diff --git a/java/src/com/android/inputmethod/latin/DictionaryFactory.java b/java/src/com/android/inputmethod/latin/DictionaryFactory.java
index 3721132..828e54f 100644
--- a/java/src/com/android/inputmethod/latin/DictionaryFactory.java
+++ b/java/src/com/android/inputmethod/latin/DictionaryFactory.java
@@ -51,7 +51,7 @@
         if (null == locale) {
             Log.e(TAG, "No locale defined for dictionary");
             return new DictionaryCollection(Dictionary.TYPE_MAIN,
-                    createBinaryDictionary(context, locale));
+                    createReadOnlyBinaryDictionary(context, locale));
         }
 
         final LinkedList<Dictionary> dictList = CollectionUtils.newLinkedList();
@@ -59,11 +59,11 @@
                 BinaryDictionaryGetter.getDictionaryFiles(locale, context);
         if (null != assetFileList) {
             for (final AssetFileAddress f : assetFileList) {
-                final BinaryDictionary binaryDictionary = new BinaryDictionary(f.mFilename,
-                        f.mOffset, f.mLength, useFullEditDistance, locale, Dictionary.TYPE_MAIN,
-                        false /* isUpdatable */);
-                if (binaryDictionary.isValidDictionary()) {
-                    dictList.add(binaryDictionary);
+                final ReadOnlyBinaryDictionary readOnlyBinaryDictionary =
+                        new ReadOnlyBinaryDictionary(f.mFilename, f.mOffset, f.mLength,
+                                useFullEditDistance, locale, Dictionary.TYPE_MAIN);
+                if (readOnlyBinaryDictionary.isValidDictionary()) {
+                    dictList.add(readOnlyBinaryDictionary);
                 }
             }
         }
@@ -89,12 +89,12 @@
     }
 
