Add event processing API

 * The EventDataHandler is a class intended for some heavy
concurrent event operations (like writing to database or file).
 * The IEventDataFactory interface is intended for adding additional
information to the event (like system info or timestamp) and also
for performing event filtering.
 * The IEventDataCache interface is intended for event cashing,
for example it can merge the EventData with same signature by
summing count.
 * It also determines the order of EventData processing. For example
it can retrive data based on the time of there signature
registration, or it can retrive data with the biggest count first.

DEVSIX-1958

Author: IdamkinI

kernel/src/main/java/com/itextpdf/kernel/counter/data/EventData.java

@@ -0,0 +1,89 @@
+/*
+
+    This file is part of the iText (R) project.
+    Copyright (c) 1998-2018 iText Group NV
+    Authors: Bruno Lowagie, Paulo Soares, et al.
+
+    This program is free software; you can redistribute it and/or modify
+    it under the terms of the GNU Affero General Public License version 3
+    as published by the Free Software Foundation with the addition of the
+    following permission added to Section 15 as permitted in Section 7(a):
+    FOR ANY PART OF THE COVERED WORK IN WHICH THE COPYRIGHT IS OWNED BY
+    ITEXT GROUP. ITEXT GROUP DISCLAIMS THE WARRANTY OF NON INFRINGEMENT
+    OF THIRD PARTY RIGHTS
+
+    This program is distributed in the hope that it will be useful, but
+    WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+    or FITNESS FOR A PARTICULAR PURPOSE.
+    See the GNU Affero General Public License for more details.
+    You should have received a copy of the GNU Affero General Public License
+    along with this program; if not, see http://www.gnu.org/licenses or write to
+    the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+    Boston, MA, 02110-1301 USA, or download the license from the following URL:
+    http://itextpdf.com/terms-of-use/
+
+    The interactive user interfaces in modified source and object code versions
+    of this program must display Appropriate Legal Notices, as required under
+    Section 5 of the GNU Affero General Public License.
+
+    In accordance with Section 7(b) of the GNU Affero General Public License,
+    a covered work must retain the producer line in every PDF that is created
+    or manipulated using iText.
+
+    You can be released from the requirements of the license by purchasing
+    a commercial license. Buying such a license is mandatory as soon as you
+    develop commercial activities involving the iText software without
+    disclosing the source code of your own applications.
+    These activities include: offering paid services to customers as an ASP,
+    serving PDFs on the fly in a web application, shipping iText with a closed
+    source product.
+
+    For more information, please contact iText Software Corp. at this
+    address: [email protected]
+ */
+package com.itextpdf.kernel.counter.data;
+
+/**
+ * The data that class that contains event signature (in simple cases it can be just event type)
+ * and count.
+ * Is used in {@link EventDataHandler} for adding some additional information to the event before processing
+ * and merging same events by increasing count.
+ *
+ * @param <T> the signature type
+ */
+public class EventData<T> {
+
+    private final T signature;
+    private long count;
+
+    public EventData(T signature) {
+        this(signature, 1);
+    }
+
+    public EventData(T signature, long count) {
+        this.signature = signature;
+        this.count = count;
+    }
+
+    /**
+     * The signature that identifies this data.
+     *
+     * @return data signature
+     */
+    public final T getSignature() {
+        return signature;
+    }
+
+    /**
+     * Number of data instances with the same signature that where merged.
+     *
+     * @return data count
+     */
+    public final long getCount() {
+        return count;
+    }
+
+    protected void mergeWith(EventData<T> data) {
+        this.count += data.getCount();
+    }
+}

kernel/src/main/java/com/itextpdf/kernel/counter/data/EventDataCacheComparatorBased.java

