本文基于最新的 OpenJDK 代码,预计发行版本为 19 。
Java 的多线程机制本质上能够完成两件事情,异步计算和并发。并发问题通过解决线程安全的一系列 API 来解决;而异步计算,常见的使用是 Runnable 和 Callable 配合线程使用。
FutureTask 是基于 Runnable 实现的一个可取消的异步调用 API 。
基本使用
- Future 代表了异步计算的结果,通过 ExecutorService 的
Future<?> submit(Runnable task)
方法,作为返回值使用:
interface ArchiveSearcher { String search(String target); }
class App {
ExecutorService executor = ...;
ArchiveSearcher searcher = ...;
void showSearch(String target) throws InterruptedException {
Callable<String> task = () -> searcher.search(target);
Future<String> future = executor.submit(task); // 获取执行结果
displayOtherThings(); // do other things while searching
try {
displayText(future.get()); // use future
} catch (ExecutionException ex) { cleanup(); return; }
}
}
- FutureTask类是实现了Runnable的Future的实现,因此可以由Executor执行。例如,上述带有submit的构造可以替换为:
class App {
ExecutorService executor = ...;
ArchiveSearcher searcher = ...;
void showSearch(String target) throws InterruptedException {
Callable<String> task = () -> searcher.search(target);
// 关键两行替换
FutureTask<String> future = new FutureTask<>(task);
executor.execute(future);
displayOtherThings(); // do other things while searching
try {
displayText(future.get()); // use future
} catch (ExecutionException ex) { cleanup(); return; }
}
}
代码分析
继承关系
Future
Future 表示异步计算的结果。定义了用于检查计算是否完成、等待计算完成以及检索计算结果的能力。只有在计算完成后,才能使用 get 方法检索结果,在必要时会阻塞线程直到 Future 计算完成。取消是由 cancel 方法执行的。还提供了其他方法来确定任务是正常完成还是被取消。一旦计算完成,就不能取消计算。如果为了可取消性而使用 Future ,但又不想提供一个可用的结果,你可以声明形式 Future<?>
并返回 null 作为任务的结果。
在介绍 Future 中定义的能力之前,先了解一下它的用来表示 Future 状态内部类,和状态检索方法:
public interface Future<V> {
enum State {
// The task has not completed.
RUNNING,
// The task completed with a result. @see Future#resultNow()
SUCCESS,
//The task completed with an exception. @see Future#exceptionNow()
FAILED,
// The task was cancelled. @see #cancel(boolean)
CANCELLED
}
default State state() {
if (!isDone()) // 根据 isDone() 判断运行中
return State.RUNNING;
if (isCancelled()) // 根据 isCancelled() 判断已取消
return State.CANCELLED;
boolean interrupted = false;
try {
while (true) { // 死循环轮询
try {
get(); // may throw InterruptedException when done
return State.SUCCESS;
} catch (InterruptedException e) {
interrupted = true;
} catch (ExecutionException e) {
return State.FAILED;
}
}
} finally {
if (interrupted) Thread.currentThread().interrupt();
}
}
}
Future 的状态检索的默认实现是根据 isDone()
、isCancelled()
和不断轮询 get()
方法获取到的返回值判断的。
当 get()
正常返回结果时, state()
返回 State.SUCCESS
; 当抛出 InterruptedException
时,最终会操作所在的线程执行尝试中断的方法;抛出其他异常时,则返回 State.FAILED
。
Future 中定义的其他方法包括:
package java.util.concurrent;
public interface Future<V> {
// 取消操作
boolean cancel(boolean mayInterruptIfRunning);
// 检查是否取消
boolean isCancelled();
// 检查是否完成
boolean isDone();
// 获取计算结果的方法
V get() throws InterruptedException, ExecutionException;
// 带有超时限制的获取计算结果的方法
V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException;
// 立刻返回结果
default V resultNow()
// 立刻抛出异常
default Throwable exceptionNow()
}
其中 resultNow()
和 exceptionNow()
是带有默认实现的:
default V resultNow() {
if (!isDone())
throw new IllegalStateException("Task has not completed");
boolean interrupted = false;
try {
while (true) {
try {
return get();
} catch (InterruptedException e) {
interrupted = true;