     /**
-     * Initializes a dictionary from a raw resource file
+     * Initializes a read-only binary dictionary from a raw resource file
      * @param context application context for reading resources
      * @param locale the locale to use for the resource
-     * @return an initialized instance of BinaryDictionary
+     * @return an initialized instance of ReadOnlyBinaryDictionary
      */
-    protected static BinaryDictionary createBinaryDictionary(final Context context,
+    protected static ReadOnlyBinaryDictionary createReadOnlyBinaryDictionary(final Context context,
             final Locale locale) {
         AssetFileDescriptor afd = null;
         try {
@@ -113,9 +113,8 @@
                 Log.e(TAG, "sourceDir is not a file: " + sourceDir);
                 return null;
             }
-            return new BinaryDictionary(sourceDir, afd.getStartOffset(), afd.getLength(),
-                    false /* useFullEditDistance */, locale, Dictionary.TYPE_MAIN,
-                    false /* isUpdatable */);
+            return new ReadOnlyBinaryDictionary(sourceDir, afd.getStartOffset(), afd.getLength(),
+                    false /* useFullEditDistance */, locale, Dictionary.TYPE_MAIN);
         } catch (android.content.res.Resources.NotFoundException e) {
             Log.e(TAG, "Could not find the resource");
             return null;
@@ -142,10 +141,10 @@
         final DictionaryCollection dictionaryCollection =
                 new DictionaryCollection(Dictionary.TYPE_MAIN);
         for (final AssetFileAddress address : dictionaryList) {
-            final BinaryDictionary binaryDictionary = new BinaryDictionary(address.mFilename,
-                    address.mOffset, address.mLength, useFullEditDistance, locale,
-                    Dictionary.TYPE_MAIN, false /* isUpdatable */);
-            dictionaryCollection.addDictionary(binaryDictionary);
+            final ReadOnlyBinaryDictionary readOnlyBinaryDictionary = new ReadOnlyBinaryDictionary(
+                    address.mFilename, address.mOffset, address.mLength, useFullEditDistance,
+                    locale, Dictionary.TYPE_MAIN);
+            dictionaryCollection.addDictionary(readOnlyBinaryDictionary);
         }
         return dictionaryCollection;
     }
diff --git a/java/src/com/android/inputmethod/latin/ExpandableBinaryDictionary.java b/java/src/com/android/inputmethod/latin/ExpandableBinaryDictionary.java
index 183f12a..7125202 100644
--- a/java/src/com/android/inputmethod/latin/ExpandableBinaryDictionary.java
+++ b/java/src/com/android/inputmethod/latin/ExpandableBinaryDictionary.java
@@ -727,7 +727,7 @@
                         holder.set(mBinaryDictionary.isValidWord(word));
                     } else {
                         holder.set(((DynamicPersonalizationDictionaryWriter) mDictionaryWriter)
-                                .isInDictionaryForTests(word));
+                                .isInBigramListForTests(word));
                     }
                 }
             }
diff --git a/java/src/com/android/inputmethod/latin/ReadOnlyBinaryDictionary.java b/java/src/com/android/inputmethod/latin/ReadOnlyBinaryDictionary.java
new file mode 100644
index 0000000..68505ce
--- /dev/null
+++ b/java/src/com/android/inputmethod/latin/ReadOnlyBinaryDictionary.java
@@ -0,0 +1,120 @@
+/*
+ * Copyright (C) 2013 The Android Open Source Project
+ *
+ * Licensed 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 com.android.inputmethod.latin;
+
+import com.android.inputmethod.keyboard.ProximityInfo;
+import com.android.inputmethod.latin.SuggestedWords.SuggestedWordInfo;
+
+import java.util.ArrayList;
+import java.util.Locale;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
+
+/**
+ * This class provides binary dictionary reading operations with locking. An instance of this class
+ * can be used by multiple threads. Note that different session IDs must be used when multiple
+ * threads get suggestions using this class.
+ */
+public final class ReadOnlyBinaryDictionary extends Dictionary {
+    /**
+     * A lock for accessing binary dictionary. Only closing binary dictionary is the operation
+     * that change the state of dictionary.
+     */
+    private final ReentrantReadWriteLock mLock = new ReentrantReadWriteLock();
+
+    private final BinaryDictionary mBinaryDictionary;
+
+    public ReadOnlyBinaryDictionary(final String filename, final long offset, final long length,
+            final boolean useFullEditDistance, final Locale locale, final String dictType) {
+        super(dictType);
+        mBinaryDictionary = new BinaryDictionary(filename, offset, length, useFullEditDistance,
+                locale, dictType, false /* isUpdatable */);
+    }
+
+    public boolean isValidDictionary() {
+        return mBinaryDictionary.isValidDictionary();
+    }
+
+    @Override
+    public ArrayList<SuggestedWordInfo> getSuggestions(final WordComposer composer,
+            final String prevWord, final ProximityInfo proximityInfo,
+            final boolean blockOffensiveWords, final int[] additionalFeaturesOptions) {
+        return getSuggestionsWithSessionId(composer, prevWord, proximityInfo, blockOffensiveWords,
+                additionalFeaturesOptions, 0 /* sessionId */);
+    }
+
+    @Override
+    public ArrayList<SuggestedWordInfo> getSuggestionsWithSessionId(final WordComposer composer,
+            final String prevWord, final ProximityInfo proximityInfo,
+            final boolean blockOffensiveWords, final int[] additionalFeaturesOptions,
+            final int sessionId) {
+        if (mLock.readLock().tryLock()) {
+            try {
+                return mBinaryDictionary.getSuggestions(composer, prevWord, proximityInfo,
+                        blockOffensiveWords, additionalFeaturesOptions);
+            } finally {
+                mLock.readLock().unlock();
+            }
+        }
+        return null;
+    }
+
+    @Override
+    public boolean isValidWord(final String word) {
+        if (mLock.readLock().tryLock()) {
+            try {
+                return mBinaryDictionary.isValidWord(word);
+            } finally {
+                mLock.readLock().unlock();
+            }
+        }
+        return false;
+    }
+
+    @Override
+    public boolean shouldAutoCommit(final SuggestedWordInfo candidate) {
+        if (mLock.readLock().tryLock()) {
+            try {
+                return mBinaryDictionary.shouldAutoCommit(candidate);
+            } finally {
+                mLock.readLock().unlock();
+            }
+        }
+        return false;
+    }
+
+    @Override
+    public int getFrequency(final String word) {
+        if (mLock.readLock().tryLock()) {
+            try {
+                return mBinaryDictionary.getFrequency(word);
+            } finally {
+                mLock.readLock().unlock();
+            }
+        }
+        return NOT_A_PROBABILITY;
+    }
+
+    @Override
+    public void close() {
+        mLock.writeLock().lock();
+        try {
+            mBinaryDictionary.close();
+        } finally {
+            mLock.writeLock().unlock();
+        }
+    }
+}
diff --git a/java/src/com/android/inputmethod/latin/personalization/DynamicPersonalizationDictionaryWriter.java b/java/src/com/android/inputmethod/latin/personalization/DynamicPersonalizationDictionaryWriter.java
index 0af028a..3050885 100644
--- a/java/src/com/android/inputmethod/latin/personalization/DynamicPersonalizationDictionaryWriter.java
+++ b/java/src/com/android/inputmethod/latin/personalization/DynamicPersonalizationDictionaryWriter.java
@@ -79,7 +79,7 @@
     public void addUnigramWord(final String word, final String shortcutTarget, final int frequency,
             final boolean isNotAWord) {
         if (mBigramList.size() > mMaxHistoryBigrams * 2) {
-            // Too many entries: just stop adding new vocabrary and wait next refresh.
+            // Too many entries: just stop adding new vocabulary and wait next refresh.
             return;
         }
         mExpandableDictionary.addWord(word, shortcutTarget, frequency);
@@ -90,7 +90,7 @@
     public void addBigramWords(final String word0, final String word1, final int frequency,
             final boolean isValid, final long lastModifiedTime) {
         if (mBigramList.size() > mMaxHistoryBigrams * 2) {
-            // Too many entries: just stop adding new vocabrary and wait next refresh.
+            // Too many entries: just stop adding new vocabulary and wait next refresh.
             return;
         }
         if (lastModifiedTime > 0) {
@@ -176,8 +176,8 @@
     }
 
