【MybatisPlus】数据库的datetime类型字段为空的时候，报错空指针？
⼀、发现经历
事情是这样的，我今天本来要演⽰系统，就去前端同学的页⾯上点⼀点。不⼩⼼点到了其他同事编写的服务，然后界⾯就报错了。这给我吓得，这还能演⽰吗这。然后，我就去服务器查看了⼀下⽇志，发现了如下景象：
看到这景象啊，我第⼀件事情就是查看堆栈，也没到⾃⼰写的代码啊，好好的咋就报错了。
于是，我第⼀件事情，复制报错信息到百度⽹站。复制粘贴，往上⼀怼！好家伙，竟然不到⼀个和我症状⼀样的。
那我只能⾃⼰处理了。
⼆、问题定位
从堆栈信息定位⼀个问题的位置其实是很简单的，但是要明⽩为什么错误，还是得多看⼀会⼉的。
我定睛看了⼀阵⼦，发现了
at org.NullableResult(LocalDateTimeTypeHandler.java:38)
这⼀⾏报错，于是我就跟进源码查看。这个源码，写的也简单。
1/**
2 * Copyright 2009-2019 the original author or authors.
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * /licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15*/
16package org.pe;
17
18import java.sql.CallableStatement;
19import java.sql.PreparedStatement;
20import java.sql.ResultSet;
21import java.sql.SQLException;
22import java.time.LocalDateTime;
23
24/**
25 * @since 3.4.5
26 * @author Tomas Rohovsky
27*/
28public class LocalDateTimeTypeHandler extends BaseTypeHandler<LocalDateTime> {
29
30 @Override
31public void setNonNullParameter(PreparedStatement ps, int i, LocalDateTime parameter, JdbcType jdbcType)
32throws SQLException {
33 ps.setObject(i, parameter);
34 }
35
36 @Override
37public LocalDateTime getNullableResult(ResultSet rs, String columnName) throws SQLException {
Object(columnName, LocalDateTime.class);
39 }
40
41 @Override
42public LocalDateTime getNullableResult(ResultSet rs, int columnIndex) throws SQLException {
Object(columnIndex, LocalDateTime.class);
44 }
45
46 @Override
47public LocalDateTime getNullableResult(CallableStatement cs, int columnIndex) throws SQLException {
Object(columnIndex, LocalDateTime.class);
49 }
50 }
⼀共也就这么⼏⾏。报错的是：
public LocalDateTime getNullableResult(ResultSet rs, String columnName) throws SQLException {
Object(columnName, LocalDateTime.class);
}
这个⽅法。这⾥⾯发⽣了空指针。
这时候，我发现还是最细的堆栈信息。于是，我到最细的那⼀层。
也就是这⼀⾏。
看到这⾥，我明⽩了。实际上就是
getTimestamp(columnIndex)
这个获取时间的时候，数据库的submit_time字段没有值，然后cast的时候，发⽣了空指针。
三、问题解决
弄清了病症，也就好下药了。我脑中浮现出两个想法：
1、让submit_time有值
这不简单吗，没值的时候有问题，我让你有值不就完事了。哈哈。开个玩笑，要结合业务处理的哈。这种⽅法明显不可取。
2、重写LocalDateTimeTypeHandler这个类，然后添加空指针判断。
这个⽅法不错，可以解决实际问题。
于是，我查看了其他类型的处理。
1/**
2 * Copyright 2009-2018 the original author or authors.
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * /licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15*/
16package org.pe;
17
18import java.sql.CallableStatement;
数据库简单吗19import java.sql.PreparedStatement;
20import java.sql.ResultSet;
21import java.sql.SQLException;
22
23/**
24 * @author Clinton Begin
25*/
26public class DoubleTypeHandler extends BaseTypeHandler<Double> {
27
28 @Override
29public void setNonNullParameter(PreparedStatement ps, int i, Double parameter, JdbcType jdbcType)
30throws SQLException {
31 ps.setDouble(i, parameter);
32 }
33
34 @Override
35public Double getNullableResult(ResultSet rs, String columnName)
36throws SQLException {
37double result = rs.getDouble(columnName);
38return result == 0 && rs.wasNull() ? null : result;
39 }
40
41 @Override
42public Double getNullableResult(ResultSet rs, int columnIndex)
43throws SQLException {
44double result = rs.getDouble(columnIndex);
45return result == 0 && rs.wasNull() ? null : result;
46 }
47
48 @Override
49public Double getNullableResult(CallableStatement cs, int columnIndex)
50throws SQLException {
51double result = cs.getDouble(columnIndex);
52return result == 0 && cs.wasNull() ? null : result;
53 }
54
55 }
看了Double的类型处理【1、wasNull设置标志位 2、获取的外⾯通过标志位判空】，我⼜看了Integer的类型处理。。。
1 /**
2 * Copyright 2009-2018 the original author or authors.
