ContentProvider 原理分析

本文目标：以MediaProvider为例，想搞清楚调用ContentResolver访问各个ContentProvider的调用过程。

Java code:

getContentResolver().query(MediaStore.Images.Thumbnails.EXTERNAL_CONTENT_URI,null,null)

具体调用过程是

1.通过ContentResolver先查找对应给定Uri的ContentProvider，返回对应的BinderProxy

如果该Provider尚未被调用进程使用过:

a.通过ServiceManager查找activityservice得到ActivityManagerService对应BinderProxy

b.调用BinderProxy的transcat方法发送GET_CONTENT_PROVIDER_TRANSACTION命令，得到对应ContentProvider的BinderProxy。

如果该Provider已被调用进程使用过，则调用进程会保留使用过provider的HashMap。此时直接从此表查询即得。

2.调用BinderProxy的query()

通过下图，可以清楚的看到，getContentResolver().query调用时首先得到Actiivity服务，再次查询Activity服务中记录的对应ContentProviderRecord.如果发现此ContentProvider尚未publish则引发publish该ContentProvider，详见分析二一文。查询到ContentProviderRecord后返回对应MediaProvider的IBinder并返回给调用者。

整个调用过程中需要经过两次Binder调用以实现跨进程访问，即:

Calling Process -> ActivityManagerService Process-> MediaProvider process

 

Detailed call sequence(If calling process doesn't ever used theProvider):

源代码调用路径：

第1,2步：

frameworks/base/core/java/android/app/ContextImpl.java

 

[java] viewplaincopyprint?

1. private static final class ApplicationContentResolver extends ContentResolver {  
2.     public ApplicationContentResolver(Context context, ActivityThread mainThread) {  
3.         super(context);  
4.         mMainThread = mainThread;  
5.     }  
6.   
7.     @Override  
8.     protected IContentProvider acquireProvider(Context context, String name) {  
9.         return mMainThread.acquireProvider(context, name);  
10.     }  

ActivityThread

 

 

[java] viewplaincopyprint?

1. public final IContentProvider acquireProvider(Context c, String name) {  
2.     IContentProvider provider = getProvider(c, name);  
3.     if(provider == null)  
4.         return null;  
5.     IBinder jBinder = provider.asBinder();  
6.     synchronized(mProviderMap) {  
7.         ProviderRefCount prc = mProviderRefCountMap.get(jBinder);  
8.         if(prc == null) {  
9.             mProviderRefCountMap.put(jBinder, new ProviderRefCount(1)); //创建对此Provider的引用计数  
10.         } else {  
11.             prc.count++; //计数+1  
12.         } //end else  
13.     } //end synchronized  
14.     return provider;  
15. }  

得到名字为name的Provider

[java] viewplaincopyprint?

1. private final IContentProvider getProvider(Context context, String name) {  
2.     IContentProvider existing = getExistingProvider(context, name);  
3.     if (existing != null) {   
4.         return existing; //Provider已经publish,直接返回  
5.     }  
6.   
7.     IActivityManager.ContentProviderHolder holder = null;  
8.     try {  
9.         holder = ActivityManagerNative.getDefault().getContentProvider(  
10.             getApplicationThread(), name);  
11.     } catch (RemoteException ex) {  
12.     }  
13.     if (holder == null) {  
14.         Slog.e(TAG, "Failed to find provider info for " + name);  
15.         return null;  
16.     }  
17.   
18.     IContentProvider prov = installProvider(context, holder.provider,  
19.             holder.info, true);  
20.     if (holder.noReleaseNeeded || holder.provider == null) {  
21.         // We are not going to release the provider if it is an external  
22.         // provider that doesn't care about being released, or if it is  
23.         // a local provider running in this process.  
24.         //Slog.i(TAG, "*** NO RELEASE NEEDED");  
25.         synchronized(mProviderMap) {  
26.             mProviderRefCountMap.put(prov.asBinder(), new ProviderRefCount(10000)); //为何holder.provider == null??  
27.         }  
28.     }  
29.     return prov;  
30. }  

得到ActivityManagerService。

frameworks/base/core/java/android/app/ActivityManagerNative.java

 

 

[java] viewplaincopyprint?