} catch (ExecutionException e) {
throw new IllegalStateException("Task completed with exception");
} catch (CancellationException e) {
throw new IllegalStateException("Task was cancelled");
}
}
} finally {
if (interrupted) Thread.currentThread().interrupt();
}
}
- Future 仍在运行中,直接抛出 IllegalStateException 。
- 执行一个轮询,调用
get()
尝试返回计算结果,如果get()
抛出异常,则根据异常抛出不同消息的 IllegalStateException 或执行中断线程的操作。
default Throwable exceptionNow() {
if (!isDone())
throw new IllegalStateException("Task has not completed");
if (isCancelled())
throw new IllegalStateException("Task was cancelled");
boolean interrupted = false;
try {
while (true) {
try {
get();
throw new IllegalStateException("Task completed with a result");
} catch (InterruptedException e) {
interrupted = true;
} catch (ExecutionException e) {
return e.getCause();
}
}
} finally {
if (interrupted) Thread.currentThread().interrupt();
}
}
- Future 仍在运行中,直接抛出 IllegalStateException 。
- Future 检查是否已取消,如果取消了抛出 IllegalStateException 。
- 执行轮询,调用
get()
方法,如果能够正常执行结束,也抛出 IllegalStateException ,消息是 "Task completed with a result" ;get()
若抛出 InterruptedException ,则执行线程中断操作;其他异常正常抛出。
这就是 Future 的全貌了。
RunnableFuture
RunnableFuture 接口同时实现了 Runnable 和 Future 接口 :
public interface RunnableFuture<V> extends Runnable, Future<V> {
void run();
}
Runnable 接口是我们常用的用来实现线程操作的,可以说是十分熟悉也十分简单了。
这个接口代表了一个可以 Runnable 的 Future ,run 方法的成功执行代表着 Future 执行完成,并可以获取它的计算结果。
这个接口是 JDK 1.6 后续才有的。
FutureTask
FutureTask 是 RunnableFuture 的直接实现类,它代表了一个可取消的异步计算任务。根据我们对 Future 的分析和 Runnable 的熟悉,就可以理解它的作用了:可取消并可以检索运行状态的一个 Runnable ,配合线程使用可以中断线程执行。当任务没有执行完成时会造成阻塞。并且它还可以配合 Executor 使用。
状态
FutureTask 内部也定义了自己的状态:
public class FutureTask<V> implements RunnableFuture<V> {
private volatile int state;
private static final int NEW = 0; // 新建
private static final int COMPLETING = 1; // 完成中
private static final int NORMAL = 2; // 正常完成
private static final int EXCEPTIONAL = 3; // 异常的
private static final int CANCELLED = 4; // 已取消
private static final int INTERRUPTING = 5; // 中断中
private static final int INTERRUPTED = 6; // 已中断
@Override
public State state() {
int s = state;
while (s == COMPLETING) {
// 等待过渡到 NORMAL 或 EXCEPTIONAL
Thread.yield();
s = state;
}
switch (s) {
case NORMAL:
return State.SUCCESS;
case EXCEPTIONAL:
return State.FAILED;
case CANCELLED:
case INTERRUPTING:
case INTERRUPTED:
return State.CANCELLED;
default:
return State.RUNNING;
}
}
}
FutureTask 的状态包括 7 种,最初为 NEW
,只有在 set、setException 和 cancel 方法中,运行状态才会转换为最终状态。在完成期间,状态可能为 COMPLETING
(当结果正在设置时) 或 INTERRUPTING
(仅当中断跑者以满足cancel(true)
)的瞬态值。
可能存在的状态转换是:
NEW -> COMPLETING -> NORMAL // 正常完成
NEW -> COMPLETING -> EXCEPTIONAL // 抛出异常
NEW -> CANCELLED // 取消
NEW -> INTERRUPTING -> INTERRUPTED // 中断
属性
下面分析一下它的属性:
private Callable<V> callable;
private Object outcome; // non-volatile, protected by state reads/writes
private volatile Thread runner;
private volatile WaitNode waiters;
内部类
先看一看这个 WaitNode ,这是一个 FutureTask 的内部类:
static final class WaitNode {
volatile Thread thread;
volatile WaitNode next;
WaitNode() { thread = Thread.currentThread(); }
}
一个链表结构,用来对等待线程进行排序。
构造方法
最后是方法的分析,首先是构造方法:
// Creates a {@code FutureTask} that will, upon running, execute the given {@code Callable}.
