Java教程
线程池的深入理解
本文主要是介绍线程池的深入理解，对大家解决编程问题具有一定的参考价值，需要的程序猿们随着小编来一起学习吧！
简单研究下线程池的执行原理。以及execute 和 submit 方法的区别。
1. execute 方法接收的是一个Runnable 参数，返回值是void 类型，也就是不接收结果， 方法签名如下：
java.util.concurrent.Executor#execute
void execute(Runnable command);
2. submit 方法三个重载的方法，如下：
对应的方法如下：‘
(1) java.util.concurrent.ExecutorService#submit(java.lang.Runnable)
Future<?> submit(Runnable task);
(2) java.util.concurrent.ExecutorService#submit(java.lang.Runnable, T)
<T> Future<T> submit(Runnable task, T result);
(3) java.util.concurrent.ExecutorService#submit(java.util.concurrent.Callable<T>)
<T> Future<T> submit(Callable<T> task);
1.java.util.concurrent.ThreadPoolExecutor#execute理解
测试代码
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(5, 5,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>(3));
// execute 方法无返回值，相当于执行不带返回结果的任务
threadPoolExecutor.execute(() -> {
log.info("--" + Thread.currentThread().getName() + "--A");
try {
Thread.sleep(2 * 1000);
} catch (InterruptedException e) {
}
log.info("--" + Thread.currentThread().getName() + "--B");
});
查看源代码如下：
(1) 接口是Executor 的方法
void execute(Runnable command);
(2) 实现类java.util.concurrent.ThreadPoolExecutor#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();
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);
}
源码跟踪：
(1) addWorker(command, true) 增加worker
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);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get(); // Re-read ctl
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 {
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());
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) {
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
worker 是一个内部类java.util.concurrent.ThreadPoolExecutor.Worker，其包含的重要属性和方法如下：
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;
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
public void run() {
runWorker(this);
}
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
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);
}
}
...
1》new Worker 的时候会创建线程，线程传入的Runnable 对象是Worker 自身，同时Worker
2》t.start(); 启动的时候会调用java.util.concurrent.ThreadPoolExecutor.Worker#run 方法
3》然后调用到java.util.concurrent.ThreadPoolExecutor#runWorker， 方法内部调用task.run(); 相当于同步的调用 传给线程池的Runnable 任务的run 方法。
4》当不是第一个任务的时候走的是java.util.concurrent.ThreadPoolExecutor#getTask， 也就是从任务队列中阻塞拿去任务
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?
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
2. submit理解
1. java.util.concurrent.AbstractExecutorService#submit(java.lang.Runnable) 理解
查看源码如下：java.util.concurrent.AbstractExecutorService#submit(java.lang.Runnable)
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}
可以理解大体的思路是对，传给线程池的Runnable 进行了下包装，java.util.concurrent.AbstractExecutorService#newTaskFor(java.lang.Runnable, T) 如下：
protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
return new FutureTask<T>(runnable, value);
}
(1) java.util.concurrent.RunnableFuture 继承了接口Runnable, Future<V>
public interface RunnableFuture<V> extends Runnable, Future<V> {
void run();
}
java.util.concurrent.Future 接口如下：
public interface Future<V> {
/**
* Attempts to cancel execution of this task. This attempt will
* fail if the task has already completed, has already been cancelled,
* or could not be cancelled for some other reason. If successful,
* and this task has not started when {@code cancel} is called,
* this task should never run. If the task has already started,
* then the {@code mayInterruptIfRunning} parameter determines
* whether the thread executing this task should be interrupted in
* an attempt to stop the task.
*
* <p>After this method returns, subsequent calls to {@link #isDone} will
* always return {@code true}. Subsequent calls to {@link #isCancelled}
* will always return {@code true} if this method returned {@code true}.
*
* @param mayInterruptIfRunning {@code true} if the thread executing this
* task should be interrupted; otherwise, in-progress tasks are allowed
* to complete
* @return {@code false} if the task could not be cancelled,
* typically because it has already completed normally;
* {@code true} otherwise
*/
boolean cancel(boolean mayInterruptIfRunning);
/**
* Returns {@code true} if this task was cancelled before it completed
* normally.
