FutureTask源码分析
FutureTask类结构如下:
/**
* A cancellable asynchronous computation. This class provides a base
* implementation of {@link Future}, with methods to start and cancel
* a computation, query to see if the computation is complete, and
* retrieve the result of the computation. The result can only be
* retrieved when the computation has completed; the {@code get}
* methods will block if the computation has not yet completed. Once
* the computation has completed, the computation cannot be restarted
* or cancelled (unless the computation is invoked using
* {@link #runAndReset}).
*
* <p>A {@code FutureTask} can be used to wrap a {@link Callable} or
* {@link Runnable} object. Because {@code FutureTask} implements
* {@code Runnable}, a {@code FutureTask} can be submitted to an
* {@link Executor} for execution.
*
* <p>In addition to serving as a standalone class, this class provides
* {@code protected} functionality that may be useful when creating
* customized task classes.
*
* @since 1.5
* @author Doug Lea
* @param <V> The result type returned by this FutureTask's {@code get} methods
*/
public class FutureTask<V> implements RunnableFuture<V> {
}
可以看到,FutrueTask实现了RunnableFuture接口,而RunnableFuture接口又继承了Future和Runnable。如下图:
上述注释大意为: Future是一个可取消的异步计算框架,提供了Future接口的基本实现,其中包括start、cancel、query以查看计算是否完成以及检索计算结果的方法。只有在计算完成之后才能检索结果,如果计算尚未完成,则get方法将阻塞,一旦计算完成,就不能重新开始或者取消计算。除非使用runAndReset。 FutureTask可以用于包装Callable或者Runnable的对象。由于FutureTask实现了Runnable接口,可以将FutureTask提交给Executor进行执行。 除了单独使用之外,此类还提供了protected方法,支持扩展,这些方法在创建自定义的任务的时候可能会很有用。
另外还有修订说明: 这与依赖AbstractQueuedSynchronizer实现的早期版本不同,此类主要是为了避免用户对取消任务期间还保留中断状态,当前设计中的同步控制依赖于通过cas更新的状态字段来跟踪完成情况,以及一个简单的Treiber堆栈来保存等待线程。 样式说明:与往常一样,我们绕过了使用AtomicXFieldUpdaters的开销。而且直接使用的是UnSafe的内部函数。
/**
* The run state of this task, initially NEW. The run state
* transitions to a terminal state only in methods set,
* setException, and cancel. During completion, state may take on
* transient values of COMPLETING (while outcome is being set) or
* INTERRUPTING (only while interrupting the runner to satisfy a
* cancel(true)). Transitions from these intermediate to final
* states use cheaper ordered/lazy writes because values are unique
* and cannot be further modified.
*
* Possible state transitions:
* NEW -> COMPLETING -> NORMAL
* NEW -> COMPLETING -> EXCEPTIONAL
* NEW -> CANCELLED
* NEW -> INTERRUPTING -> INTERRUPTED
*/
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;
此任务的运行状态中,最初为new,运行状态仅在set、setException和cancel方法中转换状态。在完成期间,状态可能会采用COMPLETING(正在设置结果时)或者INTERRUPTING(仅在满足cancel条件进行中断的时候)的瞬时值。从这些中间状态的转换使用低消耗的ordered/lazy 写入,因为值是唯一的,无法进一步修改。 我们可以看到状态的变化路径:
/** 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;
变量整理出如下表:
变量 | 类型 | 说明 |
---|---|---|
callable | Callable | 底层调用的callable,如果为空则停止 |
outcome | Object | 调用get返回的结果或者抛出的异常 |
runner | volatile Thread | 运行callable的线程,cas操作在其run()方法中 |
waiters | WaitNode | 等待线程的Treiber堆栈 |
构造函数主要有两种,分别是callable和支持runnable。
这个方法将根据传入的callable运行。
/**
* Creates a {@code FutureTask} that will, upon running, execute the
* given {@code Callable}.
*
* @param callable the callable task
* @throws NullPointerException if the callable is null
*/
public FutureTask(Callable<V> callable) {
//如果为空,则抛出NPE异常
if (callable == null)
throw new NullPointerException();
this.callable = callable;
//初始状态为NEW
this.state = NEW; // ensure visibility of callable
}
这个方法传入runnable,并返回result。runnable实际上并不支持返回值,Executors的callable方法做了特殊处理。
/**
* Creates a {@code FutureTask} that will, upon running, execute the
* given {@code Runnable}, and arrange that {@code get} will return the
* given result on successful completion.
