Merge branch 'master' of https://github.com/iluwatar/java-design-patterns

* 'master' of https://github.com/iluwatar/java-design-patterns: (27 commits)
  Remove use of coveralls-maven-plugin (sonarqube.com covers this)
  Add SonarQube.com badge
  Fix environment variable
  Add Travis instructions for SonarQube.com analysis
  Adds more criticism to Singleton pattern.
  Event Based Asynchronous pattern: Add missing license header and puml diagram
  Changed config to non-interactive
  Moved config into a separate dir
  Unused import removed.
  End process logic clause has been corrected.
  Caching pattern: Documentation and diagram
  Fixes #437. Adds criticism to Singleton pattern.
  Alter JUnit tests to run in lesser time.
  Updated version snapshot to 1.14.0
  Changes based on review feedback.
  Closes #436. Adds criticism to service locator pattern.
  Caching pattern: Implementation of Cache-Aside pattern
  Caching pattern: Style fix for null check
  Caching pattern: Refactor LRU cache to avoid NPE and unnecessary cache lookup
  Caching pattern: Refactor shutdown hook to use method reference
  ...

caching/src/main/java/com/iluwatar/caching/App.java

@@ -29,22 +29,23 @@ import org.slf4j.LoggerFactory;
  *
  * The Caching pattern describes how to avoid expensive re-acquisition of resources by not releasing
  * the resources immediately after their use. The resources retain their identity, are kept in some
- * fast-access storage, and are re-used to avoid having to acquire them again. There are three main
- * caching strategies/techniques in this pattern; each with their own pros and cons. They are:
+ * fast-access storage, and are re-used to avoid having to acquire them again. There are four main
+ * caching strategies/techniques in this pattern; each with their own pros and cons. They are;
  * <code>write-through</code> which writes data to the cache and DB in a single transaction,
- * <code>write-around</code> which writes data immediately into the DB instead of the cache, and
+ * <code>write-around</code> which writes data immediately into the DB instead of the cache,
  * <code>write-behind</code> which writes data into the cache initially whilst the data is only
- * written into the DB when the cache is full. The <code>read-through</code> strategy is also
- * included in the mentioned three strategies -- returns data from the cache to the caller <b>if</b>
- * it exists <b>else</b> queries from DB and stores it into the cache for future use. These
- * strategies determine when the data in the cache should be written back to the backing store (i.e.
- * Database) and help keep both data sources synchronized/up-to-date. This pattern can improve
- * performance and also helps to maintain consistency between data held in the cache and the data in
- * the underlying data store.
+ * written into the DB when the cache is full, and <code>cache-aside</code> which pushes the
+ * responsibility of keeping the data synchronized in both data sources to the application itself.
+ * The <code>read-through</code> strategy is also included in the mentioned four strategies --
+ * returns data from the cache to the caller <b>if</b> it exists <b>else</b> queries from DB and
+ * stores it into the cache for future use. These strategies determine when the data in the cache
+ * should be written back to the backing store (i.e. Database) and help keep both data sources
+ * synchronized/up-to-date. This pattern can improve performance and also helps to maintain
+ * consistency between data held in the cache and the data in the underlying data store.
  * <p>
  * In this example, the user account ({@link UserAccount}) entity is used as the underlying
  * application data. The cache itself is implemented as an internal (Java) data structure. It adopts
- * a Least-Recently-Used (LRU) strategy for evicting data from itself when its full. The three
+ * a Least-Recently-Used (LRU) strategy for evicting data from itself when its full. The four
  * strategies are individually tested. The testing of the cache is restricted towards saving and
  * querying of user accounts from the underlying data store ( {@link DbManager}). The main class (
  * {@link App} is not aware of the underlying mechanics of the application (i.e. save and query) and
@@ -80,6 +81,7 @@ public class App {
     app.useReadAndWriteThroughStrategy();
     app.useReadThroughAndWriteAroundStrategy();
     app.useReadThroughAndWriteBehindStrategy();
+    app.useCacheAsideStategy();
   }
 
   /**
@@ -142,4 +144,26 @@ public class App {
     AppManager.find("004");
     LOGGER.info(AppManager.printCacheContent());
   }
+
+  /**
+   * Cache-Aside
+   */
+  public void useCacheAsideStategy() {
+    System.out.println("# CachingPolicy.ASIDE");
+    AppManager.initCachingPolicy(CachingPolicy.ASIDE);
+    System.out.println(AppManager.printCacheContent());
+
+    UserAccount userAccount3 = new UserAccount("003", "Adam", "He likes food.");
+    UserAccount userAccount4 = new UserAccount("004", "Rita", "She hates cats.");
+    UserAccount userAccount5 = new UserAccount("005", "Isaac", "He is allergic to mustard.");
+    AppManager.save(userAccount3);
+    AppManager.save(userAccount4);
+    AppManager.save(userAccount5);
+
+    System.out.println(AppManager.printCacheContent());
+    AppManager.find("003");
+    System.out.println(AppManager.printCacheContent());
+    AppManager.find("004");
+    System.out.println(AppManager.printCacheContent());
+  }
 }

