利用Eclipse和NDK建立一個簡單service

NDK是Google提供的一套開發支持Native代碼的應用程序的工具。因為Android用了自己改寫的C庫(Bionic)，並應用自己的compiler和linker，所以如果想用通用的toolchain編譯Android應用中C/C++代碼就比較麻煩。之前的方法是，可以把自己的代碼整合到整個android系統中，利用Android Build System的編譯工具進行編譯(具體例子可以參看<android source>/development/samples/下的PlatformLibrary和SimpleJNI兩個例子)。但是缺點是要建立整個的Build的環境，第一次Build所有的代碼時間很長。 
NDK則提供了一個相對簡單Build System。只要按它的要求提供應用和Native代碼位置及要編譯的模塊等基本信息，然後用它提供的工具就可以很方便的生成Shared Librare，並自動將其拷貝到SDK應用開發的環境下方便生成apk。具體的介紹在<android NDK>/docs/下。 
以下是我應用NDK做的一個小實驗。其實前面基本上就是在Eclipse下建立一個Service應用，只是在Service中用到的share library是用NDK生成的。與NDK相關部分用紅色標注，在工程中加入*.mk,jni等只是為了方便編輯。 實驗環境： 
Ubuntu, SDK 1.5r2, NDK 1.5r1, Eclipse3.4 (ADT plugin) 具體步驟記錄如下： 
1.在Eclipse下新建android-Project項目MyServer，Package為ndk.sample.myservice； 
2.在MyServer/src/ndk.sample.myservice下： 
a)添加aidl文件：IMyService.aidl，之後在gen/sample.ndk.myserver下會自動生成IMyService.Java;

# package ndk.sample.myservice;
# interface IMyService {
# String getGreetings(in String toName);
# }

b)實現MyService; 

2. package ndk.sample.myservice;
3. import android.app.Service;
4. import android.content.Intent;
5. import android.os.IBinder;
6. import android.util.Log;
7. public class MyService extends Service {
8. private final IMyService.Stub binder = new IMyService.Stub() {
9.   public String getGreetings(String toName) {
10.    return get_greetings(toName);
11.   }
12. };
13.
14. @Override
15. public void onCreate() {
16.   Log.v("MyService", "Lifecycle - onCreate");
17. }
18.
19. @Override
20. public void onDestroy() {
21.   Log.v("MyService", "Lifecycle - onDestroy");
22. }
23.
24. @Override
25. public void onStart(Intent intent, int startId) {
26.   Log.v("MyService", "Lifecycle - onStart: id = " + Integer.toString(startId));
27. }
28.
29. @Override
30. public IBinder onBind(Intent intent) {
31.   Log.v("MyService", "Lifecycle - onBind");
32.   return this.binder;
33. }
34. @Override
35. public void onRebind(Intent intent) {
36.   Log.v("MyService", "Lifecycle - onRebind");
37. }
38.
39. @Override
40. public boolean onUnbind(Intent intent) {
41.   Log.v("MyService", "Lifecycle - onUnbind");
42.   return false;
43. }
44.
45. static {
46.   System.loadLibrary("mynativecodes");
47. }
48.
49. public native String get_greetings(String toName);
50. }

