Status of thread pool (5 kinds):
The AtomicInteger variable ctl is very powerful: the low 29 bits represent the number of threads in the thread pool, and the high 3 bits represent the running state of the thread pool:
- RUNNING: -1 << COUNT_ Bits, that is, the upper 3 bits are 111. The thread pool in this state will receive new tasks and process the tasks in the blocking queue;
- SHUTDOWN: 0 << COUNT_ Bits, that is, the upper 3 bits are 000. The thread pool in this state will not receive new tasks, but will process the tasks in the blocking queue;
- STOP : 1 << COUNT_ Bits, that is, the upper 3 bits are 001. The thread in this state will not receive new tasks, nor process the tasks in the blocking queue, and will interrupt the running tasks;
- TIDYING : 2 << COUNT_ Bits, that is, the upper 3 bits are 010. This status indicates that the thread pool arranges and optimizes threads;
- TERMINATED: 3 << COUNT_ Bits, that is, the upper 3 bits are 011, which indicates that the thread pool stops working;
How to create a thread pool
Executors factory class
- Executors.newCachedThreadPool(); Note: create a cacheable thread pool. If the length of the thread pool exceeds the processing needs, you can flexibly recycle idle threads. If there is no recyclable thread, you can create a new thread
- Executors.newFixedThreadPool(int); Note: create a fixed length thread pool to control the maximum concurrent number of threads. Excess threads will wait in the queue.
- Executors.newSingleThreadExecutor(); Note: create a singleton thread pool. It will only use a unique working thread to execute tasks to ensure that all tasks are executed in order.
- Executors.newScheduledThreadPool(int); Description: create a fixed length routing pool to support regular and periodic task execution.
Disadvantages:
- FixedThreadPool and SingleThreadPool:
The allowed request queue length is Integer.MAX_VALUE, which may accumulate a large number of requests, resulting in OOM. 2. CachedThreadPool and ScheduledThreadPool: the number of threads allowed to create is Integer.MAX_VALUE, a large number of threads may be created, resulting in OOM.
ThreadPoolExecutor
Although the above Executors provide tool classes, there are still problems that may lead to OOM. We generally recommend using the thread pool inside the SDK. At the same time, users can understand the meaning of each parameter to prevent problems. The SDK provides the construction method of creating thread pool, as follows:
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