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
}
FutureTask 接收一个 Callable 或一个 Runnable 作为参数,Runnable 会封装一下都保存到属性 callable
,然后更新 FutureTask 的状态为 NEW
。
从 Future 接口中实现的方法逐个分析:
检索 FutureTask 状态
public boolean isCancelled() {
return state >= CANCELLED; // 大于等于 4, 已取消、中断中、已中断
}
public boolean isDone() {
return state != NEW; // 不是 new 就代表执行结束了
}
取消操作
// mayInterruptIfRunning 表示最终的取消是通过中断还是通过取消。
public boolean cancel(boolean mayInterruptIfRunning) {
if (!(state == NEW && STATE.compareAndSet(this, NEW, mayInterruptIfRunning ? INTERRUPTING : CANCELLED))) // 尝试设置 CANCELLED 或 INTERRUPTING
return false;
try { // in case call to interrupt throws exception
if (mayInterruptIfRunning) {
try {
Thread t = runner;
if (t != null)
t.interrupt(); // 通过中断取消任务
} finally { // final state
STATE.setRelease(this, INTERRUPTED); // 更新中断状态
}
}
} finally {
finishCompletion();
}
return true;
}
这里的 finishCompletion()
的作用是通过 LockSupport 唤醒等待的全部线程并从等待列表中移除,然后调用done()
,最后把 callable 置空。相当于取消成功后释放资源的操作。
private void finishCompletion() {
// assert state > COMPLETING;
for (WaitNode q; (q = waiters) != null;) {
if (WAITERS.weakCompareAndSet(this, q, null)) {
for (;;) {
Thread t = q.thread;
if (t != null) {
q.thread = null;
LockSupport.unpark(t);
}
WaitNode next = q.next;
if (next == null)
break;
q.next = null; // unlink to help gc
q = next;
}
break;
}
}
done();
callable = null; // to reduce footprint
}
done()
是个空实现,供子类去自定义的。
protected void done() { }
计算结果
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L); // 异步结果
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);
}
这里涉及两个方法:awaitDone
方法和 report
方法 。
awaitDone 方法:
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
// The code below is very delicate, to achieve these goals:
// - if nanos <= 0L, 及时返回,不需要 allocation 或 nanoTime
// - if nanos == Long.MIN_VALUE, don't underflow
// - if nanos == Long.MAX_VALUE, and nanoTime is non-monotonic
// and we suffer a spurious wakeup, we will do no worse than
// to park-spin for a while
long startTime = 0L; // Special value 0L means not yet parked
WaitNode q = null;
boolean queued = false;
for (;;) {
int s = state;
if (s > COMPLETING) { // COMPLETING = 1
if (q != null)
q.thread = null;
return s;
}
else if (s == COMPLETING) // 瞬时态,完成中
// We may have already promised (via isDone) that we are done
// so never return empty-handed or throw InterruptedException
Thread.yield();
else if (Thread.interrupted()) {
removeWaiter(q); // 线程中断,移除等待的线程
throw new InterruptedException();
}
else if (q == null) {
if (timed && nanos <= 0L)
return s;
q = new WaitNode();
}
else if (!queued)
queued = WAITERS.weakCompareAndSet(this, q.next = waiters, q);
else if (timed) { // 设置超时时间的情况
final long parkNanos;
if (startTime == 0L) { // first time
startTime = System.nanoTime();
if (startTime == 0L)
startTime = 1L;
parkNanos = nanos;
} else {
long elapsed = System.nanoTime() - startTime;
if (elapsed >= nanos) {
removeWaiter(q);
return state;
}
parkNanos = nanos - elapsed;
}
// nanoTime may be slow; recheck before parking
if (state < COMPLETING)
LockSupport.parkNanos(this, parkNanos);
}
else
LockSupport.park(this);
}
}
通过 CAS 和 LockSupport 的挂起/唤醒操作配合,阻塞当前线程,异步地等待计算结果。
这里有个 removeWaiter
方法,内部就是遍历 waiters
,删除超时和中断的等待线程。
当异步逻辑执行完成后,调用 report 方法:
// 为完成的任务返回结果或抛出异常
private V report(int s) throws ExecutionException {
Object x = outcome;
if (s == NORMAL)
return (V)x;
if (s >= CANCELLED)
throw new CancellationException();
throw new ExecutionException((Throwable)x);
}
这里用到了一个 outcome ,它是一个 Object 类型,作为返回结果,通过 set 方法可以对它进行设置:
// 除非该 future 已被设置或取消,否则将该 future 的结果设置为给定值。
// 该方法在成功完成计算后由 run 方法在内部调用。
protected void set(V v) {
if (STATE.compareAndSet(this, NEW, COMPLETING)) {
outcome = v;
