Step-by-step learning ANDROIDNDK Programming (Java passes data to c)

This is already the fourth chapter of "Step-by-step learning ANDROIDNDK Programming" , in which we will pass code to C code in Java.

First, we create a new Android project "Ndkdata",

The first step:

The same first declares the native method, as follows:

public native int Add (int a,int b);p ublic native string Hellosir (string name);p ublic native int[] Intmethod (int[] Intarray );

Can see I've declared here three methods to pass different data types to C code

Step Two:

Since all of our methods are declared in Mainactivity.java, we need to first compile the Mainactivity.java with Javac as a. class file and then use Javah to generate the corresponding. h header file.

Compile the Mainactivity.java with Javac as a. class file : This is the Android project that you can run to generate a corresponding. class file in the Bin/classes directory.

under command line enter the directory where bin/classes is located : Execute Javah com.example.ndkdata.MainActivity to generate the corresponding com_example_ in this directory Ndkdata_mainactivity.h header file, the contents are as follows:

/* do don't EDIT this file-it was machine generated */#include <jni.h>/* Header for CLA SS Com_example_ndkdata_mainactivity */#ifndef _included_com_example_ndkdata_mainactivity#define _Included_com_     Example_ndkdata_mainactivity#ifdef __cplusplusextern "C" {#endif */* * class:com_example_ndkdata_mainactivity * Method: Add * Signature: (II) I */jniexport jint jnicall java_com_example_ndkdata_mainactivity_add (jnienv *, Jobject, Jint, J int);/* * class:com_example_ndkdata_mainactivity * Method:hellosir * Signature: (ljava/lang/string;) ljava/lang/st Ring */jniexport jstring jnicall Java_com_example_ndkdata_mainactivity_hellosir (jnienv *, Jobject, jstring);/* * CLASS:C om_example_ndkdata_mainactivity * Method:intmethod * Signature: ([i) [i */jniexport Jintarray jnicall java_com_example_ Ndkdata_mainactivity_intmethod (JNIEnv *, Jobject, jintarray); #ifdef __cplusplus} #endif #endif 

Next we create the JNI directory in the Ndkdata project, and then theCopy the Com_example_ndkdata_mainactivity.h file to the directory, and then create our C code: HELLO.C

Add the method generated in Com_example_ndkdata_mainactivity.h to the hello.c file here, you need to add a parameter, because the default is to generate only the method's parameter type, no Parameter object, and introduce the corresponding header file, as follows:

#include <stdio.h> #include <jni.h> #include "com_example_ndkdata_mainactivity.h" Jniexport jint Jnicall Java_com_example_ndkdata_mainactivity_add  (jnienv * env, Jobject obj, Jint x, Jint y) {      }jniexport jstring jnical L Java_com_example_ndkdata_mainactivity_hellosir  (jnienv * env, jobject obj, jstring jstr) {      }jniexport Jintarray jnicall java_com_example_ndkdata_mainactivity_intmethod  (jnienv * env, jobject obj, JintArray arr) {        }

The next step is to implement our C code, as follows:

#include <stdio.h> #include <jni.h> #include "com_example_ndkdata_mainactivity.h" Jniexport jint Jnicall java_com_example_ndkdata_mainactivity_add  (JNIENV * env, Jobject obj, Jint x, jint y) {    return x+y ;   }/** * return value char* This represents the first address of a char array  * &NBSP;JSTRING2CSTR converts the type of jstring in Java into a char string in the C language  */char*   JSTRING2CSTR (jnienv*   env,   jstring   jstr) {<span style= "White-space:pre" >& lt;/span> char*   RTN   =   Null;<span style= "White-space:pre" ></span> jclass   CLSSTR ing   =   (*env)->findclass (env, "java/lang/string"); String<span style= "White-space:pre" ></span> jstring   strencode   =   (*env) Newstringutf (env, "GB2312");  //get a Java string "GB2312" <span style= "White-space:pre" ></span> jmethodid   Mid   =   (* ENV)->getmethodid (env,clsstring,   "GetBytes",   "(ljava/lang/string;) [B");//[String.getbytes ("gb2312"), <span style= "White-space:pre" ></span> jbytearray   barr=   ( Jbytearray) (*env)->callobjectmethod (Env,jstr,mid,strencode); String. GetByte ("GB2312"); <span style= "White-space:pre" ></span> jsize   Alen   =   (*env)- >getarraylength (Env,barr); byte array length <span style= "White-space:pre" ></span> jbyte*   ba   =   (*env) Getbytearrayelements (env,barr,jni_false); <span style= "White-space:pre" ></span> if (Alen   >   0) <span style= "White-space:pre" ></span> {<span style= "White-space:pre" ></span>  rtn   =   (char*) malloc (alen+1);        //"<span" style= "White-space:pre" ></span>  memcpy (Rtn,ba,alen);< Span style= "White-space:pre" ></span>  rtn[alen]=0;<span style= "White-space:pre" ></span> }<span style= "White-space:pre" ></span> (*env)->releasebyteaRrayelements (env,barr,ba,0);  //<span style= "White-space:pre" ></span> return RTN;} Jniexport jstring jnicall java_com_example_ndkdata_mainactivity_hellosir  (JNIENV * env, jobject obj, jstring jstr) {   //In C language is no Java string<span style= "White-space:pre" ></span>char* CStr = Jstring2CStr (env, JSTR); <span style= "White-space:pre" ></span>//the string in the C language is the end <span style= "White-space:pre" "></span>char arr[7]= {" ', ' h ', ' e ', ' l ', ' l ', ' o ', ' white-space:pre '};<span style=, "></span> strcat (Cstr,arr); <span style= "White-space:pre" ></span>return (*env)->newstringutf (ENV,CSTR);     }jniexport jintarray jnicall java_com_example_ndkdata_mainactivity_intmethod  (JNIENV * env, Jobject obj, Jintarray arr) {    }

After writing the hello.c, you need to write android.mk, which reads as follows:

#交叉编译编译c/c++ code depends on the configuration file # Gets the path of the current android.mk  local_path: = $ (call My-dir) #变量初始化操作include $ (clear_vars) #libhello. In fact, the generated libhello.so is in front of our module name plus lib behind plus. Solocal_module    : = hellolocal_src_files: = Hello.cinclude $ (build_ Shared_library)

Now it is necessary to enter the Ndkdata root of the project under the command line of the Cygwine, execute the "ndk-build" command to generate the corresponding directly running binary files, note that each time after the "ndk-build" execution, it is best to clean the Android project.

After the "Ndk-build" succeeds, these library files need to be introduced in Java code:

static {        system.loadlibrary ("Hello");}

Note that the "Hello" here is the module name we wrote in the Android.mk file, which is the value of Local_module.

Now it's time to execute the native method of the declaration, which can be called directly in the Java code, isn't it simple?

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Step-by-step learning ANDROIDNDK Programming (Java passes data to c)