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Future & Promise

decouple computing from results compose tasks flexibly parallelize

• future: the result
• promise: computing

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• future <- functional programming
• promise <- callback hell

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JDK Future

Since 1.5

package java.util.concurrent;
/**
 * A Future represents the result of an asynchronous computation....
 */
public interface Future<V> {
    boolean cancel(boolean mayInterruptIfRunning);
    boolean isCancelled();
    boolean isDone();
    V get() throws InterruptedException, ExecutionException;
    V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException;
}

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JavaScript Promise

ES6

• prototype.then
• prototype.catch
• prototype.finally
• all
• any
• race
• …

Java Third Callback Implements

Netty

package io.netty.util.concurrent;

/**
 * The result of an asynchronous operation.
 */
public interface Future<V> extends java.util.concurrent.Future<V> {
    ...

• boolean isSuccess();
• boolean isCancellable();
• Throwable cause();
• addListener
• addListeners
• removeListener
• removeListeners
• …

Guava

package com.google.common.util.concurrent;
/**
 * A {@link Future} that accepts completion listeners....
 */
public interface ListenableFuture<V> extends Future<V> {
    void addListener(Runnable listener, Executor executor);
}

Java Third Promise Implements

Netty

package io.netty.util.concurrent;

/**
 * Special {@link Future} which is writable.
 */
public interface Promise<V> extends Future<V> {
    Promise<V> setSuccess(V result);
    boolean trySuccess(V result);
    Promise<V> setFailure(Throwable cause);
    boolean tryFailure(Throwable cause);
    ...

Guava

package com.google.common.util.concurrent;

/**
 * A {@link ListenableFuture} whose result can be set by a {@link #set(Object)}, {@link
 * #setException(Throwable)} or {@link #setFuture(ListenableFuture)} call. It can also, like any
 * other {@code Future}, be {@linkplain #cancel cancelled}....
 */
public final class SettableFuture<V> extends AbstractFuture.TrustedFuture<V> {

  public boolean set(@Nullable V value) {
    return super.set(value);
  }
  public boolean setException(Throwable throwable) {
    return super.setException(throwable);
  }
  public boolean setFuture(ListenableFuture<? extends V> future) {
    return super.setFuture(future);
  }

  ...
}

JDK CompletableFuture

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