(一)runnable 与Callable的区别
@FunctionalInterface public interface Runnable { public abstract void run(); } @FunctionalInterface public interface Callable<V> { V call() throws Exception; }
- call()方法可以有返回值，返回类型为泛型V，代表着支持所有类型的返回值。
- call()方法定义时声明了可以抛出异常：throws Exception，而run()则不行。
public interface Future<V> { /** 尝试取消执行此任务。如果任务已完成、已取消或由于其他原因无法取消，则此尝试将失败 */ boolean cancel(boolean mayInterruptIfRunning); /** 如果此任务在完成之前被取消，则返回True */ boolean isCancelled(); /** 如果此任务已完成，则返回｛true｝。完成可能是由于正常终止、异常或取消——在所有这些情况下，此方法都将返回｛ true｝ */ boolean isDone(); /** 如果需要，等待计算完成，然后检索其结果,如果计算被取消，则返回计算结果@throws CancellationException 如果计算引发异常，则返回@throws ExecutionException 如果当前线程在等待时中断，则返回异常@throws InterruptedException */ V get() throws InterruptedException, ExecutionException; /** 如果需要，最多等待给定的时间以完成计算，然后检索其结果（如果可用） @param timeout等待的最长时间 @param unit超时参数的时间单位 */ V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException; }
（二）Future与FutureTask
FutureTask实现了RunnableFuture接口，而RunnableFuture除开继承了Runnable外，还继承了Future接口.Future接口中主要提供了两类方法：获取任务执行结果的get方法以及取消任务的cancel方法.
在FutureTask出现之前，Java中的多线程编程执行任务后是不能获取执行结果的。而FutureTask整合了Runnable、Callable、Future三个接口，使得我们的多线程任务执行后可以异步获取到多线程的执行结果。FutureTask会将执行结束后的结果保存在成员变量：outcome中，等待获取执行结果的线程则读取outcome成员值即可。
public class FutureTask<V> implements RunnableFuture<V> { /** * 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; private Callable<V> callable; private Object outcome; // non-volatile, protected by state reads/writes private volatile Thread runner; private volatile WaitNode waiters; @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); } public FutureTask(Callable<V> callable) { if (callable == null) throw new NullPointerException(); this.callable = callable; this.state = NEW; // ensure visibility of callable } ... public V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException { if (unit == null) throw new NullPointerException(); int s = state; //如果状态小于COMPLETING，表示FutureTask任务还没完成，则调用awaitDone让当前线程等待 if (s <= COMPLETING && (s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING) throw new TimeoutException(); return report(s); } public void run() { //r如果状态值state不是NEW,捉着设置runner值失败，直接返回 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 { //调用callable方法，获取结果 result = c.call(); //运行成功 ran = true; } catch (Throwable ex) { result = null; ran = false; //设置异常 setException(ex); } if (ran) //设置返回结果 set(result); } } finally { runner = null; int s = state; if (s >= INTERRUPTING) handlePossibleCancellationInterrupt(s); } } protected void set(V v) { if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) { //将结果赋值给outcome outcome = v; UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state finishCompletion(); } } private int awaitDone(boolean timed, long nanos) throws InterruptedException { final long deadline = timed ? System.nanoTime() + nanos : 0L; WaitNode q = null; boolean queued = false; for (;;) { //如果当前线程设置了中断标记 if (Thread.interrupted()) { //从列表中移除节点q，并抛出中断异常 removeWaiter(q); throw new InterruptedException(); } int s = state; //如果状态已经完成，表示FutureTask任务已结束 if (s > COMPLETING) { if (q != null) q.thread = null; return s; } //还有一些后续操作没有完成，当前线程让出执行权 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, //timed为true表示需要设置超时 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); } } }
- submit与execute是否都可以接受异常，是否都有返回值
public interface Executor { /** * Executes the given command at some time in the future. The command * may execute in a new thread, in a pooled thread, or in the calling * thread, at the discretion of the {@code Executor} implementation. * * @param command the runnable task * @throws RejectedExecutionException if this task cannot be * accepted for execution * @throws NullPointerException if command is null */ void execute(Runnable command); }
execute方法是在executor接口中定义的，执行execute方法时，如果任务不可被接受，则抛出RejectedExecutionException。如果command是null，则抛出NullPointerException。但是执行runnable接口时，其run方法是不抛出异常的。
public interface ExecutorService extends Executor { <T> Future<T> submit(Callable<T> task); <T> Future<T> submit(Runnable task, T result); Future<?> submit(Runnable task); }
submit方法是在executorService接口中定义的，如果任务不可被接受，则抛出RejectedExecutionException。如果command是null，则抛出NullPointerException。submit执行时，会将Callable或者runnable接口封装成futureTask，然后调用futuretask的run方法，最后实际调用了futuretask内部的Callable成员变量的call方法。对于runnable接口封装成的futureTask，最后实际调用的是runnable的run方法。而对于Callable接口封装成的futureTask，最后实际调用的是Callable的call方法.
总结：execute方法接受的是runnable接口，是不会往外抛异常的。而submit方法，即可接受runnable接口，也可接受Callable接口。抛不抛异常看传入的接口类型。
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);
}
}