Synchronized blocks: the best approach?

I have a question concerning synchronization within a servlet. As I
          understand it, by default only one instance of a servlet will be
          instantiated by WebLogic. The impact of this is that if an instance
          variable is set in 'doGet', there is no guarantee that the value will be
          unchanged upon execution of 'doPost' (i.e. it is not "thread-safe").
          There are a few approaches to this problem, none of which are very
          appealing to me. If I utilize the "SingleThreadModel" interface, then I am
          guaranteed that the value will not change. However, this will occur at the
          expense of performance (isn't there a maximum of 5 instances?), especially
          if hundreds of users are concurrently attempting to access this same
          servlet. Even if I store a variable in the session, if the user can open
          multiple browser windows, then I could still run into a synchronization
          problem (since each window will share the same session).
          With this in mind (assuming that a user can open multiple browser windows
          and that I do not want to implement "SingleThreadModel"), is my only option
          to place synchronized blocks around the sections of code that I want
          protected?
          I appreciate any input. Thanks.

          It is possible to make this really messy - avoid that.
          Don't use intance variables, use only variables declared in the doGet method -
          each execution will have it's own variables and you won't have these problems.
          If you need data to persist from one call to the next, use the httpSession.
          If you need to manage separate browser windows - then put a hidden field or a
          parameter in the URL that identifies that window. Then store the data in the httpsession
          along with the window id -
          session.putValue("USERNAME_"+winId, username);
          Putting synchronized blocks doesn't really protect you from the multiple window
          problem - suppose a user does a search in one window - and the search results
          are stored in the http session. Then they do a second search in a second window
          - even if you protect the variables in a synchronize block - the second results
          will overwrite the first results. Then they start to page through what should
          be the first results in the first window - but they would see the second results.
          Mike Reiche
          Resume at http://home.earthlink.net/~mikereiche/resume.htm
          "Edward Mench" <[email protected]> wrote:
          > I have a question concerning synchronization within a servlet. As
          >I
          >understand it, by default only one instance of a servlet will be
          >instantiated by WebLogic. The impact of this is that if an instance
          >variable is set in 'doGet', there is no guarantee that the value will
          >be
          >unchanged upon execution of 'doPost' (i.e. it is not "thread-safe").
          >
          > There are a few approaches to this problem, none of which are very
          >appealing to me. If I utilize the "SingleThreadModel" interface, then
          >I am
          >guaranteed that the value will not change. However, this will occur
          >at the
          >expense of performance (isn't there a maximum of 5 instances?), especially
          >if hundreds of users are concurrently attempting to access this same
          >servlet. Even if I store a variable in the session, if the user can
          >open
          >multiple browser windows, then I could still run into a synchronization
          >problem (since each window will share the same session).
          >
          > With this in mind (assuming that a user can open multiple browser windows
          >and that I do not want to implement "SingleThreadModel"), is my only
          >option
          >to place synchronized blocks around the sections of code that I want
          >protected?
          >
          > I appreciate any input. Thanks.
          >
          >
          >
          >

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Hi
In a roundabout way you have really answered your own question :-)
In my opinion, the best approach, although by all means not the only approach - see below) would be to have the object loaded as two folders with one join going to the first folder and the second join to the other folder. You would of course name the folders appropriately.
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I hope this helps
Best wishes
Michael

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SQL> create table sales_orders
2 as
3 select 1 no, 'O' status, 'Y' ind_over_credit_limit, 'N' ind_overdue, 'N' ind_stock_not_available from dual union all
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6 select 4, 'O', 'N', 'Y', 'N' from dual union all
7 select 5, 'O', 'N', 'N', 'Y' from dual
8 /
Tabel is aangemaakt.
SQL> create table log_table
2 ( sales_order_no number
3 , message        varchar2(100)
4 )
5 /
Tabel is aangemaakt.
SQL> create table invoices
2 ( sales_order_no number
3 )
4 /
Tabel is aangemaakt.
SQL> select * from sales_orders
2 /
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         1 O      Y                     N           N
         2 O      N                     N           N
         3 O      N                     Y           Y
         4 O      N                     Y           N
         5 O      N                     N           Y
5 rijen zijn geselecteerd.
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2    when ind_over_credit_limit = 'Y' then
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8    else
9         into invoices (sales_order_no) values (no)
10 select * from sales_orders where status = 'O'
11 /
5 rijen zijn aangemaakt.
SQL> select * from invoices
2 /
SALES_ORDER_NO
             2
1 rij is geselecteerd.
SQL> select * from log_table
2 /
SALES_ORDER_NO MESSAGE
             1 Over credit limit
             3 Overdue
             4 Overdue
             5 Stock not available
4 rijen zijn geselecteerd.Hope this helps.
Regards,
Rob.

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