主要介绍线程池相关知识，关于线程池，首先我们思考下为什么要用线程池。如果单纯的使用线程，线程的创建和销毁都是自己来完成，如果并发请求过多，可能造成资源耗尽。线程池可以对线程进行统一分配，调优和监控。本篇文章为《图灵学院》课程笔记
- 降低资源消耗（线程无限制地创建，然后使用完毕后销毁）
- 提高响应速度（无须创建线程）
- 提高线程的可管理性
java是如何实现和管理线程池的，jdk5开始把工作单元和任务执行分离，工作单元包括callable、runnable，而执行机制由Executor提供，Executor的实现还提供了对线程生命周期的管理
相关接口
接口介绍
-
java.util.concurrent.Executor （执行器，执行方法）
-
java.util.concurrent.ExecutorService （执行服务） 包含服务的生命周期
-
java.util.concurrent.ScheduledExecutorService （调度相关的服务）
核心接口实现
- java.util.concurrent.ThreadPoolExecutor （普通的的线程池实现类）
- java.util.concurrent.ScheduledThreadPoolExecutor （调度的核心实现类）
| 名称 | 方法 | 说明 | 类型 |
|---|---|---|---|
| java.util.concurrent. Executor |
execute | 执行接口 | 接口 |
| java.util.concurrent. ExecutorService |
submit(java.util.concurrent.Callable) | 提交接口 | 接口 |
| java.util.concurrent. AbstractExecutorService |
submit(Callable |
把执行和提交接口 进行合并区别:有 返回值和无返回值 |
抽象类 |
| java.util.concurrent. ThreadPoolExecutor |
execute(Runnable command) | 调 用 runwork 方 法 getTask（从队列 拿数据） |
实现类 |
| java.util.concurrent. ScheduledExecutorService |
scheduleAtFixedRate、scheduleWithFixedDelay | 定义方法 | 接口 |
| java.util.concurrent. ScheduledThreadPoolExecutor |
delayedExecute | 具体实现 add>task>addWo rk |
实现类 |
内部类分为两种
- policy 策略
- worker 工作
内部工作原理（构造方法赋值）
- corePool：核心线程池大小
- maximumPool：最大线程池大小
- BlockingQueue：任务工作队列
- keepAliveTime：线程活跃时间，如果线程数量大于核心线程数量，多余线程空闲时间超时候被销毁
- RejectedExecutionHandler：当ThreadPoolExecutor关闭或最大线程池已经满了，executor将调用的handler
- ThreadFactory：使用ThreadFactory创建线程，默认使用defaultThreadFactory
线程池的运行思路
- 如果当前线程池中的线程数目小于corePoolSize，则每来一个任务，就会创建一个线程去执行这个任务；
- 如果当前线程池中的线程数目>=corePoolSize，则每来一个任务，会尝试将其添加到任务缓存队列当中，若添加成功，则该任务会等待空闲线程将其取出去执行；若添加失败（一般来说是任务缓存队列已满），则会尝试创建新的线程去执行这个任务；
- 如果当前线程池中的线程数目达到maximumPoolSize，则会采取任务拒绝策略进行处理；
- 如果线程池中的线程数量大于 corePoolSize时，如果某线程空闲时间超过keepAliveTime，线程将被终止，直至线程池中的线程数目不大于corePoolSize；如果允许为核心池中的线程设置存活时间，那么核心池中的线程空闲时间超过keepAliveTime，线程也会被终止
拒接策略
ThreadPoolExecutor.AbortPolicy:丢弃任务并抛出RejectedExecutionException异常(默认)
ThreadPoolExecutor.DiscardPolicy：也是丢弃任务，但是不抛出异常
ThreadPoolExecutor.DiscardOldestPolicy：丢弃队列最前面的任务，然后重新尝试执行任务（重复此过程）
ThreadPoolExecutor.CallerRunsPolicy：由调用线程处理该任务
源码解析
线程池的执行原理
初始化构造器
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();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.acc = System.getSecurityManager() == null ?
