深入分析AIDL原理

• 博客分類：
• Android

在上一篇文章（Service使用方式）中，介紹了Android進程間通信（IPC）的使用，并給出了一個示例。但并沒有深入分析aidl是怎樣可以做到進程間通信的，它的執行過程是怎樣的？

這篇文章來分析IRemoteService.aidl的執行過程，并理解aidl是怎樣跨進程通信的。

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當我們創建IRemoteService.aidl文件時，IDE會為我們在gen目錄中創建相應的文件。

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Iremoteservice.java代碼 ?

1. /**?This?file?is?auto-generated.??DO?NOT?MODIFY.??
2. ?*?Original?file:?F:\\workspace\\AndroidImprove\\src\\com\\example\\aidl\\IRemoteService.aidl??
3. ?*/??
4. package?com.example.aidl;??
5. public?interface?IRemoteService?extends?android.os.IInterface??
6. {??
7. /**?Local-side?IPC?implementation?stub?class.?*/??
8. public?static?abstract?class?Stub?extends?android.os.Binder?implements?com.example.aidl.IRemoteService??
9. {??
10. private?static?final?java.lang.String?DESCRIPTOR?=?"com.example.aidl.IRemoteService";??
11. /**?Construct?the?stub?at?attach?it?to?the?interface.?*/??
12. public?Stub()??
13. {??
14. this.attachInterface(this,?DESCRIPTOR);??
15. }??
16. /**??
17. ?*?Cast?an?IBinder?object?into?an?com.example.aidl.IRemoteService?interface,??
18. ?*?generating?a?proxy?if?needed.??
19. ?*/??
20. public?static?com.example.aidl.IRemoteService?asInterface(android.os.IBinder?obj)??
21. {??
22. if?((obj==null))?{??
23. return?null;??
24. }??
25. android.os.IInterface?iin?=?(android.os.IInterface)obj.queryLocalInterface(DESCRIPTOR);??
26. if?(((iin!=null)&&(iin?instanceof?com.example.aidl.IRemoteService)))?{??
27. return?((com.example.aidl.IRemoteService)iin);??
28. }??
29. return?new?com.example.aidl.IRemoteService.Stub.Proxy(obj);??
30. }??
31. public?android.os.IBinder?asBinder()??
32. {??
33. return?this;??
34. }??
35. @Override?public?boolean?onTransact(int?code,?android.os.Parcel?data,?android.os.Parcel?reply,?int?flags)?throws?android.os.RemoteException??
36. {??
37. switch?(code)??
38. {??
39. case?INTERFACE_TRANSACTION:??
40. {??
41. reply.writeString(DESCRIPTOR);??
42. return?true;??
43. }??
44. case?TRANSACTION_register:??
45. {??
46. data.enforceInterface(DESCRIPTOR);??
47. com.example.aidl.IRemoteCallback?_arg0;??
48. _arg0?=?com.example.aidl.IRemoteCallback.Stub.asInterface(data.readStrongBinder());??
49. this.register(_arg0);??
50. reply.writeNoException();??
51. return?true;??
52. }??
53. case?TRANSACTION_unregister:??
54. {??
55. data.enforceInterface(DESCRIPTOR);??
56. com.example.aidl.IRemoteCallback?_arg0;??
57. _arg0?=?com.example.aidl.IRemoteCallback.Stub.asInterface(data.readStrongBinder());??
58. this.unregister(_arg0);??
59. reply.writeNoException();??
60. return?true;??
61. }??
62. case?TRANSACTION_execute:??
63. {??
64. data.enforceInterface(DESCRIPTOR);??
65. this.execute();??
66. reply.writeNoException();??
67. return?true;??
68. }??
69. case?TRANSACTION_getStatus:??
70. {??
71. data.enforceInterface(DESCRIPTOR);??
72. java.lang.String?_arg0;??
73. _arg0?=?data.readString();??
74. int?_result?=?this.getStatus(_arg0);??
75. reply.writeNoException();??
76. reply.writeInt(_result);??
77. return?true;??
78. }??
79. }??
80. return?super.onTransact(code,?data,?reply,?flags);??
81. }??
82. private?static?class?Proxy?implements?com.example.aidl.IRemoteService??
83. {??
84. private?android.os.IBinder?mRemote;??
85. Proxy(android.os.IBinder?remote)??
86. {??
87. mRemote?=?remote;??
