咱们都晓得线程创立的形式有以下几种
- Thread
- Runnable
- Callable
- Executors
其中 Callable 是能获取到返回值或者异样的，callable 接口如下
@FunctionalInterface
public interface Callable<V> {
/**
* Computes a result, or throws an exception if unable to do so. * * @return computed result
* @throws Exception if unable to compute a result
*/ V call() throws Exception;}但 callable 必须要和线程池搭配应用，或者应用 FutureTask，具体用法如下
public interface Future<V> {
// 勾销工作
boolean cancel(boolean mayInterruptIfRunning);
// 是否被勾销
boolean isCancelled();
// 是否实现，已实现返回 true
boolean isDone();
// 阻塞获取返回值，如果有异样则抛出异样
V get() throws InterruptedException, ExecutionException;
// 指定阻塞工夫获取返回值
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}实现类 RunnableFuture, 该类实现了 Runnable 接口和 Future 接口
public interface RunnableFuture<V> extends Runnable, Future<V> {
// 将此 Future 设置为其计算的后果，除非工作勾销
void run();}实现类 FutureTask（重点 )
future 接口实现个别应用线程池或者 FutureTask 实现线程调用
// 构造方法传入 callable 赋值给成员变量 callable, 并设置 state 为新建
public FutureTask(Callable<V> callable) {if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}state 有以下几个状态
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 有下几个成员变量
构造方法 1
// 构造方法传入的 callable 对象
private Callable<V> callable;
// 线程执行完的返回后果
private Object outcome;
// 以后运行的线程
private volatile Thread runner;
//waiters 示意如果多个线程执行一个 callable 对象，则会存在一个单向链表中
private volatile WaitNode waiters;构造方法 2
// 带返回值的构造方法
public FutureTask(Runnable runnable, V result) {this.callable = Executors.callable(runnable, result);
this.state = NEW;
}因为 FutureTask 实现了 Runnable 接口，所以线程调度时执行 FutureTask 的 run 办法
public void run() {
// 如果状态不为 NEW 或无奈将以后线程设置进去，为 runnerOffeset，则返回
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
// 如果 FutureTask 内的 Callable 不为空且为新建状态，则执行 if 外部办法
if (c != null && state == NEW) {
V result;
boolean ran;
try {
// 执行 callale 的 call 办法，且设置 ran=true 示意执行胜利
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
// 设置 ran=false 示意执行失败
ran = false;
// 如果捕捉到异样，则传入异样
setException(ex);
}
// 如果是执行胜利，通过 set 办法设置后果
if (ran)
// 将后果设置给 set 办法
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);
}
}将 callable 的返回值通过 cas 设置进去，并
protected void set(V v) {if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
// 将状态设置失常 NORMAL
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
// 实现赋值后唤醒 UnPack get 的线程
finishCompletion();}
}如果 call 办法出现异常，则通过 cas 设置胜利后进行唤醒
protected void setException(Throwable t) {if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = t;
UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
finishCompletion();}
}实现后进行唤醒
private void finishCompletion() {
// assert state > COMPLETING;
for (WaitNode q; (q = waiters) != null;) {if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {for (;;) {
Thread t = q.thread;
if (t != null) {
q.thread = null;
// 唤醒 get 线程，且将前面所有执行的 waitNode 节点
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
}// 是否勾销, 当 state>= 勾销状态, 即等于 CANCELLED,INTERRUPTING 或者 INTERRUPTED 时返回 true
public boolean isCancelled() {return state >= CANCELLED;}
// 是否实现, 当线程不为 NEW 状态都视为已实现 (包含失常实现, 异样实现, 以及中断等)
public boolean isDone() {return state != NEW;}// 阻塞获取返回后果
public V get() throws InterruptedException, ExecutionException {
int s = state;
// 当线程小于等于 COMPLETING(NEW 新建或者 COMPLETING 筹备实现) 时, 进行无工夫闲置的阻塞
if (s <= COMPLETING)
s = awaitDone(false, 0L);
// 此处只有当后果返回时下面的阻塞才会被唤醒, 否则始终阻塞在下面的 if 中
return report(s);
}// 阻塞指定工夫获取返回后果, 当指定工夫仍旧未获取到后果, 抛出 TimeoutException 异样
public V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {if (unit == null)
// 传入的工夫单位如果为空，则抛空指针异样
throw new NullPointerException();
int s = state;
// 当后果未返回且等待时间大于指定的 timeout 工夫, 抛出 timeout 异样
if (s <= COMPLETING &&
(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
throw new TimeoutException();
// 此处只有当后果返回时下面的阻塞才会被唤醒, 否则始终阻塞在下面的 if 中
return report(s);
}report 是依据以后的 state 判断 call 办法是执行失常还是失败，失常则返回泛型的 result, 如果是勾销状态，则抛出勾销异样，否则抛出 ExecutionException 异样示意执行异样
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);
}阻塞办法