caching/src/main/java/com/iluwatar/caching/AppManager.java

@@ -64,12 +64,7 @@ public final class AppManager {
   public static void initCachingPolicy(CachingPolicy policy) {
     cachingPolicy = policy;
     if (cachingPolicy == CachingPolicy.BEHIND) {
-      Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() {
-        @Override
-        public void run() {
-          CacheStore.flushCache();
-        }
-      }));
+      Runtime.getRuntime().addShutdownHook(new Thread(CacheStore::flushCache));
     }
     CacheStore.clearCache();
   }
@@ -86,6 +81,8 @@ public final class AppManager {
       return CacheStore.readThrough(userId);
     } else if (cachingPolicy == CachingPolicy.BEHIND) {
       return CacheStore.readThroughWithWriteBackPolicy(userId);
+    } else if (cachingPolicy == CachingPolicy.ASIDE) {
+      return findAside(userId);
     }
     return null;
   }
@@ -100,10 +97,37 @@ public final class AppManager {
       CacheStore.writeAround(userAccount);
     } else if (cachingPolicy == CachingPolicy.BEHIND) {
       CacheStore.writeBehind(userAccount);
+    } else if (cachingPolicy == CachingPolicy.ASIDE) {
+      saveAside(userAccount);
     }
   }
 
   public static String printCacheContent() {
     return CacheStore.print();
   }
+
+  /**
+   * Cache-Aside save user account helper
+   */
+  private static void saveAside(UserAccount userAccount) {
+    DbManager.updateDb(userAccount);
+    CacheStore.invalidate(userAccount.getUserId());
+  }
+
+  /**
+   * Cache-Aside find user account helper
+   */
+  private static UserAccount findAside(String userId) {
+    UserAccount userAccount = CacheStore.get(userId);
+    if (userAccount != null) {
+      return userAccount;
+    }
+
+    userAccount = DbManager.readFromDb(userId);
+    if (userAccount != null) {
+      CacheStore.set(userId, userAccount);
+    }
+
+    return userAccount;
+  }
 }

caching/src/main/java/com/iluwatar/caching/CacheStore.java

@@ -45,7 +45,7 @@ public class CacheStore {
    * Init cache capacity
    */
   public static void initCapacity(int capacity) {
-    if (null == cache) {
+    if (cache == null) {
       cache = new LruCache(capacity);
     } else {
       cache.setCapacity(capacity);
@@ -126,7 +126,7 @@ public class CacheStore {
    * Clears cache
    */
   public static void clearCache() {
-    if (null != cache) {
+    if (cache != null) {
       cache.clear();
     }
   }
@@ -158,4 +158,25 @@ public class CacheStore {
     sb.append("----\n");
     return sb.toString();
   }
+
+  /**
+   * Delegate to backing cache store
+   */
+  public static UserAccount get(String userId) {
+    return cache.get(userId);
+  }
+
+  /**
+   * Delegate to backing cache store
+   */
+  public static void set(String userId, UserAccount userAccount) {
+    cache.set(userId, userAccount);
+  }
+
+  /**
+   * Delegate to backing cache store
+   */
+  public static void invalidate(String userId) {
+    cache.invalidate(userId);
+  }
 }

caching/src/main/java/com/iluwatar/caching/CachingPolicy.java

@@ -24,11 +24,11 @@ package com.iluwatar.caching;
 
 /**
  *
- * Enum class containing the three caching strategies implemented in the pattern.
+ * Enum class containing the four caching strategies implemented in the pattern.
  *
  */
 public enum CachingPolicy {
-  THROUGH("through"), AROUND("around"), BEHIND("behind");
+  THROUGH("through"), AROUND("around"), BEHIND("behind"), ASIDE("aside");
 
   private String policy;
 

caching/src/main/java/com/iluwatar/caching/DbManager.java

@@ -82,7 +82,7 @@ public final class DbManager {
       }
       return null;
     }
-    if (null == db) {
+    if (db == null) {
       try {
         connect();
       } catch (ParseException e) {
@@ -106,7 +106,7 @@ public final class DbManager {
       virtualDB.put(userAccount.getUserId(), userAccount);
       return;
     }
-    if (null == db) {
+    if (db == null) {
       try {
         connect();
       } catch (ParseException e) {
@@ -126,7 +126,7 @@ public final class DbManager {
       virtualDB.put(userAccount.getUserId(), userAccount);
       return;
     }
-    if (null == db) {
+    if (db == null) {
       try {
         connect();
       } catch (ParseException e) {
@@ -148,7 +148,7 @@ public final class DbManager {
       virtualDB.put(userAccount.getUserId(), userAccount);
       return;
     }
-    if (null == db) {
+    if (db == null) {
       try {
         connect();
       } catch (ParseException e) {

caching/src/main/java/com/iluwatar/caching/LruCache.java

@@ -78,7 +78,6 @@ public class LruCache {
   }
 
   /**
-   *
    * Remove node from linked list.
    */
   public void remove(Node node) {
@@ -95,7 +94,6 @@ public class LruCache {
   }
 
   /**
-   *
    * Move node to the front of the list.
    */
   public void setHead(Node node) {
@@ -141,10 +139,11 @@ public class LruCache {
    * Invalidate cache for user
    */
   public void invalidate(String userId) {
-    LOGGER.info("# {} has been updated! Removing older version from cache...", userId);
-    Node toBeRemoved = cache.get(userId);
-    remove(toBeRemoved);
-    cache.remove(userId);
+    Node toBeRemoved = cache.remove(userId);
+    if (toBeRemoved != null) {
+      LOGGER.info("# {} has been updated! Removing older version from cache...", userId);
+      remove(toBeRemoved);
+    }
   }
 
   public boolean isFull() {
@@ -165,7 +164,6 @@ public class LruCache {
   }
 
   /**
-   *
    * Returns cache data in list form.
    */
   public List<UserAccount> getCacheDataInListForm() {

caching/src/main/java/com/iluwatar/caching/UserAccount.java

@@ -23,9 +23,7 @@
 package com.iluwatar.caching;
 
 /**
- *
  * Entity class (stored in cache and DB) used in the application.
- *
  */
 public class UserAccount {
   private String userId;