     @UsedForTesting
-    public boolean isInDictionaryForTests(final String word) {
+    public boolean isInBigramListForTests(final String word) {
         // TODO: Use native method to determine whether the word is in dictionary or not
-        return mBigramList.containsKey(word);
+        return mBigramList.containsKey(word) || mBigramList.getBigrams(null).containsKey(word);
     }
 }
diff --git a/java/src/com/android/inputmethod/latin/utils/PrioritizedSerialExecutor.java b/java/src/com/android/inputmethod/latin/utils/PrioritizedSerialExecutor.java
index 5dc0b58..201a70d 100644
--- a/java/src/com/android/inputmethod/latin/utils/PrioritizedSerialExecutor.java
+++ b/java/src/com/android/inputmethod/latin/utils/PrioritizedSerialExecutor.java
@@ -16,8 +16,11 @@
 
 package com.android.inputmethod.latin.utils;
 
-import java.util.ArrayDeque;
 import java.util.Queue;
+import java.util.concurrent.ArrayBlockingQueue;
+import java.util.concurrent.ConcurrentLinkedQueue;
+import java.util.concurrent.ThreadPoolExecutor;
+import java.util.concurrent.TimeUnit;
 
 /**
  * An object that executes submitted tasks using a thread.
@@ -27,19 +30,20 @@
 
     private final Object mLock = new Object();
 
-    // The default value of capacities of task queues.
-    private static final int TASK_QUEUE_CAPACITY = 1000;
     private final Queue<Runnable> mTasks;
     private final Queue<Runnable> mPrioritizedTasks;
     private boolean mIsShutdown;
+    private final ThreadPoolExecutor mThreadPoolExecutor;
 
     // The task which is running now.
     private Runnable mActive;
 
     public PrioritizedSerialExecutor() {
-        mTasks = new ArrayDeque<Runnable>(TASK_QUEUE_CAPACITY);
-        mPrioritizedTasks = new ArrayDeque<Runnable>(TASK_QUEUE_CAPACITY);
+        mTasks = new ConcurrentLinkedQueue<Runnable>();
+        mPrioritizedTasks = new ConcurrentLinkedQueue<Runnable>();
         mIsShutdown = false;
+        mThreadPoolExecutor = new ThreadPoolExecutor(1 /* corePoolSize */, 1 /* maximumPoolSize */,
+                0 /* keepAliveTime */, TimeUnit.MILLISECONDS, new ArrayBlockingQueue<Runnable>(1));
     }
 
