Android111 calling C code in Java

Mainactivity:

Package Com.itheima.helloworld1;import android.os.bundle;import android.app.activity;import Android.view.View; Import Android.widget.Toast; Public classMainactivity extends Activity {Static{        //load the packaged so class librarySystem.loadlibrary ("Hello");//Hello is the name specified in Abdroid.mk. } @Overrideprotected voidonCreate (Bundle savedinstancestate) {super.oncreate (savedinstancestate);            Setcontentview (R.layout.activity_main); }    /*# #使用jni 1. Create a JNI folder under the project root directory 2. Create a C file in the Jni file (src is write Java code) 3. In Java code, create a local method HELLOFROMC public n    ative String HELLOFROMC (); 4. In Jni, define the function to implement this method, the function name must be jstring JAVA_COM_ITHEIMA_HELLOWORLD1_MAINACTIVITY_HELLOFROMC (jnienv* env, Jobject obj ) 5.    Returns a string using C to define a string char* CStr = "Hello from c";            6. Convert c string to Java string jstring jstr = (*env)->newstringutf (env, CSTR); return jstr; 7. Create the Android.mk file in Jni 8. Add the <jni.h> header file 9 to the C file. Execute the ndk-build.cmd instruction under the Jni folder (compile with cmd command under the disk Jni folder, after compiling, there is a libhello.so file under the Armeabi folder under Libs, this is the compiled C code) 10. Loading so class libraries in Java code, calling local methods*/         Public voidClick (View v) {Toast.maketext ( This, HELLOFROMC (),0). Show (); }        //defines a local method, the method body is implemented by the C language     Publicnative String Hellofromc (); }

Android.mk

# MK is what make means Local_path:= $ (call my-dir)    include $ (clear_vars)    #编译后生成的文件的类库叫什么名字    Local_ MODULE    := Hello    #要编译的c文件    local_src_files:= hello.c    include $ (build_ Shared_library)

hello.c

#include <stdio.h>#include<stdlib.h>#include<jni.h>//jstring indicates that STRING,JAVA_COM_ITHEIMA_HELLOWORLD1_MAINACTIVITY_HELLOFROMC is the package name Class name Method name//jnienv* env, Jobject obj is a must-write parameter, env is the memory address of the Java virtual machine, the Java code is running on the virtual machine and the C code is not running on the virtual machine,//env is a struct pointer, here * ENV is a two-level pointer to the Java runtime Environment, obj is a Mainactivity object, and C Jobject represents the object. Jstring JAVA_COM_ITHEIMA_HELLOWORLD1_MAINACTIVITY_HELLOFROMC (jnienv*env, Jobject obj) {    //c is not a string, is a character array, char* CStr or char cstr[], the name of the array is the address of the No. 0 element,    Char* CStr ="Hello from C"; //converts a C string into a Java string//jstring (*newstringutf) (jnienv*, const char*);//jstring jstr = (* (*env)). Newstringutf (env, CSTR);Jstring jstr = (*env)Newstringutf (env, CSTR); returnjstr;}

Android111 calling C code in Java