Android JNI learning practices (1)

I recently learned about JNI and learned how to do it. I changed the 2D engine of the game "ant financial" to JNI, encapsulated the code, and improved the running speed.

The preliminary steps are as follows: 1. Write the Java file and determine the JNI Interface Name:

package com.howfun.android.antguide.hf2d_jni;import android.app.Activity;import android.os.Bundle;import android.util.Log;import android.widget.TextView;public class hf2d extends Activity{      public void onCreate(Bundle savedInstanceState) {      Log.d("jni", "oncreate()");      super.onCreate(savedInstanceState);      /*       * Create a TextView and set its content. the text is retrieved by calling       * a native function.       */      TextView tv = new TextView(this);      tv.setText(stringFromJNI());      setContentView(tv);   }      public native String stringFromJNI() ;   static {      System.loadLibrary("hf2d");   }}

Important:

public native String stringFromJNI() ;

Confirm the interface.

2. compile the project and generate hf2d. Class to $ myproject/bin/($ myproject refers to my android project directory). Run:

$ Javah-JNI com. howfun. Android. antguide. hf2d_jni.hf2d

Generate hf2d. h In the current directory. Note that the directory should not be run. Just run it in Bin.

Test hf2d. h to $ myproject/JNI/(create this JNI directory) and write hf2d. C as follows:

#include <string.h>#include <jni.h>#include "hf2d.h"JNIEXPORT jstring JNICALL Java_com_howfun_android_antguide_hf2d_1jni_hf2d_stringFromJNI  (JNIEnv *env, jobject thiz){    return (*env)->NewStringUTF(env, "Hello from JNI !");}

3. Write the Android. mk file and place it in $ myproject/JNI/, as shown below:

LOCAL_PATH := $(call my-dir)include $(CLEAR_VARS)LOCAL_MODULE    := hf2d LOCAL_SRC_FILES := hf2d.cinclude $(BUILD_SHARED_LIBRARY)

What do I mean by specific label reference: $ ndk/android-ndk-r4b/docs/ANDROID-MK.TXT, very detailed.

4. Download the android ndk package (My R4 version ),

$ Export ndk = My/ndk/path

$ CD $ ndk/android-ndk-r4b /,

$ Export ndk_project_path = "~ /Workspace/antguide"

$./Ndk-build

The libhf2d. So file is generated here.

In step 1, call:

static {      System.loadLibrary("hf2d");   }

Load the dynamic library. Note that the preceding "lib" is removed.

Running APK will display greetings from the remote JNI.

Note: Do not underline the class path. Otherwise, you need to add "1" after the underline Of the function name, as the opposite is typical:

Package name: Package com. howfun. Android. antguide. hf2d_jni;

The result is generated as follows:

Java_com_howfun_android_antguide_hf2d_1jni_hf2d_stringfromjn has an "1", FT...

This chapter is complete. To be continued: Port hf2d Java engine to JNI