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本文接下来分析上文涉及到的ObjectIdentityCache接口及相关对象

先熟悉一下继承和依赖关系，简要UML类图如下：

我们先来了解一下ObjectIdentityCache接口的源码（泛型接口）

/**
 * An object cache for create-once-by-name-and-then-reuse objects. 
 * 
 * Objects are added, but never removed. Subsequent get()s using the 
 * same key will return the exact same object, UNLESS all such objects
 * have been forgotten, in which case a new object MAY be returned. 
 * 
 * This allows implementors (such as ObjectIdentityBdbCache or 
 * CachedBdbMap) to page out (aka 'expunge') instances to
 * persistent storage while they're not being used. However, as long as
 * they are used (referenced), all requests for the same-named object
 * will share a reference to the same object, and the object may be
 * mutated in place without concern for explicitly persisting its
 * state to disk.  
 * 
 * @param <V>
 */
public interface ObjectIdentityCache<V extends IdentityCacheable> extends Closeable {
    /** get the object under the given key/name -- but should not mutate 
     * object state*/
    public abstract V get(final String key);
    
    /** get the object under the given key/name, using (and remembering)
     * the object supplied by the supplier if no prior mapping exists 
     * -- but should not mutate object state */
    public abstract V getOrUse(final String key, Supplier<V> supplierOrNull);

    /** force the persistent backend, if any, to be updated with all 
     * live object state */ 
    public abstract void sync();
    
    /** force the persistent backend, if any, to eventually be updated with 
     * live object state for the given key */ 
    public abstract void dirtyKey(final String key);

    /** close/release any associated resources */ 
    public abstract void close();
    
    /** count of name-to-object contained */ 
    public abstract int size();

    /** set of all keys */ 
    public abstract Set<String> keySet();
}

该接口是用来管理对象缓存的，而被管理的对象必须是实现了IdentityCacheable接口的对象（泛型）

在heritrix3.1.0系统里面，有三个类实现了ObjectIdentityCache接口，分别为ObjectIdentityBdbCache、ObjectIdentityMemCache、ObjectIdentityBdbManualCache

最重要的是ObjectIdentityBdbManualCache类，我们可以在BdbModule类找到它的初始化方法

/**
     * Get an ObjectIdentityBdbCache, backed by a BDB Database of the 
     * given name, with the given value class type. If 'recycle' is true,
     * reuse values already in the database; otherwise start with an 
     * empty cache. 
     *  
     * @param <V>
     * @param dbName
     * @param recycle
     * @param valueClass
     * @return
     * @throws DatabaseException
     */
    public <V extends IdentityCacheable> ObjectIdentityBdbManualCache<V> getOIBCCache(String dbName, boolean recycle,
            Class<? extends V> valueClass) 
    throws DatabaseException {
        if (!recycle) {
            try {
                bdbEnvironment.truncateDatabase(null, dbName, false);
            } catch (DatabaseNotFoundException e) {
                // ignored
            }
        }
        ObjectIdentityBdbManualCache<V> oic = new ObjectIdentityBdbManualCache<V>();
        oic.initialize(bdbEnvironment, dbName, valueClass, classCatalog);
        oiCaches.put(dbName, oic);
        return oic;
    }

初始化方法里面传入了BDB数据库的环境变量、数据库名、要缓存的对象类名、StoredClassCatalog classCatalog变量（用于对象类型转换）

ObjectIdentityBdbManualCache类是一个泛型类，最重要的成员变量如下：

/** The BDB JE database used for this instance. */
    protected transient Database db;

    /** in-memory map of new/recent/still-referenced-elsewhere instances */
    protected transient ConcurrentMap<String,V> memMap;

    /** The Collection view of the BDB JE database used for this instance. */
    protected transient StoredSortedMap<String, V> diskMap;

    protected transient ConcurrentMap<String,V> dirtyItems;
    
    protected AtomicLong count;

上面均为泛型容器（支持同步），其中被管理的对象为V类型（实现IdentityCacheable接口），每个被管理的对象以key/value的形式存储在上面容器中，其中最关键的容器是StoredSortedMap<String, V> diskMap 

