《Android進階》之第七篇 NDK的使用

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《Android進階》之第一篇 在Java中調用C庫函數

這一篇列舉的方法是在NDK沒有出來時候用的方式

在Android發布NDK之後,可以這樣使用

一、首先下載android-ndk 下載後把壓縮包解壓出來，例如：D:\android-ndk-r10e，目錄下的ndk-build.cmd就是用來編譯的批處理命令。

 為了編譯方便,推薦添加環境變數

Path目錄下添加D:\android-ndk-r10e;

以NDK內建的例子\samples\hello-jni為例

將例子匯入安卓工程:

HelloJni.java如下:

/* * 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. */package com.example.hellojni;import android.app.Activity;import android.widget.TextView;import android.os.Bundle;public class HelloJni extends Activity{    /** Called when the activity is first created. */    @Override    public void onCreate(Bundle savedInstanceState)    {        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);    }    /* A native method that is implemented by the     * ‘hello-jni‘ native library, which is packaged     * with this application.     */    public native String  stringFromJNI();    /* This is another native method declaration that is *not*     * implemented by ‘hello-jni‘. This is simply to show that     * you can declare as many native methods in your Java code     * as you want, their implementation is searched in the     * currently loaded native libraries only the first time     * you call them.     *     * Trying to call this function will result in a     * java.lang.UnsatisfiedLinkError exception !     */    public native String  unimplementedStringFromJNI();    /* this is used to load the ‘hello-jni‘ library on application     * startup. The library has already been unpacked into     * /data/data/com.example.hellojni/lib/libhello-jni.so at     * installation time by the package manager.     */    static {        System.loadLibrary("hello-jni");    }}

觀察上面代碼會發現:

有個匯入 System.loadLibrary的過程

還定義了兩個Nativie函數

在查看其中的c源檔案

/* * 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. * */#include <string.h>#include <jni.h>/* This is a trivial JNI example where we use a native method * to return a new VM String. See the corresponding Java source * file located at: * *   apps/samples/hello-jni/project/src/com/example/hellojni/HelloJni.java */jstringJava_com_example_hellojni_HelloJni_stringFromJNI( JNIEnv* env,                                                  jobject thiz ){#if defined(__arm__)  #if defined(__ARM_ARCH_7A__)    #if defined(__ARM_NEON__)      #if defined(__ARM_PCS_VFP)        #define ABI "armeabi-v7a/NEON (hard-float)"      #else        #define ABI "armeabi-v7a/NEON"      #endif    #else      #if defined(__ARM_PCS_VFP)        #define ABI "armeabi-v7a (hard-float)"      #else        #define ABI "armeabi-v7a"      #endif    #endif  #else   #define ABI "armeabi"  #endif#elif defined(__i386__)   #define ABI "x86"#elif defined(__x86_64__)   #define ABI "x86_64"#elif defined(__mips64)  /* mips64el-* toolchain defines __mips__ too */   #define ABI "mips64"#elif defined(__mips__)   #define ABI "mips"#elif defined(__aarch64__)   #define ABI "arm64-v8a"#else   #define ABI "unknown"#endif    return (*env)->NewStringUTF(env, "Hello from JNI !  Compiled with ABI " ABI ".");}

因為我們是拿這個c源碼檔案來使用，如果遷就Java_com_example_hellojni_HelloJni_stringFromJNI函數名的話，在我們的android工程中java類的聲明就要是：com/example/hellojni/HelloJni.java。 二、編譯，開啟cmd命令列視窗

D:\>cd D:\android-ndk-r10e\samples\hello-jni\jniD:\android-ndk-r10e\samples\hello-jni\jni>ndk-build[arm64-v8a] Gdbserver      : [aarch64-linux-android-4.9] libs/arm64-v8a/gdbserver[arm64-v8a] Gdbsetup       : libs/arm64-v8a/gdb.setup[x86_64] Gdbserver      : [x86_64-4.9] libs/x86_64/gdbserver[x86_64] Gdbsetup       : libs/x86_64/gdb.setup[mips64] Gdbserver      : [mips64el-linux-android-4.9] libs/mips64/gdbserver[mips64] Gdbsetup       : libs/mips64/gdb.setup[armeabi-v7a] Gdbserver      : [arm-linux-androideabi-4.8] libs/armeabi-v7a/gdbserver[armeabi-v7a] Gdbsetup       : libs/armeabi-v7a/gdb.setup[armeabi] Gdbserver      : [arm-linux-androideabi-4.8] libs/armeabi/gdbserver[armeabi] Gdbsetup       : libs/armeabi/gdb.setup[x86] Gdbserver      : [x86-4.8] libs/x86/gdbserver[x86] Gdbsetup       : libs/x86/gdb.setup

編譯成功會在D:\~\samples\hello-jni\libs\armeabi目錄下產生libhello-jni.so檔案。

不光是armeabi 其他所有平台都會產生.so檔案

現在程式就能運行啦

stringFromJNI()其實調用的是.so檔案中的方法
程式運行結果:

《Android進階》之第七篇 NDK的使用