由于工作需要，需要进行并行查询数据库，返回结果后进行计算，也就是说各个线程要全部运行完毕，才能进行下一步的计算，这时候要用到CountDownLatch。
先上代码，实现的比较简单，但是这不是重点，哈哈哈，主要是分析Executor相关的原理。
线程池相关参数及含义等可参考下面这篇文章，内部也有部分源码分析，但是个人认为不够详尽
https://blog.csdn.net/baidu_37107022/article/details/77415936
package com.gome.mars.utils;
import java.util.concurrent.*;
/**
* @ClassName
* @Description TODO
* @Author oo
* @Date 2018/12/10 18:00
* @Version 1.0
**/
public class SimpleParallelTaskExecutor {
private CountDownLatch countDownLatch;
// 此处实现了固定大小的线程池，可根据需要进行其他实现，每次不再新建线程池实例
private static ExecutorService executor=Executors.newFixedThreadPool(20);
//构造方法参数为并行线程的数量，并且每次new CountDownLatch对象，因为不可重复使用
public SimpleParallelTaskExecutor(Integer nThreads) {
this.countDownLatch = new CountDownLatch(nThreads);
}
//调用此方法向线程池中添加任务，此处对Callable进行了简单包装，为了执行完任务调用countDownLatch.countDown();
public <V> Future<V> addTask(Callable<V> task) throws Exception {
return executor.submit(new WrapperThread<V>(task,countDownLatch));
}
//可设置超时时间，检查任务是否运行完毕
public boolean checkDone(long milliseconds) throws InterruptedException {
return countDownLatch.await(milliseconds, TimeUnit.MILLISECONDS);
}
//Callable包装类，为了执行完任务调用countDownLatch.countDown();
public class WrapperThread<V> implements Callable<V> {
private Callable<? extends V> callable;
private CountDownLatch countDownLatch;
public WrapperThread(Callable callable, CountDownLatch countDownLatch) {
this.callable = callable;
this.countDownLatch = countDownLatch;
}
@Override
public V call() throws Exception {
//此处直接调用callable.call();和直接调用thread.run()类似，没有起新的线程此处和加入的任务内部是同一个线程。
V call = callable.call();
this.countDownLatch.countDown();
return call;
}
}
public static void main(String[] args) throws Exception {
SimpleParallelTaskExecutor simpleParallelTaskExecutor = new SimpleParallelTaskExecutor(2);
Future<Integer> integerFuture = simpleParallelTaskExecutor.addTask(() -> {
//此处模拟执行数据查询等任务
Thread.sleep(2000);
return 1;
});
Future<Integer> integerFuture1 = simpleParallelTaskExecutor.addTask(() -> {
Thread.sleep(1000);
return 2;
});
simpleParallelTaskExecutor.checkDone(3000);
Integer integer = integerFuture.get();
Integer integer1 = integerFuture1.get();
System.out.println(integer);
System.out.println(integer1);
}
}
Executor框架
我们在addTask方法处打断点，进入submit方法，可以看出我们进入的是AbstractExecutorService类的submit方法，此方法接受一个callable对象，返回Future<T>对象，我们可以在Future中获取执行结果。
/**
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
/**
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}
/**
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public <T> Future<T> submit(Runnable task, T result) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task, result);
execute(ftask);
return ftask;
}
submit方法有几个重载方法，都是通过NewTaskFor方法，将任务包装成一个RunnableFuture对象，只不过Runnable没有返回结果，结果类型为null。
下面我们看看NewTaskFor都做了些什么事情。
/**
* Returns a {@code RunnableFuture} for the given runnable and default
* value.
*
* @param runnable the runnable task being wrapped
* @param value the default value for the returned future
* @param <T> the type of the given value
* @return a {@code RunnableFuture} which, when run, will run the
* underlying runnable and which, as a {@code Future}, will yield
* the given value as its result and provide for cancellation of
* the underlying task
* @since 1.6
*/
protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
return new FutureTask<T>(runnable, value);
}
/**
* Returns a {@code RunnableFuture} for the given callable task.