c)實現TestClIEnt；   

2. package ndk.sample.myservice;
3. import android.app.Activity;
4. import android.content.ComponentName;
5. import android.content.Intent;
6. import android.content.ServiceConnection;
7. import android.os.Bundle;
8. import android.os.IBinder;
9. import android.os.RemoteException;
10. import android.util.Log;
11. import android.view.VIEw;
12. import android.widget.Button;
13. import android.widget.EditText;
14. import android.widget.TextView;
15. public class TestClient extends Activity {
16.     private TextView tvGreetings;
17.     private EditText etName;
18.     private IMyService service;
19.    
20.     private ServiceConnection connection = new ServiceConnection () {
21.      @Override
22.      public void onServiceConnected(ComponentName className, IBinder iservice) {
23.       service = IMyService.Stub.asInterface(iservice);
24.      }
25.      
26.      @Override
27.      public void onServiceDisconnected(ComponentName className) {
28.       service = null;
29.      }
30.     };
31.    
32.     @Override
33.     public void onCreate(Bundle savedInstanceState) {
34.         super.onCreate(savedInstanceState);
35.         setContentView(R.layout.main);
36.         
37.         tvGreetings = (TextView)findViewById(R.id.tvGreetings);
38.         etName = (EditText)findViewById(R.id.etName);
39.         
40.         Button btnHello = (Button)findViewById(R.id.btnHello);
41.         btnHello.setOnClickListener(new View.OnClickListener () {
42.          @Override
43.          public void onClick(View view) {
44.           String toName = etName.getText().toString();
45.           try {
46.            String greetings = service.getGreetings(toName);
47.            tvGreetings.setText(greetings);
48.            etName.setText("");
49.           }
50.           catch (RemoteException e) {
51.            Log.e("TestClient", "error", e);
52.           }
53.          }
54.         });
55.     }
56.    
57.     @Override
58.     public void onStart() {
59.      super.onStart();
60.      
61.      boolean rc = bindService(
62.        new Intent(IMyService.class.getName()),
63.        connection, BIND_AUTO_CREATE);
64.      if (rc == false)
65.       Log.v("TestClIEnt", "Fail to bindService");
66.     }
67.    
68.     @Override
69.     public void onStop() {
70.      super.onStop();
71.      
72.      unbindService(connection);
73.     }
74. }

d)修改androidManifest.XML，添加service

# <?xml version="1.0" encoding="utf-8"?>
# <manifest XMLns:android="http://schemas.android.com/apk/res/android"
#       package="ndk.sample.myservice"
#       android:versionCode="1"
#       android:versionName="1.0">
#     <application android:icon="@drawable/icon" android:label="@string/app_name">
#         <activity android:name=".TestClIEnt"
#                   android:label="@string/app_name">
#             <intent-filter>
#                 <action android:name="android.intent.action.MAIN" />
#                 <category android:name="android.intent.category.LAUNCHER" />
#             </intent-filter>
#         </activity>
#         <service android:name=".MyService">
#          <intent-filter>
#           <action android:name="ndk.sample.myservice.IMyService" />
#          </intent-filter>>
#         </service>>
#     </application>
#     <uses-sdk android:minSdkVersion="3" />
# </manifest> 

e)添加Application.mk文件

  1. APP_PROJECT_PATH := $(call my-dir)
2. APP_MODULES := mynativecodes

3.在MyService下添加目錄jni： 
a)從Terminal: 
$ cd MyServer/jni 
$ Javah -jni -classpath ../bin ndk.sample.myservice.MyService 
自動生成頭文件ndk_sample_myservice_MyService.h b)在MyServer/jni下添加mynative.c，實現   

2. #include "ndk_sample_myservice_MyService.h"
3. #include <stdio.h>
4. #include <string.h>
5. JNIEXPORT JString JNICALL Java_ndk_sample_myservice_MyService_get_1greetings
6.   (JNIEnv *env, jobject obj, JString name)
7. {
8. // fixed buffer size for test
9. char buf[128];
10. const jbyte *str;
11. str = (*env)->GetStringUTFChars(env, name, NULL);
12. if (str == NULL) {
13.   return NULL;
14. }
15. if (strlen (str) == 0)
16.   sprintf (buf, "Who are you?");
17. else
18.   sprintf (buf, "Hello, %s!!!", str);
19. (*env)->ReleaseStringUTFChars(env, name, str);
20. return (*env)->NewStringUTF(env, buf);
21. }

c)在MyServer/jni下添加android.mk

# LOCAL_PATH := $(call my-dir)
# include $(CLEAR_VARS)
# LOCAL_MODULE := mynativecodes
# LOCAL_SRC_FILES := mynative.c
# include $(BUILD_SHARED_LIBRARY)

4.在Terminal下，進入NDK目錄，建立指向應用和share library的鏈接（可以不把自己的代碼放在NDK的目錄下，但必須在NDK中加入指向存放Application.mk和android.mk的路徑的鏈接） 
$ cd <androi ndk> 
$ cd sources 
$ ln -s <my service dir>/ myservice 
$ cd ../aPPS 
$ ln -s <my service dir>/jni/ mynativecodes 
$ cd ../ 
$ make APP=myservice V=1 
APP部分指明要編譯的應用名（路徑名），必須有。V=1指定顯示編譯命令，可以沒有。編譯成功後，可以在libs\armeabi下生成了libmynativecodes.so，Eclipse環境中也可以看到。 
5.最後按正常的方法生成MyService的apk即可。