java源码之Runnable、Callable和Future
2019-06-21 本文已影响18人
WangGavin
在java并发编程中，几乎都会用到Runnable、Callable、Future和FutureTask等类或接口,所以理解它们的概念和关系，对设计并发业务和源码阅读会有很大帮助。
1 Runnable
public interface Runnable {
/**
* When an object implementing interface <code>Runnable</code> is used
* to create a thread, starting the thread causes the object's
* <code>run</code> method to be called in that separately executing
* thread.
* <p>
* The general contract of the method <code>run</code> is that it may
* take any action whatsoever.
*
* @see java.lang.Thread#run()
*/
public abstract void run();
}
Runnable接口应该是被最终要被线程执行的类所实现，所以这些类必须实现无参的run()方法。
这个接口被设计为对象提供一个公共协议，这种对象会在它们活跃的时候执行一些代码块。比如Thread类就实现了Runnable接口，它在活跃的时候就会执行run方法。当然活跃时（being active）也可以当作是一个线程已被创建，然后还没有停止（stoped）的过程。
当然Runnable也为某些类提供了方法，这些类可以有活跃状态（active），但它不一定是Thread类.
当一个对象实现Runnable接口，通常是用于创建一个线程，然后线程会调用run方法，以执行这些设计在工作线程的业务代码。
2 Future
public interface Future<V> {
// 尝试取消执行任务，如果任务已完成或者已取消等其他原因会取消失败；如果任务还未开始，则能取消成功，任务会不再执行；如果任务正在执行，则可以取消成功，如果取消是可以通过中断取消的，可以设置中断，否则取消会允许任务完成。
boolean cancel(boolean mayInterruptIfRunning);
// 返回任务完成之前是否已取消
boolean isCancelled();
// 返回任务是否已完成
boolean isDone();
// 获取到结果，该方法会阻塞到获取结果，可能会抛中断异常
V get() throws InterruptedException, ExecutionException;
// 获取到结果，可以设置超时时间
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
一个Future代表一个异步计算结果，接口方法提供了检查是否完成，获取结果，取消，设置结果操作。get方法会阻塞，直到任务返回结果。
3 Callable
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;
}
跟Runable差不多，只不过它可以返回结果，也可能会抛异常。Callable是为Runnable兼容Future而设计的，比如构造一个FutureTask时，FutureTask的构造方法可以把Runable转为Callable
4 RunnableFuture
public interface RunnableFuture<V> extends Runnable, Future<V> {
// 设置计算结果，除非任务被取消
void run();
}
RunnableFuture接口就是Runnable和Future的混合体，可以把它看作是一个Runnable，但这个Runnable可以获取到计算结果。
Executor是Runnable和Callable的调度容器，Future就是对于具体的Runnable或者Callable任务执行的抽象。
简单用法：
public static final void main(String[] args){
// runnable 队列
BlockingQueue<Runnable> blockingQueue = new LinkedBlockingDeque<>();
// 线程工厂
ThreadFactory threadFactory = new ThreadFactory() {
private AtomicInteger atomicInteger = new AtomicInteger(1);
@Override
public Thread newThread(Runnable r) {
int threadId = atomicInteger.getAndIncrement();
System.out.println("thread are created:"+threadId);
return new Thread(r,"thread #"+threadId);
}
};
// cpu 核心数
final int cpuCore = Runtime.getRuntime().availableProcessors();
// 线程池核心线程数
final int coreCount = Math.min(2,Math.min(4,cpuCore-1));
// 线程池最大线程数
final int maxCount = cpuCore*2-1;
// 临时线程存活时间
final int KEEP_ALIVE_SECONDS = 30;
// 构造线程池
final ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
coreCount,maxCount,KEEP_ALIVE_SECONDS,TimeUnit.SECONDS,blockingQueue,threadFactory
);
// 提交一个callable
Future<Integer> future = threadPoolExecutor.submit(new Callable<Integer>() {
@Override
public Integer call() throws Exception {
System.out.println("thread:"+Thread.currentThread());
Thread.sleep(3000);
return 200;
}
});
System.out.println(">>>>>>>>>>>>>>>>getResult>>>>>>>>>>>>");
try {
int result = future.get();
System.out.println("result: "+result);
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
5 FutureTask
public class FutureTask<V> implements RunnableFuture<V> {
/**
* 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;
/** 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;
/**
* Returns result or throws exception for completed task.
*
* @param s completed state value
*/
@SuppressWarnings("unchecked")
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);
}
/**
* 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) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
/**
* 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) {
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;
}
/**
* @throws CancellationException {@inheritDoc}
*/
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}
/**
* @throws CancellationException {@inheritDoc}
*/
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 method invoked when this task transitions to state
* {@code isDone} (whether normally or via cancellation). The
* default implementation does nothing. Subclasses may override
* this method to invoke completion callbacks or perform
* bookkeeping. Note that you can query status inside the
* implementation of this method to determine whether this task
* has been cancelled.
*/
protected void done() { }
/**
* Sets the result of this future to the given value unless
* this future has already been set or has been cancelled.
