从源码角度理解Java并发编程：线程池设计

原创缺乏、安全感 2024-11-13 23:51 19阅读 0赞

Java并发编程中，线程池是一种非常重要的组件，它帮助我们管理线程的创建和销毁，提高资源利用率，减少开销，并提供更好的系统响应性。从源码角度理解线程池设计，我们可以从以下几个方面入手：

###1.线程池的基本概念线程池维护一组线程，这些线程可以被重复使用，而不是每次需要执行任务时都创建和销毁线程。线程池的核心接口是`ExecutorService`，它定义了线程池的基本操作。

###2.线程池的实现类Java中线程池的实现类主要是`ThreadPoolExecutor`，它是`ExecutorService`接口的一个实现。

###3.线程池的核心参数- **corePoolSize**：核心线程数，线程池中始终保持的线程数量，即使它们处于空闲状态。
- **maximumPoolSize**：最大线程数，线程池中允许的最大线程数量。
- **keepAliveTime**：非核心线程空闲时在终止前等待新任务的最长时间。
- **unit**：`keepAliveTime`的时间单位。
- **workQueue**：一个阻塞队列，用于存放任务。
- **threadFactory**：用于创建新线程的工厂。
- **handler**：饱和策略，当队列和线程池都满了，采取的策略。

###4.线程池的工作流程1. **提交任务**：当提交一个任务到线程池时，线程池会尝试添加到工作队列中。
2. **任务执行**：如果工作队列满了，并且线程数量小于最大线程数，线程池会创建新的线程来执行任务。
3. **线程复用**：如果线程池中的线程空闲，它们会被复用执行新的任务。
4. **线程销毁**：如果线程空闲时间超过`keepAliveTime`，并且线程数量超过核心线程数，这些线程会被销毁。

###5.源码分析```javapublic class ThreadPoolExecutor extends AbstractExecutorService {
 private final BlockingQueue<Runnable> workQueue;
 private final ThreadFactory threadFactory;
 private final RejectedExecutionHandler handler;

public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
 long keepAliveTime,
 TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
if (corePoolSize <0 ||
 maximumPoolSize <=0 ||
 maximumPoolSize < corePoolSize ||
 keepAliveTime <0)
 throw new IllegalArgumentException();
 this.corePoolSize = corePoolSize;
 this.maximumPoolSize = maximumPoolSize;
 this.workQueue = workQueue;
 this.keepAliveTime = unit.toNanos(keepAliveTime);
 this.threadFactory = threadFactory;
 this.handler = handler;
 }

public void execute(Runnable command) {
if (command == null)
 throw new NullPointerException();
/*
* Proceeding in3 steps:
 *
*1. If fewer than corePoolSize threads are running, try to* start a new thread with the given command as its first* task. The call to addWorker atomically checks runState and* workerCount, preventing false alarms that would add threads* when such additions are not allowed.
 *
*2. If a task can be successfully queued, then we still help* out by trying to add a new worker, as in (1), because the* existing workers are busy. If we cannot queue the task, then* we reject it.
 *
*3. If we cannot queue the task and we are sure that we are* not going to start a new thread (because we now have more* than maximumPoolSize threads running), then we reject the* task.
 */
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
 return;
c = ctl.get();
 }
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
if (!isRunning(recheck) && remove(command))
 reject(command);
 else if (workerCountOf(recheck) ==0)
addWorker(null, false);
 }
 else if (!addWorker(command, false))
 reject(command);
 }
}
```###6.总结通过源码分析，我们可以看到线程池的设计非常精巧，它通过控制线程的创建和销毁，以及任务的排队和执行，来实现高效的并发处理。理解这些机制对于编写高性能的并发程序至关重要。