*
* @return {@code true} if this task was cancelled before it completed
*/
boolean isCancelled();
/**
* Returns {@code true} if this task completed.
*
* Completion may be due to normal termination, an exception, or
* cancellation -- in all of these cases, this method will return
* {@code true}.
*
* @return {@code true} if this task completed
*/
boolean isDone();
/**
* Waits if necessary for the computation to complete, and then
* retrieves its result.
*
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread was interrupted
* while waiting
*/
V get() throws InterruptedException, ExecutionException;
/**
* Waits if necessary for at most the given time for the computation
* to complete, and then retrieves its result, if available.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread was interrupted
* while waiting
* @throws TimeoutException if the wait timed out
*/
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
java.util.concurrent.FutureTask 的重要属性和方法如下：
public class FutureTask<V> implements RunnableFuture<V> {
private volatile int state;
private static final int NEW = 0;
private static final int COMPLETING = 1;
private static final int NORMAL = 2;
private static final int EXCEPTIONAL = 3;
private static final int CANCELLED = 4;
private static final int INTERRUPTING = 5;
private static final int INTERRUPTED = 6;
/** The underlying callable; nulled out after running */
private Callable<V> callable;
/** The result to return or exception to throw from get() */
private Object outcome; // non-volatile, protected by state reads/writes
/** The thread running the callable; CASed during run() */
private volatile Thread runner;
/** Treiber stack of waiting threads */
private volatile WaitNode waiters;
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
public FutureTask(Runnable runnable, V result) {
this.callable = Executors.callable(runnable, result);
this.state = NEW; // ensure visibility of callable
}
public boolean isCancelled() {
return state >= CANCELLED;
}
public boolean isDone() {
return state != NEW;
}
public boolean cancel(boolean mayInterruptIfRunning) {
if (!(state == NEW &&
UNSAFE.compareAndSwapInt(this, stateOffset, NEW,
mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
return false;
try { // in case call to interrupt throws exception
if (mayInterruptIfRunning) {
try {
Thread t = runner;
if (t != null)
t.interrupt();
} finally { // final state
UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
}
}
} finally {
finishCompletion();
}
return true;
}
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}
public V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
if (unit == null)
throw new NullPointerException();
int s = state;
if (s <= COMPLETING &&
(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
throw new TimeoutException();
return report(s);
}
protected void done() { }
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}
...
1》java.util.concurrent.Executors#callable(java.lang.Runnable, T) 采用适配器模式，将Runnable 适配为Callable
public static <T> Callable<T> callable(Runnable task, T result) {
if (task == null)
throw new NullPointerException();
return new RunnableAdapter<T>(task, result);
}
java.util.concurrent.Executors.RunnableAdapter 如下：
static final class RunnableAdapter<T> implements Callable<T> {
final Runnable task;
final T result;
RunnableAdapter(Runnable task, T result) {
this.task = task;
this.result = result;
}
public T call() {
task.run();
return result;
}
}
可以看到是返回固定的结果， 也就是接受Runnable 的时候，返回的结果是固定的结果
2》然后同上面execute 方法一样，执行execute(ftask); 方法，这时候线程池跑的是FutureTask 对象
3》return ftask; 返回Future 对象，使得可以外部拿该对象可以获取执行结果。
4》线程跑任务会调用java.util.concurrent.FutureTask#run， 方法如下：
public void run() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
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);
