一、Runnable和Callable
经常会将一个任务放在一个子线程或者线程池中执行。主线程怎么获取在子线程执行的任务结果呢？如果任务在子线程中抛出了异常，主线程可以感知吗？
1. Runnable
看下Runnable的java定义：
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();
}
可以看到run()方法既不会返回结果，也不会抛出异常。
2. Callable
/**
* A task that returns a result and may throw an exception.
* Implementors define a single method with no arguments called
* {@code call}.
*
* <p>The {@code Callable} interface is similar to {@link
* java.lang.Runnable}, in that both are designed for classes whose
* instances are potentially executed by another thread. A
* {@code Runnable}, however, does not return a result and cannot
* throw a checked exception.
*
* <p>The {@link Executors} class contains utility methods to
* convert from other common forms to {@code Callable} classes.
*
* @see Executor
* @since 1.5
* @author Doug Lea
* @param <V> the result type of method {@code call}
*/
@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;
}
从上面的注释可以发现：Runnable既不会有返回值，也不能抛出checked exception。虽然checked exception可以通过在方法名中throws出来，但是Runnable的run方法却不行，只能通过try catch的方式。但是Callable既可以返回结果，又可以抛出异常。
3. 示例
public class FutureTest {
ExecutorService executorService = Executors.newSingleThreadExecutor();
@Test
public void testRunnableException() {
executorService.execute(new Runnable() {
@Override
public void run() {
System.out.println("running...");
throw new RuntimeException("runnable throw exception");
}
});
sleep(500);
System.out.println("main over");
}
private void sleep(long timeMs) {
try {
TimeUnit.MILLISECONDS.sleep(timeMs);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
@Test
public void testCallableException() {
Future<String> future = executorService.submit(new Callable<String>() {
@Override
public String call() throws Exception {
if (Math.random() < 0.5) {
throw new RuntimeException("callable throw exception");
}
return "callable";
}
});
try {
String res = future.get();
System.out.println(res);
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
sleep(500);
System.out.println("main over");
}
}
运行结果：
// testRunnableException
running...
// 可以看到这个异常在线程“pool-1-thread-1”中
Exception in thread "pool-1-thread-1" java.lang.RuntimeException: runnable throw exception
at FutureTest$1.run(FutureTest.java:16)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
at java.base/java.lang.Thread.run(Thread.java:834)
main over
// testCallableException
// 可以看到在main线程中捕获了ExecutionException，caused by是RuntimeException
java.util.concurrent.ExecutionException: java.lang.RuntimeException: callable throw exception
at java.base/java.util.concurrent.FutureTask.report(FutureTask.java:122)
at java.base/java.util.concurrent.FutureTask.get(FutureTask.java:191)
at FutureTest.testCallableException(FutureTest.java:45)
Caused by: java.lang.RuntimeException: callable throw exception
at FutureTest$2.call(FutureTest.java:38)
at FutureTest$2.call(FutureTest.java:34)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
at java.base/java.lang.Thread.run(Thread.java:834)
main over
// testCallableException
callable
main over
二、Future
那么Future有什么魔力可以在一个线程中获得另一个线程的执行结果？又怎么在一个线程中捕获了另一个线程的异常(虽然封装了一层ExecutionException)？先来看看Future的java定义。
1. Future定义
/**
* A {@code Future} represents the result of an asynchronous
* computation. Methods are provided to check if the computation is
* complete, to wait for its completion, and to retrieve the result of
* the computation. The result can only be retrieved using method
* {@code get} when the computation has completed, blocking if
* necessary until it is ready. Cancellation is performed by the
* {@code cancel} method. Additional methods are provided to
* determine if the task completed normally or was cancelled. Once a
* computation has completed, the computation cannot be cancelled.
* If you would like to use a {@code Future} for the sake
* of cancellability but not provide a usable result, you can
* declare types of the form {@code Future<?>} and
* return {@code null} as a result of the underlying task.
*
* <p><b>Sample Usage</b> (Note that the following classes are all
* made-up.)
*
* <pre> {@code
* interface ArchiveSearcher { String search(String target); }
* class App {
* ExecutorService executor = ...
* ArchiveSearcher searcher = ...
* void showSearch(String target) throws InterruptedException {
* Callable<String> task = () -> searcher.search(target);
* Future<String> future = executor.submit(task);
* displayOtherThings(); // do other things while searching
* try {
* displayText(future.get()); // use future
* } catch (ExecutionException ex) { cleanup(); return; }
* }
* }}</pre>
*
* The {@link FutureTask} class is an implementation of {@code Future} that
* implements {@code Runnable}, and so may be executed by an {@code Executor}.