接下来查看它的初始化方法（它的构造方法很平庸，此处忽略贴出）

/**
     * Call this method when you have an instance when you used the
     * default constructor or when you have a deserialized instance that you
     * want to reconnect with an extant bdbje environment.  Do not
     * call this method if you used the
     * {@link #CachedBdbMap(File, String, Class, Class)} constructor.
     * @param env
     * @param keyClass
     * @param valueClass
     * @param classCatalog
     * @throws DatabaseException
     */
    @SuppressWarnings("unchecked")
    public void initialize(final Environment env, String dbName,
            final Class valueClass, final StoredClassCatalog classCatalog)
    throws DatabaseException {
        // TODO: tune capacity for actual threads, expected size of key caches? 
        this.memMap = new MapMaker().concurrencyLevel(64).initialCapacity(8192).softValues().makeMap();    
        this.db = openDatabase(env, dbName);
        this.diskMap = createDiskMap(this.db, classCatalog, valueClass);
        // keep a record of items that must be persisted; auto-persist if 
        // unchanged after 5 minutes, or more than 10K would collect
        this.dirtyItems = new MapMaker().concurrencyLevel(64)
            .maximumSize(10000).expireAfterWrite(5,TimeUnit.MINUTES)
            .evictionListener(this).makeMap();
            
        this.count = new AtomicLong(diskMap.size());
    }

初始化数据库Database db、内存容器ConcurrentMap<String,V> memMap、BDB容器StoredSortedMap<String, V> diskMap、临时容器ConcurrentMap<String,V> dirtyItems等

Database openDatabase(final Environment environment,final String dbName)方法为根据BDB环境和数据库名创建数据库

protected Database openDatabase(final Environment environment,
            final String dbName) throws DatabaseException {
        DatabaseConfig dbConfig = new DatabaseConfig();
        dbConfig.setTransactional(false);
        dbConfig.setAllowCreate(true);
        dbConfig.setDeferredWrite(true);
        return environment.openDatabase(null, dbName, dbConfig);
    }

StoredSortedMap<String, V> createDiskMap(Database database,StoredClassCatalog classCatalog, Class valueClass)方法根据创建的数据库，数据项转换对象以及要存储的类型创建StoredSortedMap<String, V> diskMap对象，显然该对象依赖于BDB数据库（容器里面的项存储于BDB数据库）

@SuppressWarnings("unchecked")
    protected StoredSortedMap<String, V> createDiskMap(Database database,
            StoredClassCatalog classCatalog, Class valueClass) {
        EntryBinding keyBinding = TupleBinding.getPrimitiveBinding(String.class);
        EntryBinding valueBinding = TupleBinding.getPrimitiveBinding(valueClass);
        if(valueBinding == null) {
            valueBinding = 
                new KryoBinding<V>(valueClass);
//                new SerialBinding(classCatalog, valueClass);
//                new BenchmarkingBinding<V>(new EntryBinding[] {
//                      new KryoBinding<V>(valueClass),                   
//                      new RecyclingSerialBinding<V>(classCatalog, valueClass),
//                  }, valueClass);
        }
        return new StoredSortedMap<String,V>(database, keyBinding, valueBinding, true);
    }

 那么Heritrix3.1.0系统里面是怎样重用容器中的被缓存的对象呢？我们在BdbFrontier类的方法里面可以看到如下方法

/**
     * Return the work queue for the given classKey, or null
     * if no such queue exists.
     * 
     * @param classKey key to look for
     * @return the found WorkQueue
     */
    protected WorkQueue getQueueFor(final String classKey) {      
        WorkQueue wq = allQueues.getOrUse(
                classKey,
                new Supplier<WorkQueue>() {
                    public BdbWorkQueue get() {
                        String qKey = new String(classKey); // ensure private minimal key
                        BdbWorkQueue q = new BdbWorkQueue(qKey, BdbFrontier.this);
                        q.setTotalBudget(getQueueTotalBudget()); //-1
                        System.out.println(getQueuePrecedencePolicy().getClass().getName());
                        getQueuePrecedencePolicy().queueCreated(q);
                        return q;
                    }});
        return wq;
    }

BdbWorkQueue类即为被管理的对象，该类间接实现了IdentityCacheable接口，从上面我们可以看到，外部类通过调用ObjectIdentityBdbManualCache对象的V getOrUse(final String key, Supplier<V> supplierOrNull)方法获取被缓存的对象

