前几天上线后老大发现几台负载非常高，dump线程状态后发现多个线程死循环在同一处，于是发现了ibatis的这个bug：
https://issues.apache.org/jira/browse/IBATIS-384
https://issues.apache.org/jira/browse/IBATIS-587
在mysql数据库上，没有结果集时，stmt.getUpdateCount()会返回0，而非-1.
ibatis 2.4.3
private ResultSet getFirstResultSet(StatementScope scope, Statement stmt) throws SQLException {
ResultSet rs = null;
boolean hasMoreResults = true;
while (hasMoreResults) {
rs = stmt.getResultSet();
if (rs != null) {
break;
}
hasMoreResults = moveToNextResultsIfPresent(scope, stmt);
}
return rs;
}
private boolean moveToNextResultsIfPresent(StatementScope scope, Statement stmt) throws SQLException {
boolean moreResults;
// This is the messed up JDBC approach for determining if there are more results
moreResults = !(((moveToNextResultsSafely(scope, stmt) == false) && (stmt.getUpdateCount() == -1)));
return moreResults;
}
private boolean moveToNextResultsSafely(StatementScope scope, Statement stmt) throws SQLException {
if (forceMultipleResultSetSupport(scope) || stmt.getConnection().getMetaData().supportsMultipleResultSets()) {
return stmt.getMoreResults();
}
return false;
}
ResultSet rs = null;
boolean hasMoreResults = true;
while (hasMoreResults) {
rs = stmt.getResultSet();
if (rs != null) {
break;
}
hasMoreResults = moveToNextResultsIfPresent(scope, stmt);
}
return rs;
}
private boolean moveToNextResultsIfPresent(StatementScope scope, Statement stmt) throws SQLException {
boolean moreResults;
// This is the messed up JDBC approach for determining if there are more results
moreResults = !(((moveToNextResultsSafely(scope, stmt) == false) && (stmt.getUpdateCount() == -1)));
return moreResults;
}
private boolean moveToNextResultsSafely(StatementScope scope, Statement stmt) throws SQLException {
if (forceMultipleResultSetSupport(scope) || stmt.getConnection().getMetaData().supportsMultipleResultSets()) {
return stmt.getMoreResults();
}
return false;
}
moreResults恒为真，程序出现死循环。
在mybatis 2.5的代码里，这部分已经修改为：
private boolean moveToNextResultsIfPresent(StatementScope scope, Statement stmt) throws SQLException {
boolean moreResults;
// This is the messed up JDBC approach for determining if there are more results
boolean movedToNextResultsSafely = moveToNextResultsSafely(scope, stmt);
int updateCount = stmt.getUpdateCount();
moreResults = !(!movedToNextResultsSafely && (updateCount == -1));
//ibatis-384: workaround for mysql not returning -1 for stmt.getUpdateCount()
if (moreResults == true){
moreResults = !(!movedToNextResultsSafely && !isMultipleResultSetSupportPresent(scope, stmt));
}
return moreResults;
}
private boolean moveToNextResultsSafely(StatementScope scope, Statement stmt) throws SQLException {
if (isMultipleResultSetSupportPresent(scope, stmt)) {
return stmt.getMoreResults();
}
return false;
}
/**
* checks whether multiple result set support is present - either by direct support of the database driver or by forcing it
*/
private boolean isMultipleResultSetSupportPresent(StatementScope scope,
Statement stmt) throws SQLException {
return forceMultipleResultSetSupport(scope) || stmt.getConnection().getMetaData().supportsMultipleResultSets();
}
boolean moreResults;
// This is the messed up JDBC approach for determining if there are more results
boolean movedToNextResultsSafely = moveToNextResultsSafely(scope, stmt);
int updateCount = stmt.getUpdateCount();
moreResults = !(!movedToNextResultsSafely && (updateCount == -1));
//ibatis-384: workaround for mysql not returning -1 for stmt.getUpdateCount()
if (moreResults == true){
moreResults = !(!movedToNextResultsSafely && !isMultipleResultSetSupportPresent(scope, stmt));
}
return moreResults;
}
private boolean moveToNextResultsSafely(StatementScope scope, Statement stmt) throws SQLException {
if (isMultipleResultSetSupportPresent(scope, stmt)) {
return stmt.getMoreResults();
}
return false;
}
/**
* checks whether multiple result set support is present - either by direct support of the database driver or by forcing it
*/
private boolean isMultipleResultSetSupportPresent(StatementScope scope,
Statement stmt) throws SQLException {
return forceMultipleResultSetSupport(scope) || stmt.getConnection().getMetaData().supportsMultipleResultSets();
}
这部分条件判断实在很极致了。
新的实现当getUpdateResult是0时，moreResults恒为真，这时再进行一个判断，如果是由于isMultipleResultSetSupportPresent为false导致了moveToNextResultsSafely为false，那么实际moreResults应是false
MyBatis 2.5还有一个issue没有解决，离发布还有一些时间，这个问题只好签出新版本代码自己build了