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# Mybatis Cursor
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- Author: [HuiFer](https://github.com/huifer)
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- Description: 该文介绍 mybatis Cursor 源码
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- 源码阅读工程: [SourceHot-Mybatis](https://github.com/SourceHot/mybatis-read.git)
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## Cursor
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- 源码位置:`org.apache.ibatis.cursor.Cursor`
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- 继承`Iterable`说明是一个迭代器,继承`Closeable`说明有一个东西需要关闭
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```java
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public interface Cursor<T> extends Closeable, Iterable<T> {
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/**
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* 游标开始从数据库获取数据,返回true,反之false
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*
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* @return true if the cursor has started to fetch items from database.
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*/
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boolean isOpen();
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/**
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* 数据库元素都被获取,返回true,反之false
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*
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* @return true if the cursor is fully consumed and has returned all elements matching the query.
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*/
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boolean isConsumed();
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/**
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* 获取数据索引,从0开始,没有返回-1
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* Get the current item index. The first item has the index 0.
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*
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* @return -1 if the first cursor item has not been retrieved. The index of the current item retrieved.
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*/
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int getCurrentIndex();
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}
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```
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## DefaultCursor
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```java
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public class DefaultCursor<T> implements Cursor<T> {
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/**
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* 对象包装结果处理类
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*/
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protected final ObjectWrapperResultHandler<T> objectWrapperResultHandler = new ObjectWrapperResultHandler<>();
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// ResultSetHandler stuff
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/**
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* ResultSet 处理器
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*/
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private final DefaultResultSetHandler resultSetHandler;
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/**
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* 结果映射
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*/
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private final ResultMap resultMap;
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/**
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* ResultSet 包装对象
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*/
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private final ResultSetWrapper rsw;
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/**
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* 分页的
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*/
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private final RowBounds rowBounds;
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/**
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* 游标的迭代器
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*/
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private final CursorIterator cursorIterator = new CursorIterator();
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/**
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* 游标开启判断
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*/
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private boolean iteratorRetrieved;
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/**
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* 游标状态,默认是创建未使用
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*/
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private CursorStatus status = CursorStatus.CREATED;
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/**
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* 分页索引,默认-1
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*/
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private int indexWithRowBound = -1;
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/**
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* 构造方法
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*
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* @param resultSetHandler
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* @param resultMap
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* @param rsw
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* @param rowBounds
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*/
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public DefaultCursor(DefaultResultSetHandler resultSetHandler, ResultMap resultMap, ResultSetWrapper rsw, RowBounds rowBounds) {
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this.resultSetHandler = resultSetHandler;
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this.resultMap = resultMap;
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this.rsw = rsw;
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this.rowBounds = rowBounds;
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}
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@Override
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public boolean isOpen() {
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return status == CursorStatus.OPEN;
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}
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@Override
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public boolean isConsumed() {
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return status == CursorStatus.CONSUMED;
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}
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/**
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* 当前索引
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* @return
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*/
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@Override
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public int getCurrentIndex() {
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return rowBounds.getOffset() + cursorIterator.iteratorIndex;
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}
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/**
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* 迭代器获取
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* @return
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*/
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@Override
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public Iterator<T> iterator() {
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// 是否获取过
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if (iteratorRetrieved) {
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throw new IllegalStateException("Cannot open more than one iterator on a Cursor");
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}
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// 是否关闭
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if (isClosed()) {
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throw new IllegalStateException("A Cursor is already closed.");
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}
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iteratorRetrieved = true;
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return cursorIterator;
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}
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/**
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* {@link Closeable} 关闭{@link ResultSet}
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*/
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@Override
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public void close() {
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// 判断是否关闭
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if (isClosed()) {
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return;
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}
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ResultSet rs = rsw.getResultSet();
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try {
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if (rs != null) {
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rs.close();
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}
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} catch (SQLException e) {
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// ignore
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} finally {
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// 设置游标状态
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status = CursorStatus.CLOSED;
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}
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}
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/**
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* 去到真正的数据行
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* @return
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*/
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protected T fetchNextUsingRowBound() {
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T result = fetchNextObjectFromDatabase();
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while (objectWrapperResultHandler.fetched && indexWithRowBound < rowBounds.getOffset()) {
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result = fetchNextObjectFromDatabase();
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}
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return result;
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}
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/**
