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鎖是在執(zhí)行多線程時用于強行限定資源訪問的同步機制，數(shù)據(jù)庫鎖根據(jù)鎖的粒度可分為行級鎖，表級鎖和頁級鎖

行級鎖

行級鎖是mysql中粒度最細的一種鎖機制，表示只對當前所操作的行進行加鎖，行級鎖發(fā)生沖突的概率很低，其粒度最小，但是加鎖的代價最大。行級鎖分為共享鎖和排他鎖。

特點：

開銷大，加鎖慢，會出現(xiàn)死鎖；鎖定粒度最小，發(fā)生鎖沖突的概率最大，并發(fā)性也高；

實現(xiàn)原理：

InnoDB行鎖是通過給索引項加鎖來實現(xiàn)的，這一點mysql和oracle不同，后者是通過在數(shù)據(jù)庫中對相應的數(shù)據(jù)行加鎖來實現(xiàn)的，InnoDB這種行級鎖決定，只有通過索引條件來檢索數(shù)據(jù)，才能使用行級鎖，否則，直接使用表級鎖。特別注意:使用行級鎖一定要使用索引

舉個栗子：

創(chuàng)建表結構

CREATE TABLE `developerinfo` (
 `userID` bigint(20) NOT NULL,
 `name` varchar(255) DEFAULT NULL,
 `passWord` varchar(255) DEFAULT NULL,
 PRIMARY KEY (`userID`),
 KEY `PASSWORD_INDEX` (`passWord`) USING BTREE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

插入數(shù)據(jù)

INSERT INTO `developerinfo` VALUES ('1', 'liujie', '123456');
INSERT INTO `developerinfo` VALUES ('2', 'yitong', '123');
INSERT INTO `developerinfo` VALUES ('3', 'tong', '123456');

（1）通過主鍵索引來查詢數(shù)據(jù)庫使用行鎖

打開三個命令行窗口進行測試

命令行窗口1	命令行窗口2	命令行窗口3

mysql> set autocommit = 0;
Query OK, 0 rows affected
mysql> select * from developerinfo where userid = '1' for update;
+--------+--------+----------+
| userID | name | passWord |
+--------+--------+----------+
| 1 | liujie | 123456 |
+--------+--------+----------+
1 row in set 
|mysql> set autocommit = 0;
Query OK, 0 rows affected
mysql> select * from developerinfo where userid = '1' for update;
等待
|mysql> set autocommit = 0;
Query OK, 0 rows affected
mysql> select * from developerinfo where userid = '3' for update;
+--------+------+----------+
| userID | name | passWord |
+--------+------+----------+
| 3 | tong | 123456 |
+--------+------+----------+
1 row in set
|mysql> commit;
Query OK, 0 rows affected mysql> select * from developerinfo where userid = '1' for update;
+--------+--------+----------+
| userID | name | passWord |
+--------+--------+----------+
| 1 | liujie | 123456 |
+--------+--------+----------+
1 row in set

（2）查詢非索引的字段來查詢數(shù)據(jù)庫使用行鎖

打開兩個命令行窗口進行測試

命令行窗口1	命令行窗口2

|mysql> set autocommit=0;
Query OK, 0 rows affected
mysql> select * from developerinfo where name = 'liujie' for update;
+--------+--------+----------+
userID name passWord
+--------+--------+----------+
1 liujie 123456
+--------+--------+----------+
1 row in set |mysql> set autocommit=0;
Query OK, 0 rows affected
mysql> select * from developerinfo where name = 'tong' for update;
等待|
mysql> commit;
Query OK, 0 rows affected mysql> select * from developerinfo where name = 'liujie' for update;
+--------+--------+----------+
| userID | name | passWord |
+--------+--------+----------+
| 1 | liujie | 123456 |
+--------+--------+----------+
1 row in set

##### （3）查詢非唯一索引字段來查詢數(shù)據(jù)庫使用行鎖鎖住多行

mysql的行鎖是針對索引假的鎖，不是針對記錄，所以可能會出現(xiàn)鎖住不同記錄的場景

打開三個命令行窗口進行測試

命令行窗口1命令行窗口2命令行窗口3

mysql> set autocommit=0;
Query OK, 0 rows affected
mysql> select * from developerinfo where password = '123456
' for update;
+--------+--------+----------+
| userID | name | passWord |
+--------+--------+----------+
| 1 | liujie | 123456 |
| 3 | tong | 123456 |
+--------+--------+----------+
2 rows in set mysql> set autocommit =0 ;
Query OK, 0 rows affected
mysql> select * from developerinfo where userid = '1' for update;

等待

mysql> set autocommit = 0;
Query OK, 0 rows affected
mysql> select * from developerinfo where userid = '2
' for update;
+--------+--------+----------+
| userID | name | passWord |
+--------+--------+----------+
| 2 | yitong | 123 |
+--------+--------+----------+
1 row in set
commit; mysql> select * from developerinfo where userid = '1' for update;
+--------+--------+----------+
| userID | name | passWord |
+--------+--------+----------+
| 1 | liujie | 123456 |
+--------+--------+----------+
1 row in set