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Simple showcases of Java concurrency problems, seeing π is believing π΅.
- The actual phenomenon that can be observed π is more intuitive and more trustworthy than the concurrency principle mentioned π.
- The
Javalanguage standard library supports threads, multithreading is heavily used in the language itself (such asGC) and applications (the server side). - Concurrency program design, in the analysis and implementation, the complexity is greatly increased. If you do not fully understand and systematically analyze the concurrent logic, and write code at will, it is not an exaggeration to describe such a program as "accidentally" running with the correct result.
- The demos here do not give explanations and discussions, and they are all entry-level
. For more information, please loop up the concurrency materials by yourself.
- The demos here do not give explanations and discussions, and they are all entry-level
Examples of concurrency problems you encountered in development are welcome to provide (submit Issue) or share (pull request after Fork)! π
- πΊ Update without synchronization cannot be read in another thread
- πΊ Infinite loop of
HashMap - πΊ Combined state read invalid combination
- πΊ
longvariable read invalid value - πΊ the result of concurrency count without synchronization is wrong
- πΊ Synchronization on mutable fields
- πΊ Deadlock caused by the symmetric locks
- πΊ Livelock caused by reentrant locks
- πΊ Instruction reordering causes non-final field variable read error
Demo class NoPublishDemo.
Set the field stop to true in the main thread to control the exit of the task thread started in main.
After the main thread field stop is true, the task thread continues to run, that is, no new value has been read in the task thread.
./mvnw compile exec:java -Dexec.mainClass=fucking.concurrency.demo.NoPublishDemoThis problem has been explained in many places.
The Demo class HashMapHangDemo can reproduce this problem.
Two task threads are opened in the main thread to perform the put operation of HashMap. The main thread does the get operation.
The main thread Block is determined by no continuous output, that is, the endless loop of HashMap appears.
./mvnw compile exec:java -Dexec.mainClass=fucking.concurrency.demo.HashMapHangDemoWhen programming, multiple state records will be required (state can be a POJO object or ints, etc.).
It is often seen that the multi-state read and write code is not synchronized, and the person who write it will naturally ignore the issue of thread safety.
Invalid combinations are combinations that have never been set.
The main thread modifies multiple states. For the convenience of checking, each write has a fixed relationship: the second state is twice the value of the first state. Read multiple states in a task thread.
Demo class InvalidCombinationStateDemo.
The second state read in the task thread is not twice the value of the first state, that is, an invalid value.
./mvnw compile exec:java -Dexec.mainClass=fucking.concurrency.demo.InvalidCombinationStateDemoAn invalid value is a value that has never been set.
Reading and writing of long variables is not atomic and will be divided into two 4-byte operations.
Demo class InvalidLongDemo.
The main thread modifies the long variable. For the convenience of checking, the upper 4 bytes and the lower 4 bytes of the long value written each time are the same. Read the long variable in the task thread.
In the task thread, a long variable whose upper 4 bytes and lower 4 bytes are different is read, which is an invalid value.
./mvnw compile exec:java -Dexec.mainClass=fucking.concurrency.demo.InvalidLongDemoDemo class WrongCounterDemo.
Two task threads are opened in the main thread to execute concurrent incrementing counts. Main thread final result check.
The count value is incorrect.
./mvnw compile exec:java -Dexec.mainClass=fucking.concurrency.demo.WrongCounterDemoIt is common to see synchronization code on a volatile field, and the person who write it will naturally feel that this is safe and correct.
# For problem analysis, see the article Synchronization on mutable fields.
Demo class SynchronizationOnMutableFieldDemo.
Two task threads are opened in the main thread to execute addListener. Main thread final result check.
The final count of Listeners is incorrect.
./mvnw compile exec:java -Dexec.mainClass=fucking.concurrency.demo.SynchronizationOnMutableFieldDemo# For problem analysis, see the article Synchronization on mutable fields
Demo class SymmetricLockDeadlockDemo.
Two task threads are opened in the main thread for execution.
Task thread deadlocked.
./mvnw compile exec:java -Dexec.mainClass=fucking.concurrency.demo.SymmetricLockDeadlockDemo# For a problem description, see the paragraph about livelocks in this article
Demo class ReentrantLockLivelockDemo.
Two task threads are trying to acquire a lock that the other thread holds while holding their own lock.
While the threads are releasing their own lock constantly, they are also re-locking it immediately, denying the other thread a chance to acquire both locks. Since both threads are not blocked from executing but blocked from doing meaningful work, this is a livelock.
./mvnw compile exec:java -Dexec.mainClass=fucking.concurrency.demo.ReentrantLockLivelockDemoDemo class FinalInitialDemo.
The writer thread calls the constructor of the class, and the reader thread obtains the member variables of the non-final domain of the class.
When calling the constructor, instruction reordering may occur, placing non-final domain variables outside the constructor, causing the writer and reader threads to obtain the default initial values of the variables.(Instruction ordering does not necessarily occur and requires specific hardware and JVM environments).
./mvnw compile exec:java -Dexec.mainClass=fucking.concurrency.demo.FinalInitialDemo