MapDB provides concurrent Maps, Sets and Queues backed by disk storage or off-heap-memory. It is a fast and easy to use embedded Java database engine. This project is more than 12 years old, you may know it under name JDBM. MapDB is free as speech and free as beer under Apache License 2.0.

News

2013-06-18 Now I work on MapDB full time.
2013-06-02 MapDB 0.9.3 was released. Fixed data corruption in Write Ahead Log and improved serialization release notes
2013-05-19 MapDB 0.9.2 was released. It fixes bugs and introduces Data Pump, release notes
2013-04-22 Blogpost outlining my future plans with MapDB has been posted
2013-04-14 MapDB 0.9.1 was just released. It fixes dozen of critical bugs and upgrade is highly recommended. Release notes are here

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Features

Concurrent - MapDB has record level locking and state-of-art concurrent engine. Its performance scales nearly linearly with number of cores. Data can be written by multiple parallel threads.
Fast - MapDB has outstanding performance rivaled only by native DBs. It is result of more than a decade of optimizations and rewrites.
ACID - MapDB optionally supports ACID transactions with full MVCC isolation. MapDB uses write-ahead-log or append-only store for great write durability.
Flexible - MapDB can be used everywhere from in-memory cache to multi-terabyte database. It also has number of options to trade durability for write performance. This makes it very easy to configure MapDB to exactly fit your needs.
Hackable - MapDB is component based, most features (instance cache, async writes, compression) are just class wrappers. It is very easy to introduce new functionality or component into MapDB.
SQL Like - MapDB was developed as faster alternative to SQL engine. It has number of features which makes transition from relational database easier: secondary indexes/collections, autoincremental sequential ID, joints, triggers, composite keys...
Low disk-space usage - MapDB has number of features (serialization, delta key packing...) to minimize disk used by its store. It also has very fast compression and custom serializers. We take disk-usage seriously and do not waste single byte.

Intro

MapDB has very power-full API, but for 99% cases you need just two classes: DBMaker is builder style factory for configuring and opening a database. It has handful of static 'newXXX' methods for particular storage mode. DB represents storage. It has methods for accessing Maps and other collections. It also controls DB life-cycle with commit, rollback and close methods.

Best place to checkout various features of MapDB are Examples. There is also screencast which describes most aspects of MapDB.

MapDB is in Maven Central. Just add code bellow to your pom file to use it. You may also download jar files directly from repo.

<dependency>
    <groupId>org.mapdb</groupId>
    <artifactId>mapdb</artifactId>
    <version>0.9.3</version>
</dependency>

There is also repository with daily builds:

<repositories>
    <repository>
        <id>sonatype-snapshots</id>
        <url>https://oss.sonatype.org/content/repositories/snapshots</url>
    </repository>
</repositories>

<dependencies>
    <dependency>
        <groupId>org.mapdb</groupId>
        <artifactId>mapdb</artifactId>
        <version>0.9.4-SNAPSHOT</version>
    </dependency>
</dependencies>

Hereafter is a simple example. It opens TreeMap backed by file in temp directory, file is discarded after JVM exit:

import org.mapdb.*;
ConcurrentNavigableMap treeMap = DBMaker.newTempTreeMap()

// and now use disk based Map as any other Map
treeMap.put(111,"some value")

More advanced example with configuration and write-ahead-log transaction.

import org.mapdb.*;

// configure and open database using builder pattern.
// all options are available with code auto-completion.
DB db = DBMaker.newFileDB(new File("testdb"))
               .closeOnJvmShutdown()
               .encryptionEnable("password")
               .make();

// open existing an collection (or create new)
ConcurrentNavigableMap<Integer,String> map = db.getTreeMap("collectionName");

map.put(1, "one");
map.put(2, "two");
// map.keySet() is now [1,2]

db.commit();  //persist changes into disk

map.put(3, "three");
// map.keySet() is now [1,2,3]
db.rollback(); //revert recent changes
// map.keySet() is now [1,2]

db.close();

What you should know

Transactions (write-ahead-log) can be disabled with DBMaker.writeAheadLogDisable(), this will speedup writes. However without transactions store gets corrupted easily when not closed correctly.
Keys and values must be immutable. MapDB may serialize them on background thread, put them into instance cache... Modifying an object after it was stored is a bad idea.
MapDB relies on memory mapped files. On 32bit JVM you will need DBMaker.randomAccessFileEnable() configuration option to access files larger than 2GB. This introduces overhead compared to memory mapped files.
MapDB does not run defrag on background. You need to call DB.compact() from time to time.
MapDB uses unchecked exceptions. All IOException are wrapped into unchecked IOError. MapDB has weak error handling and assumes disk failure can not be recovered at runtime. However this does not affects data safety, if you use durable commits.

Benchmark

MapDB has outstanding performance rivaled only by native db engines. There is no proper benchmark yet, so here is just small example.

Insert/read/update 100 000 000 records into Map in single thread. Source code is here

Sequential insert took 144 seconds. That is 695 000 inserts per second
Randomly read 1e8 entries from previous set takes 264 seconds. That is 379 000 reads per second
Randomly update 1e8 entries from previous set takes 620 seconds. That is 161 000 random updates per second.

Support

There is mail-group with archive. Ask here support questions such as 'how do I open db'.

Anything with stack-trace should go to new bug report.

Before creating report:

check if it is not already fixed in snapshot repository
try alternative configurations (disable cache, disable async write thread...)
try to use your own serializer, to make sure problem is not in serializer

MapDB is a hobby project and my time is limited.Please always attach code to illustrate/reproduce your problem, so I can fix it efficiently. For hard to reproduce problems I would strongly suggest to record JVM execution with Chronon and submit the record together with a bug report. This will make sure your bug will get fixed promptly.