STATE.setRelease(this, NORMAL); // final state
finishCompletion();
}
}
立刻获取结果或异常
这两个方法都是通过 outcome
预设的返回值,返回预期的结果或异常。
public V resultNow() {
switch (state()) { // Future.State
case SUCCESS:
@SuppressWarnings("unchecked")
V result = (V) outcome;
return result;
case FAILED:
throw new IllegalStateException("Task completed with exception");
case CANCELLED:
throw new IllegalStateException("Task was cancelled");
default:
throw new IllegalStateException("Task has not completed");
}
}
@Override
public Throwable exceptionNow() {
switch (state()) { // Future.State
case SUCCESS:
throw new IllegalStateException("Task completed with a result");
case FAILED:
Object x = outcome;
return (Throwable) x;
case CANCELLED:
throw new IllegalStateException("Task was cancelled");
default:
throw new IllegalStateException("Task has not completed");
}
}
run 方法组
最后是实现了 Runnable 的 run 方法:
public void run() {
// 保证 NEW 状态和 RUNNER 成功设置当前线程
if (state != NEW ||
!RUNNER.compareAndSet(this, null, Thread.currentThread()))
return;
try {
Callable<V> c = callable; // 待执行的 Callable
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call(); // 执行 Callable
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result);
}
} finally {
// 为了防止并发调用 run ,直到 state 确定之前, runner 必须是非空的
runner = null;
// 状态必须在 runner 置空后重新读取,以防止泄漏中断
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
这里涉及两个方法,第一个是 setException(ex)
:
// 导致该future报告一个{@link ExecutionException},并将给定的可抛出对象作为其原因,除非该future已经被设置或取消。
protected void setException(Throwable t) {
if (STATE.compareAndSet(this, NEW, COMPLETING)) {
outcome = t;
STATE.setRelease(this, EXCEPTIONAL); // final state
finishCompletion();
}
}
另一个是 handlePossibleCancellationInterrupt 方法:
private void handlePossibleCancellationInterrupt(int s) {
// It is possible for our interrupter to stall before getting a
// chance to interrupt us. Let's spin-wait patiently.
if (s == INTERRUPTING)
while (state == INTERRUPTING)
Thread.yield(); // wait out pending interrupt
// assert state == INTERRUPTED;
// 我们想清除可能从cancel(true)接收到的所有中断。
// 然而,允许使用中断作为任务与其调用者通信的独立机制,并没有办法只清除取消中断。
// Thread.interrupted();
}
最后是 runAndReset 方法:
protected boolean runAndReset() {
if (state != NEW || !RUNNER.compareAndSet(this, null, Thread.currentThread()))
return false;
boolean ran = false; // flag 表示正常执行结束
int s = state;
try {
Callable<V> c = callable;
if (c != null && s == NEW) {
try {
c.call(); // don't set result
ran = true;
} catch (Throwable ex) {
setException(ex);
}
}
} 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
s = state; //
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
return ran && s == NEW; // 当正常执行结束,且 state 一开始就是 NEW 时,表示可以运行并重置。
}
执行计算时不设置其结果,然后将该 future 重置为初始状态,如果计算遇到异常或被取消,则不这样做。这是为本质上执行多次的任务设计的。
run 和 runAndReset 都用到了一个 RUNNER
, 最后就来揭秘一下这个东西:
private static final VarHandle STATE;
private static final VarHandle RUNNER;
private static final VarHandle WAITERS;
static {
try {
MethodHandles.Lookup l = MethodHandles.lookup();
STATE = l.findVarHandle(FutureTask.class, "state", int.class);
RUNNER = l.findVarHandle(FutureTask.class, "runner", Thread.class);
WAITERS = l.findVarHandle(FutureTask.class, "waiters", WaitNode.class);
} catch (ReflectiveOperationException e) {
throw new ExceptionInInitializerError(e);
}
// Reduce the risk of rare disastrous classloading in first call to
// LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
Class<?> ensureLoaded = LockSupport.class;
}
MethodHandles.lookup()
创建一个MethodHandles.Lookup
对象,该对象可以创建所有访问权限的方法,包括public
,protected
,private
,default
。
VarHandle
主要用于动态操作数组的元素或对象的成员变量。VarHandle
通过 MethodHandles
来获取实例,然后调用 VarHandle
的方法即可动态操作指定数组的元素或指定对象的成员变量。
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