Mr. Cooooooooooooode

We all know if you want to communicate with your dirver in Android Environment.

You write the Java code, you also must write C/C++ code.

The problem is that how java communicate with C/C++.

JNI (Java Native Language) is the key to do this.

There is a sample for use JNI.

You can get the SimpleJNI code in Android/development/samples/SimpleJNI

[Java Code - SimpleJNI.java]

package com.example.android.simplejni;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class SimpleJNI extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        TextView tv = new TextView(this);
        int sum = Native.add(2, 3);
        tv.setText("2 + 3 = " + Integer.toString(sum));
        setContentView(tv);
    }
}

class Native {
    static {
        // The runtime will add "lib" on the front and ".o" on the end of
        // the name supplied to loadLibrary.
        System.loadLibrary("simplejni");
    }

    static native int add(int a, int b);
}

[C/C++ Code - Native.cpp]

/*
 * Copyright (C) 2008 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#define LOG_TAG "simplejni native.cpp"
#include <utils/Log.h>

#include <stdio.h>

#include "jni.h"

//JNI Function
static jint
add(JNIEnv *env, jobject thiz, jint a, jint b) {
int result = a + b;
    LOGI("%d + %d = %d", a, b, result);
    return result;
}

static const char *classPathName = "com/example/android/simplejni/Native";

static JNINativeMethod methods[] = {
  {"add", "(II)I", (void*)add },
};

/*
 * Register several native methods for one class.
 */
static int registerNativeMethods(JNIEnv* env, const char* className,
    JNINativeMethod* gMethods, int numMethods)
{
    jclass clazz;

    clazz = env->FindClass(className);
    if (clazz == NULL) {
        LOGE("Native registration unable to find class '%s'", className);
        return JNI_FALSE;
    }
    if (env->RegisterNatives(clazz, gMethods, numMethods) < 0) {
        LOGE("RegisterNatives failed for '%s'", className);
        return JNI_FALSE;
    }

    return JNI_TRUE;
}

/*
 * Register native methods for all classes we know about.
 *
 * returns JNI_TRUE on success.
 */
static int registerNatives(JNIEnv* env)
{
  if (!registerNativeMethods(env, classPathName,
                 methods, sizeof(methods) / sizeof(methods[0]))) {
    return JNI_FALSE;
  }

  return JNI_TRUE;
}


// ----------------------------------------------------------------------------

/*
 * This is called by the VM when the shared library is first loaded.
 */
 
typedef union {
    JNIEnv* env;
    void* venv;
} UnionJNIEnvToVoid;

jint JNI_OnLoad(JavaVM* vm, void* reserved)
{
    UnionJNIEnvToVoid uenv;
    uenv.venv = NULL;
    jint result = -1;
    JNIEnv* env = NULL;
   
    LOGI("JNI_OnLoad");

    if (vm->GetEnv(&uenv.venv, JNI_VERSION_1_4) != JNI_OK) {
        LOGE("ERROR: GetEnv failed");
        goto bail;
    }
    env = uenv.env;

    if (registerNatives(env) != JNI_TRUE) {
        LOGE("ERROR: registerNatives failed");
        goto bail;
    }
   
    result = JNI_VERSION_1_4;
   
bail:
    return result;
}

[Compile]

01. First, you should download all Android code

02. In Android Folder, type make SimpleJNI

03. SimpleJNI is the package name, it will help to compile out application and library

04. When compiled completely, you will get two file - libsimplejni.so and simplejni.apk

[Run]

01. First get the two file - libsimplejni.so and simplejni.apk

02. Start adb

03. #adb push libsimplejni.so /system/lib  (Put the library into your device/emulator)

04. #adb install simplejni.apk (install your application)

05. WOW, amazing!!