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * /licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15*/
16package org.pe;
17
18import java.sql.CallableStatement;
19import java.sql.PreparedStatement;
20import java.sql.ResultSet;
21import java.sql.SQLException;
22
23/**
24 * @author Clinton Begin
25*/
26public class IntegerTypeHandler extends BaseTypeHandler<Integer> {
27
28 @Override
29public void setNonNullParameter(PreparedStatement ps, int i, Integer parameter, JdbcType jdbcType)
30throws SQLException {
31 ps.setInt(i, parameter);
32 }
33
34 @Override
35public Integer getNullableResult(ResultSet rs, String columnName)
36throws SQLException {
37int result = rs.getInt(columnName);
38return result == 0 && rs.wasNull() ? null : result;
39 }
40
41 @Override
42public Integer getNullableResult(ResultSet rs, int columnIndex)
43throws SQLException {
44int result = rs.getInt(columnIndex);
45return result == 0 && rs.wasNull() ? null : result;
46 }
47
48 @Override
49public Integer getNullableResult(CallableStatement cs, int columnIndex)
50throws SQLException {
51int result = cs.getInt(columnIndex);
52return result == 0 && cs.wasNull() ? null : result;
53 }
54 }
Integer的类型处理也是【1、wasNull设置标志位 2、获取的外⾯通过标志位判空】。
这真是好家伙，坐不住了。其他的类型都处理了，合着就我⽤的LocalDateTime没做空指针处理。这哪能⾏。重写，必须重写。
1package com.vsofo;
2
3import org.pe.LocalDateTimeTypeHandler;
4import org.springframework.stereotype.Component;
5
6import java.sql.ResultSet;
7import java.sql.SQLException;
8import java.time.LocalDateTime;
9
10 @Component
11public class LocalDateTimeTypeHandlerPlus extends LocalDateTimeTypeHandler {
12
13 @Override
14public LocalDateTime getResult(ResultSet rs, String columnName) throws SQLException {
15 Object object = rs.getObject(columnName);
16if(object==null){
17return null;
18 }
Result(rs, columnName);
20 }
21 }
我直接继承，然后重写获取逻辑。
1、先获取数据库的值
2、判空，如果值为空，直接返回。
3、如果，值不为空，进⾏原来的强转逻辑。
重写之后，问题完美解决了。
=============================================================================
2021-08-18 最近在做另外⼀个项⽬的时候，也发现了这个问题，然⽽⼀样的步骤失效了，原因了，没有使⽤到这个⾃定义的LocalDateTime处理器。后⾯，我使⽤类替换的形式代替，解决了该问题。复制出原来的LocalDateTimeTypeHandler放在同⼀个包下，依旧是重写getResult(ResultSet rs, String columnName)⽅法解决了该问题，这使⽤的⽅式是直接替换了源码中的类。特此记录！
1/**
2 * Copyright 2009-2019 the original author or authors.
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * /licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15*/
16package org.pe;
17
18import java.sql.CallableStatement;
19import java.sql.PreparedStatement;
20import java.sql.ResultSet;
21import java.sql.SQLException;
22import java.time.LocalDateTime;
23
24/**
25 * @since 3.4.5
26 * @author Tomas Rohovsky
27*/
28public class LocalDateTimeTypeHandler extends BaseTypeHandler<LocalDateTime> {
29
30 @Override
31public LocalDateTime getResult(ResultSet rs, String columnName) throws SQLException {
32 Object object = rs.getObject(columnName);
33if(object==null){
34return null;
35 }
Result(rs, columnName);
37 }
38
39 @Override
40public void setNonNullParameter(PreparedStatement ps, int i, LocalDateTime parameter, JdbcType jdbcType)
41throws SQLException {
42 ps.setObject(i, parameter);
43 }
44
45 @Override
46public LocalDateTime getNullableResult(ResultSet rs, String columnName) throws SQLException {
Object(columnName, LocalDateTime.class);
48 }
49
50 @Override
51public LocalDateTime getNullableResult(ResultSet rs, int columnIndex) throws SQLException {
Object(columnIndex, LocalDateTime.class);
53 }
54
55 @Override
56public LocalDateTime getNullableResult(CallableStatement cs, int columnIndex) throws SQLException {
Object(columnIndex, LocalDateTime.class);
58 }
59 }
或者，是在字段上的注解@TableField设置该字段的类型转换器
/**
* 下发时间
*/
@TableField(value = "oper_time",typeHandler =LocalDateTimeTypeHandlerPlus.class)
private LocalDateTime operTime;
如果是xml的话，也有该配置
<resultMap id="myParam" type="com.ity.UserStaticUserinfo" >
<result column="plat_oper_time" property="platOperTime" typeHandler="cn.lxw.LocalDateTimeTypeHandlerPlus"/>
</resultMap>
最后，还有⼀种⽅式，那就是把对应映射的字段类型改成java.util.Date类型，因为Date类型的源码已经做了特殊处理，只是这样要动到⼀些涉及的代码⽽增加⼀些风险。
四、⼼得体会
我觉着吧，框架是个好东西，能让我们开发项⽬事半功倍。但是如果给你埋了⼀个⼤雷，也可能让你事倍功半。
所以，我们⽤框架开发的过程中，应该多⽤多测多实践。出影响项⽬的问题，然后解决它。
分享结束，谢谢⼤家观看哈！
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