     /**
@@ -59,7 +63,16 @@
     public void execute(final Runnable r) {
         synchronized(mLock) {
             if (!mIsShutdown) {
-                mTasks.offer(r);
+                mTasks.offer(new Runnable() {
+                    @Override
+                    public void run() {
+                        try {
+                            r.run();
+                        } finally {
+                            scheduleNext();
+                        }
+                    }
+                });
                 if (mActive == null) {
                     scheduleNext();
                 }
@@ -74,45 +87,36 @@
     public void executePrioritized(final Runnable r) {
         synchronized(mLock) {
             if (!mIsShutdown) {
-                mPrioritizedTasks.offer(r);
-                if (mActive ==  null) {
+                mPrioritizedTasks.offer(new Runnable() {
+                    @Override
+                    public void run() {
+                        try {
+                            r.run();
+                        } finally {
+                            scheduleNext();
+                        }
+                    }
+                });
+                if (mActive == null) {
                     scheduleNext();
                 }
             }
         }
     }
 
-    private boolean fetchNextTasks() {
-        synchronized(mLock) {
-            mActive = mPrioritizedTasks.poll();
-            if (mActive == null) {
-                mActive = mTasks.poll();
-            }
-            return mActive != null;
+    private boolean fetchNextTasksLocked() {
+        mActive = mPrioritizedTasks.poll();
+        if (mActive == null) {
+            mActive = mTasks.poll();
         }
+        return mActive != null;
     }
 
     private void scheduleNext() {
         synchronized(mLock) {
-            if (!fetchNextTasks()) {
-                return;
+            if (fetchNextTasksLocked()) {
+                mThreadPoolExecutor.execute(mActive);
             }
-            new Thread(new Runnable() {
-                @Override
-                public void run() {
-                    try {
-                        do {
-                            synchronized(mLock) {
-                                if (mActive != null) {
-                                    mActive.run();
-                                }
-                            }
-                        } while (fetchNextTasks());
-                    } finally {
-                        scheduleNext();
-                    }
-                }
-            }).start();
         }
     }
 
diff --git a/tests/src/com/android/inputmethod/latin/personalization/UserHistoryDictionaryTests.java b/tests/src/com/android/inputmethod/latin/personalization/UserHistoryDictionaryTests.java
index d605cdb..06c4271 100644
--- a/tests/src/com/android/inputmethod/latin/personalization/UserHistoryDictionaryTests.java
+++ b/tests/src/com/android/inputmethod/latin/personalization/UserHistoryDictionaryTests.java
@@ -84,29 +84,24 @@
     }
 
     /**
-     * @param checksContents if true, checks whether written words are actually in the dictionary
+     * @param checkContents if true, checks whether written words are actually in the dictionary
      * or not.
      */
     private void addAndWriteRandomWords(final String testFilenameSuffix, final int numberOfWords,
-            final Random random, final boolean checksContents) {
+            final Random random, final boolean checkContents) {
         final List<String> words = generateWords(numberOfWords, random);
         final UserHistoryPredictionDictionary dict =
                 PersonalizationHelper.getUserHistoryPredictionDictionary(getContext(),
                         testFilenameSuffix /* locale */, mPrefs);
         // Add random words to the user history dictionary.
         addToDict(dict, words);
-        if (checksContents) {
+        if (checkContents) {
             try {
                 Thread.sleep(TimeUnit.MILLISECONDS.convert(5L, TimeUnit.SECONDS));
             } catch (InterruptedException e) {
             }
-            // Limit word count to check when using a Java on memory dictionary.
-            final int wordCountToCheck =
-                    ExpandableBinaryDictionary.ENABLE_BINARY_DICTIONARY_DYNAMIC_UPDATE ?
-                            numberOfWords : 10;
-            for (int i = 0; i < wordCountToCheck; ++i) {
+            for (int i = 0; i < numberOfWords; ++i) {
                 final String word = words.get(i);
-                // This may fail as long as we use tryLock on inserting the bigram words
                 assertTrue(dict.isInDictionaryForTests(word));
             }
         }