NDK 使用流程

本文通過一個簡單的java調用c語言的例子來介紹NDK的使用流程

使用流程

創建一個Android項目
創建native方法
編譯生成頭文件 .h 文件
編寫c文件
編譯c生成so文件，調用

1.創建一個Android項目

2.創建native方法：

public class MainActivity extends Activity {

	public static native String getStringFromC();

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		// TODO Auto-generated method stub
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		
	}
}

3.利用NDK編譯生成頭文件，先cd到Android項目的路徑中，用javah命令生成頭文件，格式爲：

javah -classpath 路徑1 -d 路徑2

例子：

javah -classpath bin/classes;E:\Eclipse\sdk\platforms\android-19\android.jar -d jni com.example.myhellojni.MainActivity

bin/classes;E:\Eclipse\sdk\platforms\android-19\android.jar classes文件加指定一個android.jar包

jni com.example.myhellojni.MainActivity jni 目錄加 java文件的包名+類名

圖片是我自己的例子，敲回車不報錯就說明頭文件生成了

頭文件如下：

結尾處是自動創建的相應的c函數，格式是Java_包名_類名_方法名

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_example_myhellojni_MainActivity */

#ifndef _Included_com_example_myhellojni_MainActivity
#define _Included_com_example_myhellojni_MainActivity
#ifdef __cplusplus
extern "C" {
#endif
#undef com_example_myhellojni_MainActivity_MODE_PRIVATE
#define com_example_myhellojni_MainActivity_MODE_PRIVATE 0L
#undef com_example_myhellojni_MainActivity_MODE_WORLD_READABLE
#define com_example_myhellojni_MainActivity_MODE_WORLD_READABLE 1L
#undef com_example_myhellojni_MainActivity_MODE_WORLD_WRITEABLE
#define com_example_myhellojni_MainActivity_MODE_WORLD_WRITEABLE 2L
#undef com_example_myhellojni_MainActivity_MODE_APPEND
#define com_example_myhellojni_MainActivity_MODE_APPEND 32768L
#undef com_example_myhellojni_MainActivity_MODE_MULTI_PROCESS
#define com_example_myhellojni_MainActivity_MODE_MULTI_PROCESS 4L
#undef com_example_myhellojni_MainActivity_MODE_ENABLE_WRITE_AHEAD_LOGGING
#define com_example_myhellojni_MainActivity_MODE_ENABLE_WRITE_AHEAD_LOGGING 8L
#undef com_example_myhellojni_MainActivity_BIND_AUTO_CREATE
#define com_example_myhellojni_MainActivity_BIND_AUTO_CREATE 1L
#undef com_example_myhellojni_MainActivity_BIND_DEBUG_UNBIND
#define com_example_myhellojni_MainActivity_BIND_DEBUG_UNBIND 2L
#undef com_example_myhellojni_MainActivity_BIND_NOT_FOREGROUND
#define com_example_myhellojni_MainActivity_BIND_NOT_FOREGROUND 4L
#undef com_example_myhellojni_MainActivity_BIND_ABOVE_CLIENT
#define com_example_myhellojni_MainActivity_BIND_ABOVE_CLIENT 8L
#undef com_example_myhellojni_MainActivity_BIND_ALLOW_OOM_MANAGEMENT
#define com_example_myhellojni_MainActivity_BIND_ALLOW_OOM_MANAGEMENT 16L
#undef com_example_myhellojni_MainActivity_BIND_WAIVE_PRIORITY
#define com_example_myhellojni_MainActivity_BIND_WAIVE_PRIORITY 32L
#undef com_example_myhellojni_MainActivity_BIND_IMPORTANT
#define com_example_myhellojni_MainActivity_BIND_IMPORTANT 64L
#undef com_example_myhellojni_MainActivity_BIND_ADJUST_WITH_ACTIVITY
#define com_example_myhellojni_MainActivity_BIND_ADJUST_WITH_ACTIVITY 128L
#undef com_example_myhellojni_MainActivity_CONTEXT_INCLUDE_CODE
#define com_example_myhellojni_MainActivity_CONTEXT_INCLUDE_CODE 1L
#undef com_example_myhellojni_MainActivity_CONTEXT_IGNORE_SECURITY
#define com_example_myhellojni_MainActivity_CONTEXT_IGNORE_SECURITY 2L
#undef com_example_myhellojni_MainActivity_CONTEXT_RESTRICTED
#define com_example_myhellojni_MainActivity_CONTEXT_RESTRICTED 4L
#undef com_example_myhellojni_MainActivity_RESULT_CANCELED
#define com_example_myhellojni_MainActivity_RESULT_CANCELED 0L
#undef com_example_myhellojni_MainActivity_RESULT_OK
#define com_example_myhellojni_MainActivity_RESULT_OK -1L
#undef com_example_myhellojni_MainActivity_RESULT_FIRST_USER
#define com_example_myhellojni_MainActivity_RESULT_FIRST_USER 1L
#undef com_example_myhellojni_MainActivity_DEFAULT_KEYS_DISABLE
#define com_example_myhellojni_MainActivity_DEFAULT_KEYS_DISABLE 0L
#undef com_example_myhellojni_MainActivity_DEFAULT_KEYS_DIALER
#define com_example_myhellojni_MainActivity_DEFAULT_KEYS_DIALER 1L
#undef com_example_myhellojni_MainActivity_DEFAULT_KEYS_SHORTCUT
#define com_example_myhellojni_MainActivity_DEFAULT_KEYS_SHORTCUT 2L
#undef com_example_myhellojni_MainActivity_DEFAULT_KEYS_SEARCH_LOCAL
#define com_example_myhellojni_MainActivity_DEFAULT_KEYS_SEARCH_LOCAL 3L
#undef com_example_myhellojni_MainActivity_DEFAULT_KEYS_SEARCH_GLOBAL
#define com_example_myhellojni_MainActivity_DEFAULT_KEYS_SEARCH_GLOBAL 4L
/*
 * Class:     com_example_myhellojni_MainActivity
 * Method:    getStringFromC
 * Signature: ()Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_example_myhellojni_MainActivity_getStringFromC
  (JNIEnv *, jclass);

#ifdef __cplusplus
}
#endif
#endif

4.編寫c文件

返回一個字符串

hello.c

#include<stdio.h>
#include<stdlib.h>
#include "com_example_myhellojni_MainActivity.h"

JNIEXPORT jstring JNICALL Java_com_example_myhellojni_MainActivity_getStringFromC(
		JNIEnv *env, jclass jclass) {
	return (*env)->NewStringUTF(env, "Hello NDK");
}

在Android.mk指定編譯名

# Copyright (C) 2009 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.
#
LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE    := hello    //生成的so文件名
LOCAL_SRC_FILES := hello.c  //目標文件

include $(BUILD_SHARED_LIBRARY)

Appliction.mk 指定編譯的平臺

APP_ABI := all

意思是編譯全部的平臺

5.編譯so

在Android項目的目錄下執行ndk-build

不出錯so文件就生成了。

調用：

public class MainActivity extends Activity {

	public static native String getStringFromC();

	private TextView mTextView;
	static {
		// 調用庫文件
		System.loadLibrary("hello");
	}

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		// TODO Auto-generated method stub
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		mTextView = (TextView) findViewById(R.id.textview1);
		// 調用方法
		mTextView.setText(getStringFromC());
	}
}