从源码看Android中sqlite是怎么读DB的（转）

执行query

执行SQLiteDatabase类中query系列函数时，只会构造查询信息，不会执行查询。

（query的源码追踪路径）

执行move（里面的fillwindow是真正打开文件句柄并分配内存的地方）

当执行Cursor的move系列函数时，第一次执行，会为查询结果集创建一块共享内存，即cursorwindow

moveToPosition源码路径

fillWindow----真正耗时的地方

然后会执行sql语句，向共享内存中填入数据，

fillWindow源码路径

在SQLiteCursor.java中可以看到

 1 @Override
 2 public boolean onMove(int oldPosition, int newPosition) {
 3     // Make sure the row at newPosition is present in the window
 4     if (mWindow == null || newPosition < mWindow.getStartPosition() ||
 5             newPosition >= (mWindow.getStartPosition() + mWindow.getNumRows())) {
 6         fillWindow(newPosition);
 7     }
 8
 9     return true;
10 }

如果请求查询的位置在cursorWindow的范围内，不会执行fillWindow，

而超出cursorwindow的范围，会调用fillWindow，

而在nativeExecuteForCursorWindow中，

获取记录时，如果要请求的位置超出窗口范围，会发生CursorWindow的清空：

 1 CopyRowResult cpr = copyRow(env, window, statement, numColumns, startPos, addedRows);
 2 if (cpr == CPR_FULL && addedRows && startPos + addedRows < requiredPos) {
 3 // We filled the window before we got to the one row that we really wanted.
 4 // Clear the window and start filling it again from here.
 5 // TODO: Would be nicer if we could progressively replace earlier rows.
 6 window->clear();
 7 window->setNumColumns(numColumns);
 8 startPos += addedRows;
 9 addedRows = 0;
10 cpr = copyRow(env, window, statement, numColumns, startPos, addedRows);
11 }  

CursorWindow的清空机制会影响到多线程读（通常认为不可以并发读写，sqlite的并发实际上是串行执行的，但可以并发读，这里要强调的是多线程读也可能有问题），具体见稍后一篇文章“listview并发读写数据库”。

Cursor关闭（显式调用close()的理由）

追踪源码看关闭

 1  //SQLiteCursor
 2
 3 super.close();
 4 synchronized (this) {
 5     mQuery.close();
 6     mDriver.cursorClosed();
 7 }
 8
 9
10 //AbstractCursor
11
12 public void close() {
13     mClosed = true;
14     mContentObservable.unregisterAll();
15     onDeactivateOrClose();
16 }
17
18 protected void onDeactivateOrClose() {
19     if (mSelfObserver != null) {
20         mContentResolver.unregisterContentObserver(mSelfObserver);
21         mSelfObserverRegistered = false;
22     }
23     mDataSetObservable.notifyInvalidated();
24 }
25
26
27 //AbstractWindowedCursor
28
29 /** @hide */
30 @Override
31 protected void onDeactivateOrClose() {
32     super.onDeactivateOrClose();
33     closeWindow();
34 }
35
36 protected void closeWindow() {
37     if (mWindow != null) {
38         mWindow.close();
39         mWindow = null;
40     }
41 }
42
43
44
45 //SQLiteClosable
46
47 public void close() {
48     releaseReference();
49 }
50
51 public void releaseReference() {
52     boolean refCountIsZero = false;
53     synchronized(this) {
54         refCountIsZero = --mReferenceCount == 0;
55     }
56     if (refCountIsZero) {
57         onAllReferencesReleased();
58     }
59 }
60
61 //CursorWindow
62
63 @Override
64 protected void onAllReferencesReleased() {
65     dispose();
66 }
67
68 private void dispose() {
69     if (mCloseGuard != null) {
70         mCloseGuard.close();
71     }
72     if (mWindowPtr != 0) {
73         recordClosingOfWindow(mWindowPtr);
74         nativeDispose(mWindowPtr);
75         mWindowPtr = 0;
76     }
77 }

跟CursorWindow有关的路径里，最终调用nativeDispose()清空cursorWindow;

当Cursor被GC回收时，会调用finalize：

 1 @Override
 2 protected void finalize() {
 3     try {
 4         // if the cursor hasn‘t been closed yet, close it first
 5         if (mWindow != null) {
 6             if (mStackTrace != null) {
 7                 String sql = mQuery.getSql();
 8                 int len = sql.length();
 9                 StrictMode.onSqliteObjectLeaked(
10                     "Finalizing a Cursor that has not been deactivated or closed. " +
11                     "database = " + mQuery.getDatabase().getLabel() +
12                     ", table = " + mEditTable +
13                     ", query = " + sql.substring(0, (len > 1000) ? 1000 : len),
14                     mStackTrace);
15             }
16             close();
17         }
18     } finally {
19         super.finalize();
20     }
21 }

然而finalize()并没有释放CursorWindow，而super.finalize();里也只是解绑了观察者，没有去释放cursorwindow

所以不调用cursor.close(),最终会导致cursorWindow所在的共享内存(1M或2M)泄露。

http://www.cnblogs.com/hellocwh/p/4924732.html