客户反映将JFY用户下的某个表的字段由VARCHAR2(50)调整为VARCHAR2(100)之后，执行exp导出时收到如下的报错：
Connected to: Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
Export done in ZHS16GBK character set and AL16UTF16 NCHAR character set
Note: grants on tables/views/sequences/roles will not be exported
About to export specified users ...
. exporting pre-schema procedural objects and actions
. exporting foreign function library names for user JFY
. exporting PUBLIC type synonyms
EXP-00008: ORACLE error 904 encountered
ORA-00904: : 标识符无效
<<<< 两个冒号之间是空格，并没有具体的标识符，这跟以前遇到过的ORA-00904报错不同。
EXP-00000: Export terminated unsuccessfully
客户的exp导出操作是在数据库服务器段完成的，且调度脚本已经稳定运行了1年多，在调整字段长度后出现该报错。脚本中exp的语句如下：
exp jfy/xxx BUFFER=8192 FILE=$bakdir$myfile.dmp ROWS=y COMPRESS=y CONSTRAINTS=y GRANTS=n INDEXES=y LOG=$bakdir$mylogfile
jfy是个DBA权限的用户。
根据以上的错误情况，在MOS中找到如下符合描述的文章：
Errors EXP-8 ORA-904 During Export (Doc ID 1091927.1)
In this Document
| Symptoms |
| Cause |
| Solution |
Applies to:
Oracle Database - Enterprise Edition - Version 10.2.0.4 and later
Information in this document applies to any platform.
***Checked for relevance on 27-Jun-2013***
Symptoms
You are attempting to perform an export as the SYSTEM user or a DBA. During export, the following errors occurred:
EXP-00008: ORACLE error 904 encountered
ORA-00904: : invalid identifier <<<< 两个冒号之间是一个空格，这和上面的报错情况一致。
When performing a trace on the export, we see that the ORA-904 error is caused by the following statement:
PARSE ERROR #9:len=302 dep=0 uid=5 oct=3 lid=5 tim=778463694169 err=904
SELECT SYNNAM, DBMS_JAVA.LONGNAME(SYNNAM), DBMS_JAVA.LONGNAME(SYNTAB), TABOWN, TABNODE, PUBLIC$, SYNOWN, SYNOWNID, TABOWNID, SYNOBJNO FROM SYS.EXU9PTS WHERE SYNOBJNO IN ( SELECT SYNOBJNO FROM SYS.EXU9TYPT WHERE TABOBJNO = :1 ) ORDER BY SYNTIME
Cause
The PUBLIC privilege has been revoked from the DBMS_JAVA package.
Privileges to the DBMS_JAVA package are needed for the export to succeed. However, as there is concern about security issues due to privileges on the DBMS_JAVA many have revoked the execute privilege to this package to keep the database secure.
Solution
Explicitly grant EXECUTE on the DBMS_JAVA package to the DBA role:
SQL> grant execute on DBMS_JAVA to dba;
This will allow members of the DBA role to have the rights to perform the export while keeping the DBMS_JAVA package out of the PUBLIC role.
按照文章的提示，执行grant execute on DBMS_JAVA to dba;语句之后exp操作恢复正常。
出现这个问题的原因是PUBLIC的DBMS_JAVA包权限被回收，要想
执行exp导出成功需要DBMS_JAVA的execute权限，然而担心到安全的问题，回收了DBMS_JAVA包的执行权限，以保证数据库的安全。
按照Oracle的解释，那么一定是某种不安全的操作触发了Oracle因为安全的原因
回收了PUBLIC的DBMS_JAVA execute权限，从而导致的该报错。
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