悲观锁的实现，往往依靠数据库提供的锁机制（也只有数据库层提供的锁机制才能真正保证数据访问的排他性，否则，即使在应用层中实现了加锁机制，也无法保证外部系统不会修改数据）。需要注意的是，在事务中，只有SELECT … FOR UPDATE 或LOCK IN SHARE MODE 相同数据时会等待其它事务结束后才执行，一般SELECT … 则不受此影响。
拿上面的实例来说，当我执行select status from t_goods where id=1 for update;后。我在另外的事务中如果再次执行select status from t_goods where id=1 for update;则第二个事务会一直等待第一个事务的提交，此时第二个查询处于阻塞的状态，但是如果我是在第二个事务中执行select status from t_goods where id=1;则能正常查询出数据，不会受第一个事务的影响。
补充：MySQL select…for update的Row Lock与Table Lock
上面我们提到，使用select…for update会把数据给锁住，不过我们需要注意一些锁的级别，MySQL InnoDB默认Row-Level Lock，所以只有「明确」地指定主键，MySQL 才会执行Row lock (只锁住被选取的数据) ，否则MySQL 将会执行Table Lock (将整个数据表单给锁住)。
测试悲观锁：要使用悲观锁，我们必须关闭mysql数据库的自动提交属性。
-
在 Console 1:
a. 执行 set autocmmit = 0; 即关闭mysql数据库的自动提交属性,其实这步不是必须的(参考：MySQL 的事务隔离级别)
b. 执行 begin; 即开启事务(这个 begin 不是必须的，好像设置了 autocmmit 为 0 就会开启事务)
c. 执行 select * from student where id = 3 for update; 即锁住数据
d. update student set name = concat(name,‘3’) where id = 3; -
在 Console 2:
e. 执行 update student set name = concat(name,‘3’) where id = 3; 可以看到会被 block 住 -
在 Console 1:
f. 执行 commit; 即提交事务 -
在 Console 2:
g. 可以看到 步骤 2 的 block 解除了
示例：
Console1:
mysql>
mysql> show variables like '%autocommit%';
+-----------------------------------------+-------+
| Variable_name | Value |
+-----------------------------------------+-------+
| autocommit | ON |
+-----------------------------------------+-------+
8 rows in set, 1 warning (0.00 sec)
mysql>
mysql> set autocommit = 0 ;
Query OK, 0 rows affected (0.00 sec)
mysql>
mysql> show variables like '%autocommit%';
+-----------------------------------------+-------+
| Variable_name | Value |
+-----------------------------------------+-------+
| autocommit | OFF |
+-----------------------------------------+-------+
8 rows in set, 1 warning (0.01 sec)
mysql>
mysql> use robertdb;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
mysql>
mysql>
mysql> select * from student;
+-----+--------------+--------+--------------+----------------------------+----------------------------+------------+
| id | name | gender | address | create_date | update_date | country_id |
+-----+--------------+--------+--------------+----------------------------+----------------------------+------------+
| 1 | c'ppp\'ppp"x | 2 | HeNan | 2018-11-12 22:15:48.000000 | 2019-10-30 15:50:50.452264 | 1 |
| 3 | rob | 4 | BeiJing | 2018-11-12 22:16:01.000000 | 2018-11-12 22:16:01.000000 | 1 |
| 4 | gww | 1 | BeiJing | 2018-11-12 22:16:06.000000 | 2020-01-10 14:58:19.914886 | 1 |
| 5 | sxn | 2 | HeNan | 2018-11-12 22:16:15.000000 | 2018-11-12 22:16:15.000000 | 1 |
| 7 | syy | 2 | MoNaShi | 2018-11-14 10:21:24.179000 | 2018-11-14 10:21:24.179000 | 3 |
| 8 | cjs | 2 | ShanXi | 2018-11-14 13:52:41.193000 | 2018-11-14 13:52:41.193000 | 1 |
+-----+--------------+--------+--------------+----------------------------+----------------------------+------------+
13 rows in set (0.00 sec)
mysql>
mysql>
mysql>
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql>
mysql> select * from student where id = 3 for update;
+----+------+--------+---------+----------------------------+----------------------------+------------+
| id | name | gender | address | create_date | update_date | country_id |
