Effective java 摘選條目分享 3 - concurrency

1. 2015/04/28 讀書會分享
Effective Java
摘選條目分享 3
- Concurrency
Kane

2. 大綱
Concurrency issues?
atomic
volatile
synchronized
常用類別
wait / notify
Thread Safety

3. You should already know ...
http://stackoverflow.com/questions/200469/what-is-the-difference-between-a-process-and-a-thread

4. You should already know ...
http://nerds.weddingpartyapp.com/tech/2014/06/20/primer-threading-handlers-android/

5. You should already know...
https://www3.ntu.edu.sg/home/ehchua/programming/java/J5e_multithreading.html

6. Concurrency issues
What is the most frequent concurrency issue you've encountered in Java?
http://stackoverflow.com/questions/461896/what-is-the-most-frequent-
concurrency-issue-youve-encountered-in-java

7. Problem
class MyThread extends Thread {
private boolean stop = false;
public void run() {
while(!stop) {
doSomeWork();
}
}
public void setStop() {
this.stop = true;
}
調用了setStop()，
卻可能仍然不會停下。

8. 暫存
http://javarevisited.blogspot.tw/2011/06/volatile-keyword-java-example-tutorial.html

9. volatile
http://www.slideshare.net/caroljmcdonald/java-concurrency-memory-model-and-trends-4961797

10. volatile 注意事項
對該變數的所有操作皆為原子操作？
http://stackoverflow.com/questions/3519664/difference-between-volatile-and-synchronized-in-java

11. 考慮以下這段程式碼
public void updateCounter() {
if (counter == 1000) {
counter = 0;
} else {
counter++;
}
}

12. 考慮以下這段程式碼
public void updateCounter() {
if (counter == 1000) {
counter = 0;
} else {
counter++;
}
}
從 main memory 讀取至 cache
在 cache 中更新值
在某個時機點寫回 main memory

13. Case: Two threads
if (counter == 1000) {
counter = 0;
} else {
counter++;
}
if (counter == 1000) {
counter = 0;
} else {
counter++;
}
thread 1 thread 2

14. atomic
● 不可切割
● a++; 讀取 a
更新 a

15. synchronized
http://www.ibm.com/developerworks/library/j-jtp03304/
waiting for M’s lock

16. object locking / monitor
https://www.artima.com/insidejvm/ed2/threadsynchP.html

17. synchronized 注意事項
● 避免調用外部方法
i.e. Template Method Pattern, callback, sub-class’s
method, ...
● 重入 (reentrant)

18. 常用類別 - 資料、容器
AtomicInteger, AtomicLong …
CopyOnWriteArrayList
ConcurrentHashMap
...

19. 常用類別 - 資料、容器
AtomicInteger, AtomicLong …
CopyOnWriteArrayList
ConcurrentHashMap
...
putIfAbsent(k, v)
-----------------------------
if (containsKey(k)) {
return put(k, v);
} else {
return get(k);
}

20. 常用類別 - 控制
Executor, ExecutorService, …
CountDownLatch
Semaphore
CyclicBarrier

21. 常用類別 - 控制
Executor, ExecutorService, …
CountDownLatch
Semaphore
CyclicBarrier
更方便控制的 thread pool

22. 常用類別 - 控制
Executor, ExecutorService, …
CountDownLatch
Semaphore
CyclicBarrier
等到扣完才動
同時間只有 n 個能進入
等到有 n 個到達 await() 後才動

23. wait / notify
1. 應優先使用 java.util.concurrent 提供的類別
2. wait 必在 loop 中使用
3. 優先考慮 notifyAll

24. https://codexplo.wordpress.com/2012/10/20/state-diagram-of-java-thread/

25. WHY: wait 必在 loop 中使用
1. notify 已經做了，先檢查條件可免 wait
2. 別人做了 notifyAll
3. spurious wakeup
synchronized (obj) {
while (<需等待的條件>) {
obj.wait();
}
}

26. spurious wake
scheduler可能因為一些原因而blackout
中間可能錯失信號
故回復時會喚醒 thread，避免無限 waiting
關鍵字：POSIX Threads, Windows API,
pthread_cond_wait, futex

27. 優先考慮 notifyAll
1. 避免沒有收到 notify 的情況
2. 保護：wait 在 loop 中

28. Thread Safety
1. immutable 不需要外部同步，因為instance狀態不變
2. unconditionally thread-safe 不需要外部同步
3. conditionally thread-safe 有些條件下必須使用外部同步
4. not thread-safe 使用者要自己控制同步
5. thread-hostile 就算都做了外部同步，也不行

29. Q&A