* For example, the above construction with {@code submit} could be replaced by:
* <pre> {@code
* FutureTask<String> future = new FutureTask<>(task);
* executor.execute(future);}</pre>
*
* <p>Memory consistency effects: Actions taken by the asynchronous computation
* <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a>
* actions following the corresponding {@code Future.get()} in another thread.
*
* @see FutureTask
* @see Executor
* @since 1.5
* @author Doug Lea
* @param <V> The result type returned by this Future's {@code get} method
*/
public interface Future<V>
正如注释所说：Future表示异步执行的结果。Future提供的这些方法用于检测任务是否完成、等待任务完成、获取计算结果。通过get方法可以获得结果，但是有可能会等待任务计算完成。cancel方法可以取消任务（但是任务一旦完成，就不能被取消了）。如果不想要返回值，但是却想cancel任务，可以通过Future<?>并返回null的方式实现。
FutureTask是Future的一个实现。
注释写的太清楚了，直接搬运过来了。
/**
* Attempts to cancel execution of this task. This attempt will
* fail if the task has already completed, has already been cancelled,
* or could not be cancelled for some other reason. If successful,
* and this task has not started when {@code cancel} is called,
* this task should never run. If the task has already started,
* then the {@code mayInterruptIfRunning} parameter determines
* whether the thread executing this task should be interrupted in
* an attempt to stop the task.
*
* <p>After this method returns, subsequent calls to {@link #isDone} will
* always return {@code true}. Subsequent calls to {@link #isCancelled}
* will always return {@code true} if this method returned {@code true}.
*
* @param mayInterruptIfRunning {@code true} if the thread executing this
* task should be interrupted; otherwise, in-progress tasks are allowed
* to complete
* @return {@code false} if the task could not be cancelled,
* typically because it has already completed normally;
* {@code true} otherwise
*/
boolean cancel(boolean mayInterruptIfRunning);
/**
* Returns {@code true} if this task was cancelled before it completed
* normally.
*
* @return {@code true} if this task was cancelled before it completed
*/
boolean isCancelled();
/**
* Returns {@code true} if this task completed.
*
* Completion may be due to normal termination, an exception, or
* cancellation -- in all of these cases, this method will return
* {@code true}.
*
* @return {@code true} if this task completed
*/
boolean isDone();
/**
* Waits if necessary for the computation to complete, and then
* retrieves its result.
*
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread was interrupted
* while waiting
*/
V get() throws InterruptedException, ExecutionException;
/**
* Waits if necessary for at most the given time for the computation
* to complete, and then retrieves its result, if available.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread was interrupted
* while waiting
* @throws TimeoutException if the wait timed out
*/
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
测试代码：
@Test
public void testCallableCancel() {
Future<?> future = executorService.submit(new Callable<Object>() {
@Override
public Object call() throws Exception {
System.out.println("callable running");
sleep(1000);
return null;
}
});
// 注释这句sleep有不同的输出
sleep(5);
System.out.println("task state, isDone: " + future.isDone() + "; isCancelled: " + future.isCancelled());
boolean canceled = future.cancel(true);
System.out.println("future cancel result: " + canceled);
System.out.println("task state, isDone: " + future.isDone() + "; isCancelled: " + future.isCancelled());
canceled = future.cancel(true);
System.out.println("again, future cancel result: " + canceled);
System.out.println("task state, isDone: " + future.isDone() + "; isCancelled: " + future.isCancelled());
}
执行结果：
// 有sleep(5);
callable running
task state, isDone: false; isCancelled: false
future cancel result: true
// 已经取消的任务，isDone返回的也是true
task state, isDone: true; isCancelled: true
// 已经取消的任务，再次取消是失败的，返回false
again, future cancel result: false
task state, isDone: true; isCancelled: true
java.lang.InterruptedException: sleep interrupted
at java.base/java.lang.Thread.sleep(Native Method)
at java.base/java.lang.Thread.sleep(Thread.java:339)
at java.base/java.util.concurrent.TimeUnit.sleep(TimeUnit.java:446)
at FutureTest.sleep(FutureTest.java:26)
at FutureTest.access$000(FutureTest.java:7)
at FutureTest$3.call(FutureTest.java:65)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
at java.base/java.lang.Thread.run(Thread.java:834)
// 没有sleep(5);
// 可以看到task没有执行
task state, isDone: false; isCancelled: false
future cancel result: true
task state, isDone: true; isCancelled: true
again, future cancel result: false
task state, isDone: true; isCancelled: true
这篇文章有点长了，再写一篇文章分析Future的原理：怎么获取的结果，怎么抛出的异常。
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