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本文次要钻研一下 cheddar 的 MessageHandlerExecutor
MessageHandlerExecutor
Cheddar/cheddar/cheddar-messaging/src/main/java/com/clicktravel/cheddar/infrastructure/messaging/pooled/listener/MessageHandlerExecutor.java
public class MessageHandlerExecutor extends ThreadPoolExecutor {public MessageHandlerExecutor(final String queueName, final int numThreads) {super(numThreads, numThreads, 0L, TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>(),
new MessageHandlerWorkerThreadFactory(queueName));
}
}MessageHandlerExecutor 继承了 ThreadPoolExecutor，其应用的 queue 为 LinkedBlockingQueue；threadFactory 为 MessageHandlerWorkerThreadFactory
MessageHandlerWorkerThreadFactory
Cheddar/cheddar/cheddar-messaging/src/main/java/com/clicktravel/cheddar/infrastructure/messaging/pooled/listener/MessageHandlerWorkerThreadFactory.java
public class MessageHandlerWorkerThreadFactory implements ThreadFactory {private final AtomicInteger threadSequenceNumber = new AtomicInteger();
private final String queueName;
public MessageHandlerWorkerThreadFactory(final String queueName) {this.queueName = queueName;}
@Override
public Thread newThread(final Runnable r) {final int seq = threadSequenceNumber.incrementAndGet();
return new Thread(r, "MessageHandler:" + queueName + ":" + seq);
}
}MessageHandlerWorkerThreadFactory 实现了 ThreadFactory 接口，其 newThread 办法应用 queueName 及 threadSequenceNumber 来命名 thread
MessageHandlerWorker
Cheddar/cheddar/cheddar-messaging/src/main/java/com/clicktravel/cheddar/infrastructure/messaging/pooled/listener/MessageHandlerWorker.java
public class MessageHandlerWorker<T extends Message> implements Runnable {private final Logger logger = LoggerFactory.getLogger(getClass());
private final T message;
private final MessageHandler<T> messageHandler;
private final PooledMessageListener<T> pooledMessageListener;
public MessageHandlerWorker(final PooledMessageListener<T> pooledMessageListener, final T message,
final MessageHandler<T> messageHandler) {
this.message = message;
this.messageHandler = messageHandler;
this.pooledMessageListener = pooledMessageListener;
}
@Override
public void run() {
try {messageHandler.handle(message);
} catch (final Exception e) {logger.error("Error handling message:" + message, e);
} finally {
try {pooledMessageListener.completeMessageProcessing(message);
} catch (final InterruptedException e) {Thread.currentThread().interrupt();}
}
}
}MessageHandlerWorker 实现了 Runnable 接口，其 run 办法执行 messageHandler.handle 办法，最初执行 pooledMessageListener.completeMessageProcessing(message)
PooledMessageListener
Cheddar/cheddar/cheddar-messaging/src/main/java/com/clicktravel/cheddar/infrastructure/messaging/pooled/listener/PooledMessageListener.java
public abstract class PooledMessageListener<T extends Message> implements MessageListener, Runnable {
/**
* Maximum number of messages to receive from the queue at a time. Using larger numbers decreases the number of
* calls to receive and thus increases throughput, at the possible expense of latency.
*/
protected static final int DEFAULT_MAX_RECEIVED_MESSAGES = 10;
/**
* Controls when messages are received from the queue by setting an ideal minimum number of runnable tasks for each
* thread. This minimum includes the currently executing tasks and those on the thread pool work queue. When the
* number of runnable tasks dips below the ideal, more messages are received.
*/
protected static final int IDEAL_RUNNABLES_PER_THREAD = 2; // Each thread has 1 executing + 1 queued runnable
/**
* The default number of worker threads to use in a fixed size thread pool
*/
protected static final int DEFAULT_NUM_WORKER_THREADS = 10;
/**
* Maximum duration (in seconds) to wait for messages on the queue during normal processing. If there is at least
* one message on the queue, the actual duration will be shorter.
*/
private static final int LONG_POLL_DURATION_SECONDS = 20;
/**
* Maximum duration (in seconds) to wait for messages on the queue during handing over to a new application instance
* in a blue-green deployment. This is shorter to enable prompt termination of this message processor.