*
* @param callable the callable task being wrapped
* @param <T> the type of the callable's result
* @return a {@code RunnableFuture} which, when run, will call the
* underlying callable and which, as a {@code Future}, will yield
* the callable's result as its result and provide for
* cancellation of the underlying task
* @since 1.6
*/
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
return new FutureTask<T>(callable);
}
可以看出，直接创建了FutureTask对象，并且返回。所以，加入线程池的任务都被包装成FutureTask对象，没有返回值的返回值为空。
下面主要看execute(ftask);方法
ThreadPoolExecutor中的execute方法。
用一个32位数的高3位表示线程池状态，低29位表示正在运行的线程数量
public class ThreadPoolExecutor extends AbstractExecutorService {
//初始状态为Running状态且运行线程数为0，所以是-1和0按位取或
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
//32-3为29
private static final int COUNT_BITS = Integer.SIZE - 3;
//1左移29位再减1，低29位全为1，高位位0；
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
// runState is stored in the high-order bits
private static final int RUNNING = -1 << COUNT_BITS;
private static final int SHUTDOWN = 0 << COUNT_BITS;
private static final int STOP = 1 << COUNT_BITS;
private static final int TIDYING = 2 << COUNT_BITS;
private static final int TERMINATED = 3 << COUNT_BITS;
// Packing and unpacking ctl
//低29位取反都为0，高三位都为1，再和c进行按位与，只留下高三位，从而获取线程池状态
private static int runStateOf(int c) { return c & ~CAPACITY; }
//获取工作线程数量，与上面类似，取得低29位。
private static int workerCountOf(int c) { return c & CAPACITY; }
private static int ctlOf(int rs, int wc) { return rs | wc; }
}
在程序运行中，反复使用了这几个方法，用来获取工作线程数或线程池状态
/**
* Executes the given task sometime in the future. The task
* may execute in a new thread or in an existing pooled thread.
*
* If the task cannot be submitted for execution, either because this
* executor has been shutdown or because its capacity has been reached,
* the task is handled by the current {@code RejectedExecutionHandler}.
*
* @param command the task to execute
* @throws RejectedExecutionException at discretion of
* {@code RejectedExecutionHandler}, if the task
* cannot be accepted for execution
* @throws NullPointerException if {@code command} is null
*/
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.
*/
//获取c的值，判断工作线程数是否小于设定的核心线程数
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) {
//如果小于核心线程数，直接新建线程，新建成功则返回
if (addWorker(command, true))
return;
c = ctl.get();
}
//如果上变判断不成立，那么工作线程数量大于等于核心线程数，
//或者新建线程没有成功，那么如果线程池接受新任务，则往队列里添加一个任务
//如果线程池中工作线程数已经达到了核心线程数，并且没有设置空闲销毁时间
//新任务到来就只是向阻塞队列里增加一条新任务，线程池是如何拿到这个任务并执行的呢？
if (isRunning(c) && workQueue.offer(command)) {
//再次检查，防止并发状态下，线程池状态等有变化
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);
}
重点方法是addWorker方法下面进一步分析
/**
* 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
*/
//第二个参数表示是否是核心线程，比较工作线程数目时，分别和corePoolSize 或者maximumPoolSize进行比较
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
//此处为何还要有第二个判断？
//我们分析一下当rs==SHUTDOWN 时，什么情况会返回false
//1.线程池shutdown， 此时firstTask为null 并且workQueue为空时
//2.线程池shutdown，firstTask不为null这时 workQueue状态已经没有用了
//SHUTDOWN状态虽然不接受新任务，但是队列里的任务会执行完，
//也就是说当线程池为SHUTDOWN时，为了执行完队列中的任务，
//会不断添加firstTask为null的任务，firstTask为null代表要取队列中的任务
//第一种情况表示队列中的任务已经清空了，无需再循环了，线程池可能将要进入stop状态了
//第二种情况表示新任务到来，线程池已经不再接受了，所以返回false
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
//内层循环主要工作就是cas为增加一个工作线程
for (;;) {
int wc = workerCountOf(c);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
//增加成功就退出外层循环
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get(); // Re-read ctl
//线程池状态状态有变化就继续执行外层循环
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
}
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
//包含一个Thread对象，传入的是Worker对象本身，后边有详细解释。
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
int rs = runStateOf(ctl.get());
//正常运行状态或者队列中还有未执行的任务
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();
}
if (workerAdded) {
//如果线程添加成功，就启动线程，执行任务。实际是执行runWorker方法。
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
Worker实现了Runnable接口，继承AbstractQueuedSynchronizer类，所以本身就是一个线程类，有自己的run方法。
/**
* Class Worker mainly maintains interrupt control state for
* threads running tasks, along with other minor bookkeeping.
