bilibili-Java併發學習筆記11 Future
基於 java 1.8.0
P41_Future模式示例剖析與源碼詳解
package java.util.concurrent;
/**
* Future 表示異步計算的結果。它提供了檢查計算是否完成的方法，以等待計算的完成，並獲取計算的結果。
* 計算完成後只能使用 get 方法來獲取結果，如有必要，計算完成前可以阻塞此方法。取消則由 cancel 方法來執行。
* 還提供了其他方法，以確定任務是正常完成還是被取消了。一旦計算完成，就不能再取消計算。
* 如果爲了可取消性而使用 Future 但又不提供可用的結果，則可以聲明 Future<?> 形式類型、並返回 null 作爲底層任務的結果。
*
* 用法示例（注意，下列各類都是構造好的。）
*
* interface ArchiveSearcher { String search(String target); }
* class App {
* ExecutorService executor = ...
* ArchiveSearcher searcher = ...
* void showSearch(final String target)
* throws InterruptedException {
* Future<String> future
* = executor.submit(new Callable<String>() {
* public String call() {
* return searcher.search(target);
* }});
* displayOtherThings(); // do other things while searching
* try {
* displayText(future.get()); // use future
* } catch (ExecutionException ex) { cleanup(); return; }
* }
* }}
*
* FutureTask 類是 Future 的一個實現，Future 可實現 Runnable，所以可通過 Executor 來執行。
* 例如，可用下列內容替換上面帶有 submit 的構造：
*
* FutureTask<String> future =
* new FutureTask<String>(new Callable<String>() {
* public String call() {
* return searcher.search(target);
* }});
* executor.execute(future);}</pre>
*
* 內存一致性效果：異步計算採取的操作 happen-before 另一線程中緊跟在相應的 Future.get() 之後的操作。
*
* @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> {
/**
* 試圖取消對此任務的執行。如果任務已完成、或已取消，或者由於某些其他原因而無法取消，則此嘗試將失敗。
* 當調用 cancel 時，如果調用成功，而此任務尚未啓動，則此任務將永不運行。
* 如果任務已經啓動，則 mayInterruptIfRunning 參數確定是否應該以試圖停止任務的方式來中斷執行此任務的線程。
*
* 此方法返回後，對 isDone() 的後續調用將始終返回 true。如果此方法返回 true，
* 則對 isCancelled() 的後續調用將始終返回 true。
*
* @param mayInterruptIfRunning 如果應該中斷執行此任務的線程，則爲 true；否則允許正在運行的任務運行完成
* @return 如果無法取消任務，則返回 false，這通常是由於它已經正常完成；否則返回 true
*/
boolean cancel(boolean mayInterruptIfRunning);
/**
* 如果在任務正常完成前將其取消，則返回 true。
*
* @return 如果任務完成前將其取消，則返回 true
*/
boolean isCancelled();
/**
* 如果任務已完成，則返回 true。 可能由於正常終止、異常或取消而完成，在所有這些情況中，此方法都將返回 true。
*
* @return 如果任務已完成，則返回 true
*/
boolean isDone();
/**
* 如有必要，等待計算完成，然後獲取其結果。
*
* @return 計算的結果
* @throws CancellationException 如果計算被取消
* @throws ExecutionException 如果計算拋出異常
* @throws InterruptedException 如果當前的線程在等待時被中斷
*/
V get() throws InterruptedException, ExecutionException;
/**
* 如有必要，最多等待爲使計算完成所給定的時間之後，獲取其結果（如果結果可用）。
*
* @param timeout 等待的最大時間
* @param unit timeout 參數的時間單位
* @return 計算的結果
* @throws CancellationException 如果計算被取消
* @throws ExecutionException 如果計算拋出異常
* @throws InterruptedException 如果當前的線程在等待時被中斷
* @throws TimeoutException 如果等待超時
*/
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
package java.util.concurrent;
import java.util.concurrent.locks.LockSupport;
/**
* 可取消的異步計算。利用開始和取消計算的方法、查詢計算是否完成的方法和獲取計算結果的方法，此類提供了對 Future 的基本實現。
* 僅在計算完成時才能獲取結果；如果計算尚未完成，則阻塞 get 方法。一旦計算完成，就不能再重新開始或取消計算。
*
* 可使用 FutureTask 包裝 Callable 或 Runnable 對象。因爲 FutureTask 實現了 Runnable，所以可將 FutureTask 提交給 Executor 執行。
*
* 除了作爲一個獨立的類外，此類還提供了 protected 功能，這在創建自定義任務類時可能很有用。
*
* @since 1.5
* @author Doug Lea
* @param <V> 此 FutureTask 的 get 方法所返回的結果類型。
*/
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);
}
/**
* 創建一個 FutureTask，一旦運行就執行給定的 Callable。
*
* @param callable 可調用的任務
* @throws NullPointerException 如果 callable 爲 null