*/
private static final int SHORT_POLL_DURATION_SECONDS = 2;
/**
* Time (in milliseconds) to pause when receive message request returns an error
*/
private static final long RECEIVE_MESSAGE_ERROR_PAUSE_MILLIS = 500;
/**
* Maximum number of attempts to delete message from queue
*/
private static final int MAX_DELETE_MESSAGE_ATTEMPTS = 5;
/**
* Time (in milliseconds) to pause when delete message request returns an error
*/
private static final long DELETE_MESSAGE_ERROR_PAUSE_MILLIS = 1500;
private final Logger logger = LoggerFactory.getLogger(getClass());
private final MessageQueue<T> messageQueue;
private final ThreadPoolExecutor threadPoolExecutor;
private final RateLimiter rateLimiter;
private final Semaphore semaphore;
private final int maxReceivedMessages;
private volatile boolean started;
private volatile boolean shutdownRequested;
private volatile boolean shutdownRequestImminent;
public PooledMessageListener(final MessageQueue<T> messageQueue, final RateLimiter rateLimiter,
final ThreadPoolExecutor threadPoolExecutor, final Semaphore semaphore, final int maxReceivedMessages) {
this.messageQueue = messageQueue;
this.rateLimiter = rateLimiter;
this.threadPoolExecutor = threadPoolExecutor;
this.semaphore = semaphore;
this.maxReceivedMessages = maxReceivedMessages;
}
protected abstract MessageHandler<T> getHandlerForMessage(T message);
protected abstract void listenerStarted();
@Override
public void start() {new Thread(this).start();}
@Override
public void run() {
try {
started = true;
listenerStarted();
final String limiterSummary = rateLimiter != null ? ("using" + rateLimiter.toString())
: "not rate limited";
logger.debug(String.format("Listener for queue [%s] has pool of %d threads and is %s", queueName(),
threadPoolExecutor.getMaximumPoolSize(), limiterSummary));
processMessagesUntilShutdownRequested();} catch (final InterruptedException e) {Thread.currentThread().interrupt();} catch (final Throwable e) {logger.error(e.getMessage(), e);
throw e;
} finally {
logger.debug(String.format("Message listener for queue [%s] has stopped receiving messages. Initiating shutdown of task executor",
queueName()));
threadPoolExecutor.shutdown();}
}
private void processMessagesUntilShutdownRequested() throws InterruptedException {while (!shutdownRequested) {
// Block until there is capacity to handle up to maxReceivedMessages
semaphore.acquire(maxReceivedMessages);
List<T> messages = Collections.emptyList();
try {if (!shutdownRequested) {
final int pollSeconds = shutdownRequestImminent ? SHORT_POLL_DURATION_SECONDS
: LONG_POLL_DURATION_SECONDS;
try {messages = messageQueue.receive(pollSeconds, maxReceivedMessages);
} catch (final MessageReceiveException e) {logger.warn("Error receiving messages on queue:[" + queueName() + "]", e);
Thread.sleep(RECEIVE_MESSAGE_ERROR_PAUSE_MILLIS);
}
}
} finally {
// Release over-allocated permits
semaphore.release(maxReceivedMessages - messages.size());
}
for (final T message : messages) {processMessage(message); // Must complete processing each message to release permit
}
}
}
/**
* Processes a message by getting the appropriate message handler and scheduling a task to execute the handler. In
* case of problems (e.g. the message cannot be parsed), the message processing is completed to ensure the message
* is deleted from the queue and the associated permit is released.
* @param message {@link Message} to process
*/
private void processMessage(final T message) throws InterruptedException {
boolean workerAssigned = false;
try {final MessageHandler<T> messageHandler = getHandlerForMessage(message);
if (messageHandler != null) {applyRateLimiter();
threadPoolExecutor.execute(new MessageHandlerWorker<T>(this, message, messageHandler));
workerAssigned = true;
}
} catch (final Exception e) {logger.error("Unable to process received message", e);
}
if (!workerAssigned) {completeMessageProcessing(message);
}
}
/**
* Completes message processing by deleting it from the queue and releasing the associated permit.
* @param message {@link Message} to complete processing
*/
public void completeMessageProcessing(final T message) throws InterruptedException {deleteMessage(message);
semaphore.release();}
private void deleteMessage(final T message) throws InterruptedException {for (int attempts = 0; attempts < MAX_DELETE_MESSAGE_ATTEMPTS; attempts++) {
try {messageQueue.delete(message);
return;
} catch (final MessageDeleteException e) {logger.warn(String.format("Failed attempt to delete message with id [%s] from queue [%s]",
message.getMessageId(), queueName()), e);
Thread.sleep(DELETE_MESSAGE_ERROR_PAUSE_MILLIS);
}
}
logger.error(String.format("Failed all attempts to delete message with id [%s] from queue [%s]",
message.getMessageId(), queueName()));
}
private void applyRateLimiter() {if (rateLimiter != null) {
try {rateLimiter.takeToken();
} catch (final InterruptedException e) {Thread.currentThread().interrupt();}
}
}
protected String queueName() {return messageQueue.getName();
}
@Override
public void prepareForShutdown() {
logger.debug(String.format("Message listener for queue [%s] is preparing for imminent shutdown. Reducing queue poll time.",
queueName()));
shutdownRequestImminent = true;
}
@Override
public void shutdownListener() {logger.debug(String.format("Message listener for queue [%s] is shutting down", queueName()));
shutdownRequested = true;
if (!started) {threadPoolExecutor.shutdown();
}
}
@Override
public boolean awaitShutdownComplete(final long timeoutMillis) {
boolean terminated = false;
try {terminated = threadPoolExecutor.awaitTermination(timeoutMillis, TimeUnit.MILLISECONDS);
if (terminated) {logger.debug(String.format("Message listener for queue [%s] shutdown has completed", queueName()));
} else {
logger.warn(String.format("Message listener for queue [%s] has not shutdown as message handler worker threads have not completed",
queueName()));
}
} catch (final InterruptedException e) {Thread.currentThread().interrupt();}
return terminated;
}
}PooledMessageListener 的 run 办法先标记 started 为 true，而后执行 processMessagesUntilShutdownRequested；processMessagesUntilShutdownRequested 办法循环 messageQueue.receive(pollSeconds, maxReceivedMessages) 拉取音讯；而后遍历执行 processMessage；processMessage 办法先获取 messageHandler，而后创立 MessageHandlerWorker 放到线程池执行
小结
cheddar 的 MessageHandlerExecutor 继承了 ThreadPoolExecutor，其应用的 queue 为 LinkedBlockingQueue；threadFactory 为 MessageHandlerWorkerThreadFactory；MessageHandlerWorker 实现了 Runnable 接口，其 run 办法执行 messageHandler.handle 办法，最初执行 pooledMessageListener.completeMessageProcessing(message)。
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