This section describes what is synchronization - a programming technique used in multi-threading applications with a synchronization lock to control execution of synchronized code.
Synchronization is a programming technique that involves 3 elements:
Lock: An object with two states: locked and unlocked.
Synchronized Block: A block of statements that is associated
with a lock.
Synchronization Rule: When a synchronized block is encountered in
a thread of execution, the associated lock will be checked. If the lock
locked, the execution will be stopped until the lock is unlocked. If the lock
is unlocked, the lock will be locked, and the synchronized
block of statements will be executed. When the execution reaches the end of
the synchronized block, the lock will be unlocked. With this rule, two synchronized
blocks associated with same lock will never be executed at the same time.
Now let's see if we can use the synchronization technique in the bank application
program to help the bank. Let's define a
synchronization block starting from the "Get Account Balance" action to the
"Set Account Balance" action in each thread, and associate the block with
a lock. With this change, both you and your friend can still withdraw $50.00,
but your account will have nothing left:
Time 01:01 02:01 03:01 04:01
+----------+---------+---------+--------
Thread 1 Get Set
Action You Account Account You
Withdraw Balance Balance 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 Friend Account Account Account Friend
Withdraw Balance Balance Balance Receive
$50.00 Stopped $50.00 $0.00 $50.00