*
* <p>This method is invoked internally by the {@link #run} method
* upon successful completion of the computation.
*
* @param v the value
*/
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}
/**
* Causes this future to report an {@link ExecutionException}
* with the given throwable as its cause, unless this future has
* already been set or has been cancelled.
*
* <p>This method is invoked internally by the {@link #run} method
* upon failure of the computation.
*
* @param t the cause of failure
*/
protected void setException(Throwable t) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = t;
UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
finishCompletion();
}
}
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);
}
}
/**
* Executes the computation without setting its result, and then
* resets this future to initial state, failing to do so if the
* computation encounters an exception or is cancelled. This is
* designed for use with tasks that intrinsically execute more
* than once.
*
* @return {@code true} if successfully run and reset
*/
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;
}
/**
* Ensures that any interrupt from a possible cancel(true) is only
* delivered to a task while in run or runAndReset.
*/
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;
// We want to clear any interrupt we may have received from
// cancel(true). However, it is permissible to use interrupts
// as an independent mechanism for a task to communicate with
// its caller, and there is no way to clear only the
// cancellation interrupt.
//
// Thread.interrupted();
}
/**
* 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;
volatile WaitNode next;
WaitNode() { thread = Thread.currentThread(); }
}
/**
* Removes and signals all waiting threads, invokes done(), and
* nulls out callable.
*/
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;
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
}
/**
* Awaits completion or aborts on interrupt or timeout.
*
* @param timed true if use timed waits
* @param nanos time to wait, if timed
* @return state upon completion
*/
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);
}
}
/**
* Tries to unlink a timed-out or interrupted wait node to avoid
* accumulating garbage. Internal nodes are simply unspliced
* without CAS since it is harmless if they are traversed anyway
* by releasers. To avoid effects of unsplicing from already
* removed nodes, the list is retraversed in case of an apparent
* race. This is slow when there are a lot of nodes, but we don't
* expect lists to be long enough to outweigh higher-overhead
* schemes.
*/
private void removeWaiter(WaitNode node) {
if (node != null) {
node.thread = null;
retry:
for (;;) { // restart on removeWaiter race
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;
}
}
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long stateOffset;
private static final long runnerOffset;
private static final long waitersOffset;
static {
try {
UNSAFE = sun.misc.Unsafe.getUnsafe();
Class<?> k = FutureTask.class;
stateOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("state"));
runnerOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("runner"));
waitersOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("waiters"));
} catch (Exception e) {
throw new Error(e);
}
}
}
FutureTask是实现了RunableFuture接口，提供了一个任务执行的操作实现，比如取消，获取结果等。
它可以通过构造方法包装Runnable和Callable
由于FutureTask实现了Runnable，因此它既可以通过Thread来直接执行，也可以提交给ExecuteService来执行。
所以上面的线程池使用还可以这样：
public static final void main(String[] args){
// runnable 队列
BlockingQueue<Runnable> blockingQueue = new LinkedBlockingDeque<>();
// 线程工厂
ThreadFactory threadFactory = new ThreadFactory() {
private AtomicInteger atomicInteger = new AtomicInteger(1);
@Override
public Thread newThread(Runnable r) {
int threadId = atomicInteger.getAndIncrement();
System.out.println("thread are created:"+threadId);
return new Thread(r,"thread #"+threadId);
}
};
// cpu 核心数
final int cpuCore = Runtime.getRuntime().availableProcessors();
// 线程池核心线程数
final int coreCount = Math.min(2,Math.min(4,cpuCore-1));
// 线程池最大线程数
final int maxCount = cpuCore*2-1;
// 临时线程存活时间
final int KEEP_ALIVE_SECONDS = 30;
// 构造线程池
final ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
coreCount,maxCount,KEEP_ALIVE_SECONDS,TimeUnit.SECONDS,blockingQueue,threadFactory
);
RunnableFuture<Integer> future = new FutureTask<Integer>(new Callable<Integer>() {
@Override
public Integer call() throws Exception {
System.out.println("thread:"+Thread.currentThread());
Thread.sleep(3000);
return 200;
}
});
// 提交一个runnableFuture
threadPoolExecutor.submit(future);
System.out.println(">>>>>>>>>>>>>>>>getResult>>>>>>>>>>>>");
try {
int result = future.get();
System.out.println("result: "+result);
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
FutureTask最终执行任务其实是执行它里面callable成员的call方法，所以在构造FutureTask时，他会把Runable转换callable。
public FutureTask(Runnable runnable, V result) {
this.callable = Executors.callable(runnable, result);
this.state = NEW; // ensure visibility of callable
}
然后转换实现又是通过一个适配器实现的。
public static <T> Callable<T> callable(Runnable task, T result) {
if (task == null)
throw new NullPointerException();
return new RunnableAdapter<T>(task, result);
}
/**
* A callable that runs given task and returns given result
*/
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;
}
}
6 总的关系
image
7 深入：FutureTask如果实现Future操作？
待研究
