uk.co.codera.lang.concurrent.PriorityTaskExecutor Maven / Gradle / Ivy
package uk.co.codera.lang.concurrent;
import java.util.Comparator;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.Executor;
import uk.co.codera.lang.concurrent.Tasks.CancellableTask;
import uk.co.codera.lang.concurrent.Tasks.CancellingTask;
/**
*
* A task executor that will process tasks in priority order. It allows tasks
* that overtake to cancel other tasks in the queue.
*
*
* Currently the implementation only allows for serial execution of tasks, i.e.
* it is single threaded. This is to prevent tasks with the same correlation id
* being processed concurrently. In future an executor that allows multiple
* threads whilst keeping tasks with the same correlation id in order may be
* provided.
*
*
* @author andystewart
*/
public class PriorityTaskExecutor {
private final Executor executor;
private final ConcurrentMap> cancelledTasks;
private final RunPolicyFactory runPolicies;
private PriorityTaskExecutor(Builder builder) {
this.executor = SequencedPriorityExecutor.singleThreadedExecutor(new PriorityTaskComparator(
builder.normalTaskBehaviour));
this.cancelledTasks = new ConcurrentHashMap<>();
this.runPolicies = new RunPolicyFactory(this.cancelledTasks);
}
public static Builder aTaskExecutor() {
return new Builder();
}
public void submit(Task task) {
this.executor.execute(new TaskRunner(runPolicyFor(task), task));
}
@SuppressWarnings("unchecked")
private RunPolicy runPolicyFor(Task task) {
return (RunPolicy) this.runPolicies.policyFor(task);
}
public static class Builder {
private NormalTaskBehaviour normalTaskBehaviour = NormalTaskBehaviour.DOES_NOT_OVERTAKE;
private Builder() {
super();
}
public Builder allowNormalTasksToOvertakeCancellableTasks() {
return normalTaskBehaviour(NormalTaskBehaviour.OVERTAKE_CANCELLABLE);
}
public Builder allowNormalTasksToOvertakeAllTasks() {
return normalTaskBehaviour(NormalTaskBehaviour.OVERTAKE_ALL);
}
private Builder normalTaskBehaviour(NormalTaskBehaviour behaviour) {
this.normalTaskBehaviour = behaviour;
return this;
}
public PriorityTaskExecutor build() {
return new PriorityTaskExecutor(this);
}
}
private enum NormalTaskBehaviour {
DOES_NOT_OVERTAKE, OVERTAKE_CANCELLABLE, OVERTAKE_ALL;
}
private static class PriorityTaskComparator implements Comparator {
private final NormalTaskBehaviour normalTaskBehaviour;
public PriorityTaskComparator(NormalTaskBehaviour normalTaskBehaviour) {
this.normalTaskBehaviour = normalTaskBehaviour;
}
@Override
public int compare(Runnable o1, Runnable o2) {
return priority(o2).compareTo(priority(o1));
}
private Integer priority(Runnable o) {
Class clazz = ((TaskRunner) o).task.getClass();
if (clazz == CancellableTask.class) {
return 0;
} else if (clazz == CancellingTask.class) {
return 1;
}
return this.normalTaskBehaviour.ordinal();
}
}
private static class TaskRunner implements Runnable {
private final RunPolicy runPolicy;
private final Task task;
private TaskRunner(RunPolicy runPolicy, Task task) {
this.runPolicy = runPolicy;
this.task = task;
}
@Override
public void run() {
if (this.runPolicy.shouldRun(this.task)) {
this.task.execute();
}
}
}
private class RunPolicyFactory {
private final ConcurrentMap> cancelledTasks;
private RunPolicyFactory(ConcurrentMap> cancelledTasks) {
this.cancelledTasks = cancelledTasks;
}
private RunPolicy policyFor(Task task) {
Class clazz = task.getClass();
if (clazz == CancellableTask.class) {
return new CheckNotCancelled(this.cancelledTasks);
} else if (clazz == CancellingTask.class) {
return new RecordsCancellation(this.cancelledTasks);
}
return new AlwaysRun();
}
}
@FunctionalInterface
private interface RunPolicy {
boolean shouldRun(T task);
}
private class AlwaysRun implements RunPolicy {
@Override
public boolean shouldRun(Task task) {
return true;
}
}
private class RecordsCancellation implements RunPolicy {
private final ConcurrentMap> cancelledTasks;
public RecordsCancellation(ConcurrentMap> cancelledTasks) {
this.cancelledTasks = cancelledTasks;
}
@Override
public boolean shouldRun(CancellingTask task) {
this.cancelledTasks.put(task.getCorrelationId(), task.getSequence());
return true;
}
}
private class CheckNotCancelled implements RunPolicy {
private final ConcurrentMap> cancelledTasks;
public CheckNotCancelled(ConcurrentMap> cancelledTasks) {
this.cancelledTasks = cancelledTasks;
}
@Override
public boolean shouldRun(CancellableTask task) {
Object correlationId = task.getCorrelationId();
if (this.cancelledTasks.containsKey(correlationId)) {
boolean reachedCancelledSequence = currentTaskSequenceExceededCancelledSequence(task, correlationId);
if (reachedCancelledSequence) {
this.cancelledTasks.remove(correlationId);
}
return reachedCancelledSequence;
}
return true;
}
@SuppressWarnings("unchecked")
private boolean currentTaskSequenceExceededCancelledSequence(CancellableTask task, Object correlationId) {
Comparable cancellingSequence = this.cancelledTasks.get(correlationId);
return ((Comparable