概述

Application Context是内存中的一组name-value对，application context从属于某个命名空间(namespace)。
用 户只能通过一个自定义procedure调用dbms_session.set_context来设置application context的值。用户使用sys_context(<namespace>,<name>)来获取某个 application context的值。
Application context分为三种

• dabase session-based application context，又称为local application context。Local application context存储在UGA中，dedicated server mode,UGA在PGA中，shared server mode，UGA在SGA中。local application context是会话/server process级别的。只有本会话能够访问。当会话/server process终止时，local application context的生命周期也结束了。另一篇博客Oracle VPD http://blog.csdn.net/chncaesar/article/details/18550029 给出了一个local application context的例子。
  • global application context，Global application context存储在SGA中，只要SGA不消亡，global application context就一直存在。global application context常常用于跨会话，与会话无关的场景。dbms_session.set_context有两个默认值为NULL的参数：
    username
    client_id
    username = null,client_id=null。所有用户都能访问。
    username = null,client_id=<client_id>。 只要session 的client_id =<client_id>，而不管username，都能访问。
    username = <database username>,client_id=null。只要session使用指定的oracle schema登陆，不管client_id，都能访问。
    username = <database username>,client_id=<client_id>. 常用于statless web session ，如http。
    username = <non-database username>,client_id=<client_id> . 该username将用于数据库连接池的owner。
• client session-based application context。通常被OCI使用，存储在客户端内存，而不是Oracle服务器端，也由OCI程序管理。

实例

例子1： 所有用户都能访问的application context

CREATE OR REPLACE CONTEXT global_hr_ctx USING hr_ctx_pkg ACCESSED GLOBALLY;
CREATE OR REPLACE PACKAGE hr_ctx_pkg
AS
PROCEDURE set_hr_ctx(sec_level IN VARCHAR2);
PROCEDUREclear_hr_ctx;
END;
/
create or replace package body hr_ctx_pkg
as
procedure set_hr_ctx(sec_level in VARCHAR2)
BEGIN
DBMS_SESSION.SET_CONTEXT(
NAMESPACE => 'global_hr_ctx',
attribute => 'job_role',0)"> value => sec_level);
end set_hr_ctx;
procedure clear_hr_ctx
as
begin
dbms_session.clear_context('global_hr_ctx','job_role');
end clear_hr_ctx;
end;
/

sql> BEGIN
2 hr_ctx_pkg.set_hr_ctx('clerk');
3 END;
4 /

PL/sql procedure successfully completed.

sql> SELECT SYS_CONTEXT('global_hr_ctx','job_role') job_role FROM DUAL;
JOB_ROLE
------------------------------------
clerk

例子2: 跨session/application，但是使用同一个数据库schema的application context。

Setting a Global Context for Database Users Who Move Between Applications
CREATE OR REPLACE PACKAGE hr_ctx1_pkg
AS
PROCEDURE set_hr_ctx(sec_level IN VARCHAR2,user_name IN VARCHAR2);
PROCEDURE clear_hr_context;
END;
/
CREATE OR REPLACE PACKAGE BODY hr_ctx1_pkg
AS
PROCEDURE set_hr_ctx(sec_level IN VARCHAR2,user_name IN VARCHAR2)
AS
BEGIN
DBMS_SESSION.SET_CONTEXT(
namespace => 'global_hr_ctx',
attribute => 'job',
value => sec_level,
username => user_name); --- 这里的user_name表示用户为数据库用户
END set_hr_ctx;
PROCEDURE clear_hr_context
AS
BEGIN
DBMS_SESSION.CLEAR_CONTEXT('global_hr_ctx');
END clear_hr_context;
END hr_ctx1_pkg;
/
例3 Tutorial: Creating a Global Application Context That Uses a Client Session ID

1. Log on as the security administrator@H_404_173@sysadmin_ctx.

   CONNECT sysadmin_ctx
   Enter password: password
   

2. Create the@H_404_173@cust_ctxglobal application context.

   CREATE CONTEXT global_cust_ctx USING cust_ctx_pkg ACCESSED GLOBALLY;
   

   The@H_404_173@cust_ctxcontext is created and associated with the schema of the security administrator@H_404_173@sysadmin_ctx. However,the@H_404_173@SYSschema owns the application context.