Tutorial Addendum On Java - Synchronization
Why Synchronization
The better problem of acceptance assorted accoutrement administration the aforementioned data set
is that one operation in one cilia could bang with addition operation
in addition accoutrement on the aforementioned data. If this happens, the aftereffect is un-desirable.
Let s use a coffer appliance program as an example. Bold that the program
has assorted accoutrement running, with anniversary cilia abutting an ATM system, and
you accept a extenuative annual in the coffer with $100.00, now you and your acquaintance
are traveling to two altered ATMs at about the aforementioned time, and aggravating to abjure
$50.00 from your account, what do you anticipate it will happen?
If the accoutrement are active independently, the afterward could happen:
Time 01:01 02:01 03:01 04:01
+----------+---------+---------+-------
Thread 1 Get Set
Action You Annual Annual You
Abjure Antithesis Antithesis Receive
$50.00 $100.00 $50.00 $50.00
Time 01:02 02:02 03:02 04:02
-+----------+---------+---------+------
Thread 2 Get Set
Action Acquaintance Annual Annual Friend
Abjure Antithesis Antithesis Receive
$50.00 $100.00 $50.00 $50.00
Time 01:01 02:01 03:01 04:01
-----------++--------++--------++------
Account $100.00 $100.00 $50.00 $50.00
Both you and your acquaintance will accept $50.00 each, and your annual will
still accept $50.00. The coffer could lose $50.00.
The band-aid this problem is synchronization.
What Is Synchronization
Synchronization is a programming address that involves 3 elements:
- Lock: An item with two states: bound and unlocked.
- Synchronized Block: A block of statements that is associated
with a lock.
- Synchronization Rule: If a synchronized block is encountered in
a cilia of execution, the associated lock will be checked. If the lock
locked, the beheading will be chock-full until the lock is unlocked. If the lock
is unlocked, the lock will be locked, and the synchronized
block of statements will be executed. If the beheading alcove the end of
the synchronized block, the lock will be unlocked. With this rule, two synchronized
blocks associated with aforementioned lock will never be accomplished at the aforementioned time.
Now let s see if we can use the synchronization address in the coffer application
program to advice the bank. Let s ascertain a
synchronization block starting from the "Get Annual Balance" activity to the
"Set Annual Balance" activity in anniversary thread, and accessory the block with
a lock. With this change, both you and your acquaintance can still abjure $50.00,
but your annual will accept annihilation left:
Time 01:01 02:01 03:01 04:02
-----------+---------++--------++-------
Lock Apart Bound Bound Unlocked
Time 01:01 02:01 03:01 04:01
+----------+---------+---------+--------
Thread 1 Get Set
Action You Annual Annual You
Abjure Antithesis Antithesis Receive
$50.00 $100.00 $50.00 $50.00
Time 01:02 02:02 03:02 04:02 05:02
-+----------+---------+---------+---------+------
Thread 2 Get Get Set
Action Acquaintance Annual Annual Annual Friend
Abjure Antithesis Antithesis Antithesis Receive
$50.00 Chock-full $50.00 $0.00 $50.00
Time 01:01 02:01 03:01 04:02 05:02
-----------++--------++--------++---------+------
Account $100.00 $100.00 $50.00 $0.00 $0.00
The synchronization address did advice the coffer from accident money.
But it aswell added the absolute transaction time.
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