*
* @param runnable the runnable task
* @param result the result to return on successful completion. If
* you don't need a particular result, consider using
* constructions of the form:
* {@code Future<?> f = new FutureTask<Void>(runnable, null)}
* @throws NullPointerException if the runnable is null
*/
public FutureTask(Runnable runnable, V result) {
//callable 通过Executors的callable方法来产生。
this.callable = Executors.callable(runnable, result);
//初始方法。
this.state = NEW; // ensure visibility of callable
}
public V get() throws InterruptedException, ExecutionException {
int s = state;
//如果不是完成状态
if (s <= COMPLETING)
//通过awaitDone进入等待
s = awaitDone(false, 0L);
//返回report方法
return report(s);
}
get方法将会进入wait状态,具体实现是LockSupport的park方法。
这个方法只是增加了一个超时时间。
public V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
//为空则抛出异常
if (unit == null)
throw new NullPointerException();
int s = state;
if (s <= COMPLETING &&
//指定时间wait
(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
throw new TimeoutException();
return report(s);
}
根据传入时间进行wait,如果时间为0,则无期限等待。
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
//根据timed得到deadline
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
boolean queued = false;
//死循环
for (;;) {
//如果线程被打断,则移除等待,并抛出异常
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
//s为状态
int s = state;
//如果s大于COMPLETING说明执行完成
if (s > COMPLETING) {
if (q != null)
q.thread = null;
return s;
}
//反之 让出当前线程
else if (s == COMPLETING) // cannot time out yet
Thread.yield();
//初始化waitNode
else if (q == null)
q = new WaitNode();
//采用cas的方式设置waiters
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);
else if (timed) {
//如果超时时间已到
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
//固定时间阻塞
LockSupport.parkNanos(this, nanos);
}
else
//阻塞
LockSupport.park(this);
}
}
可以看到FutureTask内部是采用 LockSupport.park来对提交任务的线程进行阻塞,使其进入WAIT状态。
此方法将取消task。
public boolean cancel(boolean mayInterruptIfRunning) {
//判断state状态
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;
//通过interrupt来解除阻塞
if (t != null)
t.interrupt();
} finally { // final state
//cas的方式修改state
UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
}
}
} finally {
//完成后操作
finishCompletion();
}
return true;
}
执行完毕之后,删除并通知所有等待线程。
private void finishCompletion() {
// assert state > COMPLETING;
//循环遍历
for (WaitNode q; (q = waiters) != null;) {
//采用cas的方式设置waiters为null
if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
//死循环
for (;;) {
Thread t = q.thread;
if (t != null) {
q.thread = null;
//执行完毕之后unpark
LockSupport.unpark(t);
}
//清理链表 便于gc回收
WaitNode next = q.next;
if (next == null)
break;
q.next = null; // unlink to help gc
q = next;
}
break;
}
}
//这个方法提供给后续回掉或者什么也不做,是一个扩展方法。
done();
callable = null; // to reduce footprint
}
尝试取消链接超时或中断的等待节点,以避免积累垃圾。内部节点在没有cas的情况下,不会拼接在一起,因为如果释放者无论如何遍历它们,都是无害的。为了避免从已删除的节点取消拆分的影响,在出现明显竞争的情况下将重新遍历该列表。当节点很多时,会导致非常慢。
private void removeWaiter(WaitNode node) {
if (node != null) {
node.thread = null;
retry:
//死循环
for (;;) { // restart on removeWaiter race
//遍历链表
for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
s = q.next;
if (q.thread != null)
pred = q;
else if (pred != null) {
pred.next = s;
if (pred.thread == null) // check for race
continue retry;
}
else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
q, s))
continue retry;
}
break;
}
}
}
这个方法是执行FrutureTask的主体方法。
public void run() {
//如果不为new状态或者不为当前线程则返回
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
//必须要callback不为空,且为new状态
if (c != null && state == NEW) {
V result;
boolean ran;
try {
//执行call方法
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
//设置异常
setException(ex);
}
if (ran)
//设置result
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;
//处理取消的中断状态
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
在不设置计算结果的情况下执行计算,然后将此状态重置为初始状态,如果遇到异常和cancel,则会失败。它设计用于本质上执行多次的任务。
protected boolean runAndReset() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return false;
boolean ran = false;
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;
}
此方法在每一此执行完之后会清理状态,重置为new,被用来设计为重复执行。
WaitNode是一个链表节点,将等待的线程进行记录。
/**
* Simple linked list nodes to record waiting threads in a Treiber
* stack. See other classes such as Phaser and SynchronousQueue
* for more detailed explanation.
*/
static final class WaitNode {
volatile Thread thread;//线程
volatile WaitNode next;//指针
WaitNode() { thread = Thread.currentThread(); }
}
FutureTask并不复杂,实现了Future设计模式,实际上本质是做一个异步的处理,但是会让任务的提交者进入等待过程,直至任务执行完毕。之后获取到执行完毕的结果。