Android中的sqlite Cursor操作详解

2014-02-10 12:20 2408人阅读评论(0) 收藏举报

目录(?)[-]

使用query的方式
query的操作
Cursor的操作
总结

就像 Android SQLiteStatement 编译、执行分析 中所说的，SQLite中所有SQL语句都需要先编译为stmt，然后执行。
上述文章介绍了SQLiteStatement在android层面的编译执行。然而，类SQLiteStatement只能用以执行无返回值或者只有一行一列（1X1）的sql语句，例如INSERT ALERT 等，而像SELECT这种返回结果可能多行多列的则不适用。
android对select提供了专门的执行方法rawQuery()，对其也有特殊的SQLiteQuery类，以及相关的Cursor类。这篇文章我们可以看到，其实SQLiteQuery与SQLiteStatement本质是相同的，android针对select的特殊性做了特殊的执行流程。

1、使用query的方式

[java] view plain copy

SQLiteDatabase db = mOpenHelper.getWritableDatabase();
Cursor cr;
cr = db.rawQuery("select * from person where age=20", null);
if (cr.moveToFirst()) {
for (int i = 0; i < cr.getCount(); i++) {
cr.getString();
cr.moveToNext();
}
}

2、query的操作

[java] view plain copy

//SQLiteDatabase.java
public Cursor rawQuery(String sql, String[] selectionArgs) {
return rawQueryWithFactory(null, sql, selectionArgs, null, null);
}
public Cursor rawQueryWithFactory(
CursorFactory cursorFactory, String sql, String[] selectionArgs,
String editTable, CancellationSignal cancellationSignal) {
acquireReference();
try {
SQLiteCursorDriver driver = new SQLiteDirectCursorDriver(this, sql, editTable,
cancellationSignal); // ①
return driver.query(cursorFactory != null ? cursorFactory : mCursorFactory,
selectionArgs); // ②
} finally {
releaseReference();
}
}

这里出现了两步操作，构建一个driver，通过driver执行。那么 SQLiteDirectCursorDriver 是什么呢？

[java] view plain copy

// SQLiteDirectCursorDriver.java
public final class SQLiteDirectCursorDriver implements SQLiteCursorDriver {
private final SQLiteDatabase mDatabase;
private final String mEditTable;
private final String mSql;
private final CancellationSignal mCancellationSignal;
private SQLiteQuery mQuery;
public SQLiteDirectCursorDriver(SQLiteDatabase db, String sql, String editTable,
CancellationSignal cancellationSignal) {
mDatabase = db;
mEditTable = editTable;
mSql = sql;
mCancellationSignal = cancellationSignal;
}
public Cursor query(CursorFactory factory, String[] selectionArgs) {
final SQLiteQuery query = new SQLiteQuery(mDatabase, mSql, mCancellationSignal); //③
final Cursor cursor;
try {
query.bindAllArgsAsStrings(selectionArgs);
if (factory == null) {
cursor = new SQLiteCursor(this, mEditTable, query); // ④
} else {
cursor = factory.newCursor(mDatabase, this, mEditTable, query);
}
} catch (RuntimeException ex) {
query.close();
throw ex;
}
mQuery = query;
return cursor;
}

可以看到，driver的一个陌生成员变量为 SQLiteQuery，并且它在构造函数中并未出现，在执行driver.query时才出现并被赋值③。接着又构造了我们熟悉的cursor并将其返回④。

首先看下SQLiteQuery：

[java] view plain copy

// SQLiteQuery.java
public final class SQLiteQuery extends SQLiteProgram {
private static final String TAG = "SQLiteQuery";
private final CancellationSignal mCancellationSignal;
SQLiteQuery(SQLiteDatabase db, String query, CancellationSignal cancellationSignal) {
super(db, query, null, cancellationSignal);
mCancellationSignal = cancellationSignal;
}

