java-design-patterns | :bulb:设计模式Java实现 | Architecture library
kandi X-RAY | java-design-patterns Summary
kandi X-RAY | java-design-patterns Summary
:bulb:设计模式Java实现
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- Clone a shape
- Creates a clone of this object
- Starts playing the media
- Load map
- The main method
- Prints the shearana
- Main entry point
- Validates given string
- Move the cursor to console
- Move to next time
- Notify all observers of all observers
- Sorts the destination
- Register an observer
- Draw the square
- Removes an observer
- Draw the rectangle
- Removes an Employee
- Add an Employee
- Implements the sleep
- Process the discount
- Do transaction
- Main method
- Package private for testing
- Create a Car
- Get a circle with the given color
- Print the move
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Trending Discussions on java-design-patterns
QUESTION
I am a little confused as to why Optimistic Locking is actually safe. If I am checking the version at the time of retrieval with the version at the time of update, it seems like I can still have two requests enter the update block if the OS issues an interrupt and swaps the processes before the commit actually occurs. For example:
...ANSWER
Answered 2021-Mar-03 at 22:15Well... that's the optimistic part. The optimism is that it is safe. If you have to be certain it's safe, then that's not optimistic.
The example you show definitely is susceptible to a race condition. Not only because of thread scheduling, but also due to transaction isolation level.
A simple read in MySQL, in the default transaction isolation level of REPEATABLE READ, will read the data that was committed at the time your transaction started.
Whereas updating data will act on the data that is committed at the time of the update. If some other concurrent session has updated the row in the database in the meantime, and committed it, then your update will "see" the latest committed row, not the row viewed by your get method.
The way to avoid the race condition is to not be optimistic. Instead, force exclusive access to the record. Doveryai, no proveryai.
If you only have one app instance, you might use a critical section for this.
If you have multiple app instances, critical sections cannot coordinate other instances, so you need to coordinate in the database. You can do this by using pessimistic locking. Either read the record using a locking read query, or else you can use MySQL's user-defined locks.
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Install java-design-patterns
You can use java-design-patterns like any standard Java library. Please include the the jar files in your classpath. You can also use any IDE and you can run and debug the java-design-patterns component as you would do with any other Java program. Best practice is to use a build tool that supports dependency management such as Maven or Gradle. For Maven installation, please refer maven.apache.org. For Gradle installation, please refer gradle.org .
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