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