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Microsoft JDBC Driver for SQL Server.
/*
* Microsoft JDBC Driver for SQL Server Copyright(c) Microsoft Corporation All rights reserved. This program is made
* available under the terms of the MIT License. See the LICENSE file in the project root for more information.
*/
package com.microsoft.sqlserver.jdbc;
import java.io.Serializable;
/**
* ScrollWindow provides an efficient way to scroll around within a limited number of rows, typically the ResultSet
* fetch size, by saving and restoring row state, such as starting point in the response and updated/deleted status, on
* movement within the window.
*
* Without a scroll window, scrolling backward through a result set would be very costly, requiring reindexing the fetch
* buffer up to row N-1 for each move to the previous row.
*/
final class ScrollWindow implements Serializable {
/**
* Always update serialVersionUID when prompted.
*/
private static final long serialVersionUID = 3028807583846251111L;
/** Set of marks for the rows in the window */
private TDSReaderMark[] rowMark;
/** Set of flags indicating which rows have been updated through the ResultSet */
private boolean[] updatedRow;
/** Set of flags indicating which rows have been deleted through the ResultSet */
private boolean[] deletedRow;
/** Set of enums indicating the types of rows in a ResultSet */
private RowType[] rowType;
/** Size (in rows) of this scroll window */
private int size = 0;
/** Max number of rows in the window (less than or equal to size) */
private int maxRows = 0;
final int getMaxRows() {
return maxRows;
}
/** Current row in the window (1-indexed) */
private int currentRow;
final int getRow() {
return currentRow;
}
ScrollWindow(int size) {
setSize(size);
reset();
}
private void setSize(int size) {
assert this.size != size;
this.size = size;
this.maxRows = size;
this.rowMark = new TDSReaderMark[size];
this.updatedRow = new boolean[size];
this.deletedRow = new boolean[size];
this.rowType = new RowType[size];
for (int i = 0; i < size; i++) {
rowType[i] = RowType.UNKNOWN;
}
}
final void clear() {
for (int i = 0; i < rowMark.length; ++i) {
rowMark[i] = null;
updatedRow[i] = false;
deletedRow[i] = false;
rowType[i] = RowType.UNKNOWN;
}
assert size > 0;
maxRows = size;
reset();
}
final void reset() {
currentRow = 0;
}
final void resize(int newSize) {
assert newSize > 0;
if (newSize != size)
setSize(newSize);
}
final String logCursorState() {
return " currentRow:" + currentRow + " maxRows:" + maxRows;
}
final boolean next(SQLServerResultSet rs) throws SQLServerException {
if (SQLServerResultSet.logger.isLoggable(java.util.logging.Level.FINER))
SQLServerResultSet.logger.finer(rs.toString() + logCursorState());
// Precondition:
// Current position should always be on a row in the window or
// just before the first row or just after the last row.
assert 0 <= currentRow && currentRow <= maxRows + 1;
// If the position is already beyond the end of the window,
// then it can move no farther forward.
if (maxRows + 1 == currentRow)
return false;
// Otherwise, we are going to attempt to move the current
// position to the next row. First, save off the row
// updated/deleted status for the current row so it
// can be restored later if we ever move to this row again.
if (currentRow >= 1) {
updatedRow[currentRow - 1] = rs.getUpdatedCurrentRow();
deletedRow[currentRow - 1] = rs.getDeletedCurrentRow();
rowType[currentRow - 1] = rs.getCurrentRowType();
}
// Start on the next row
++currentRow;
// If we were on the last row of the window then make sure
// the move past the last row consumes the remainder of that
// row from the fetch buffer. The fetch buffer should be
// left pointing beyond the last row of the window, which
// is most likely the end of the fetch buffer as well.
if (maxRows + 1 == currentRow) {
rs.fetchBufferNext();
return false;
}
// We weren't on the last row of the window. If we already
// know that there was another row in the fetch buffer,
// then restore the response buffer position and updated/deleted
// status for the new row.
if (null != rowMark[currentRow - 1]) {
rs.fetchBufferReset(rowMark[currentRow - 1]);
rs.setCurrentRowType(rowType[currentRow - 1]);
rs.setUpdatedCurrentRow(updatedRow[currentRow - 1]);
rs.setDeletedCurrentRow(deletedRow[currentRow - 1]);
return true;
}
// We weren't on the last row of the window and we don't
// know whether there are additional rows in the fetch
// buffer, so try to read another row now. If we find
// one then keep track of its position in the response
// buffer.
if (rs.fetchBufferNext()) {
rowMark[currentRow - 1] = rs.fetchBufferMark();
rowType[currentRow - 1] = rs.getCurrentRowType();
if (SQLServerResultSet.logger.isLoggable(java.util.logging.Level.FINEST))
SQLServerResultSet.logger.finest(rs.toString() + " Set mark " + rowMark[currentRow - 1] + " for row "
+ currentRow + " of type " + rowType[currentRow - 1]);
return true;
}
// We weren't on the last row of the window and we now
// know that there are no more rows in the fetch buffer,
// so adjust maxRows down accordingly.
maxRows = currentRow - 1;
return false;
}
final void previous(SQLServerResultSet rs) throws SQLServerException {
if (SQLServerResultSet.logger.isLoggable(java.util.logging.Level.FINER))
SQLServerResultSet.logger.finer(rs.toString() + logCursorState());
// Precondition:
// Current position should always be on a row in the window or
// just before the first row or just after the last row.
assert 0 <= currentRow && currentRow <= maxRows + 1;
// If the position is already before the start of the window,
// then it can move no farther back.
if (0 == currentRow)
return;
// Otherwise, we are going to attempt to move the current
// position to the previous row. First, save off the row
// updated/deleted status for the current row so it
// can be restored later if we ever move to this row again.
if (currentRow <= maxRows) {
assert currentRow >= 1;
updatedRow[currentRow - 1] = rs.getUpdatedCurrentRow();
deletedRow[currentRow - 1] = rs.getDeletedCurrentRow();
rowType[currentRow - 1] = rs.getCurrentRowType();
}
// Start on the previous row
--currentRow;
// If we were on the first row of the window before moving,
// then we're now before the first row and we're done.
if (0 == currentRow)
return;
// If we weren't on the first row before moving then we
// are now on the previous row. Restore the saved
// position in the response buffer and updated/deleted
// state for the now current row.
assert null != rowMark[currentRow - 1];
rs.fetchBufferReset(rowMark[currentRow - 1]);
rs.setCurrentRowType(rowType[currentRow - 1]);
rs.setUpdatedCurrentRow(updatedRow[currentRow - 1]);
rs.setDeletedCurrentRow(deletedRow[currentRow - 1]);
}
}