正如文章开头所说，SQLiteQuery继承自SQLiteProgram，和SQLiteStatement相同。由（ Android SQLiteStatement 编译、执行分析）可以知道，在其构造函数中，经历了sql语句的prepare过程，在某个连接池的某个connection中已经含有了相应的stmt。
在④中可以看到，如果factory == null，即没有自定义的cursor工厂类（我们一般不会自定义的），会直接构造一个SQLiteCursor。具体看下 SQLiteCursor类。
[java] view plain copy

// SQLiteCursor.java
public class SQLiteCursor extends AbstractWindowedCursor {
static final String TAG = "SQLiteCursor";
static final int NO_COUNT = -1;
private final String mEditTable;
private final String[] mColumns;
private final SQLiteQuery mQuery;
private final SQLiteCursorDriver mDriver;
private int mCount = NO_COUNT;
private int mCursorWindowCapacity;
private Map mColumnNameMap;
private final Throwable mStackTrace;
public SQLiteCursor(SQLiteCursorDriver driver, String editTable, SQLiteQuery query) {
……
mDriver = driver;
mEditTable = editTable;
mColumnNameMap = null;
mQuery = query;
mColumns = query.getColumnNames();
mRowIdColumnIndex = DatabaseUtils.findRowIdColumnIndex(mColumns);
}

可以看到，SQLiteCursor维护的也是一些元信息，但其继承自 AbstractWindowedCursor，后者又继承自AbstractCursor。

[java] view plain copy

// AbstractWindowedCursor.java
public abstract class AbstractWindowedCursor extends AbstractCursor {
protected CursorWindow mWindow;
}
// AbstractCursor.java
public abstract class AbstractCursor implements CrossProcessCursor {
protected int mPos;
......
}

在AbstractWindowedCursor中，我们看到了 CursorWindow，在数据库中cursor window是很重要的概念。

[java] view plain copy

// CursorWindow.java
public class CursorWindow extends SQLiteClosable implements Parcelable {
public int mWindowPtr; // ！！！
private int mStartPos;
private final String mName;
private final CloseGuard mCloseGuard = CloseGuard.get();
private static native int nativeCreate(String name, int cursorWindowSize);
private static native void nativeClear(int windowPtr);
private static native int nativeGetNumRows(int windowPtr);
private static native double nativeGetDouble(int windowPtr, int row, int column);
……
}

mWindowPtr 目测是指向native层sqlite相应window的指针。并且该类含有不少native方法，部分对sqlite中window的操作，应该是通过这个类实现的。
通过继承，SQLiteCursor有了指向cursor window的能力，但是在构造函数中并未体现，并且driver.query时，直接将new处的cursor返回了。此时，尚未通过native实际执行过select语句。

3、Cursor的操作

[java] view plain copy

//AbstractCursor.java
public final boolean moveToFirst() {
return moveToPosition(0);
}
public final boolean moveToNext() {
return moveToPosition(mPos + 1);
}
public final boolean moveToPosition(int position) {
final int count = getCount(); // ⑤
if (position >= count) {
mPos = count;
return false;
}
if (position < 0) {
mPos = -1;
return false;
}
if (position == mPos) {
return true;
}
boolean result = onMove(mPos, position); /// ⑨
if (result == false) {
mPos = -1;
} else {
mPos = position;
if (mRowIdColumnIndex != -1) {
mCurrentRowID = Long.valueOf(getLong(mRowIdColumnIndex));
}
}
return result;
}
// SQLiteCursor.java
@Override
public int getCount() {
if (mCount == NO_COUNT) {
fillWindow(0);
}
return mCount;
}
@Override
public boolean onMove(int oldPosition, int newPosition) {
if (mWindow == null || newPosition < mWindow.getStartPosition() ||
newPosition >= (mWindow.getStartPosition() + mWindow.getNumRows())) {
fillWindow(newPosition);
}
return true;
}

