线程池
为什么使用线程池
- 将执行机制和工作单元分离
- 减少创建和销毁线程对象的开销
- 可控性强，可对线程池进行配置，监控
- 可扩展性强，如优先级，可延迟，定时线程池
流程
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部分源码
ThreadPoolExecutor
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execute
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
/*
* Proceed in 3 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, and so prevents false alarms that would add
* threads when it shouldn't, by returning false.
*
* 2. If a task can be successfully queued, then we still need
* to double-check whether we should have added a thread
* (because existing ones died since last checking) or that
* the pool shut down since entry into this method. So we
* recheck state and if necessary roll back the enqueuing if
* stopped, or start a new thread if there are none.
*
* 3. If we cannot queue task, then we try to add a new
* thread. If it fails, we know we are shut down or saturated
* and so reject the task.
*/
int c = ctl.get();
// 小于核心线程，尝试添加线程
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
// offer 不阻塞的那个，尝试加入队列
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
// double-check 如果线程池停了，移除，并拒绝
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
// 无法排队，check最大线程数，开新线程
else if (!addWorker(command, false))
reject(command);
}
addWorker
/**
* 尝试创建新线程
*
*
* Checks if a new worker can be added with respect to current
* pool state and the given bound (either core or maximum). If so,
* the worker count is adjusted accordingly, and, if possible, a
* new worker is created and started, running firstTask as its
* first task. This method returns false if the pool is stopped or
* eligible to shut down. It also returns false if the thread
* factory fails to create a thread when asked. If the thread
* creation fails, either due to the thread factory returning
* null, or due to an exception (typically OutOfMemoryError in
* Thread.start()), we roll back cleanly.
*
* @param firstTask the task the new thread should run first (or
* null if none). Workers are created with an initial first task
* (in method execute()) to bypass queuing when there are fewer
* than corePoolSize threads (in which case we always start one),
* or when the queue is full (in which case we must bypass queue).
* Initially idle threads are usually created via
* prestartCoreThread or to replace other dying workers.
*
* @param core if true use corePoolSize as bound, else
* maximumPoolSize. (A boolean indicator is used here rather than a
* value to ensure reads of fresh values after checking other pool
* state).
* @return true if successful
*/
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
for (;;) {
int wc = workerCountOf(c);
// 数量达到上限
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
// cas增加线程数 成功就跳出循环
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get(); // Re-read ctl
// 状态发生变化，重新循环
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
}
// 成功+1之后
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
// 尝试加入到workers
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
int rs = runStateOf(ctl.get());
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
workers.add(w);
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
// 线程启动
if (workerAdded) {
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
runWorker 任务的run的委托方法
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
// getTask 很重要，如果当前线程数大于核心线程数，就返回null，就销毁了该线程，执行完成了
// 当前线程数小于核心线程数的话，就一直循环，执行两个钩子函数
while (task != null || (task = getTask()) != null) {
w.lock();
// If pool is stopping, ensure thread is interrupted;
// if not, ensure thread is not interrupted. This
// requires a recheck in second case to deal with
// shutdownNow race while clearing interrupt
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
try {
beforeExecute(wt, task);
Throwable thrown = null;
try {
task.run();
} catch (RuntimeException x) {
thrown = x; throw x;
} catch (Error x) {
thrown = x; throw x;
} catch (Throwable x) {
thrown = x; throw new Error(x);
} finally {
afterExecute(task, thrown);
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}
getTask
/**
* Performs blocking or timed wait for a task, depending on
* current configuration settings, or returns null if this worker
* must exit because of any of:
* 1. There are more than maximumPoolSize workers (due to
* a call to setMaximumPoolSize).
* 2. The pool is stopped.
* 3. The pool is shutdown and the queue is empty.
* 4. This worker timed out waiting for a task, and timed-out
* workers are subject to termination (that is,
* {@code allowCoreThreadTimeOut || workerCount > corePoolSize})
* both before and after the timed wait, and if the queue is
* non-empty, this worker is not the last thread in the pool.
*
* @return task, or null if the worker must exit, in which case
* workerCount is decremented
*/
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
int wc = workerCountOf(c);
// Are workers subject to culling?
// allowCoreThreadTimeOut，一般都是false，除非方法改过
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
// 空闲，并且上次在队列里没拿到worker，淘汰
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
// 开始从队列里取任务了
try {
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
状态
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参数
corePoolSize
核心线程数，线程池中始终维护的，存活的线程数，核心线程数,核心线程一旦创建就不销毁 存活是指不断调用阻塞队列的take方法，如果队列里也没有值就会阻塞
maximumPoolSize
最大线程数，在corePooSize已满、队列也满的情况下，扩充线程至此值。
keepAliveTime
最大线程数可以存活的时间，当线程中没有任务执行时，最大线程就会销毁一部分，最终保持核心线程数量的线程。
unit:
单位
workQueue（BlockingQueue）：
阻塞队列，当新任务来的时候会先判断当前运行的线程数量是否达到核心线程数，如果达到的话，任务就会被存放在队列中。
- BlockingQueue：
- put和take：阻塞式的
- ArrayBlockingQueue ：是一个用数组实现的环形队列，在构造函数中，会要求传入数组的容量
- LinkedBlockingQueue：是一种基于单向链表的阻塞队列。
- 可以无界
- PriorityQueue：是按照元素的优先级从小到大出队列的二叉堆。正因为如此，PriorityQueue中的2个元素之间需要可以比较大小，并实现Comparable接口（在构造方法传比较器），当元素个数超出数组长度时，执行扩容操作。
- DelayQueue：即延迟队列，也就是一个按延迟时间从小到大出队的PriorityQueue。
- SynchronousQueue：一个不存储元素的阻塞队列
- 相当于把元素先存在TransferQueue或者TransferStack
- put和take：会阻塞调用线程，知道有对应的存或取操作
- offer和poll：如果有对应的存取操作在等待，则消费完返回true，如果没有就直接返回false
- 仅使用无法阻塞的offer和poll，是无效的，都会失败
- 直接转发给maxPoolSize策略
- LinkedTransferQueue：一个由链表结构组成的无界阻塞队列。与SynchronousQueue类似，还含有非阻塞方法。
- LinkedBlockingDeque：一个由链表结构组成的双向阻塞队列。
threadFactory
线程工厂，主要用来创建线程，默认为正常优先级、非守护线程。
handler
线程池任 务队列超过 maxinumPoolSize 之后的拒绝策略
- AbortPolicy：拒绝并抛出异常。
- CallerRunsPolicy：使用当前调用的线程来执行此任务。
- DiscardOldestPolicy：抛弃队列头部（最旧）的一个任务，并执行当前任务。
- DiscardPolicy：忽略并抛弃当前任务。
Callable与Future
Callable
@FunctionalInterface
public interface Callable<V> {
/**
* Computes a result, or throws an exception if unable to do so.