1. static public IActivityManager getDefault()  
2. {  
3.     if (gDefault != null) {  
4.         return gDefault;  
5.     }  
6.     IBinder b = ServiceManager.getService("activity");  
7.     gDefault = asInterface(b);  
8.     return gDefault;  
9. }  

frameworks/base/services/java/com/android/server/am/ActivityManagerService.java

 

 

[java] viewplaincopyprint?

1.     public final ContentProviderHolder getContentProvider(  
2.             IApplicationThread caller, String name) {  
3.         if (caller == null) {  
4.             String msg = "null IApplicationThread when getting content provider "  
5.                     + name;  
6.              throw new SecurityException(msg);  
7.         }  
8.   
9.         return getContentProviderImpl(caller, name);  
10.     }  
11.   
12.     private final ContentProviderHolder getContentProviderImpl(  
13.         IApplicationThread caller, String name) {  
14.         ContentProviderRecord cpr;  
15.         ProviderInfo cpi = null;  
16.   
17.         synchronized(this) {  
18.             ProcessRecord r = null;  
19.             if (caller != null) {  
20.                 r = getRecordForAppLocked(caller); //caller app must be registered  
21.                 if (r == null) {  
22.                     throw new SecurityException(  
23.                             "Unable to find app for caller " + caller  
24.                           + " (pid=" + Binder.getCallingPid()  
25.                           + ") when getting content provider " + name);  
26.                 }  
27.             }  
28.   
29.             // First check if this content provider has been published...  
30.             cpr = mProvidersByName.get(name);  
31.             if (cpr != null) {  
32.                 cpi = cpr.info;  
33.                 String msg;  
34.                 if ((msg=checkContentProviderPermissionLocked(cpi, r)) != null) { //检查app是否有访问权限  
35.                     throw new SecurityException(msg);  
36.                 }  
37.   
38.                 if (r != null && cpr.canRunHere(r)) {  
39.                     // This provider has been published or is in the process  
40.                     // of being published...  but it is also allowed to run  
41.                     // in the caller's process, so don't make a connection  
42.                     // and just let the caller instantiate its own instance.  
43.                     if (cpr.provider != null) {  
44.                         // don't give caller the provider object, it needs  
45.                         // to make its own.  
46.                         cpr = new ContentProviderRecord(cpr);  
47.                     }  
48.                     return cpr;  
49.                 }  
50.   
51.                 final long origId = Binder.clearCallingIdentity();  
52.   
53.                 // In this case the provider instance already exists, so we can  
54.                 // return it right away.  
55.                 if (r != null) {  
56.                     if (DEBUG_PROVIDER) Slog.v(TAG,  
57.                             "Adding provider requested by "  
58.                             + r.processName + " from process "  
59.                             + cpr.info.processName);  
60.                     Integer cnt = r.conProviders.get(cpr);  
61.                     if (cnt == null) {  
62.                         r.conProviders.put(cpr, new Integer(1));  
63.                     } else {  
64.                         r.conProviders.put(cpr, new Integer(cnt.intValue()+1));  
65.                     }  
66.                     cpr.clients.add(r);  
67.                     if (cpr.app != null && r.setAdj <= PERCEPTIBLE_APP_ADJ) {  
68.                         // If this is a perceptible app accessing the provider,  
69.                         // make sure to count it as being accessed and thus  
70.                         // back up on the LRU list.  This is good because  
71.                         // content providers are often expensive to start.  
72.                         updateLruProcessLocked(cpr.app, false, true);  
73.                     }  
74.                 } else {  
75.                     cpr.externals++;  
76.                 }  
77.   
78.                 if (cpr.app != null) {  
79.                     updateOomAdjLocked(cpr.app);  
80.                 }  
81.   
82.                 Binder.restoreCallingIdentity(origId);  
83.   
84.             }   
85. ...  
86.         }  
87.   
88.         // Wait for the provider to be published...  
89.         synchronized (cpr) {  
90.             while (cpr.provider == null) {  
91.                 if (cpr.launchingApp == null) {  
92.                     Slog.w(TAG, "Unable to launch app "  
93.                             + cpi.applicationInfo.packageName + "/"  
94.                             + cpi.applicationInfo.uid + " for provider "  
95.                             + name + ": launching app became null");  
96.                     EventLog.writeEvent(EventLogTags.AM_PROVIDER_LOST_PROCESS,  
97.                             cpi.applicationInfo.packageName,  
98.                             cpi.applicationInfo.uid, name);  
99.                     return null;  
100.                 }  
101.                 try {  
102.                     cpr.wait(); //publishContentProvider结束后会notify  
103.                 } catch (InterruptedException ex) {  
104.                 }  
105.             }  
106.         }  
107.         return cpr;  
108.     }  

 

frameworks/base/core/java/android/content/ContentProviderNative.java

 

[java] viewplaincopyprint?