null :
AccessController.getContext();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}
ThreadPoolExecutor#execute
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
//判断是否小于核心数量，是直接新增work成功后直接退出
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
// 增加失败后继续获取标记
c = ctl.get();
}
//判断是运行状态并且扔到workQueue里成功后
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
//再次check判断运行状态如果是非运行状态就移除出去&reject掉
if (! isRunning(recheck) && remove(command))
reject(command);
//否则发现可能运行线程数是0那么增加一个null的worker
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
//直接增加worker如果不成功直接reject
else if (!addWorker(command, false))
reject(command);
}
ThreadPoolExecutor#addWorker
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()))
// 两种情况
//1.如果非运行状态
//2.不是这种情况（停止状态并且是null对象并且workQueue不等于null）
return false;
for (;;) {
int wc = workerCountOf(c);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;// 判断是否饱和容量了
if (compareAndIncrementWorkerCount(c)) //增加一个work数量 然后跳出去
break retry;
c = ctl.get(); // Re-read ctl 增加work失败后继续递归
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 {
//增加一个worker
w = new Worker(firstTask);
final Thread t = w.thread;
//判断是否 为null
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. 锁定后并重新检查下 是否存在线程工厂的失败或者锁定前的关闭
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); //增加work
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) { //本次要是新增加work成功就调用start运行
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
ThreadPoolExecutor#runWorker
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();//1.取到当前线程
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
while (task != null || (task = getTask()) != null) { //获取任务 看看是否能拿到
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();// 确保线程是能中断的
try {
beforeExecute(wt, task); //开始任务前的钩子
Throwable thrown = null;
try {
task.run();//执行任务
} 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); //任务后的钩子
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}
ThreadPoolExecutor#processWorkerExit
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); //移除work
} finally {
mainLock.unlock();
}
tryTerminate();
int c = ctl.get();
if (runStateLessThan(c, STOP)) { //判断是否还有任务
if (!completedAbruptly) {
int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
if (min == 0 && ! workQueue.isEmpty())
min = 1;
if (workerCountOf(c) >= min)
return; // replacement not needed
}
addWorker(null, false);
}
}
线程池调度原理
调度核心构造器
public ScheduledThreadPoolExecutor(int corePoolSize) {
super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
new DelayedWorkQueue());
}
ScheduledThreadPoolExecutor#delayedExecute
private void delayedExecute(RunnableScheduledFuture<?> task) {
if (isShutdown())
reject(task);
else {
super.getQueue().add(task);//增加任务
if (isShutdown() &&
!canRunInCurrentRunState(task.isPeriodic()) &&
remove(task))
task.cancel(false);
else
ensurePrestart();
}
}
通过DelayedWorkQueue 延迟队列实现 offer获取对象的延迟
ScheduledThreadPoolExecutor.DelayedWorkQueue#offer
public boolean offer(Runnable x) {
if (x == null)
throw new NullPointerException();
RunnableScheduledFuture<?> e = (RunnableScheduledFuture<?>)x; //当前对象
final ReentrantLock lock = this.lock;
lock.lock();
try {
int i = size;
if (i >= queue.length) //扩容
grow();
size = i + 1;
if (i == 0) {
queue[0] = e;
setIndex(e, 0); //第一个直接设置索引和下标0
} else {
siftUp(i, e); //筛选到上边
}
if (queue[0] == e) {
leader = null;
available.signal(); //唤醒所有的被挤压的wait线程
}
} finally {
lock.unlock();
}
return true;
}
ScheduledThreadPoolExecutor.DelayedWorkQueue#siftUp
private void siftUp(int k, RunnableScheduledFuture<?> key) {
while (k > 0) {
int parent = (k - 1) >>> 1;
RunnableScheduledFuture<?> e = queue[parent];
if (key.compareTo(e) >= 0)
break;
queue[k] = e;
setIndex(e, k);
k = parent;
}
queue[k] = key;
setIndex(key, k);
}
ScheduledThreadPoolExecutor.ScheduledFutureTask#compareTo
public int compareTo(Delayed other) {
