If you want a persistent, less heap and concurrent maps, then Chronicle Hash Map is the best one to use. Chronicle Map also helps in sharing the data between the JVMs. Earlier version of ChronicleMap is SharedHashMap.
Download
Add the following repository to your pom.xml with the following dependency.
<!-- Repository. Add inside repositories -->
<repository>
<id>Snapshot Repository</id>
<name>Snapshot Repository</name>
<url>https://oss.sonatype.org/content/repositories/snapshots</url>
<snapshots>
<enabled>true</enabled>
</snapshots>
</repository>
<!-- Add the following dependency -->
<dependency>
<groupId>net.openhft</groupId>
<artifactId>chronicle-map</artifactId>
<version>2.1.4</version>
</dependency>
Points to be noted
There are few points to be noted about a ChronicleMap.
- It stores the data onto a file for persistency.
- In order to implement, the object (which has to be stored) must implement Serializable/Externalizable
- Two programs can share the same file (Used while creating the map) to share the same data across JVMs.
- Replication can be done using Chronicle Map using TCP/IP. (not detailed in the current post).
- No need to depend on JVM Heap Memory as the data is store Off-Heap (on a file).
- And the last important point is, the performance of the Map is purely dependent on how the Operating System allocates the memory to the file.
If we clearly observe, all these advantages or features are because of storing a data off-heap using a file.
How to create
ChronicleMap is created using ChronicleMapBuilder. See the sample code for creating the map.
Map<Integer,String> map = ChronicleMapBuilder.of(Integer.class,
String.class).create();
If you want to persist the data to a file. Use like this
File f = new File("/tmp/file.map");
Map<Integer, String> map = ChronicleMapBuilder.of(Integer.class,
String.class).
createPersistedTo(f);
Here, we are creating map and persisting to a file /tmp/file.map. As its storing the data onto a file, we can limit the size of the Map by specific number of entries it can store. See below
Map<Integer,String> map = ChronicleMapBuilder.of(Integer.class,
String.class).
entries(2000).
createPersistedTo(new File("/tmp/file.map"));
There is a difference between Concurrent Map and Chronicle Map in size. It can't be re-sized once we fix the entries size as re-sizing is a costly operation on Map.
ChronicleMap Interface
There are few specials methods which are provided by ChronicleMap. See below
- V getUsing(K key, V value); getUsing is same as get(key) but getUsing will return the value in value parameter without creating a new object whereas get will create a new object for returning the value with key.
- V acquireUsing(K key, V value); acquireUsing is again same as getUsing but if there is no value defined with key, it will insert a new entry with key and returns the same value.
ChronicleMap can read and write the data from/to a JSON object. The following methods can be used to do so
- void getAll(File toFile) throws IOException; To read map from the file which was created by another ChronicleMap using JSON format.
- void putAll(File fromFile) throws IOException; To dump the entire map into a file using a JSON format.
- void close(); As the data is stored off-heap, its recommended to close the map to release the heap data and persist the data.
All other methods are same as Map as it implements java.util.Map. Explore more on ChronicleMap
Happy Learning!!!!
