Java Concurrency Gotchas

http://www.slideshare.net/alexmiller/java-concurrency-gotchas-3666977

 

并发编程的几个基本topic：

互斥（共享变量，竞争区域）

同步

性能

 

互斥：

在共享变量的使用上，一是避免共享(threadLocal 变量，local variable即每次新建实例)，二是正确的使用lock，注意读写都要加锁(我们常常仅仅对写加锁)

java 中共享变量还有两个issue，一是可见性，为避免OS/JVM优化，每个线程拥有不同副本(?)，使用volatile关键字确保仅有一个副本；

二是原子性，注意点，++操作不是原子操作，long(64位变量)的修改非原子性 - 多个线程对同一个long非同步条件下进行++, --，会出现不可预见的结果。

另外高并发条件下，safe publication也是一个问题，常见的错误是在constructor中将this发布出去。保证安全的做法是使用factory method或者避免在construcotr 发布this对象

 

同步：

最基本的wait, notify, 在loop中wait，必须synchronize即获得对象的锁，才能在对象上wait或者唤醒

1.5后的Lock/Condition，synchronizer(CountDownLatch, CylicBarrier, Semaphore)更灵活简洁

 

性能：

避免大量线程/状态，使用一个锁，造成锁的竞争。

多使用异步操作。

Topic

• Shared Data 
  • Locking
  • Visibility
  • Atomicity
  • Safe Publication
• Coordination 
• Performance

Shared Data

Lock

Shared Mutable statics

• instance per call
• thread local
• lock

java

• Danger: DateFormat, Calendar, Matcher
• Safe: Random, Pattern

do not synchronize on

• null
• string literals (for scope)
• autoboxed vals (for scope) 

Visibility

volatile

Atomicity

Assignment of 64 bit values - volatile

Atomic read for long value

public class LongAssignment { private volatile long x; public void setLong(long val) { x = val; }  

Safe publication

register listener(this) in constructor / starting thread(this) in constructor, use factory method ?

public class Taxi { public Taxi(Dispatcher d) { d.register(this); } } 

 

Coordination

Threads

resume()

 

Wait. Notify

• must synchronize
• Always wait in a loop

private final Lock lock = new ReentrantLock(); private final Condition condition = lock.newCondition(); public void waitTillChange() { lock.lock(); try { while(! someCondition()) condition.await(); } finally { lock.unlock(); } } public void change() { lock.lock(); try { satisfyCondition(); condition.signalAll(); } finally { lock.unlock(); } } 

Performance

DeadLock avoidance

• lock splitting
• lock ordering
• lock timeout
• tryLock

Spin wait

private volatile boolean flag = false; public void waitTillChange() { while(! flag) { Thread.sleep(100); //Spin on flag, waiting for a change. } } public void change() { flag = true; } 

Condition

private final Lock lock = new ReentrantLock(); private final Condition condition = lock.newCondition(); private boolean flag = false; public void waitTillChange() { lock.lock(); try { while(! flag) condition.await(); } finally { lock.unlock(); // Better but longer. } } public void change() { lock.lock(); try { flag = true; condition.signalAll(); } finally { lock.unlock(); } } 

 

CountDownLatch private final CountDownLatch latch = new CountDownLatch(1); public void waitTillChange() { latch.await(); } public void change() { latch.countDown(); } 

Lock contention

Lock striping

Question

both write and read shard value need synchronization ?

 

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