到这里可以看到，第一次 moveToPosition时，因为此时mCount为-1，fillWindow(0)。
[java] view plain copy

// SQLiteCursor.java
private void fillWindow(int requiredPos) {
clearOrCreateWindow(getDatabase().getPath()); // ⑥
if (mCount == NO_COUNT) {
int startPos = DatabaseUtils.cursorPickFillWindowStartPosition(requiredPos, 0); // ⑦
mCount = mQuery.fillWindow(mWindow, startPos, requiredPos, true); // ⑧
mCursorWindowCapacity = mWindow.getNumRows();
if (Log.isLoggable(TAG, Log.DEBUG)) {
Log.d(TAG, "received count(*) from native_fill_window: " + mCount);
}
} else {
int startPos = DatabaseUtils.cursorPickFillWindowStartPosition(requiredPos,
mCursorWindowCapacity);
mQuery.fillWindow(mWindow, startPos, requiredPos, false);
}
}
protected void clearOrCreateWindow(String name) {
if (mWindow == null) { // 建立CursorWindow
mWindow = new CursorWindow(name);
} else {
mWindow.clear();
}
}

在第⑥中，new出CursorWindow，将其赋值给mWindow，此时，由SQLiteCursor掌管。如下， new CursorWindow的过程，调用了nativeCreate，并使mWindowPtr指向native层的window。

[java] view plain copy

// CursorWindow.java
public CursorWindow(String name) {
mStartPos = 0;
mName = name != null && name.length() != 0 ? name : "";
mWindowPtr = nativeCreate(mName, sCursorWindowSize); // ！！！
if (mWindowPtr == 0) {
throw new CursorWindowAllocationException("Cursor window allocation of " +
(sCursorWindowSize / 1024) + " kb failed. " + printStats());
}
mCloseGuard.open("close");
recordNewWindow(Binder.getCallingPid(), mWindowPtr);
}

先看第⑧中， fillWindow()

[java] view plain copy

// SQLiteQuery.java
int fillWindow(CursorWindow window, int startPos, int requiredPos, boolean countAllRows) {
....
int numRows = getSession().executeForCursorWindow(getSql(), getBindArgs(),
window, startPos, requiredPos, countAllRows, getConnectionFlags(),
mCancellationSignal);
return numRows;
}
// SQLiteSeesion.java
public int executeForCursorWindow(String sql, Object[] bindArgs,
CursorWindow window, int startPos, int requiredPos, boolean countAllRows,
int connectionFlags, CancellationSignal cancellationSignal) {
acquireConnection(sql, connectionFlags, cancellationSignal);
try {
return mConnection.executeForCursorWindow(sql, bindArgs,
window, startPos, requiredPos, countAllRows,
cancellationSignal);
} finally {
releaseConnection();
}
}
// SQLiteConnection.java
public int executeForCursorWindow(String sql, Object[] bindArgs,
CursorWindow window, int startPos, int requiredPos, boolean countAllRows,
CancellationSignal cancellationSignal) {
final PreparedStatement statement = acquirePreparedStatement(sql);
final long result = nativeExecuteForCursorWindow( // !!!
mConnectionPtr, statement.mStatementPtr, window.mWindowPtr,
startPos, requiredPos, countAllRows);
actualPos = (int)(result >> 32);
countedRows = (int)result;
filledRows = window.getNumRows();
window.setStartPosition(actualPos);
return countedRows;
.....
}

可以看到，最终仍是通过SQLiteConnection连接到native来执行。

剩下的getString就比较简单了，一直会调用到到mWindow的getString

[java] view plain copy

public String getString(int row, int column) {
acquireReference();
try {
return nativeGetString(mWindowPtr, row - mStartPos, column);
} finally {
releaseReference();
}
}