+----+------+--------+---------+----------------------------+----------------------------+------------+
| 3 | rob | 4 | BeiJing | 2018-11-12 22:16:01.000000 | 2018-11-12 22:16:01.000000 | 1 |
+----+------+--------+---------+----------------------------+----------------------------+------------+
1 row in set (0.00 sec)
mysql>
mysql>
mysql>
mysql> update student set name = concat(name,'3') where id = 3;
Query OK, 1 row affected (0.06 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql>
mysql> commit;
Query OK, 0 rows affected (0.10 sec)
mysql>
Console 2: 可以看到 一个简单的 update SQL 居然执行了 34.70 sec
mysql>
mysql> show variables like '%autocommit%';
+-----------------------------------------+-------+
| Variable_name | Value |
+-----------------------------------------+-------+
| autocommit | ON |
+-----------------------------------------+-------+
8 rows in set, 1 warning (0.09 sec)
mysql>
mysql> use robertdb;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
mysql>
mysql> select * from students;
ERROR 1146 (42S02): Table 'robertdb.students' doesn't exist
mysql> select * from student;
+-----+--------------+--------+--------------+----------------------------+----------------------------+------------+
| id | name | gender | address | create_date | update_date | country_id |
+-----+--------------+--------+--------------+----------------------------+----------------------------+------------+
| 1 | c'ppp\'ppp"x | 2 | HeNan | 2018-11-12 22:15:48.000000 | 2019-10-30 15:50:50.452264 | 1 |
| 3 | rob | 4 | BeiJing | 2018-11-12 22:16:01.000000 | 2018-11-12 22:16:01.000000 | 1 |
| 4 | gww | 1 | BeiJing | 2018-11-12 22:16:06.000000 | 2020-01-10 14:58:19.914886 | 1 |
| 5 | sxn | 2 | HeNan | 2018-11-12 22:16:15.000000 | 2018-11-12 22:16:15.000000 | 1 |
| 7 | syy | 2 | MoNaShi | 2018-11-14 10:21:24.179000 | 2018-11-14 10:21:24.179000 | 3 |
| 8 | cjs | 2 | ShanXi | 2018-11-14 13:52:41.193000 | 2018-11-14 13:52:41.193000 | 1 |
+-----+--------------+--------+--------------+----------------------------+----------------------------+------------+
13 rows in set (0.01 sec)
mysql>
mysql>
mysql> update student set name = concat(name,'3') where id = 3;
Query OK, 1 row affected (34.70 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from student;
+-----+--------------+--------+--------------+----------------------------+----------------------------+------------+
| id | name | gender | address | create_date | update_date | country_id |
+-----+--------------+--------+--------------+----------------------------+----------------------------+------------+
| 1 | c'ppp\'ppp"x | 2 | HeNan | 2018-11-12 22:15:48.000000 | 2019-10-30 15:50:50.452264 | 1 |
| 3 | rob33 | 4 | BeiJing | 2018-11-12 22:16:01.000000 | 2018-11-12 22:16:01.000000 | 1 |
| 4 | gww | 1 | BeiJing | 2018-11-12 22:16:06.000000 | 2020-01-10 14:58:19.914886 | 1 |
| 5 | sxn | 2 | HeNan | 2018-11-12 22:16:15.000000 | 2018-11-12 22:16:15.000000 | 1 |
| 7 | syy | 2 | MoNaShi | 2018-11-14 10:21:24.179000 | 2018-11-14 10:21:24.179000 | 3 |
| 8 | cjs | 2 | ShanXi | 2018-11-14 13:52:41.193000 | 2018-11-14 13:52:41.193000 | 1 |
+-----+--------------+--------+--------------+----------------------------+----------------------------+------------+
13 rows in set (0.00 sec)
mysql>
mysql>
