Android invoke. So file Jni

Android Call so

The Android virtual machine cannot call the underlying device directly, and we need to use so if we want to call the underlying device.

So use C language or C + + writing complete, using NDK to compile, run directly in the Linux kernel.

By JNI call So when the basic type can be directly interactive, jstring use a bit cumbersome, so I do a jstring and char* to convert the example.

First step:

Create a JNI directory under the engineering root directory. The Libs directory is not built manually.

Note: Use NDK_R7 here so there is no need to use JDK to generate C file headers.

Part II:

Java Authoring Interface file (Device.java)

[Java] view plain copy public class Device {static {system.loadlibrary ("Device");   Public native string deviceteststring (string test); The method name must be declared with the native keyword, and the Class C library must be hosted using System.loadlibrary ("so filename").

Part III:

Preparation of the C document (DEVICES.C)

The C code written here belongs to the Linux C category.

[CPP]  View plain copy #include  <string.h>   #include  <jni.h>       CHAR*&NBSP;JSTRINGTOSTRM (JNIENV*&NBSP;ENV,&NBSP;JSTRING&NBSP;JSTR)    {               char* pStr = NULL;           jclass     jstrObj   =  (*env)-> Findclass (env,  "java/lang/string");       jstring     encode    =  (*env)->newstringutf (env,  "Utf-8");        jmethodID  methodId  =  (*env)->getmethodid (env, jstrobj,  "getBytes ", " (ljava/lang/string;) [B];       jbyteArray byteArray =  ( Jbytearray) (*env)->callobjectmethod (env, jstr, methodid, encode);        jsize      strlen    =  (*env)->GetArrayLength (env,  bytearray);       jbyte      *jBuf      =  (*env)->getbytearrayelements (env, bytearray, jni_false);           if  (jbuf > 0)        {            pStr =  (char*) malloc (strlen +  1);              if  (!pstr)             {                return NULL;           }              memcpy (Pstr, jbuf, strlen);      &Nbsp;       pstr[strlen] = 0;       }            (*env)->releasebytearrayelements (env, bytearray,  jbuf, 0);          return pStr;  }   Jstring java_com_jack_device_deviceteststring (Jnienv* env,jclass clazz,jstring path) {        //system ("echo devices.so test > /sdcard/log/ Log.txt ");       char * test = jstringtostrm (Env,path);        return  (*env)->newstringutf (env, test);  }  
Note: C's function naming rules: Java is the JNI standard must have, Com_jack_device is the full name of the Device.java file, and then down is the C function name

The JSTRINGTOSTRM function must be written before the java_com_jack_device_deviceteststring function, if it does not have to be declared at the beginning of the C file.

Declaration Code:

[CPP] view plain copy char* JSTRINGTOSTRM (jnienv* env, jstring jstr); Fourth step: Writing android.mk and compiling

ANDROID.MK files are as follows: