leastrecentlyusedcache.java

Sequoia ERP是一个真正的企业级开源ERP解决方案。它提供的模块包括：电子商务应用(e-commerce), POS系统(point of sales),知识管理,存货与仓库管理

/*
 * Licensed under the X license (see http://www.x.org/terms.htm)
 */
package org.ofbiz.minerva.pool.cache;

import java.io.PrintWriter;
import java.util.*;

/**
 * A Least Recently Used cache implementation.  The object in the
 * cache that was least recently used is dropped when the cache is
 * full and a new request comes in.  The implementation uses a linked
 * list (to track recentness) and a HashMap (to navigate quickly).
 * @author Aaron Mulder [e-mail]
 */
public class LeastRecentlyUsedCache implements ObjectCache {

    private Object lock = new Object();
    private HashMap keyMap = new HashMap();
    private PrintWriter log = null;
    private Node mostRecentNode, leastRecentNode;
    private int size;
    private int maxSize;
    private CachedObjectFactory factory;

    public LeastRecentlyUsedCache(CachedObjectFactory factory, int maxSize) {
        this.factory = factory;
        this.maxSize = maxSize;
    }

    public Object getObject(Object key) {
        return getObject(key, false);
    }

    public Object useObject(Object key) {
        return getObject(key, true);
    }

    public void returnObject(Object key, Object value) {
        Object pooled = keyMap.get(key);
        Object source = factory.translateObject(value);

        if (pooled == null) {
            factory.deleteObject(source);
        } else if (pooled instanceof Node) {
            Node node = (Node) pooled;
            if (node.data == source) {
                try {
                    node.setUsed(false);
                } catch (ConcurrentModificationException e) {
                }
            } else {
                factory.deleteObject(source);
            }
        } else if (pooled instanceof LinkedList) {
            for (Iterator it = ((LinkedList) pooled).iterator(); it.hasNext();) {
                Node node = (Node) it.next();
                if (node.data == source) {
                    try {
                        node.setUsed(false);
                    } catch (ConcurrentModificationException e) {
                    }
                    return;
                }
            }
            factory.deleteObject(source);
        } else
            throw new Error("LRU Cache Assertion Failure: Wrong class '" + pooled.getClass().getName() + "' in keyMap!");
    }

    public void removeObjects(Object key) {
        synchronized (lock) {
            Object value = keyMap.get(key);
            if (value == null) {
                return;
            } else if (value instanceof Node) {
                removeNode((Node) value);
            } else if (value instanceof LinkedList) {
                LinkedList list = (LinkedList) value;
                int max = list.size();
                for (int i = 0; i < max; i++) {
                    removeNode((Node) list.get(0));
                }
            }
        }
    }

    public void setSize(int maxSize) {
        this.maxSize = maxSize;
        checkMaxSize();
    }

    public void close() {
        synchronized (lock) {
            Node current = leastRecentNode;
            Node next = current == null ? null : current.moreRecent;
            while (current != null) {
                removeNode(current);
                current = next;
                next = current == null ? null : current.moreRecent;
            }
        }
    }

    private Object getObject(Object key, boolean use) {
        Node node = null;
        try {
            Object value = keyMap.get(key);
            if (value == null) {
                node = addObject(key);
            } else if (value instanceof Node) {
                node = (Node) value;
                if (!node.used) {
                    makeMostRecent(node);
                } else {
                    node = addObject(key);
                }
            } else if (value instanceof LinkedList) {
                for (Iterator it = ((LinkedList) value).iterator(); it.hasNext();) {
                    node = (Node) it.next();
                    if (!node.used) {
                        makeMostRecent(node);
                        break;
                    } else {
                        node = null;
                    }
                }
                if (node == null) {
                    node = addObject(key);
                }
            } else
                throw new Error("LRU Cache Assertion Failure: Wrong class '" + value.getClass().getName() + "' in keyMap!");
            if (use) {
                node.setUsed(true);
            }
        } catch (ConcurrentModificationException e) {
            return getObject(key, use);
        }
        return factory.prepareObject(node.data);
    }

    private Node addObject(Object key) {
        Object data;
        try {
            data = factory.createObject(key);
        } catch (Exception e) {
            if (log != null)
                e.printStackTrace(log);
            return null;
        }
        Node result;
        synchronized (lock) {
            if (mostRecentNode == null) {
                result = new Node(null, null, data);
                mostRecentNode = result;
                leastRecentNode = result;
            } else {
                result = new Node(mostRecentNode, null, data);
                mostRecentNode.moreRecent = result;
                mostRecentNode = result;
            }
            ++size;
            checkMaxSize();
        }

