Java concurrency can often seem like a dense jungle of concepts and principles. However, with the right examples, we can illuminate this complex topic. In this guide, we will explore various Java concurrency issues through simple demonstrations, allowing you to see the problems in action and understand their implications.

Why Organize Demos?

• Real-time phenomena are more intuitive and trustworthy than theoretical discussions.
• Java’s standard library supports threads and multithreading, making it essential to grasp these concepts for practical applications.
• Concurrency design can be perplexing; without systematic understanding, code can accidentally run without appropriate logic, leading to unreliable results.

Feel free to contribute your examples of concurrency issues you’ve encountered in your development journey by submitting an issue or pull request!

Demo Categories

• Update without Synchronization
• Infinite Loop of HashMap
• Invalid Combined State
• Invalid Long Variable Read
• Concurrency Count Issues
• Synchronization on Mutable Fields
• Deadlock due to Symmetric Locks
• Livelock due to Reentrant Locks
• Instruction Reordering Issues

Update without Synchronization Cannot be Read in Another Thread

Demo Description: This demo illustrates how setting a field in one thread may not be visible to another thread without proper synchronization.

Problem Statement: After the main thread sets the field `stop` to `true`, the task thread may not see this change, leading to an incorrect thread termination.

Quickly Run:

bash.mvnw compile exec:java -Dexec.mainClass=fucking.concurrency.demo.NoPublishDemo

Infinite Loop of HashMap

Demo Description: This demo reveals a scenario where multiple threads may lead to a blocking state when attempting to perform operations on a HashMap.

Problem Statement: A main thread trying to read from the HashMap can get stuck in an infinite loop when other threads are writing to it concurrently.

Quickly Run:

bash.mvnw compile exec:java -Dexec.mainClass=fucking.concurrency.demo.HashMapHangDemo

Combined State Read Invalid Combination

Demo Description: This demo showcases issues when multiple states are modified without synchronization.

Problem Statement: The read from different states may yield invalid combinations if not properly synchronized.

Quickly Run:

bash.mvnw compile exec:java -Dexec.mainClass=fucking.concurrency.demo.InvalidCombinationStateDemo

Long Variable Read Invalid Value

Demo Description: Demonstrates the problem when a `long` variable is read improperly across threads.

Problem Statement: A long variable may be split into two parts, leading to an invalid read value if not handled correctly.

Quickly Run:

bash.mvnw compile exec:java -Dexec.mainClass=fucking.concurrency.demo.InvalidLongDemo

Concurrency Count Issues

Demo Description: Shows how concurrent updates to a counter can yield incorrect results without synchronization.

Problem Statement: Final count value of concurrent increments ends up incorrect.

Quickly Run:

bash.mvnw compile exec:java -Dexec.mainClass=fucking.concurrency.demo.WrongCounterDemo

Synchronization on Mutable Fields

Demo Description: Explores the implications of using synchronized blocks on mutable fields.

Problem Statement: Expecting correct outcomes while the synchronized block does not guarantee thread safety for mutable fields.

Quickly Run:

bash.mvnw compile exec:java -Dexec.mainClass=fucking.concurrency.demo.SynchronizationOnMutableFieldDemo

Deadlock Caused by Symmetric Locks

Demo Description: A demo illustrating how improper management of locks can lead to deadlocks.

Problem Statement: Shows how two threads can end up in a deadlock state, waiting on each other.

Quickly Run:

bash.mvnw compile exec:java -Dexec.mainClass=fucking.concurrency.demo.SymmetricLockDeadlockDemo

Livelock Caused by Reentrant Locks

Demo Description: This demo shows how reentrant locks can lead to a livelock scenario.

Problem Statement: Demonstrates how two threads holding locks attempt to acquire the other’s lock repeatedly, leading to livelock.

Quickly Run:

bash.mvnw compile exec:java -Dexec.mainClass=fucking.concurrency.demo.ReentrantLockLivelockDemo

Instruction Reordering Causes Non-final Field Variable Read Error

Demo Description: This demo shows how instruction reordering can create issues with reads across threads.

Problem Statement: When a constructor is called, instruction reordering may expose non-final fields erroneously to reader threads.

Quickly Run:

bash.mvnw compile exec:java -Dexec.mainClass=fucking.concurrency.demo.FinalInitialDemo

Troubleshooting

If you encounter issues while testing the demos or have questions, consider the following tips:

• Ensure you are using the appropriate Java version, as compatibility may vary.
• Verify that any dependencies are properly resolved.
• Examine the command output for any error messages, which might guide you towards the specific problem.

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