effective java 摘選條目分享 3 - concurrency

2015/04/28 讀書會分享

Effective Java摘選條目分享 3

- Concurrency

Kane

大綱

Concurrency issues?atomicvolatilesynchronized 常用類別

wait / notifyThread Safety

You should already know ...

http://stackoverflow.com/questions/200469/what-is-the-difference-between-a-process-and-a-thread

You should already know ...

http://nerds.weddingpartyapp.com/tech/2014/06/20/primer-threading-handlers-android/

You should already know...

https://www3.ntu.edu.sg/home/ehchua/programming/java/J5e_multithreading.html

Concurrency issuesWhat 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

Problemclass 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。

暫存

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

volatile

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

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

考慮以下這段程式碼

public void updateCounter() {

if (counter == 1000) {

counter = 0;

} else {

counter++;

}

}

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

Case: Two threads

if (counter == 1000) { counter = 0; } else { counter++; }

if (counter == 1000) {

counter = 0; } else { counter++; }

thread 1 thread 2

atomic

● 不可切割

● a++; 讀取 a

更新 a

synchronized

http://www.ibm.com/developerworks/library/j-jtp03304/

waiting for M’s lock

object locking / monitor

https://www.artima.com/insidejvm/ed2/threadsynchP.html

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)

常用類別 - 資料、容器

AtomicInteger, AtomicLong …

CopyOnWriteArrayList

ConcurrentHashMap

...

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);}

常用類別 - 控制

Executor, ExecutorService, …

CountDownLatch

Semaphore

CyclicBarrier

常用類別 - 控制

Executor, ExecutorService, …

CountDownLatch

Semaphore

CyclicBarrier

更方便控制的 thread pool

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

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

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

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 wakeupsynchronized (obj) {

while (<需等待的條件>) {

obj.wait();

}

}

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

優先考慮 notifyAll

1. 避免沒有收到 notify 的情況

2. 保護：wait 在 loop 中

Thread Safety1. 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

Q&A