88. }??
89. public?android.os.IBinder?asBinder()??
90. {??
91. return?mRemote;??
92. }??
93. public?java.lang.String?getInterfaceDescriptor()??
94. {??
95. return?DESCRIPTOR;??
96. }??
97. //注冊回調??
98. ??
99. public?void?register(com.example.aidl.IRemoteCallback?callback)?throws?android.os.RemoteException??
100. {??
101. android.os.Parcel?_data?=?android.os.Parcel.obtain();??
102. android.os.Parcel?_reply?=?android.os.Parcel.obtain();??
103. try?{??
104. _data.writeInterfaceToken(DESCRIPTOR);??
105. _data.writeStrongBinder((((callback!=null))?(callback.asBinder()):(null)));??
106. mRemote.transact(Stub.TRANSACTION_register,?_data,?_reply,?0);??
107. _reply.readException();??
108. }??
109. finally?{??
110. _reply.recycle();??
111. _data.recycle();??
112. }??
113. }??
114. //取消注冊回調??
115. ??
116. public?void?unregister(com.example.aidl.IRemoteCallback?callback)?throws?android.os.RemoteException??
117. {??
118. android.os.Parcel?_data?=?android.os.Parcel.obtain();??
119. android.os.Parcel?_reply?=?android.os.Parcel.obtain();??
120. try?{??
121. _data.writeInterfaceToken(DESCRIPTOR);??
122. _data.writeStrongBinder((((callback!=null))?(callback.asBinder()):(null)));??
123. mRemote.transact(Stub.TRANSACTION_unregister,?_data,?_reply,?0);??
124. _reply.readException();??
125. }??
126. finally?{??
127. _reply.recycle();??
128. _data.recycle();??
129. }??
130. }??
131. //執行回調??
132. ??
133. public?void?execute()?throws?android.os.RemoteException??
134. {??
135. android.os.Parcel?_data?=?android.os.Parcel.obtain();??
136. android.os.Parcel?_reply?=?android.os.Parcel.obtain();??
137. try?{??
138. _data.writeInterfaceToken(DESCRIPTOR);??
139. mRemote.transact(Stub.TRANSACTION_execute,?_data,?_reply,?0);??
140. _reply.readException();??
141. }??
142. finally?{??
143. _reply.recycle();??
144. _data.recycle();??
145. }??
146. }??
147. //獲取狀態??
148. ??
149. public?int?getStatus(java.lang.String?flag)?throws?android.os.RemoteException??
150. {??
151. android.os.Parcel?_data?=?android.os.Parcel.obtain();??
152. android.os.Parcel?_reply?=?android.os.Parcel.obtain();??
153. int?_result;??
154. try?{??
155. _data.writeInterfaceToken(DESCRIPTOR);??
156. _data.writeString(flag);??
157. mRemote.transact(Stub.TRANSACTION_getStatus,?_data,?_reply,?0);??
158. _reply.readException();??
159. _result?=?_reply.readInt();??
160. }??
161. finally?{??
162. _reply.recycle();??
163. _data.recycle();??
164. }??
165. return?_result;??
166. }??
167. }??
168. static?final?int?TRANSACTION_register?=?(android.os.IBinder.FIRST_CALL_TRANSACTION?+?0);??
169. static?final?int?TRANSACTION_unregister?=?(android.os.IBinder.FIRST_CALL_TRANSACTION?+?1);??
170. static?final?int?TRANSACTION_execute?=?(android.os.IBinder.FIRST_CALL_TRANSACTION?+?2);??
171. static?final?int?TRANSACTION_getStatus?=?(android.os.IBinder.FIRST_CALL_TRANSACTION?+?3);??
172. }??
173. //注冊回調??
174. ??
175. public?void?register(com.example.aidl.IRemoteCallback?callback)?throws?android.os.RemoteException;??
176. //取消注冊回調??
177. ??
178. public?void?unregister(com.example.aidl.IRemoteCallback?callback)?throws?android.os.RemoteException;??
179. //執行回調??
180. ??
181. public?void?execute()?throws?android.os.RemoteException;??
182. //獲取狀態??
183. ??
184. public?int?getStatus(java.lang.String?flag)?throws?android.os.RemoteException;??
185. }??

