Android Ndk:② First NDK program (Create steps & some questions)

Android Ndk:② First NDK program (Create steps & some questions)

This section is: Demo to build the first NDK program

• Android NDK First NDK program creation steps some questions
  • Create an Android Project
  • Declaring the native method in Java code
  • Create a JNI folder write Ccpp code
  • Add NDK support to your project
  • New Configuration A Builder
  • Loading libraries where the native method was called
  • All right, run the program.
  • Summarize
  • Problems encountered

1. Create an Android Project

Create a new Android application project directly

The native method is declared in the 2.Java code:

 PackageCOM.JAY.EXAMPLE.FIRSTNDK;Importandroid.app.Activity;ImportAndroid.os.Bundle;ImportAndroid.view.View;ImportAndroid.view.View.OnClickListener;ImportAndroid.widget.Button;ImportAndroid.widget.Toast; Public  class mainactivity extends Activity {    PrivateButton btnshow;//Declaration native Method     Public nativeStringJavafromjni();@Override    protected void onCreate(Bundle savedinstancestate) {Super. OnCreate (Savedinstancestate);        Setcontentview (R.layout.activity_main);        Btnshow = (Button) Findviewbyid (r.id.btnshow); Btnshow.setonclicklistener (NewOnclicklistener () {@Override             Public void OnClick(View v) {Toast.maketext (mainactivity). This, Javafromjni (), Toast.length_short). Show ();    }        }); }}

3. Create the Jni folder and write the C/cpp code

Here are two places to note:

① return value type

Jni.h file in the pre-defined data type, we can find in the Ndk/platforms/android-xx/arch-arm/user/include directory, ~xx represents the version number
Open Jni.h We'll see a lot of data types, and here's the string:

② method Name: In accordance with the following rules:

Return value type Java_ Package Name _ Class name _native method name (jnienv* env, Jobject obj)

And the parameters inside are fixed, in the jni.h file we can also find that he is a struct called jninativeinterface, the struct defines a lot of data types, we find that return string:

OK, so then we'll finish writing our C code, and we'll create a file called first.c in the JNI directory:

#include<stdio.h>  #include<jni.h>  jstring Java_com_jay_example_firstndk_MainActivity_javaFromJNI(JNIEnv* env, jobject obj) {    return(*env)->"First Blood~");}

4. Add NDK support for your project

Project Right-click->android Tools->add Native support input so library name:

Name the so library yourself:

This time will see that the JNI directory in addition to our own written C files, there are more than two east, Xx.cpp and android.mk files, the CPP can be deleted directly, and then open android.mk pieces, modify the following things:

LOCAL_PATH:$(call my-dir)include$(CLEAR_VARS)LOCAL_MODULE    :FirstNDKLOCAL_SRC_FILES:= first.cinclude$(BUILD_SHARED_LIBRARY)

PS: In fact, this step can be done directly in 3, will directly generate a. cpp and ANDROID.MK Files ~

5. New & Configure a Builder

Right-project->properties->builders->new, create a new builder, and click Program,ok! on the popup dialog box.

Double-click to open to configure:

Click OK, OK, OK, then you can customize the compilation code ~ For example, we modify the code after saving, he will recompile.

6. The native method is called where the library is loaded:

7. Well, run the program up ~

8. Summary:

Well, the first NDK program is completed, halfway bump encountered a lot of problems, and finally the smooth run up, the following summary of the approximate steps:

• Create a common NDK project
• Declaring the native method
• To create a JNI folder, you can write your own. C or. cpp file, or directly generate a. cpp and android.mk file directly in step Fourth
• Add NDK support to your project
• New & Configure a Builder
• Loading library (system.loadlibrary (XXX)) where the native method was called
• Run the program ~ ~

Well, probably the process as described above ~ There may be other quick way to create, follow-up encountered in the introduction of HA ~ finally attached to this program encountered some problems and solutions!

9. Problems encountered

1. After the operation of the program directly died--, and then found that the log was C where the method name is wrong, the direct report method can not find!
Solution: Write in strict accordance with the following provisions
Return value type Java_ Package Name _ Class name _native method name (jnienv* env, Jobject obj)

2. This prompt appears:
Android NDK:WARNING:APP_PLATFORM android-17 is larger than android:minsdkversion
Workaround:
Android-ndk-r8e/build/core/add-application.mk the 128th line to change the __ndk_warning to __ndk_info, and then rebuild the project again to eliminate the error.
Or, just write it like this ... Our company is doing this ... I feel like this is not an error, but not good ...

3. The appearance of writing in C: Method ' Newstringutf ' could not being resolved
Just can't find the method, really wonderful ...
Workaround:

For example, my path is:
E:\NDK\android-ndk-r10d\platforms\android-17\arch-arm\usr\include

4. If you're using. cpp, there's a 3 problem
Workaround: Add the extern "C" modifier before the C + + function for Java calls
Also the statement of the output string is not the same:
(*env)->method (env, XXX) can be changed to Env->method (XXX).

Android Ndk:② First NDK program (Create steps & some questions)