/* (non-Javadoc)
     * @see org.archive.util.ObjectIdentityCache#get(java.lang.String, org.archive.util.ObjectIdentityBdbCache)
     */
    public V getOrUse(final String key, Supplier<V> supplierOrNull) {
        countOfGets.incrementAndGet();
        
        if (countOfGets.get() % 10000 == 0) {
            logCacheSummary();
        }
        
        // check mem cache
        V val = memMap.get(key);
        if(val != null) {
            // the concurrent garden path: in memory and valid
            cacheHit.incrementAndGet();
            val.setIdentityCache(this); 
            return val;
        }
        val = diskMap.get(key);
        V prevVal; 
        if(val == null) {
            // never yet created, consider creating
            if(supplierOrNull==null) {
                return null;
            }
            val = supplierOrNull.get();
            supplierUsed.incrementAndGet();
            // putting initial value directly into diskMap
            // (rather than just the memMap until page-out)
            // ensures diskMap.keySet() provides complete view
            prevVal = diskMap.putIfAbsent(key, val); 
            if(prevVal!=null) {
                // we lost a race; discard our local creation in favor of disk version
                diskHit.incrementAndGet();
                val = prevVal;
            } else {
                // we uniquely added a new key
                count.incrementAndGet();
            }
        } else {
            diskHit.incrementAndGet();
        }
        
        prevVal = memMap.putIfAbsent(key, val); // fill memMap or lose race gracefully
        if(prevVal != null) {
            val = prevVal; 
        }
        val.setIdentityCache(this); 
        return val; 
    }

上述方法跟我们以前的缓存管理有点类似，首先根据key从缓存获取对象，如果没有则将新对象加入缓存（以后复用）

接下来看后面的方法

void dirtyKey(String key)方法为将指定key的V类型对象从memMap容器同时添加到dirtyItems容器

@Override
    public void dirtyKey(String key) {
       V val = memMap.get(key);
       if(val==null) {
           logger.severe("dirty key not in memory should be impossible");
       }
       dirtyItems.put(key,val); 
    }

void onEviction(String key, V val)方法将key/value对象添加到diskMap容器（MapEvictionListener接口方法）

 @Override
    public void onEviction(String key, V val) {
        evictions.incrementAndGet();
        diskMap.put(key, val);
    }

void sync()方法将dirtyItems容器中的对象同步到BDB数据库

/**
     * Sync all in-memory map entries to backing disk store.
     */
    public synchronized void sync() {
        String dbName = null;
        // Sync. memory and disk.
        useStatsSyncUsed.incrementAndGet();
        long startTime = 0;
        if (logger.isLoggable(Level.FINE)) {
            dbName = getDatabaseName();
            startTime = System.currentTimeMillis();
            logger.fine(dbName + " start sizes: disk " + this.diskMap.size() +
                ", mem " + this.memMap.size());
        }
        
        Iterator<Entry<String, V>> iter = dirtyItems.entrySet().iterator();
        while(iter.hasNext()) {
            Entry<String, V> entry = iter.next(); 
            iter.remove();
            diskMap.put(entry.getKey(), entry.getValue());
        }
        
        try {
            this.db.sync();
        } catch (DatabaseException e) {
            throw new RuntimeException(e);
        }
        
        
        if (logger.isLoggable(Level.FINE)) {
            logger.fine(dbName + " sync took " +
                (System.currentTimeMillis() - startTime) + "ms. " +
                "Finish sizes: disk " +
                this.diskMap.size() + ", mem " + this.memMap.size());
        }
    }

接下来分析IdentityCacheable接口及相关类， IdentityCacheable接口声明的方法很简单，其源码如下：

/**
 * Common interface for objects held in ObjectIdentityCaches. 
 * 
 * @contributor gojomo
 */
public interface IdentityCacheable extends Serializable {
    public void setIdentityCache(ObjectIdentityCache<?> cache);
    public String getKey();
    public void makeDirty(); 
}

实现该接口的类必须实现上面三方法，我这里主要介绍WorkQueue类（抽象类），它的实现上面方法的相关代码如下

 //
    // IdentityCacheable support
    //
    transient private ObjectIdentityCache<?> cache;
    @Override
    public String getKey() {
        return getClassKey();
    }

    @Override
    public void makeDirty() {
        cache.dirtyKey(getKey());
    }

    @Override
    public void setIdentityCache(ObjectIdentityCache<?> cache) {
        this.cache = cache; 
    } 

从这里可以看出，void setIdentityCache(ObjectIdentityCache<?> cache)方法是设置管理当前被缓存的对象的缓存管理类（ObjectIdentityCache类型对象） 

在void makeDirty()方法里面是回调ObjectIdentityCache类型对象的方法

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本系列Heritrix 3.1.0 源码解析系本人原创

转载请注明出处 博客园 刺猬的温驯

本文链接 http://www.cnblogs.com/chenying99/archive/2013/04/18/3027679.html