1. final class ContentProviderProxy implements IContentProvider  
2. {  
3.     public Cursor query(Uri url, String[] projection, String selection,  
4.             String[] selectionArgs, String sortOrder) throws RemoteException {  
5.         //TODO make a pool of windows so we can reuse memory dealers  
6.         CursorWindow window = new CursorWindow(false );  
7.         BulkCursorToCursorAdaptor adaptor = new BulkCursorToCursorAdaptor();  
8.         IBulkCursor bulkCursor = bulkQueryInternal(  
9.             url, projection, selection, selectionArgs, sortOrder,  
10.             adaptor.getObserver(), window,  
11.             adaptor);  
12.         return adaptor;  
13.     }  
14.     private IBulkCursor bulkQueryInternal(  
15.         Uri url, String[] projection,  
16.         String selection, String[] selectionArgs, String sortOrder,  
17.         IContentObserver observer, CursorWindow window,  
18.         BulkCursorToCursorAdaptor adaptor) throws RemoteException {  
19.         Parcel data = Parcel.obtain();  
20.         Parcel reply = Parcel.obtain();  
21.         data.writeInterfaceToken(IContentProvider.descriptor);  
22.   
23.         url.writeToParcel(data, 0);  
24.         int length = 0;  
25.         if (projection != null) {  
26.             length = projection.length;  
27.         }  
28.         data.writeInt(length);  
29.         for (int i = 0; i < length; i++) {  
30.             data.writeString(projection[i]);  
31.         }  
32.         data.writeString(selection);  
33.         if (selectionArgs != null) {  
34.             length = selectionArgs.length;  
35.         } else {  
36.             length = 0;  
37.         }  
38.         data.writeInt(length);  
39.         for (int i = 0; i < length; i++) {  
40.             data.writeString(selectionArgs[i]);  
41.         }  
42.         data.writeString(sortOrder);  
43.         data.writeStrongBinder(observer.asBinder());  
44.         window.writeToParcel(data, 0);  
45.   
46.         // Flag for whether or not we want the number of rows in the  
47.         // cursor and the position of the "_id" column index (or -1 if  
48.         // non-existent).  Only to be returned if binder != null.  
49.         final boolean wantsCursorMetadata = (adaptor != null);  
50.         data.writeInt(wantsCursorMetadata ? 1 : 0);  
51.         mRemote.transact(IContentProvider.QUERY_TRANSACTION, data, reply, 0);  
52.   
53.         DatabaseUtils.readExceptionFromParcel(reply);  
54.   
55.         IBulkCursor bulkCursor = null;  
56.         IBinder bulkCursorBinder = reply.readStrongBinder();  
57.         if (bulkCursorBinder != null) {  
58.             bulkCursor = BulkCursorNative.asInterface(bulkCursorBinder);  
59.   
60.             if (wantsCursorMetadata) {  
61.                 int rowCount = reply.readInt();  
62.                 int idColumnPosition = reply.readInt();  
63.                 if (bulkCursor != null) {  
64.                     adaptor.set(bulkCursor, rowCount, idColumnPosition);  
65.                 }  
66.             }  
67.         }  
68.   
69.         data.recycle();  
70.         reply.recycle();  
71.   
72.         return bulkCursor;  
73.     }  

frameworks/base/core/java/android/content/ContentProvider.java

 

[java] viewplaincopyprint?

1. class Transport extends ContentProviderNative {  
2.   
3.     public Cursor query(Uri uri, String[] projection,  
4.             String selection, String[] selectionArgs, String sortOrder) {  
5.         enforceReadPermission(uri);  
6.         return ContentProvider.this.query(uri, projection, selection,  
7.                 selectionArgs, sortOrder);  
8.     }  

MediaProvider.java

 

 

[java] viewplaincopyprint?

1. public Cursor query(Uri uri, String[] projectionIn, String selection,  
2.         String[] selectionArgs, String sort) {   //调用到真正做事情的地方