}
}
2. java.util.concurrent.AbstractExecutorService#submit(java.lang.Runnable, T) 理解
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;
}
根据上面的理解，是返回固定的结果result， 只不过java.util.concurrent.AbstractExecutorService#submit(java.lang.Runnable) 的返回结果是null。
3. java.util.concurrent.AbstractExecutorService#submit(java.util.concurrent.Callable<T>) 理解
(1) 测试代码
Future<String> submit1 = threadPoolExecutor.submit(new Callable<String>() {
@Override
public String call() throws Exception {
log.info("--" + Thread.currentThread().getName() + "--E");
try {
Thread.sleep(2 * 1000);
} catch (InterruptedException e) {
}
log.info("--" + Thread.currentThread().getName() + "--F");
return "123456";
}
});
(2) 源码：java.util.concurrent.AbstractExecutorService#submit(java.util.concurrent.Callable<T>)
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
1》java.util.concurrent.AbstractExecutorService#newTaskFor(java.util.concurrent.Callable<T>) 创建 FutureTask
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
return new FutureTask<T>(callable);
}
接着调用：java.util.concurrent.FutureTask#FutureTask(java.util.concurrent.Callable<V>)
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
2》然后调用java.util.concurrent.ThreadPoolExecutor#execute 跑任务，只不过Runnable 对象是java.util.concurrent.FutureTask， 内部的run 方法是调用成员属性Callable 的call 方法，方法执行完之后记录其返回结果， 所以FutureTask 可以拿到返回的结果。
补充：Future 两个重要的方法
(1) java.util.concurrent.FutureTask#isDone 判断是否执行完成
public boolean isDone() {
return state != NEW;
}
(2) 获取结果：
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}
java.util.concurrent.FutureTask#awaitDone 阻塞获取结果：
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()) {
removeWaiter(q);
throw new InterruptedException();
}
int s = state;
if (s > COMPLETING) {
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();
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);
else if (timed) {
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
LockSupport.parkNanos(this, nanos);
}
else
LockSupport.park(this);
}
}
可以看到这里使用了 java.util.concurrent.locks.LockSupport#park(java.lang.Object) 进行阻塞线程，下面使用的是UNSAFE 类的相关API。
补充：关于LockSupport 的使用
参考：https://www.jianshu.com/p/8e6b1942ae39
测试代码：
package org.example;
import java.util.concurrent.locks.LockSupport;
public class PlainTest2 {
public static void main(String[] args) {
Thread t = new Thread(new Runnable() {
@Override
public void run() {
System.out.println("111222");
LockSupport.park();
System.out.println("333444");
//LockSupport.park(obj);
}
});
t.start();
try {
Thread.sleep(10000);
} catch (InterruptedException e) {
}
System.out.println("555555");
LockSupport.unpark(t);
System.out.println("555555");
}
}
查看LockSupport 源码：
public static void park() {
UNSAFE.park(false, 0L);
}
public static void unpark(Thread thread) {
if (thread != null)
UNSAFE.unpark(thread);
}
strace linux环境下面监测如下：
1》park
clock_gettime(CLOCK_MONOTONIC, {tv_sec=273, tv_nsec=622979062}) = 0
futex(0xf6797ab8, FUTEX_WAIT_PRIVATE, 1, NULL) = 0
futex(0xf6797a9c, FUTEX_WAIT_PRIVATE, 2, NULL) = 0
futex(0xf6797a9c, FUTEX_WAKE_PRIVATE, 1) = 0
clock_gettime(CLOCK_MONOTONIC, {tv_sec=373, tv_nsec=621209093}) = 0
clock_gettime(CLOCK_MONOTONIC, {tv_sec=373, tv_nsec=621237803}) = 0
futex(0xf6797344, FUTEX_WAIT_PRIVATE, 1, {tv_sec=0, tv_nsec=971290}) = 0
futex(0xf6797328, FUTEX_WAIT_PRIVATE, 2, NULL) = 0
futex(0xf6797328, FUTEX_WAKE_PRIVATE, 1) = 0
2》unpark
write(1, "555555", 6) = 6
write(1, "\n", 1) = 1
clock_gettime(CLOCK_MONOTONIC, {tv_sec=373, tv_nsec=620209800}) = 0
futex(0xf6797ab8, FUTEX_WAKE_OP_PRIVATE, 1, 1, 0xf6797ab4, FUTEX_OP_SET<<28|0<<12|FUTEX_OP_CMP_GT<<24|0x1) = 1
futex(0xf6797a9c, FUTEX_WAKE_PRIVATE, 1) = 1
write(1, "555555", 6) = 6
可以看到到内核调用的也是futex 指令，和synchronized 一样。
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