log4j2异步日志解读（一）AsyncAppender

 

log4j、logback、log4j2 历史和关系，我们就在这里不展开讲了。直接上干货，log4j2突出于其他日志的优势，异步日志实现。

看一个东西，首先看官网文档， ，因为前面文章已经讲解了disruptor源码【】，本文主要展开说说异步日志asyncappender和asynclogger（基于disruptor实现）。

asynclogger笔者下文展开讲。

 

一、asyncappender

 

我们先来看看asyncapperder核心，就是logger将数据通过append方法放入到阻塞队列中，随后后台线程从队列中取出数据然后进行后续的操作。

那这样看来，就很简单了，一个append()方法，一个后台线程执行就是我们要看的核心代码了。围绕我们要看的类asyncappender，来看看类关系图。

一、放入队列

主要实现就是logger将数据通过append方法放入到阻塞队列中。

//asyncappender.java
    /**
     * actual writing occurs here.
     *
     * @param logevent the logevent.
     */
    @override
    public void append(final logevent logevent) {
        if (!isstarted()) {
            throw new illegalstateexception("asyncappender " + getname() + " is not active");
        }
        //创建log4jlogevent的对象memento
        final log4jlogevent memento = log4jlogevent.creatememento(logevent, includelocation);
        internalasyncutil.makemessageimmutable(logevent.getmessage());
        //transfer(memento)将event放入队列
        //默认arrayblockingqueuefactory 大小1024
        if (!transfer(memento)) {
            if (blocking) {
                if (abstractlogger.getrecursiondepth() > 1) { // log4j2-1518, log4j2-2031
                    // if queue is full and we are in a recursive call, call appender directly to prevent deadlock
                    asyncqueuefullmessageutil.logwarningtostatuslogger();
                    logmessageincurrentthread(logevent);
                } else {
                    // delegate to the event router (which may discard, enqueue and block, or log in current thread)
                 
                    final eventroute route = asyncqueuefullpolicy.getroute(thread.getid(), memento.getlevel());
                    route.logmessage(this, memento);
                }
            } else {
                error("appender " + getname() + " is unable to write primary appenders. queue is full");
                logtoerrorappenderifnecessary(false, memento);
            }
        }
    }

    private boolean transfer(final logevent memento) {
        return queue instanceof transferqueue
            ? ((transferqueue<logevent>) queue).trytransfer(memento)
            : queue.offer(memento);
    }

如流程图所示，首先会判断用户是否设置了blocking选项，默认是true，如果设置为false，则appender直接会toerrorappender，如果用户没有配置或者配置为true，则会按照一定的策略来处理这些消息。策略可以分为2种，他们分别为：

1、defaultasyncqueuefullpolicy---等待队列，转为同步操作策略

public class defaultasyncqueuefullpolicy implements asyncqueuefullpolicy {
    @override
    public eventroute getroute(final long backgroundthreadid, final level level) {

        // log4j2-471: prevent deadlock when ringbuffer is full and object
        // being logged calls logger.log() from its tostring() method
        if (thread.currentthread().getid() == backgroundthreadid) {
            return eventroute.synchronous;
        }
        return eventroute.enqueue;
    }

2、discardingasyncqueuefullpolicy---按照日志等级抛弃日志策略

//discardingasyncqueuefullpolicy.java
    @override
    public eventroute getroute(final long backgroundthreadid, final level level) {
        if (level.islessspecificthan(thresholdlevel)) {
            if (discardcount.getandincrement() == 0) {
                logger.warn("async queue is full, discarding event with level {}. " +
                        "this message will only appear once; future events from {} " +
                        "are silently discarded until queue capacity becomes available.",
                        level, thresholdlevel);
            }
            return eventroute.discard;
        }
        return super.getroute(backgroundthreadid, level);
    }

二、后台线程执行后续操作。

主要就是后台线程从队列中取出数据然后进行后续的操作。

//asyncappender.java
private class asyncthread extends log4jthread {

        private volatile boolean shutdown = false;
        private final list<appendercontrol> appenders;
        private final blockingqueue<logevent> queue;

        public asyncthread(final list<appendercontrol> appenders, final blockingqueue<logevent> queue) {
            super("asyncappender-" + thread_sequence.getandincrement());
            this.appenders = appenders;
            this.queue = queue;
            setdaemon(true);
        }

        @override
        public void run() {
            while (!shutdown) {
                logevent event;
                try {
                    event = queue.take();
                    if (event == shutdown_log_event) {
                        shutdown = true;
                        continue;
                    }
                } catch (final interruptedexception ex) {
                    break; // log4j2-830
                }
                event.setendofbatch(queue.isempty());
                final boolean success = callappenders(event);
                if (!success && errorappender != null) {
                    try {
                        errorappender.callappender(event);
                    } catch (final exception ex) {
                        // silently accept the error.
                    }
                }
            }
            // process any remaining items in the queue.
            logger.trace("asyncappender.asyncthread shutting down. processing remaining {} queue events.",
                queue.size());
            int count = 0;
            int ignored = 0;
            while (!queue.isempty()) {
                try {
                    final logevent event = queue.take();
                    if (event instanceof log4jlogevent) {
                        final log4jlogevent logevent = (log4jlogevent) event;
                        logevent.setendofbatch(queue.isempty());
                        callappenders(logevent);
                        count++;
                    } else {
                        ignored++;
                        logger.trace("ignoring event of class {}", event.getclass().getname());
                    }
                } catch (final interruptedexception ex) {
                    // may have been interrupted to shut down.
                    // here we ignore interrupts and try to process all remaining events.
                }
            }
            logger.trace("asyncappender.asyncthread stopped. queue has {} events remaining. "
                + "processed {} and ignored {} events since shutdown started.", queue.size(), count, ignored);
        }

    ...
}

该线程会一直尝试从阻塞队列中获取logevent，如果获取成功，调用appenderref所引用appender的append方法。我们也可以看到，asyncappender实际上主要是类似于中转，日志异步化，当消息放入阻塞队列，返回成功，这样能够大幅提高日志记录的吞吐。用户可以在权衡性能与日志收集质量上进行权衡配置策略(设置blocking选项)，当然也可以设置不同类型的阻塞队列已到达更好的日志记录吞吐。

asyncappender配置参数  

https://logging.apache.org/log4j/2.x/manual/appenders.html#asyncappender