最后看下第⑦，即window fill 的控制。

这里有涉及fill策略，一般无需考虑。如果结果集大于window怎么办？如果所需某个元素不在window中怎么办？尚未详细分析了，贴下代码。

若是第一次fill，required row 为0,即从第一条记录开始fill满window。

window将会包含所需的row及其周围的一些row。例如，想要结果集的第120个元素，window大小为90，则将结果集第90-180的元素填充至window，120之前30个，之后60个。如果window中没有，将其放置在window的第10个位置。

[java] view plain copy

// DatabaseUtils.java
public static int cursorPickFillWindowStartPosition(
int cursorPosition, int cursorWindowCapacity) {
return Math.max(cursorPosition - cursorWindowCapacity / 3, 0);
}

[cpp] view plain copy

static jlong nativeExecuteForCursorWindow(JNIEnv* env, jclass clazz,
jint connectionPtr, jint statementPtr, jint windowPtr,
jint startPos, jint requiredPos, jboolean countAllRows) {
......
int retryCount = 0;
int totalRows = 0;
int addedRows = 0;
bool windowFull = false;
bool gotException = false;
while (!gotException && (!windowFull || countAllRows)) {
int err = sqlite3_step(statement);
if (err == SQLITE_ROW) {
LOG_WINDOW("Stepped statement %p to row %d", statement, totalRows);
retryCount = 0;
totalRows += 1;
// Skip the row if the window is full or we haven't reached the start position yet.
if (startPos >= totalRows || windowFull) {
continue;
}
CopyRowResult cpr = copyRow(env, window, statement, numColumns, startPos, addedRows);
if (cpr == CPR_FULL && addedRows && startPos + addedRows < requiredPos) {
// We filled the window before we got to the one row that we really wanted.
// Clear the window and start filling it again from here.
// TODO: Would be nicer if we could progressively replace earlier rows.
window->clear();
window->setNumColumns(numColumns);
startPos += addedRows;
addedRows = 0;
cpr = copyRow(env, window, statement, numColumns, startPos, addedRows);
}
if (cpr == CPR_OK) {
addedRows += 1;
} else if (cpr == CPR_FULL) {
windowFull = true;
} else {
gotException = true;
}
} else if (err == SQLITE_DONE) {
// All rows processed, bail
LOG_WINDOW("Processed all rows");
break;
} else if (err == SQLITE_LOCKED || err == SQLITE_BUSY) {
// The table is locked, retry
LOG_WINDOW("Database locked, retrying");
if (retryCount > 50) {
ALOGE("Bailing on database busy retry");
throw_sqlite3_exception(env, connection->db, "retrycount exceeded");
gotException = true;
} else {
// Sleep to give the thread holding the lock a chance to finish
usleep(1000);
retryCount++;
}
} else {
throw_sqlite3_exception(env, connection->db);
gotException = true;
}
}
LOG_WINDOW("Resetting statement %p after fetching %d rows and adding %d rows"
"to the window in %d bytes",
statement, totalRows, addedRows, window->size() - window->freeSpace());
sqlite3_reset(statement);
// Report the total number of rows on request.
if (startPos > totalRows) {
ALOGE("startPos %d > actual rows %d", startPos, totalRows);
}
jlong result = jlong(startPos) << 32 | jlong(totalRows);
return result;
}

4、总结

① query的执行同普通sql语句相同，都需经过sql语句的编译及执行。

② 编译后为SQLiteQuery，执行后返回SQLiteCursor，SQLiteCursor的mWindow指向native层的cursor window。

③ 通过SQLiteCursor对返回结果进行控制。

④ 执行的过程，是构建SQLiteCursor的过程，并未将结果集写入相应window。

⑤ 结果集写入window，发生在第一次类似cursor.moveToFirst()操作中。这是android中处处体现的惰性策略。

⑥ sqlite本身对结果集与window的关系做了优化，android在此基础上再次优化，以应对结果集过大、跳跃式读取结果等问题。尚未分析。

1、使用query的方式

2、query的操作

3、Cursor的操作

4、总结

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