1 Java thread pool Profile
Java1.5 master Doug Lea's work. Thread pool thread can be unified distribution, responsible for processing tasks through a fixed number of threads, to avoid the frequent create and destroy objects , so threads can be repeated use , perform multiple tasks.
2 Java types of thread pool
Executors to achieve the kind of thread factory
| kind | achieve | Blocking queue based | Explanation |
|---|---|---|---|
| Fixed size thread pool | FixedThreadPool | LinkedBlockingQueue | Fixed number of threads, task queue unbounded |
| Delivery model thread pool | CachedThreadPool | SynchronousQueue | Without limiting the number of threads, task queue does not store data |
| Parallel thread pool | WorkStealingPool | FIFO_QUEUE或LIFO_QUEUE | Without limiting the number of threads, dividing the task execution |
| Timing task thread pool | ScheduledThreadPool | DelayedWorkQueue | Without limiting the number of threads, task execution time priority |
| Singleton fixed thread pool | SingleThreadExecutor | LinkedBlockingQueue | A number of threads, task execution order of addition |
Achieve 3 Java thread pool
3.1 blocking queue
| queue | achieve | Explanation |
|---|---|---|
| LinkedBlockingQueue | And a display lock queue link list based on the realization | Similar support concurrent LinkedList |
| SynchronousQueue | Queue-based dual-stack delivery / double lock achieved and a display queue | Dual-stack / queue algorithm based on double |
| DelayedWorkQueue | Based on the priority queue and the queue delay locks for display | Similarly with the priority queue, the algorithm implemented by a binary heap |
3.2 thread factory Executors
The main process thread pool
3.2.1 CachedThreadPool
- Prior to reuse threads (threads in the thread pool is not destroyed)
- If no thread, a new thread is created and added to the pool
- The default thread pool is not used for the 60s is terminated and removed
- Long-term idle pool will not consume any resources
When the execute method, will first offer the first implementation of the method SynchronousQueue submit tasks and queries whether idle threads in the thread pool to execute SynchronousQueue poll method to remove tasks. If so, then the pairing is successful, the task to the idle threads. Otherwise, the pairing fails, create a new thread to handle the task; When a thread pool idle threads will wait for the implementation of the poll method SynchronousQueue SynchronousQueue perform tasks in the new submission. If more than 60s is still not committed to the task SynchronousQueue, the idle thread will terminate; because maximumPoolSize is unbounded, so the speed of the submitted jobs> thread pool thread speed processing tasks will continue to create new threads; each submission task will there are threads to deal with immediately, so CachedThreadPool applicable to handle large, less time-consuming task.
Advantages: immediate task execution, the thread automatic recovery
Disadvantages: will appear instantly at the moment is too high concurrency create thousands of threads, resulting in paralysis of the system
scenes to be used
OKHTTP framework
3.2.2 FixedThreadPool
- Reuse a fixed number of threads to create a thread pool
- When all threads are active, if submitted other tasks, they will wait for a thread in the queue available
- When the maximum number of tasks, plus membership be rejected
- Thread will always exist until you call shutdown
FixedThreadPool only the core thread, and a fixed number, no non-core thread. keepAliveTime set to 0L, on behalf of the excess threads will be terminated immediately. Because no extra thread, keepAliveTime is invalid parameter; using the task queue unbounded blocking queue LinkedBlockingQueue (capacity default Integer.MAX_VALUE).
Advantages: Controlled relatively fixed number of threads
Cons: tasks performed under high concurrency will become slow, will be waiting in the queue.
scenes to be used
Tomcat worker threads
3.2.3 ScheduledThreadPool
- Set the delay time to perform regular
- Idle thread will be reserved.
When scheduleAtFixedRate or scheduleWithFixedDelay ScheduledThreadPoolExecutor method of execution, the task will be added to achieve a wrapper class ScheduledFutureTask RunnableScheduledFuture interface to DelayedWorkQueue, thread and check whether the number of cores running threads corePoolSize. If you do not create a new thread and start. But the task is not executed immediately, but to take the task in DelayedWorkQueue packaging ScheduledFutureTask, and then to perform the task; if the thread runs reached corePoolSize, add tasks to put in the task queue DelayedWorkQueue; DelayedWorkQueue task will be sorted, first performed the task in front of the queue. After task execution, ScheduledFutureTask variables in time changed the next time you want to perform, and put back in DelayedWorkQueue.
3.3 rejection policy
| Tactics | Explanation |
|---|---|
| AbortPolicy | When the task is added to the thread pool is rejected, it will throw an exception RejectedExecutionException. |
| CallerRunsPolicy | When the task is added to the thread pool is rejected, processing tasks to be rejected in Thread thread pool thread pool that are currently running. |
| DiscardOldestPolicy | When the task is added to the thread pool is rejected, the thread pool will give up waiting in the queue oldest unprocessed tasks, and then add the rejected task to the waiting queue. |
| DiscardPolicy | When the task is added to the thread pool is rejected, discarded thread pool rejected task. |
4 Description
Reference material
ScheduledThreadPool thread pool of learning blog.csdn.net/qq_36299025...
Compared with using five thread pool
other information
Java priority queue DelayedWorkQueue principle analysis www.jianshu.com/p/587901245...
Detailed SynchronousQueue principle - fair mode www.cnblogs.com/dwlsxj/p/Th...
SynchronousQueue Detailed principles - non-equity modes www.cnblogs.com/dwlsxj/p/sy...