        // Put the key/value(s) and node/key in the map
        Object value = keyMap.get(key);
        if (value == null) {
            keyMap.put(key, result);
        } else if (value instanceof LinkedList) {
            ((LinkedList) value).add(result);
        } else if (value instanceof Node) {
            LinkedList list = new LinkedList();
            list.add(value);
            list.add(result);
            keyMap.put(key, list);
        } else
            throw new Error("LRU Cache Assertion Failure: Wrong class '" + value.getClass().getName() + "' in keyMap!");
        keyMap.put(result, key);
        return result;
    }

    private void checkMaxSize() {
        if (maxSize <= 0)
            return;
        while (size > maxSize) {
            Node drop = leastRecentNode;
            leastRecentNode = drop.moreRecent;
            leastRecentNode.lessRecent = null;
            --size;

            removeNode(drop);
        }
    }

    private void makeMostRecent(Node node) {
        synchronized (lock) {
            if (node.moreRecent == null) {
                if (mostRecentNode != node)
                    throw new ConcurrentModificationException();
                return;
            }

            // Prepare surrounding nodes
            Node previous = node.moreRecent;
            Node next = node.lessRecent;
            previous.lessRecent = next;
            if (next == null) {
                leastRecentNode = previous;
            } else {
                next.moreRecent = previous;
            }

            // Prepare node and new 2nd most recent
            node.moreRecent = null;
            node.lessRecent = mostRecentNode;
            mostRecentNode.moreRecent = node;
            mostRecentNode = node;
        }
    }

    // This should be called while holding the lock
    private void removeNode(Node node) {
        boolean used = node.used;
        if (!used) {
            node.used = true;
        }
        Object key = keyMap.remove(node);


        Object value = keyMap.get(key);
        if (value instanceof Node) {
            keyMap.remove(key);
        } else if (value instanceof LinkedList) {
            LinkedList list = (LinkedList) value;
            Iterator it = list.iterator();
            while (it.hasNext()) {
                Node current = (Node) it.next();
                if (current == node) {
                    it.remove();
                    break;
                }
            }
            if (list.size() == 1) {
                keyMap.put(key, list.get(0));
            }
        } else
            throw new Error("LRU Cache Assertion Failure: Wrong class '" + value.getClass().getName() + "' in keyMap!");
        if (!used) {
            factory.deleteObject(node.data);
        }
        node.moreRecent = null;
        node.lessRecent = null;
    }

    private class Node {

        Node lessRecent;
        Node moreRecent;
        Object data;
        boolean used = false;

        public Node(Node lessRecent, Node moreRecent, Object data) {
            this(lessRecent, moreRecent, data, false);
        }

        public Node(Node lessRecent, Node moreRecent, Object data, boolean used) {
            this.lessRecent = lessRecent;
            this.moreRecent = moreRecent;
            this.data = data;
            this.used = used;
        }

        public synchronized void setUsed(boolean used) {
            if (this.used == used)
                throw new ConcurrentModificationException();
            this.used = used;
        }
    }

/*  This is for testing only

    private void printList() {
        System.out.print("Fwd Nodes:");
        Node node = mostRecentNode;
        for(int i=0; i<size; i++) {
            System.out.print(" "+node.data);
            node = node.lessRecent;
        }
        System.out.println();
        Stack stack = new Stack();
        node = leastRecentNode;
        for(int i=0; i<size; i++) {
            stack.push(node.data);
            node = node.moreRecent;
        }
        System.out.print("Rev Nodes:");
        while(!stack.isEmpty())
            System.out.print(" "+stack.pop());
        System.out.println();
    }

    public static void main(String args[]) {
        try {
            java.io.BufferedReader in = new java.io.BufferedReader(new java.io.InputStreamReader(System.in));
            LeastRecentlyUsedCache cache = new LeastRecentlyUsedCache(new CachedObjectFactory() {
                public Object createObject(Object identifier) {
                    return "v"+identifier;
                }
                public void deleteObject(Object object) {
                    System.out.println("Deleting Object "+object);
                }
            }, 5);

            * *** THIS TESTS getObject ****

            while(true) {
                cache.printList();
                System.out.print("> ");
                System.out.flush();
                String key = in.readLine();
                if(key.equalsIgnoreCase("quit") || key.equalsIgnoreCase("exit"))
                    return;
                Object value = cache.getObject(key);
                System.out.println("Got Value: "+value);
            }

            * ********* *

            **** THIS TESTS useObject AND returnObject ****

            Object lastKey = null, last = null;
            while(true) {
                cache.printList();
                System.out.print("> ");
                System.out.flush();
                String key = in.readLine();
                if(key.equalsIgnoreCase("quit") || key.equalsIgnoreCase("exit"))
                    return;
                Object value = cache.useObject(key);
                if(last != null) {
                    cache.returnObject(lastKey, last);
                }
                lastKey = key;
                last = value;
                System.out.println("Got Value: "+value);
            }

            ********** *
        } catch(Throwable e) {
            e.printStackTrace();
        }
    }
*/
}