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* 从数据库获取数据
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* @return
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*/
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protected T fetchNextObjectFromDatabase() {
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if (isClosed()) {
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return null;
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}
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try {
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objectWrapperResultHandler.fetched = false;
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// 游标状态设置
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status = CursorStatus.OPEN;
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if (!rsw.getResultSet().isClosed()) {
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// 处理数据结果放入，objectWrapperResultHandler
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resultSetHandler.handleRowValues(rsw, resultMap, objectWrapperResultHandler, RowBounds.DEFAULT, null);
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}
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} catch (SQLException e) {
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throw new RuntimeException(e);
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}
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// 获取处理结果
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T next = objectWrapperResultHandler.result;
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// 结果不为空
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if (objectWrapperResultHandler.fetched) {
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// 索引+1
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indexWithRowBound++;
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}
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// No more object or limit reached
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// 如果没有数据, 或者 当前读取条数= 偏移量+限额量
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if (!objectWrapperResultHandler.fetched || getReadItemsCount() == rowBounds.getOffset() + rowBounds.getLimit()) {
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// 关闭游标
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close();
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status = CursorStatus.CONSUMED;
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}
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// 设置结果为null
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objectWrapperResultHandler.result = null;
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return next;
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}
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/**
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* 是否关闭状态判断
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*
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* @return
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*/
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private boolean isClosed() {
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return status == CursorStatus.CLOSED || status == CursorStatus.CONSUMED;
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}
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/**
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* 下一个索引
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* @return
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*/
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private int getReadItemsCount() {
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return indexWithRowBound + 1;
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}
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/**
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* 游标的状态
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*/
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private enum CursorStatus {
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/**
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* 新创建的游标, ResultSet 还没有使用过
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* A freshly created cursor, database ResultSet consuming has not started.
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*/
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CREATED,
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/**
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* 游标使用过, ResultSet 被使用
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* A cursor currently in use, database ResultSet consuming has started.
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*/
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OPEN,
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/**
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* 游标关闭, 可能没有被消费完全
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* A closed cursor, not fully consumed.
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*/
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CLOSED,
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/**
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* 游标彻底消费完毕, 关闭了
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* A fully consumed cursor, a consumed cursor is always closed.
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*/
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CONSUMED
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}
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/**
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* 对象处理结果的包装类
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* @param <T>
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*/
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protected static class ObjectWrapperResultHandler<T> implements ResultHandler<T> {
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/**
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* 数据结果
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*/
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protected T result;
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/**
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* 是否null
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*/
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protected boolean fetched;
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/**
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* 从{@link ResultContext} 获取结果对象
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* @param context
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*/
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@Override
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public void handleResult(ResultContext<? extends T> context) {
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this.result = context.getResultObject();
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context.stop();
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fetched = true;
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}
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}
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/**
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* 游标迭代器
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*/
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protected class CursorIterator implements Iterator<T> {
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/**
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* 下一个数据
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* Holder for the next object to be returned.
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*/
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T object;
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/**
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* 下一个的索引
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* Index of objects returned using next(), and as such, visible to users.
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*/
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int iteratorIndex = -1;
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/**
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* 是否有下一个值
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* @return
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*/
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@Override
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public boolean hasNext() {
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if (!objectWrapperResultHandler.fetched) {
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object = fetchNextUsingRowBound();
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}
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return objectWrapperResultHandler.fetched;
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}
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/**
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* 下一个值
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* @return
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*/
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@Override
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public T next() {
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// Fill next with object fetched from hasNext()
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T next = object;
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if (!objectWrapperResultHandler.fetched) {
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next = fetchNextUsingRowBound();
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}
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if (objectWrapperResultHandler.fetched) {
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objectWrapperResultHandler.fetched = false;
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object = null;
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iteratorIndex++;
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return next;
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}
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throw new NoSuchElementException();
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}
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/**
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* 不可执行抛出异常
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*/
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@Override
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public void remove() {
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throw new UnsupportedOperationException("Cannot remove element from Cursor");
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}
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}
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}
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```
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