@@ -0,0 +1,106 @@
+/*
+
+    This file is part of the iText (R) project.
+    Copyright (c) 1998-2018 iText Group NV
+    Authors: Bruno Lowagie, Paulo Soares, et al.
+
+    This program is free software; you can redistribute it and/or modify
+    it under the terms of the GNU Affero General Public License version 3
+    as published by the Free Software Foundation with the addition of the
+    following permission added to Section 15 as permitted in Section 7(a):
+    FOR ANY PART OF THE COVERED WORK IN WHICH THE COPYRIGHT IS OWNED BY
+    ITEXT GROUP. ITEXT GROUP DISCLAIMS THE WARRANTY OF NON INFRINGEMENT
+    OF THIRD PARTY RIGHTS
+
+    This program is distributed in the hope that it will be useful, but
+    WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+    or FITNESS FOR A PARTICULAR PURPOSE.
+    See the GNU Affero General Public License for more details.
+    You should have received a copy of the GNU Affero General Public License
+    along with this program; if not, see http://www.gnu.org/licenses or write to
+    the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+    Boston, MA, 02110-1301 USA, or download the license from the following URL:
+    http://itextpdf.com/terms-of-use/
+
+    The interactive user interfaces in modified source and object code versions
+    of this program must display Appropriate Legal Notices, as required under
+    Section 5 of the GNU Affero General Public License.
+
+    In accordance with Section 7(b) of the GNU Affero General Public License,
+    a covered work must retain the producer line in every PDF that is created
+    or manipulated using iText.
+
+    You can be released from the requirements of the license by purchasing
+    a commercial license. Buying such a license is mandatory as soon as you
+    develop commercial activities involving the iText software without
+    disclosing the source code of your own applications.
+    These activities include: offering paid services to customers as an ASP,
+    serving PDFs on the fly in a web application, shipping iText with a closed
+    source product.
+
+    For more information, please contact iText Software Corp. at this
+    address: [email protected]
+ */
+package com.itextpdf.kernel.counter.data;
+
+import java.util.ArrayList;
+import java.util.Comparator;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.TreeSet;
+
+/**
+ * Comparator-based implementation of {@link IEventDataCache}.
+ * Merges data with the same signature by increasing its count.
+ * Retrieve the smallest element based on comparator.
+ *
+ * Not thread safe.
+ *
+ * @param <T> the data signature type
+ * @param <V> the data type
+ */
+public class EventDataCacheComparatorBased<T, V extends EventData<T>> implements IEventDataCache<T, V> {
+
+    private Map<T, V> map = new HashMap<>();
+    private Set<V> orderedCache;
+
+    public EventDataCacheComparatorBased(Comparator<V> comparator) {
+        orderedCache = new TreeSet<>(comparator);
+    }
+
+    @Override
+    public void put(V data) {
+        if (data != null) {
+            V old = map.put(data.getSignature(), data);
+            if (old != null) {
+                orderedCache.remove(old);
+                data.mergeWith(old);
+                orderedCache.add(data);
+            } else {
+                orderedCache.add(data);
+            }
+        }
+    }
+
+    @Override
+    public V retrieveNext() {
+        for (V data : orderedCache) {
+            if (data != null) {
+                map.remove(data.getSignature());
+                orderedCache.remove(data);
+                return data;
+            }
+        }
+        return null;
+    }
+
+    @Override
+    public List<V> clear() {
+        ArrayList<V> result = new ArrayList<>(map.values());
+        map.clear();
+        orderedCache.clear();
+        return result;
+    }
+}

kernel/src/main/java/com/itextpdf/kernel/counter/data/EventDataCacheQueueBased.java

@@ -0,0 +1,94 @@
+/*
+
+    This file is part of the iText (R) project.
+    Copyright (c) 1998-2018 iText Group NV
+    Authors: Bruno Lowagie, Paulo Soares, et al.
+
+    This program is free software; you can redistribute it and/or modify
+    it under the terms of the GNU Affero General Public License version 3
+    as published by the Free Software Foundation with the addition of the
+    following permission added to Section 15 as permitted in Section 7(a):
+    FOR ANY PART OF THE COVERED WORK IN WHICH THE COPYRIGHT IS OWNED BY
+    ITEXT GROUP. ITEXT GROUP DISCLAIMS THE WARRANTY OF NON INFRINGEMENT
+    OF THIRD PARTY RIGHTS
+
+    This program is distributed in the hope that it will be useful, but
+    WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+    or FITNESS FOR A PARTICULAR PURPOSE.
+    See the GNU Affero General Public License for more details.
+    You should have received a copy of the GNU Affero General Public License
+    along with this program; if not, see http://www.gnu.org/licenses or write to
+    the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+    Boston, MA, 02110-1301 USA, or download the license from the following URL:
+    http://itextpdf.com/terms-of-use/
+
+    The interactive user interfaces in modified source and object code versions
+    of this program must display Appropriate Legal Notices, as required under
+    Section 5 of the GNU Affero General Public License.
+
+    In accordance with Section 7(b) of the GNU Affero General Public License,
+    a covered work must retain the producer line in every PDF that is created
+    or manipulated using iText.
+
+    You can be released from the requirements of the license by purchasing
+    a commercial license. Buying such a license is mandatory as soon as you
+    develop commercial activities involving the iText software without
+    disclosing the source code of your own applications.
+    These activities include: offering paid services to customers as an ASP,
+    serving PDFs on the fly in a web application, shipping iText with a closed
+    source product.
+
+    For more information, please contact iText Software Corp. at this
+    address: [email protected]
+ */
+package com.itextpdf.kernel.counter.data;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * Queue-based implementation of {@link IEventDataCache}.
+ * Merges data with the same signature by increasing its count.
+ * Will retrieve the first elements by the time of its signature registration.
+ *
+ * Not thread safe.
+ *
+ * @param <T> the data signature type
+ * @param <V> the data type
+ */
+public class EventDataCacheQueueBased<T, V extends EventData<T>> implements IEventDataCache<T, V> {
+
+    private Map<T, V> map = new HashMap<>();
+    private LinkedList<T> signatureQueue = new LinkedList<>();
+
+    @Override
+    public void put(V data) {
+        if (data != null) {
+            V old = map.put(data.getSignature(), data);
+            if (old != null) {
+                data.mergeWith(old);
+            } else {
+                signatureQueue.addLast(data.getSignature());
+            }
+        }
+    }
+
+    @Override
+    public V retrieveNext() {
+        if (!signatureQueue.isEmpty()) {
+            return map.remove(signatureQueue.pollFirst());
+        }
+        return null;
+    }
+
+    @Override
+    public List<V> clear() {
+        ArrayList<V> result = new ArrayList<>(map.values());
+        map.clear();
+        signatureQueue.clear();
+        return result;
+    }
+}