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MaxMind DB Reader


This is the Java API for reading MaxMind DB files. MaxMind DB is a binary file format that stores data indexed by IP address subnets (IPv4 or IPv6).



We recommend installing this package with Maven. To do this, add the dependency to your pom.xml:



Add the following to your build.gradle file:

repositories {
dependencies {
    compile 'com.maxmind.db:maxmind-db:3.0.0'


Note: For accessing MaxMind GeoIP2 databases, we generally recommend using the GeoIP2 Java API rather than using this package directly.

To use the API, you must first create a Reader object. The constructor for the reader object takes a File representing your MaxMind DB. Optionally you may pass a second parameter with a FileMode with a value of MEMORY_MAP or MEMORY. The default mode is MEMORY_MAP, which maps the file to virtual memory. This often provides performance comparable to loading the file into real memory with MEMORY.

To look up an IP address, pass the address as an InetAddress to the get method on Reader, along with the class of the object you want to deserialize into. This method will create an instance of the class and populate it. See examples below.

We recommend reusing the Reader object rather than creating a new one for each lookup. The creation of this object is relatively expensive as it must read in metadata for the file.


import com.maxmind.db.MaxMindDbConstructor;
import com.maxmind.db.MaxMindDbParameter;
import com.maxmind.db.Reader;
import com.maxmind.db.DatabaseRecord;


public class Lookup {
    public static void main(String[] args) throws IOException {
        File database = new File("/path/to/database/GeoIP2-City.mmdb");
        try (Reader reader = new Reader(database)) {
            InetAddress address = InetAddress.getByName("");

            // get() returns just the data for the associated record
            LookupResult result = reader.get(address, LookupResult.class);


            // getRecord() returns a DatabaseRecord class that contains both
            // the data for the record and associated metadata.
            DatabaseRecord<LookupResult> record
                = reader.getRecord(address, LookupResult.class);


    public static class LookupResult {
        private final Country country;

        public LookupResult (
            @MaxMindDbParameter(name="country") Country country
        ) {
   = country;

        public Country getCountry() {

    public static class Country {
        private final String isoCode;

        public Country (
            @MaxMindDbParameter(name="iso_code") String isoCode
        ) {
            this.isoCode = isoCode;

        public String getIsoCode() {
            return this.isoCode;

You can also use the reader object to iterate over the database. The reader.networks() and reader.networksWithin() methods can be used for this purpose.

Reader reader = new Reader(file);
Networks networks = reader.networks(Map.class);

while(networks.hasNext()) {
    DatabaseRecord<Map<String, String>> iteration =;

    // Get the data.
    Map<String, String> data = iteration.getData();

    // The IP Address
    InetAddress ipAddress = InetAddress.getByName(data.get("ip"));

    // ...


The database API supports pluggable caching (by default, no caching is performed). A simple implementation is provided by com.maxmind.db.CHMCache. Using this cache, lookup performance is significantly improved at the cost of a small (~2MB) memory overhead.


Reader reader = new Reader(database, new CHMCache());

Please note that the cache will hold references to the objects created during the lookup. If you mutate the objects, the mutated objects will be returned from the cache on subsequent lookups.

Multi-Threaded Use

This API fully supports use in multi-threaded applications. In such applications, we suggest creating one Reader object and sharing that among threads.

Common Problems

File Lock on Windows

By default, this API uses the MEMORY_MAP mode, which memory maps the file. On Windows, this may create an exclusive lock on the file that prevents it from being renamed or deleted. Due to the implementation of memory mapping in Java, this lock will not be released when the DatabaseReader is closed; it will only be released when the object and the MappedByteBuffer it uses are garbage collected. Older JVM versions may also not release the lock on exit.

To work around this problem, use the MEMORY mode or try upgrading your JVM version. You may also call System.gc() after dereferencing the DatabaseReader object to encourage the JVM to garbage collect sooner.

Packaging Database in a JAR

If you are packaging the database file as a resource in a JAR file using Maven, you must disable binary file filtering. Failure to do so will result in InvalidDatabaseException exceptions being thrown when querying the database.


The MaxMind DB format is an open format for quickly mapping IP addresses to records. The specification is available, as is our Perl writer for the format.

Bug Tracker

Please report all issues with this code using the GitHub issue tracker.

If you are having an issue with a MaxMind database or service that is not specific to this reader, please contact MaxMind support.


This API requires Java 11 or greater.


Patches and pull requests are encouraged. Please include unit tests whenever possible.


The MaxMind DB Reader API uses Semantic Versioning.

Copyright and License

This software is Copyright (c) 2014-2022 by MaxMind, Inc.

This is free software, licensed under the Apache License, Version 2.0.