*
* @return computed result
* @throws Exception if unable to compute a result
*/
V call() throws Exception;
}
- 有返回值的Runnable，本身是一个相对独立的函数式接口
Future
FutureTask
implements RunnableFuture<V> interface RunnableFuture<V> extends Runnable, Future<V>
- FutureTask 接收一个Callable，并且在run的时候调用call，接收返回值。用get去拿返回值
- FutureTask可以直接传递给Thread
CompletableFuture
implements Future<T>, CompletionStage<T>
- 处理异步任务的结果,多任务
- 各种方法
- supplyAsync: 异步立刻执行
- completedFuture: 不执行任务，直接返回 T
- thenApplyAsync: 在当前线程中异步处理上个任务，并返回结果
- thenApply: 同步
- henAccept: 执行完成后，接收返回结果，不返回
- 上个任务抛出异常就不执行下一个then*
- get: 阻塞获取异步结果
- complete(T): 主动完成
- completeExceptionally: 主动抛出异常
- exceptionally((ex) -> ()): thenApplyAsync等如果异常了会进入
- handle((result, fail) -> ()): 都会进入
- thenComplete: 没有返回值
几种Executor
ForkJoinPool
例子
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveTask;
public class FibonacciCalculator extends RecursiveTask<Integer> {
private final int n;
public FibonacciCalculator(int n) {
this.n = n;
}
@Override
protected Integer compute() {
// 终止条件
if (n <= 1) {
return n;
} else {
FibonacciCalculator fib1 = new FibonacciCalculator(n - 1);
fib1.fork(); // 在新的线程中异步执行 fib(n-1)
FibonacciCalculator fib2 = new FibonacciCalculator(n - 2);
// 合并任务，在当前线程中计算 fib(n-2)，然后等待 fib(n-1) 完成并获取其结果
return fib2.compute() + fib1.join();
}
}
public static void main(String[] args) {
int n = 10;
ForkJoinPool forkJoinPool = ForkJoinPool.commonPool();
FibonacciCalculator calculator = new FibonacciCalculator(n);
int result = forkJoinPool.invoke(calculator);
System.out.println("The " + n + "th Fibonacci number is: " + result);
}
}
ScheduledThreadPoolExecutor
用了DelayedWorkQueue 延迟队列
- 延迟队列无界，maximumPoolSize没用到
- DelayedWorkQueue take的时候会调用Condition.await，等待延迟时间
- 方法：
- schedule
- scheduleAtFixedRate(Runnable command, long initialDelay,long period,TimeUnit unit)
- 不考虑任务时间（任务时间小于设置的间隔时间），如：任务3s,设置间隔2s -> 实际间隔3s
- scheduleWithFixedDelay(Runnable command,long initialDelay, ong delay,TimeUnit unit)
- 考虑任务时间：如：任务3s,设置间隔2s -> 实际间隔5s
Executors大家庭
工具类，调用的ThreadPoolExecutor,ForkJoinPool的构造函数
FixedThreadPool
可配置的，固定大小的，无界队列的
- corePoolSize
- nThreads 固定大小（fixed），线程数有上限
- maximumPoolSize
- nThreads 没用，反正会进队列
- keepAliveTime
- workQueue
- LinkedBlockingQueue 无界的， 多出来的会一直塞到队列
- handler
- AbortPolicy 没有触发条件
SingleThreadScheduled
只有一个线程，固定大小的，无界队列的
- corePoolSize
- 1 最多就一个线程
- maximumPoolSize
- 1 没用，反正会进队列
- keepAliveTime
- workQueue
- LinkedBlockingQueue 无界的，多出来的会一直塞到队列
- handler
- AbortPolicy 没有触发条件
CachedThreadPool
如果已创建线程没跑完，后面提交任务会创建一个线程。如果没生产者了，线程60s后会被淘汰
- corePoolSize
- 0
- maximumPoolSize
- Integer.MAX_VALUE
- keepAliveTime
- 60s
- workQueue
- SynchronousQueue
- 开始先offer肯定是失败的，所以开始的时候用不到这个队列，直接addWorkers了
- 后面有线程在poll(timeout)队列了，再offer的话会直接让等待的线程取走任务
- SynchronousQueue
- handler
- AbortPolicy 没有触发条件