if (other == this) // compare zero if same object
return 0;
if (other instanceof ScheduledFutureTask) {
ScheduledFutureTask<?> x = (ScheduledFutureTask<?>)other;
long diff = time - x.time; //判断time
if (diff < 0)
return -1;
else if (diff > 0)
return 1;
else if (sequenceNumber < x.sequenceNumber)
return -1;
else
return 1;
}
long diff = getDelay(NANOSECONDS) - other.getDelay(NANOSECONDS);
return (diff < 0) ? -1 : (diff > 0) ? 1 : 0;
}
ThreadPoolExecutor#ensurePrestart
确保有work执行
void ensurePrestart() {
int wc = workerCountOf(ctl.get());
if (wc < corePoolSize)
addWorker(null, true);
else if (wc == 0)
addWorker(null, false);
}
ScheduledThreadPoolExecutor.DelayedWorkQueue#take
work运行的时候调用queue的take方法
public RunnableScheduledFuture<?> take() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
for (;;) {
RunnableScheduledFuture<?> first = queue[0];//获取第一个对象
if (first == null)
available.await();
else {
long delay = first.getDelay(NANOSECONDS);//延迟时间
if (delay <= 0)//到时间了
return finishPoll(first);
first = null; // don't retain ref while waiting
if (leader != null)
available.await();//因为没有执行线程初始化，所以等等什么时候有了自己被他人唤醒
else {
Thread thisThread = Thread.currentThread();
leader = thisThread;
try {
available.awaitNanos(delay); //各种condition的awaitNanos
} finally {
if (leader == thisThread)
leader = null;
}
}
}
}
} finally {
if (leader == null && queue[0] != null)
available.signal();
lock.unlock();
}
}
ScheduledThreadPoolExecutor.DelayedWorkQueue#finishPoll
private RunnableScheduledFuture<?> finishPoll(RunnableScheduledFuture<?> f) {
int s = --size;
RunnableScheduledFuture<?> x = queue[s]; //重排序队列
queue[s] = null;
if (s != 0)
siftDown(0, x);
setIndex(f, -1);
return f
}
ScheduledThreadPoolExecutor.ScheduledFutureTask#run
public void run() {
boolean periodic = isPeriodic();
if (!canRunInCurrentRunState(periodic))
cancel(false);
else if (!periodic)
ScheduledFutureTask.super.run();
else if (ScheduledFutureTask.super.runAndReset()) {//有period的要执行成功设置下次执行时间和增加额外任务
setNextRunTime();
reExecutePeriodic(outerTask);
}
}
异步结果源码分析
怎么拿到的异步任务结果？
FutureTask#awaitDone
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
boolean queued = false;
for (;;) {
if (Thread.interrupted()) { //check线程中断
removeWaiter(q);
throw new InterruptedException();
}
int s = state;
if (s > COMPLETING) { //判断是否完成
if (q != null)
q.thread = null;
return s;
}
else if (s == COMPLETING) // cannot time out yet
Thread.yield();
else if (q == null)
q = new WaitNode(); //生成一个waint对象
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);//链表的对象下一个置成当前的waitNode
else if (timed) {
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
LockSupport.parkNanos(this, nanos); //等待时间阻塞
}
else
LockSupport.park(this); //一直阻塞
}
}
什么时候回填的结果
FutureTask#run
public void run() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread())) //如果状态不是new 或者 runner状态置不成功直接退出
return;
try {
Callable<V> c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call();//运行ok 的时候返回result
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)//正常成功set result对象
set(result);
}
} finally {
// runner must be non-null until state is settled to
// prevent concurrent calls to run()
runner = null;
// state must be re-read after nulling runner to prevent
// leaked interrupts
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
FutureTask#cancel
public boolean cancel(boolean mayInterruptIfRunning) {
if (!(state == NEW &&
UNSAFE.compareAndSwapInt(this, stateOffset, NEW,
mayInterruptIfRunning ? INTERRUPTING : CANCELLED))) //CAS 置成stateOffset 的中断或者取消
return false;
try { // in case call to interrupt throws exception
if (mayInterruptIfRunning) { //如果线程运行中，可能中断
try {
Thread t = runner;
if (t != null)
t.interrupt();
} finally { // final state
UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
}
}
} finally {
finishCompletion();
}
return true;
}
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