* This class opportunistically extends AbstractQueuedSynchronizer
* to simplify acquiring and releasing a lock surrounding each
* task execution. This protects against interrupts that are
* intended to wake up a worker thread waiting for a task from
* instead interrupting a task being run. We implement a simple
* non-reentrant mutual exclusion lock rather than use
* ReentrantLock because we do not want worker tasks to be able to
* reacquire the lock when they invoke pool control methods like
* setCorePoolSize. Additionally, to suppress interrupts until
* the thread actually starts running tasks, we initialize lock
* state to a negative value, and clear it upon start (in
* runWorker).
*/
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable
{
/**
* This class will never be serialized, but we provide a
* serialVersionUID to suppress a javac warning.
*/
private static final long serialVersionUID = 6138294804551838833L;
/** Thread this worker is running in. Null if factory fails. */
final Thread thread;
/** Initial task to run. Possibly null. */
Runnable firstTask;
/** Per-thread task counter */
volatile long completedTasks;
/**
* Creates with given first task and thread from ThreadFactory.
* @param firstTask the first task (null if none)
*/
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
//新建Thread对象，传入自身
this.thread = getThreadFactory().newThread(this);
}
/** Delegates main run loop to outer runWorker */
//当调用Thread.start()时，新线程启动，调用runWorker方法，传入自身
public void run() {
runWorker(this);
}
// Lock methods
//
// The value 0 represents the unlocked state.
// The value 1 represents the locked state.
protected boolean isHeldExclusively() {
return getState() != 0;
}
protected boolean tryAcquire(int unused) {
if (compareAndSetState(0, 1)) {
setExclusiveOwnerThread(Thread.currentThread());
return true;
}
return false;
}
protected boolean tryRelease(int unused) {
setExclusiveOwnerThread(null);
setState(0);
return true;
}
public void lock() { acquire(1); }
public boolean tryLock() { return tryAcquire(1); }
public void unlock() { release(1); }
public boolean isLocked() { return isHeldExclusively(); }
void interruptIfStarted() {
Thread t;
if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
try {
t.interrupt();
} catch (SecurityException ignore) {
}
}
}
}
runWorker方法是真正执行用户传进来的任务的地方，并且可以重写beforeExecute以及afterExecute方法，再任务执行前后加入自定义操作。
/**
* Main worker run loop. Repeatedly gets tasks from queue and
* executes them, while coping with a number of issues:
*
* 1. We may start out with an initial task, in which case we
* don't need to get the first one. Otherwise, as long as pool is
* running, we get tasks from getTask. If it returns null then the
* worker exits due to changed pool state or configuration
* parameters. Other exits result from exception throws in
* external code, in which case completedAbruptly holds, which
* usually leads processWorkerExit to replace this thread.
*
* 2. Before running any task, the lock is acquired to prevent
* other pool interrupts while the task is executing, and then we
* ensure that unless pool is stopping, this thread does not have
* its interrupt set.
*
* 3. Each task run is preceded by a call to beforeExecute, which
* might throw an exception, in which case we cause thread to die
* (breaking loop with completedAbruptly true) without processing
* the task.
*
* 4. Assuming beforeExecute completes normally, we run the task,
* gathering any of its thrown exceptions to send to afterExecute.
* We separately handle RuntimeException, Error (both of which the
* specs guarantee that we trap) and arbitrary Throwables.
* Because we cannot rethrow Throwables within Runnable.run, we
* wrap them within Errors on the way out (to the thread's
* UncaughtExceptionHandler). Any thrown exception also
* conservatively causes thread to die.
