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package com.fireflysource.common.concurrent;
import com.fireflysource.common.sys.Result;
import java.io.IOException;
import java.util.function.Consumer;
/**
* This specialized callback implements a pattern that allows
* a large job to be broken into smaller tasks using iteration
* rather than recursion.
*
* A typical example is the write of a large content to a socket,
* divided in chunks. Chunk C1 is written by thread T1, which
* also invokes the callback, which writes chunk C2, which invokes
* the callback again, which writes chunk C3, and so forth.
*
*
* The problem with the example is that if the callback thread
* is the same that performs the I/O operation, then the process
* is recursive and may result in a stack overflow.
* To avoid the stack overflow, a thread dispatch must be performed,
* causing context switching and cache misses, affecting performance.
*
*
* To avoid this issue, this callback uses an AtomicReference to
* record whether success callback has been called during the processing
* of a sub task, and if so then the processing iterates rather than
* recurring.
*
*
* Subclasses must implement method {@link #process()} where the sub
* task is executed and a suitable {@link Action} is
* returned to this callback to indicate the overall progress of the job.
* This callback is passed to the asynchronous execution of each sub
* task and a call the success result on this callback represents
* the completion of the sub task.
*
*/
public abstract class IteratingCallback implements Consumer> {
/**
* The internal states of this callback
*/
private enum State {
/**
* This callback is IDLE, ready to iterate.
*/
IDLE,
/**
* This callback is iterating calls to {@link #process()} and is dealing with
* the returns. To get into processing state, it much of held the lock state
* and set iterating to true.
*/
PROCESSING,
/**
* Waiting for a schedule callback
*/
PENDING,
/**
* Called by a schedule callback
*/
CALLED,
/**
* The overall job has succeeded as indicated by a {@link Action#SUCCEEDED} return
* from {@link IteratingCallback#process()}
*/
SUCCEEDED,
/**
* The overall job has failed as indicated by a call to {@link IteratingCallback#failed(Throwable)}
*/
FAILED,
/**
* This callback has been closed and cannot be reset.
*/
CLOSED
}
/**
* The indication of the overall progress of the overall job that
* implementations of {@link #process()} must return.
*/
protected enum Action {
/**
* Indicates that {@link #process()} has no more work to do,
* but the overall job is not completed yet, probably waiting
* for additional events to trigger more work.
*/
IDLE,
/**
* Indicates that {@link #process()} is executing asynchronously
* a sub task, where the execution has started but the callback
* may have not yet been invoked.
*/
SCHEDULED,
/**
* Indicates that {@link #process()} has completed the overall job.
*/
SUCCEEDED
}
private final AutoLock _lock = new AutoLock();
private State _state;
private boolean _iterate;
protected IteratingCallback() {
_state = State.IDLE;
}
protected IteratingCallback(boolean needReset) {
_state = needReset ? State.SUCCEEDED : State.IDLE;
}
/**
* Method called by {@link #iterate()} to process the sub task.
*
* Implementations must start the asynchronous execution of the sub task
* (if any) and return an appropriate action:
*
*
* - {@link Action#IDLE} when no sub tasks are available for execution
* but the overall job is not completed yet
* - {@link Action#SCHEDULED} when the sub task asynchronous execution
* has been started
* - {@link Action#SUCCEEDED} when the overall job is completed
*
*
* @return the appropriate Action
* @throws Throwable if the sub task processing throws
*/
protected abstract Action process() throws Throwable;
/**
* Invoked when the overall task has completed successfully.
*
* @see #onCompleteFailure(Throwable)
*/
protected void onCompleteSuccess() {
}
/**
* Invoked when the overall task has completed with a failure.
*
* @param cause the throwable to indicate cause of failure
* @see #onCompleteSuccess()
*/
protected void onCompleteFailure(Throwable cause) {
}
/**
* This method must be invoked by applications to start the processing
* of sub tasks. It can be called at any time by any thread, and it's
* contract is that when called, then the {@link #process()} method will
* be called during or soon after, either by the calling thread or by
* another thread.
*/
public void iterate() {
boolean process = false;
try (AutoLock lock = _lock.lock()) {
switch (_state) {
case PENDING:
case CALLED:
// process will be called when callback is handled
break;
case IDLE:
_state = State.PROCESSING;
process = true;
break;
case PROCESSING:
_iterate = true;
break;
case FAILED:
case SUCCEEDED:
break;
case CLOSED:
default:
throw new IllegalStateException(toString());
}
}
if (process)
processing();
}
private void processing() {
// This should only ever be called when in processing state, however a failed or close call
// may happen concurrently, so state is not assumed.
boolean onCompleteSuccess = false;
// While we are processing
processing:
while (true) {
// Call process to get the action that we have to take.