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在ClientActivity綁定遠程Service并建立連接時會調用ServiceConnection.onServiceConnected(ComponentName name, IBinder service)

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Java代碼 ?

1. public?void?onServiceConnected(ComponentName?name,?IBinder?service)?{??
2. ????????????remoteService?=?IRemoteService.Stub.asInterface(service);??
3. ????????????//注冊回調??
4. ????????????try?{??
5. ????????????????remoteService.register(remoteCallback);??
6. ????????????}?catch?(RemoteException?e)?{??
7. ????????????????e.printStackTrace();??
8. ????????????}??
9. }??

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?IBinder service是從RemoteService返回的IRemoteService.Stub iBinder，這個對象是Server應用進程中的對象。

IRemoteService.Stub.asInterface(service)在本地創建了一個代理

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public static com.example.aidl.IRemoteService asInterface(android.os.IBinder obj)

{

if ((obj==null)) {

return null;

}

android.os.IInterface iin = (android.os.IInterface)obj.queryLocalInterface(DESCRIPTOR);//這里肯定返回null

if (((iin!=null)&&(iin instanceof com.example.aidl.IRemoteService))) {

return ((com.example.aidl.IRemoteService)iin);

}

return new com.example.aidl.IRemoteService.Stub.Proxy(obj);//創建一個本地代理

}

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當使用remoteService調用方法時，其實是調用了本地com.example.aidl.IRemoteService.Stub.Proxy對象的方法，從Proxy方法中可以看到，每個方法都執行了mRemote.transact(Stub.TRANSACTION_xxx, _data, _reply, 0);。

如：

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Java代碼 ?

1. //獲取狀態??
2. ??
3. public?int?getStatus(java.lang.String?flag)?throws?android.os.RemoteException??
4. {??
5. android.os.Parcel?_data?=?android.os.Parcel.obtain();??
6. android.os.Parcel?_reply?=?android.os.Parcel.obtain();??
7. int?_result;??
8. try?{??
9. _data.writeInterfaceToken(DESCRIPTOR);??
10. _data.writeString(flag);??
11. mRemote.transact(Stub.TRANSACTION_getStatus,?_data,?_reply,?0);??
12. _reply.readException();??
13. _result?=?_reply.readInt();??
14. }??
15. finally?{??
16. _reply.recycle();??
17. _data.recycle();??
18. }??
19. return?_result;??
20. }??

?這一過程是把Client端的參數轉換成Parcel（_data）傳遞到Server端，而在Server端又會把返回數據保存到_reply中，這就形成了一次交互。

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mRemote是遠程對象，transact方法會執行onTransact方法

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Java代碼 ?

1. public?final?boolean?transact(int?code,?Parcel?data,?Parcel?reply,??
2. ????????????int?flags)?throws?RemoteException?{??
3. ????????if?(Config.LOGV)?Log.v("Binder",?"Transact:?"?+?code?+?"?to?"?+?this);??
4. ????????if?(data?!=?null)?{??
5. ????????????data.setDataPosition(0);??
6. ????????}??
7. ????????boolean?r?=?onTransact(code,?data,?reply,?flags);??
8. ????????if?(reply?!=?null)?{??
9. ????????????reply.setDataPosition(0);??
10. ????????}??
11. ????????return?r;??
12. ????}??

?這樣就會執行遠程的onTransact方法，

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Java代碼 ?

1. case?TRANSACTION_getStatus:??
2. {??
3. data.enforceInterface(DESCRIPTOR);??
4. java.lang.String?_arg0;??
5. _arg0?=?data.readString();??
6. int?_result?=?this.getStatus(_arg0);??
7. reply.writeNoException();??
8. reply.writeInt(_result);??
9. return?true;??
10. }??

注意 int _result = this.getStatus(_arg0);，這就調用了Server端的getStatus(String flag)，并把返回結果寫到Client端的代理Proxy對象的_reply中

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到此，aidl通信過程就完成了。

PS: aidl通信有點復雜，但仔細分析并不是很難

總結

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