中午一开发说更新了一个package body里几个procedure之后编译一直不成功，PL/SQL Developer无响应不报错，没碰到过这种问题，看了alert也没有发现异常，检查了代码没有问题，猜测是不是有可能procedure中涉及到的对象被锁从而导致编译不成功，查询v$lock没有看到blocker，然后编译的那个会话就报错了：
ORA-04021: timeout occurred while waiting to lock object
[oracle@ASM ~]$ oerr ora 04021
04021, 00000, "timeout occurred while waiting to lock object %s%s%s%s%s"
// *Cause: While waiting to lock a library object, a timeout occurred.
// *Action: Retry the operation later.
google了一把，一般ORA-04021是因为这个包正在被其他会话调用，编译时申请不到library lock导致的，通过dba_ddl_lock 找到调用这个包的会话kill之后据能够正常编译，按照这个方法问题得到解决。
下面把这个错误重现了一下：
SCOTT@ora11g> create or replace procedure p1 as
2 begin
3 while true loop
4 null;
5 end loop;
6 end;
7 /
Procedure created.
SYS@ora11g> alter procedure scott.p1 compile;
Procedure altered.
SCOTT@ora11g> exec p1
SYS@ora11g> alter procedure scott.p1 compile;
alter procedure scott.p1 compile
*
ERROR at line 1:
ORA-04021: timeout occurred while waiting to lock object
SYS@ora11g> select * from dba_ddl_locks where name='P1';
SESSION_ID OWNER NAME TYPE MODE_HELD MODE_REQU
---------- ----- ----- ---------------------------------------- --------- ---------
21 SCOTT P1 Table/Procedure/Type Null None
SYS@ora11g> select sid,serial# from v$session where sid=21;
SID SERIAL#
---------- ----------
21 149
SYS@ora11g> alter system kill session '21,149';
System altered.
SCOTT@ora11g> exec p1
BEGIN p1; END;
*
ERROR at line 1:
ORA-00028: your session has been killed
ORA-00028: your session has been killed
SCOTT@ora11g>
SYS@ora11g> alter procedure scott.p1 compile;
Procedure altered.