Action action;
try {
action = process();
} catch (Throwable x) {
failed(x);
break;
}
// acted on the action we have just received
try (AutoLock lock = _lock.lock()) {
switch (_state) {
case PROCESSING: {
switch (action) {
case IDLE: {
// Has iterate been called while we were processing?
if (_iterate) {
// yes, so skip idle and keep processing
_iterate = false;
_state = State.PROCESSING;
continue processing;
}
// No, so we can go idle
_state = State.IDLE;
break processing;
}
case SCHEDULED: {
// we won the race against the callback, so the callback has to process and we can break processing
_state = State.PENDING;
break processing;
}
case SUCCEEDED: {
// we lost the race against the callback,
_iterate = false;
_state = State.SUCCEEDED;
onCompleteSuccess = true;
break processing;
}
default:
break;
}
throw new IllegalStateException(String.format("%s[action=%s]", this, action));
}
case CALLED: {
if (action != Action.SCHEDULED)
throw new IllegalStateException(String.format("%s[action=%s]", this, action));
// we lost the race, so we have to keep processing
_state = State.PROCESSING;
continue processing;
}
case SUCCEEDED:
case FAILED:
case CLOSED:
break processing;
case IDLE:
case PENDING:
default:
throw new IllegalStateException(String.format("%s[action=%s]", this, action));
}
}
}
if (onCompleteSuccess)
onCompleteSuccess();
}
@Override
public void accept(Result result) {
if (result.isSuccess()) {
this.succeeded();
} else {
this.failed(result.getThrowable());
}
}
/**
* Invoked when the sub task succeeds.
* Subclasses that override this method must always remember to call
* {@code super.succeeded()}.
*/
public void succeeded() {
boolean process = false;
try (AutoLock lock = _lock.lock()) {
switch (_state) {
case PROCESSING: {
_state = State.CALLED;
break;
}
case PENDING: {
_state = State.PROCESSING;
process = true;
break;
}
case CLOSED:
case FAILED: {
// Too late!
break;
}
default: {
throw new IllegalStateException(toString());
}
}
}
if (process)
processing();
}
/**
* Invoked when the sub task fails.
* Subclasses that override this method must always remember to call
* {@code super.failed(Throwable)}.
*/
public void failed(Throwable x) {
boolean failure = false;
try (AutoLock lock = _lock.lock()) {
switch (_state) {
case SUCCEEDED:
case FAILED:
case IDLE:
case CLOSED:
case CALLED:
// too late!.
break;
case PENDING:
case PROCESSING: {
_state = State.FAILED;
failure = true;
break;
}
default:
throw new IllegalStateException(toString());
}
}
if (failure)
onCompleteFailure(x);
}
public void close() {
String failure = null;
try (AutoLock lock = _lock.lock()) {
switch (_state) {
case IDLE:
case SUCCEEDED:
case FAILED:
_state = State.CLOSED;
break;
case CLOSED:
break;
default:
failure = String.format("Close %s in state %s", this, _state);
_state = State.CLOSED;
}
}
if (failure != null)
onCompleteFailure(new IOException(failure));
}
/*
* only for testing
* @return whether this callback is idle and {@link #iterate()} needs to be called
*/
boolean isIdle() {
try (AutoLock lock = _lock.lock()) {
return _state == State.IDLE;
}
}
public boolean isClosed() {
try (AutoLock lock = _lock.lock()) {
return _state == State.CLOSED;
}
}
/**
* @return whether this callback has failed
*/
public boolean isFailed() {
try (AutoLock lock = _lock.lock()) {
return _state == State.FAILED;
}
}
/**
* @return whether this callback has succeeded
*/
public boolean isSucceeded() {
try (AutoLock lock = _lock.lock()) {
return _state == State.SUCCEEDED;
}
}
/**
* Resets this callback.
*
* A callback can only be reset to IDLE from the
* SUCCEEDED or FAILED states or if it is already IDLE.
*
*
* @return true if the reset was successful
*/
public boolean reset() {
try (AutoLock lock = _lock.lock()) {
switch (_state) {
case IDLE:
return true;
case SUCCEEDED:
case FAILED:
_iterate = false;
_state = State.IDLE;
return true;
default:
return false;
}
}
}
@Override
public String toString() {
return String.format("%s@%x[%s]", getClass().getSimpleName(), hashCode(), _state);
}
}