New parent class AbstractContentDigestHistory for implementations to extend

* AbstractContentDigestHistory.java
    abstract methods load(), store(); non-abstract persistKeyFor(); javadocs
* BdbContentDigestHistory.java
    extend AbstractContentDigestHistory
* ContentDigestHistoryLoader.java, ContentDigestHistoryStorer.java
    use AbstractContentDigestHistory

modules/src/main/java/org/archive/modules/recrawl/AbstractContentDigestHistory.java

@@ -0,0 +1,59 @@
+/*
+ *  This file is part of the Heritrix web crawler (crawler.archive.org).
+ *
+ *  Licensed to the Internet Archive (IA) by one or more individual 
+ *  contributors. 
+ *
+ *  The IA licenses this file to You under the Apache License, Version 2.0
+ *  (the "License"); you may not use this file except in compliance with
+ *  the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package org.archive.modules.recrawl;
+
+import org.archive.modules.CrawlURI;
+
+/**
+ * Represents a store of information, presumably persistent, keyed by content
+ * digest.
+ * 
+ * @contributor nlevitt
+ */
+public abstract class AbstractContentDigestHistory {
+    /**
+     * Looks up the history by key {@code persistKeyFor(curi)} and loads it into
+     * {@code curi.getContentDigestHistory()}.
+     * 
+     * @param curi
+     */
+    public abstract void load(CrawlURI curi);
+    
+    /**
+     * Stores {@code curi.getContentDigestHistory()} for the key
+     * {@code persistKeyFor(curi)}.
+     * 
+     * @param curi
+     */
+    public abstract void store(CrawlURI curi);
+
+    /**
+     * 
+     * @param curi
+     * @return {@code curi.getContentDigestSchemeString()}
+     * @throws IllegalStateException if {@code curi.getContentDigestSchemeString()} is null
+     */
+    protected String persistKeyFor(CrawlURI curi) {
+        String key = curi.getContentDigestSchemeString();
+        if (key == null) {
+            throw new IllegalStateException("cannot load content digest history, CrawlURI does not have content digest value for " + curi);
+        }
+        return key;
+    }
+}

modules/src/main/java/org/archive/modules/recrawl/BdbContentDigestHistory.java

@@ -35,7 +35,7 @@ import com.sleepycat.je.Database;
 import com.sleepycat.je.DatabaseException;
 
 /** Needs to be a toplevel bean for Lifecyle? */
-public class BdbContentDigestHistory implements Lifecycle {
+public class BdbContentDigestHistory extends AbstractContentDigestHistory implements Lifecycle {
 
     private static final Logger logger = 
             Logger.getLogger(BdbContentDigestHistory.class.getName());
@@ -58,10 +58,6 @@ public class BdbContentDigestHistory implements Lifecycle {
     protected StoredSortedMap<String, Map> store;
     protected Database historyDb;
 
-    protected String persistKeyFor(CrawlURI curi) {
-        return curi.getContentDigestSchemeString();
-    }
-    
     @Override
     @SuppressWarnings({"rawtypes"})
     public void start() {

modules/src/main/java/org/archive/modules/recrawl/ContentDigestHistoryLoader.java

@@ -24,10 +24,10 @@ import org.springframework.beans.factory.annotation.Autowired;
 
 public class ContentDigestHistoryLoader extends Processor {
     
-    protected BdbContentDigestHistory contentDigestHistory;
+    protected AbstractContentDigestHistory contentDigestHistory;
     @Autowired
     public void setContentDigestHistory(
-            BdbContentDigestHistory contentDigestHistory) {
+            AbstractContentDigestHistory contentDigestHistory) {
         this.contentDigestHistory = contentDigestHistory;
     }
 

modules/src/main/java/org/archive/modules/recrawl/ContentDigestHistoryStorer.java

@@ -24,10 +24,10 @@ import org.springframework.beans.factory.annotation.Autowired;
 
 public class ContentDigestHistoryStorer extends Processor {
     
-    protected BdbContentDigestHistory contentDigestHistory;
+    protected AbstractContentDigestHistory contentDigestHistory;
     @Autowired
     public void setContentDigestHistory(
-            BdbContentDigestHistory contentDigestHistory) {
+            AbstractContentDigestHistory contentDigestHistory) {
         this.contentDigestHistory = contentDigestHistory;
     }