这篇 blog 我们来讲一下 FutureTask 的源码，FutureTask 是 java 实现异步编程的基础
public class FutureTask<V> implements RunnableFuture<V>
...
public interface RunnableFuture<V> extends Runnable, Future<V> {
void run();
}
从⬆️我们能够看到，FutureTask 实现了 Runnable 接口，因此 FutureTask 实例能够被提交到线程池上执行
FutureTask 的运行方式：
- 将一个 Callable 置为 FutureTask 的内置成员
- 执行 Callable 中的 call 方法
- 调用 get() 或者 get(timeout, TimeUnit) 方法，获取 call 方法返回的结果
下面我们一个一个来看看
内部属性
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;
FutureTask 使用 state 变量表示当前的执行状态
- NEW：任务处于新建状态
- COMPLETING：任务正在完成中
- NORMAL：任务正常完成
- EXCEPTIONAL：call 方法执行过程中抛出了异常
- CANCELLED：调用 cancel 方法，取消任务
- INTERRUPTING：任务正在中断
- INTERRUPTED：thread.interrupt() 执行完毕
private Callable<V> callable;
private Object outcome;
private volatile Thread runner;
private volatile WaitNode waiters;
- callable：创建 FutureTask 实栗的时候传入的，用来异步执行获取结果
- outcome：执行 callable 的 call 方法返回的结果，可能是结果或者执行过程中抛出的异常
- runner：执行 FutureTask run 方法的线程
- waiters：WaitNode 组成的单向链表，用于存储调用 get() 或者 get(timeout, TimeUnit) 方法的线程
构造函数
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW;
}
run 方法
public void run() {
// CAS 设置 runner
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 {
// 执行 callable 的 call 方法
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
// 执行中抛异常, 更新 state 状态, 释放等待的线程(调用 finishCompletion)
setException(ex);
}
if (ran)
// 执行成功, 进行赋值操作，释放等待的线程(调用 finishCompletion)
set(result);
}
} finally {
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
set
protected void set(V v) {
// CAS 更新 state 为 COMPLETING
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v; // 设置结果
UNSAFE.putOrderedInt(this, stateOffset, NORMAL);
// 唤醒等待的线程
finishCompletion();
}
}
finishCompletion
遍历链表，唤醒等待的线程
static final class WaitNode {
volatile Thread thread;
volatile WaitNode next;
WaitNode() { thread = Thread.currentThread(); }
}
private void finishCompletion() {
for (WaitNode q; (q = waiters) != null;) {
// CAS 将链表头设置为 null
if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
// 遍历链表
for (;;) {
Thread t = q.thread;
if (t != null) {
// 帮助 GC
q.thread = null;
// 唤醒等待的线程
LockSupport.unpark(t);
}
WaitNode next = q.next;
if (next == null)
break;
// 帮助 GC
q.next = null;
q = next;
}
break;
}
}
done();
callable = null;
}
get
public V get() throws InterruptedException, ExecutionException {
int s = state;
// 当 FutureTask 没有执行完毕，调用 awaitDone 进入 链表等待
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);
}
report
private V report(int s) throws ExecutionException {
Object x = outcome;
// 当状态为正常的时候，转型返回
if (s == NORMAL)
return (V)x;
// 失败的时候抛出异常
if (s >= CANCELLED)
throw new CancellationException();
throw new ExecutionException((Throwable)x);
}
awaitDone
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
// 计算等待的最长时间
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
boolean queued = false;
for (;;) {
// 线程中断的时候，调用 removeWaiter 从链表中删除节点
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
int s = state;
// 执行完毕，直接返回状态
if (s > COMPLETING) {
if (q != null)
q.thread = null;
return s;
}
// 当 state 为 COMPLETING，状态马上会更改，先 yield 一下
else if (s == COMPLETING) // cannot time out yet
Thread.yield();
// 初始化一个 WaitNode
else if (q == null)
q = new WaitNode();
// CAS 头插法，设置 WaitNode 头部节点
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);
else if (timed) {
nanos = deadline - System.nanoTime();
// 挂起超时，执行 removeWaiter
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
// 限时挂起
LockSupport.parkNanos(this, nanos);
}
else
// 挂起
LockSupport.park(this);
}
}
removeWaiter
private void removeWaiter(WaitNode node) {
if (node != null) {
node.thread = null; // 将移除的节点的thread＝null, 为移除做标示
retry:
for (;;) {
// q 为链表中的当前节点，pred 为 q 之前的节点，s 为 q 的后继节点
for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
s = q.next;
if (q.thread != null)
pred = q;
// prev.next -> s
else if (pred != null) {
pred.next = s;
if (pred.thread == null)
continue retry;
}
// 头部节点就是需要删除的
else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
q, s))
continue retry;
}
break;
}
}
}
总结
- 需要实现一个链表(每个节点包含当前线程的引用)
- 通过 LockSupport.park 对线程进行阻塞
- 有个公共方法进行节点的唤醒(task 完成, task 执行异常，线程 Interrupt, 或 await 超时), 并且次方法要线程安全