with '/0 ' +     Chars1[7]={' ','h','e','L','L','o',' /'}; A strcat (S,S1); the     return(*env)Newstringutf (env,s); + } -  $ jniexport jintarray jnicall java_com_forsta_ndk_nativeclass_array $(JNIENV *env, Jobject obj, Jintarray arr) { -     //1. Get array length -     //2. Modify each data the     intSize= (*env)getarraylength (Env,arr); -     int* array= (*env)->getintarrayelements (Env,arr,0);Wuyi     intI=0; the      for(; i<size;i++){ -array[i]+=Ten; Wu     } -     returnarr; About  $ } - jniexport jint jnicall java_com_forsta_ndk_nativeclass_sub -(JNIENV *env, Jclass Clazz, Jint x, Jint y) { -     returnX-y; A}

Add a click event in 4.MainActivity

1 @Override2      Public voidOnClick (View v) {3         intID =V.getid ();4         Switch(ID) {5          CaseR.ID.BT1:6             intResult=nativeclass.add (10, 90);7Toast.maketext (Getapplicationcontext (), "Show results:" +result, Toast.length_short). Show ();8              Break;9          CaseR.ID.BT2:TenToast.maketext (Getapplicationcontext (), nativeclass.showstring ("Forsta"), Toast.length_short). Show (); One              Break; A          CaseR.ID.BT3: -             int[] arr={1,2,3,4,5}; - Nativeclass.array (arr); theString s = ""; -              for(intI:arr) { -s+=i+ ""; -             } + Toast.maketext (Getapplicationcontext (), S,toast.length_short). Show (); -              Break; +          CaseR.ID.BT4: A             intResult1=nativeclass.sub (90, 10); atToast.maketext (Getapplicationcontext (), "Subtraction display result:" +result1, Toast.length_short). Show (); -              Break; -         default: -              Break; -         } -          in}

5. Demo