*/
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
/**
* 創建一個 FutureTask，一旦運行就執行給定的 Runnable，並安排成功完成時 get 返回給定的結果 。
*
* @param runnable 可運行的任務。
* @param result 成功完成時要返回的結果。如果不需要特定的結果，則考慮使用下列形式的構造：Future<?> f = new FutureTask<Object>(runnable, null)
* @throws NullPointerException 如果 runnable 爲 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);
}
/**
* 當此任務轉換到狀態 isDone（不管是正常地還是通過取消）時，調用受保護的方法。默認實現不執行任何操作。子類可以重寫此方法，以調用完成回調或執行簿記。注意，可以查詢此方法的實現內的狀態，從而確定是否已取消了此任務。
*/
protected void done() { }
/**
* 除非已經設置了此 Future 或已將其取消，否則將其結果設置爲給定的值。在計算成功完成時通過 run 方法內部調用此方法。
*
* @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();
}
}
/**
* 除非已經設置了此 Future 或已將其取消，否則它將報告一個 ExecutionException，並將給定的 throwable 作爲其原因。在計算失敗時通過 run 方法內部調用此方法。
*
* @param t 失敗的原因
*/
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);
}
}
/**
* 執行計算而不設置其結果，然後將此 Future 重置爲初始狀態，如果計算遇到異常或已取消，則該操作失敗。本操作被設計用於那些本質上要執行多次的任務。
*
* @return 如果成功運行並重置，則返回 true。
*/
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);
}
}
}
案例：
package new_package.thread.p41;
import java.util.Random;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;
public class FutureTest {
public static void main(String[] args) {
Callable<Integer> callable = () -> {
System.out.println("futureTask start");
Thread.sleep(5000);
int c = new Random().nextInt(1000);
System.out.println("futureTask over");
return c;
};
FutureTask<Integer> futureTask = new FutureTask<>(callable);
new Thread(futureTask).start();
System.out.println("main thread start");
try {
Thread.sleep(2000);
// get 阻塞方法
System.out.println(futureTask.get());
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
}
P42_CompletableFuture示例剖析與源碼解讀
/**
* @author Doug Lea
* @since 1.8
*/
public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
}
/**
* @author Doug Lea
* @since 1.8
*/
public interface CompletionStage<T> {
}
CompletableFutureTest 示例：
package new_package.thread.p41;
import java.util.concurrent.CompletableFuture;
public class CompletableFutureTest {
public static void main(String[] args) {
String value = CompletableFuture.supplyAsync(() -> "hello").thenApplyAsync(v -> v + " world").join();
System.out.println(value);
CompletableFuture.supplyAsync(() -> "hello").thenAccept(v -> System.out.println("www " + v));
String v2 = CompletableFuture.supplyAsync(() -> {
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
return "hello ";
}).thenCombine(
CompletableFuture.supplyAsync(() -> {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
return "world";
})
,
(s1, s2) -> s1 + s2
).join();
System.out.println(v2);
}
}
package new_package.thread.p41;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.TimeUnit;
public class CompletableFutureTest2 {
public static void main(String[] args) {
CompletableFuture<Void> completableFuture = CompletableFuture.runAsync(() -> {
try {
TimeUnit.SECONDS.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("task finished");
});
completableFuture.whenComplete((t, action) -> System.out.println("task over"));
System.out.println("main thread over");
try {
TimeUnit.SECONDS.sleep(5);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}