Oracle:TOM实验DBMS_LOCK

创建一个带主键的表，和一个触发器，触发器会防止两个或多个会话同事插入相同的值。这个触发器使用DBMS_UTILITY.GET_ HASH_VALUE 来计算主键的散列值，得到一个0～1 073 741 823 之间的数（这也是Oracle 允许我们使用的锁ID 号的范围）。然后使用DBMS_LOCK.REQUEST 根据这个ID 分配一个排他锁（也称独占锁，exclusive lock）。一次只有一个会话能做这个工作。
grant execute on dbms_lock to scott;
create table demo (x int primary key);

create or replace trigger demo_bifer
before insert on demo
for each row
declare
l_lock_id number;
resource_busy exception;
pragma exception_init(resource_busy,-54);
begin
l_lock_id :=
dbms_utility.get_hash_value(to_char( :new.x),0,1024);
if ( dbms_lock.request
( id => l_lock_id,
lockmode => dbms_lock.x_mode,
timeout => 0,
release_on_commit => TRUE ) <> 0 )
then
raise resource_busy;
end if;
end;
/


SQL> grant execute on dbms_lock to scott;

Grant succeeded.

SQL> conn scott/tiger
Connected.

SQL> create table demo (x int primary key);

Table created.

SQL> create or replace trigger demo_bifer
2 before insert on demo
3 for each row
4 declare
5 l_lock_id number;
6 resource_busy exception;
7 pragma exception_init(resource_busy,-54);
8 begin
9 l_lock_id :=
10 dbms_utility.get_hash_value(to_char( :new.x),0,1024);
11 if ( dbms_lock.request
12 ( id => l_lock_id,
13 lockmode => dbms_lock.x_mode,
14 timeout => 0,
15 release_on_commit => TRUE ) <> 0 )
16 then
17 raise resource_busy;
18 end if;
19 end;
20 /

Trigger created.

SQL> insert into demo values(1);

1 row created.

SQL> insert into demo values(1);
insert into demo values(1)
*
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified
ORA-06512: at "SCOTT.DEMO_BIFER", line 14
ORA-04088: error during execution of trigger 'SCOTT.DEMO_BIFER'