*
* 5. After task.run completes, we call afterExecute, which may
* also throw an exception, which will also cause thread to
* die. According to JLS Sec 14.20, this exception is the one that
* will be in effect even if task.run throws.
*
* The net effect of the exception mechanics is that afterExecute
* and the thread's UncaughtExceptionHandler have as accurate
* information as we can provide about any problems encountered by
* user code.
*
* @param w the worker
*/
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
//如果task!=null 就执行当前任务（工作线程数小于核心现场数），
//如果为null（队列中有任务）就在队列中获取一个任务，
//此处可以看出，如果队列中有任务，会一直while循环，直到队列为空，
//队列为空时，由于是阻塞队列，线程将阻塞在这里，直到又有任务添加进队列
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 {
//空方法
beforeExecute(wt, task);
Throwable thrown = null;
try {
//真正执行我们的任务
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 {
//空方法
afterExecute(task, thrown);
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
//此方法用于处理当前工作线程退出的相关事宜
processWorkerExit(w, completedAbruptly);
}
}
当此工作线程退出以后，相关清理及记录工作，当程序抛出异常，或队列不为空而没有工作线程时或工作线程数少于核心线程数时，会继续addWorker(null, false);替换当前工作线程。
/**
* Performs cleanup and bookkeeping for a dying worker. Called
* only from worker threads. Unless completedAbruptly is set,
* assumes that workerCount has already been adjusted to account
* for exit. This method removes thread from worker set, and
* possibly terminates the pool or replaces the worker if either
* it exited due to user task exception or if fewer than
* corePoolSize workers are running or queue is non-empty but
* there are no workers.
*
* @param w the worker
* @param completedAbruptly if the worker died due to user exception
*/
private void processWorkerExit(Worker w, boolean completedAbruptly) {
if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
decrementWorkerCount();
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
completedTaskCount += w.completedTasks;
workers.remove(w);
} finally {
mainLock.unlock();
}
//一个工作线程退出后会尝试终止线程池，通过判断当前线程池的状态，如果终止成功则不会进入下边的if判断，否则进入if判断
tryTerminate();
int c = ctl.get();
//判断线程池是否是running或shutdown状态，再次判断completedAbruptly，
//这个变量表示是否被打断，正常执行完毕一般为false，如果满足是就继续判断是否继续addWorker
if (runStateLessThan(c, STOP)) {
if (!completedAbruptly) {
int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
if (min == 0 && ! workQueue.isEmpty())
min = 1;
//1.如果设置了allowCoreThreadTimeOut 参数且队列为非空，则工作线程数为0的时候才addWorker
//如果设置了allowCoreThreadTimeOut 且队列为空，则直接返回，不addWorker
//如果没有设置allowCoreThreadTimeOut ，只要工作线程数小于核心线程数，都addWorker
if (workerCountOf(c) >= min)
return; // replacement not needed
}
//关键这在这，加入一条firstTask为null的非核心线程任务，
addWorker(null, false);
}
}
总结
线程池执行过程：
- submit(Callable<T> task)方法后，将task包装成一个FutureTask对象；
- 执行execute(ftask)方法
if(工作线程数<核心线程数){
addWorker(Runnable firstTask, boolean core）
添加成功 return；
}
核心线程数已经达到最大
if(线程池是running状态){
向队列中添加一个任务
workQueue.offer(command)
} - addWorker(Runnable firstTask, boolean core)
自旋尝试改变workerCount数量
compareAndIncrementWorkerCount(c)
成功
new Worker(firstTask);
并启动线程 - runWorker(Worker w)
while(如果当前Worker中task！=null 执行此任务
否则从队列中task = getTask())
调用task.run();
队列为空时getTask()方法中根据参数设定判断何时返回null何时阻塞 - 当一个工作线程退出后执行processWorkerExit(w, completedAbruptly);方法中还会尝试终止线程池，如果线程池终止成功，则直接return
否则判断是否继续addWorker(null, false)替换当前线程
over
有不正确的地方欢迎指正！
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