org.h2.util.LazyFuture Maven / Gradle / Ivy
/*
* Copyright 2004-2014 H2 Group. Multiple-Licensed under the MPL 2.0,
* and the EPL 1.0 (http://h2database.com/html/license.html).
* Initial Developer: H2 Group
*/
package org.h2.util;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
import org.h2.message.DbException;
/**
* Single threaded lazy future.
*
* @param
* @author Sergi Vladykin
*/
public abstract class LazyFuture implements Future {
private static final int S_READY = 0;
private static final int S_DONE = 1;
private static final int S_ERROR = 2;
private static final int S_CANCELED = 3;
private int state = S_READY;
private T result;
private Exception error;
/**
* Reset this future to the initial state.
*
* @return {@code false} if it was already in initial state
*/
public boolean reset() {
if (state == S_READY) {
return false;
}
state = S_READY;
result = null;
error = null;
return true;
}
/**
* Run computation and produce the result.
*
* @return the result of computation
*/
protected abstract T run() throws Exception;
@Override
public boolean cancel(boolean mayInterruptIfRunning) {
if (state != S_READY) {
return false;
}
state = S_CANCELED;
return true;
}
@Override
public T get() throws InterruptedException, ExecutionException {
switch (state) {
case S_READY:
try {
result = run();
state = S_DONE;
} catch (Exception e) {
error = e;
if (e instanceof InterruptedException) {
throw (InterruptedException) e;
}
throw new ExecutionException(e);
} finally {
if (state != S_DONE) {
state = S_ERROR;
}
}
return result;
case S_DONE:
return result;
case S_ERROR:
throw new ExecutionException(error);
case S_CANCELED:
throw new CancellationException();
default:
throw DbException.throwInternalError();
}
}
@Override
public T get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException {
return get();
}
@Override
public boolean isCancelled() {
return state == S_CANCELED;
}
@Override
public boolean isDone() {
return state != S_READY;
}
}