聊聊logback的DuplicateMessageFilter

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本文主要研究一下logback的DuplicateMessageFilter

TurboFilter

ch/qos/logback/classic/turbo/TurboFilter.java

代码语言：javascript复制

public abstract class TurboFilter extends ContextAwareBase implements LifeCycle {

    private String name;
    boolean start = false;

    /**
     * Make a decision based on the multiple parameters passed as arguments. The
     * returned value should be one of <code>{@link FilterReply#DENY}</code>,
     * <code>{@link FilterReply#NEUTRAL}</code>, or
     * <code>{@link FilterReply#ACCEPT}</code>.
     * 
     * @param marker
     * @param logger
     * @param level
     * @param format
     * @param params
     * @param t
     * @return
     */
    public abstract FilterReply decide(Marker marker, Logger logger, Level level, String format, Object[] params,
            Throwable t);

    public void start() {
        this.start = true;
    }

    public boolean isStarted() {
        return this.start;
    }

    public void stop() {
        this.start = false;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

TurboFilter继承了ContextAwareBase，声明实现LifeCycle接口，它定义了decide方法由子类实现

DuplicateMessageFilter

ch/qos/logback/classic/turbo/DuplicateMessageFilter.java

代码语言：javascript复制

public class DuplicateMessageFilter extends TurboFilter {

    /**
     * The default cache size.
     */
    public static final int DEFAULT_CACHE_SIZE = 100;
    /**
     * The default number of allows repetitions.
     */
    public static final int DEFAULT_ALLOWED_REPETITIONS = 5;

    public int allowedRepetitions = DEFAULT_ALLOWED_REPETITIONS;
    public int cacheSize = DEFAULT_CACHE_SIZE;

    private LRUMessageCache msgCache;

    @Override
    public void start() {
        msgCache = new LRUMessageCache(cacheSize);
        super.start();
    }

    @Override
    public void stop() {
        msgCache.clear();
        msgCache = null;
        super.stop();
    }

    @Override
    public FilterReply decide(Marker marker, Logger logger, Level level, String format, Object[] params, Throwable t) {
        int count = msgCache.getMessageCountAndThenIncrement(format);
        if (count <= allowedRepetitions) {
            return FilterReply.NEUTRAL;
        } else {
            return FilterReply.DENY;
        }
    }

    public int getAllowedRepetitions() {
        return allowedRepetitions;
    }

    /**
     * The allowed number of repetitions before
     * 
     * @param allowedRepetitions
     */
    public void setAllowedRepetitions(int allowedRepetitions) {
        this.allowedRepetitions = allowedRepetitions;
    }

    public int getCacheSize() {
        return cacheSize;
    }

    public void setCacheSize(int cacheSize) {
        this.cacheSize = cacheSize;
    }

}

DuplicateMessageFilter继承了TurboFilter，它使用了LRUMessageCache(默认大小100)来缓存format，decide方法会执行getMessageCountAndThenIncrement，若超出allowedRepetitions(默认5)则返回FilterReply.DENY，否则返回FilterReply.NEUTRAL

LRUMessageCache

ch/qos/logback/classic/turbo/LRUMessageCache.java

代码语言：javascript复制

class LRUMessageCache extends LinkedHashMap<String, Integer> {

    private static final long serialVersionUID = 1L;
    final int cacheSize;

    LRUMessageCache(int cacheSize) {
        super((int) (cacheSize * (4.0f / 3)), 0.75f, true);
        if (cacheSize < 1) {
            throw new IllegalArgumentException("Cache size cannot be smaller than 1");
        }
        this.cacheSize = cacheSize;
    }

    int getMessageCountAndThenIncrement(String msg) {
        // don't insert null elements
        if (msg == null) {
            return 0;
        }

        Integer i;
        // LinkedHashMap is not LinkedHashMap. See also LBCLASSIC-255
        synchronized (this) {
            i = super.get(msg);
            if (i == null) {
                i = 0;
            } else {
                i = i   1;
            }
            super.put(msg, i);
        }
        return i;
    }

    // called indirectly by get() or put() which are already supposed to be
    // called from within a synchronized block
    protected boolean removeEldestEntry(Map.Entry<String, Integer> eldest) {
        return (size() > cacheSize);
    }

    @Override
    synchronized public void clear() {
        super.clear();
    }
}

LRUMessageCache继承了LinkedHashMap，其初始size为cacheSize * (4.0f / 3)，getMessageCountAndThenIncrement方法内部通过synchronized加锁获取指定msg的次数，不存在则设置为0，存在则递增；其removeEldestEntry方法判断size() > cacheSize

小结

DuplicateMessageFilter继承了TurboFilter，它使用了LRUMessageCache(默认大小100)来缓存format，decide方法会执行getMessageCountAndThenIncrement，若超出allowedRepetitions(默认5)则返回FilterReply.DENY，否则返回FilterReply.NEUTRAL。

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