假设我有一个来自Executors静态工厂方法的ExecutorService实例。
如果我提交了一个Callable,其中RetVal不是某个线程的线程安全的本地实例化对象,那么当我从同一线程中获取RetVals的完整性时,我是否需要担心它的完整性?人们说局部变量是线程安全的,但是当您返回本地实例化的Object并从其他线程接收它时,我不确定它是否适用。
这是与我的情况类似的示例:
ExecutorService executor = Executors.newFixedThreadPool(5);
Future<List<String>> fut = executor.submit(() -> {
List<String> ret = new ArrayList<>();
ret.add("aasdf");
ret.add("dfls");
return ret;
});
List<String> myList = fut.get();
在上面的示例中,我正在检索在另一个线程中创建的ArrayList -由执行程序创建的一个。我认为上面的代码不是线程安全的,但是我找不到关于我的特定情况的太多信息。
现在,我在计算机上尝试了上面的代码,实际上它100%地返回了预期的结果,我什至尝试使用自己的ExecutorService实现,到目前为止,我只得到了预期的结果。因此,除非我非常幸运,否则我很确定它会起作用,但我不确定如何。我在另一个线程中创建了一个非线程安全的对象,并在另一个线程中接收了该对象。我是否应该有机会收到部分构造的对象-在我的情况下,该列表不包含2个字符串?
以下是我自定义的实现,仅供测试。您可以忽略EType枚举。
class MyExecutor {
enum EType {
NoHolder, Holder1, Holder2
}
private ConcurrentLinkedQueue<MyFutureTask<?>> tasksQ;
private final Thread thread;
private final EType eType;
public MyExecutor(EType eType) {
eType = Objects.requireNonNull(eType);
tasksQ = new ConcurrentLinkedQueue<>();
thread = new Thread(new MyRunnable());
thread.start();
}
public <T> Future<T> submit(Callable<T> c) {
MyFutureTask<T> task = new MyFutureTask<T>(c, eType);
tasksQ.add(task);
return task;
}
class MyRunnable implements Runnable {
@Override
public void run() {
while (true) {
if (tasksQ.isEmpty()) {
try {
Thread.sleep(1);
continue;
} catch (InterruptedException ite) {
Thread.interrupted();
break;
}
}
MyFutureTask<?> task = tasksQ.poll();
try {
task.run();
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
class MyFutureTask<T> implements RunnableFuture<T> {
final Callable<?> cb;
volatile Object outcome;
static final int STATE_PENDING = 1;
static final int STATE_EXECUTING = 2;
static final int STATE_DONE = 3;
final AtomicInteger atomicState = new AtomicInteger(STATE_PENDING);
final EType eType;
public MyFutureTask(Callable<?> cb, EType eType) {
cb = Objects.requireNonNull(cb);
eType = Objects.requireNonNull(eType);
}
@Override
public boolean cancel(boolean mayInterruptIfRunning) {
throw new NotImplementedException();
}
@Override
public boolean isCancelled() {
return false;
}
@Override
public boolean isDone() {
return atomicState.get() == STATE_DONE;
}
@SuppressWarnings("unchecked")
@Override
public T get() throws InterruptedException, ExecutionException {
while (true) {
switch (atomicState.get()) {
case STATE_PENDING:
case STATE_EXECUTING:
// Thread.sleep(1);
break;
case STATE_DONE:
return (T)outcome;
default:
throw new IllegalStateException();
}
}
}
@Override
public T get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
throw new NotImplementedException();
}
void set(T t) {
outcome = t;
}
@Override
public void run() {
if (atomicState.compareAndSet(STATE_PENDING, STATE_EXECUTING)) {
Object result;
try {
switch (eType) {
case NoHolder:
result = cb.call();
break;
case Holder1:
throw new NotImplementedException();
case Holder2:
throw new NotImplementedException();
default:
throw new IllegalStateException();
}
} catch (Exception e) {
e.printStackTrace();
result = null;
}
outcome = result;
atomicState.set(STATE_DONE);
}
}
}
}
class MyTask implements Callable<List<Integer>> {
@Override
public List<Integer> call() throws Exception {
List<Integer> ret = new ArrayList<>(100);
IntStream.range(0, 100).boxed().forEach(ret::add);
return ret;
}
}
重要的是happens-before关系。从ExecutorService
API文档:
内存一致性影响:线程之前的操作向
Runnable
提交Callable
或ExecutorService
任务happen-before该任务执行的任何操作,依次执行happen-before通过Future.get()
检索结果。
因此您可以安全地传输这样的可变对象。