MaxMind DB Reader#

Description#

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).

Installation#

Maven#

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

    <dependency>
        <groupId>com.maxmind.db</groupId>
        <artifactId>maxmind-db</artifactId>
        <version>4.1.0</version>
    </dependency>

Gradle#

Add the following to your build.gradle file:

repositories {
    mavenCentral()
}
dependencies {
    implementation 'com.maxmind.db:maxmind-db:4.1.0'
}

Usage#

Note: For accessing MaxMind GeoIP 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_MAPPED or MEMORY. The default mode is MEMORY_MAPPED, 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.

Example#

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

import java.io.File;
import java.io.IOException;
import java.net.InetAddress;

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("24.24.24.24");

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

            System.out.println(result.getCountry().getIsoCode());

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

            System.out.println(record.data().getCountry().getIsoCode());
            System.out.println(record.network());
        }
    }

    public static class LookupResult {
        private final Country country;

        @MaxMindDbConstructor
        public LookupResult (
            @MaxMindDbParameter(name="country") Country country
        ) {
            this.country = country;
        }

        public Country getCountry() {
            return this.country;
        }
    }

    public static class Country {
        private final String isoCode;

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

        public String getIsoCode() {
            return this.isoCode;
        }
    }
}

Constructor and parameter selection#

• Preferred: annotate a constructor with @MaxMindDbConstructor and its parameters with @MaxMindDbParameter(name = "...").
• Records: if no constructor is annotated, the canonical record constructor is used automatically. Record component names are used as field names.
• Classes with a single public constructor: if no constructor is annotated, that constructor is used automatically.
• Unannotated parameters: when a parameter is not annotated, the reader falls back to the parameter name. For records, this is the component name; for classes, this is the Java parameter name. To use Java parameter names at runtime, compile your model classes with the -parameters flag (Maven: maven-compiler-plugin with <parameters>true</parameters>). If Java parameter names are unavailable (no -parameters) and there is no @MaxMindDbParameter annotation, the reader throws a ParameterNotFoundException with guidance.

Defaults for missing values

• Provide a default with @MaxMindDbParameter(name = "...", useDefault = true, defaultValue = "...").

• Supports primitives, boxed types, and String. If defaultValue is empty and useDefault is true, Java defaults are used (0, false, 0.0, empty string).

• Example:

  @MaxMindDbConstructor
  Example(
      @MaxMindDbParameter(name = "count", useDefault = true, defaultValue = "0")
      int count,
      @MaxMindDbParameter(
          name = "enabled",
          useDefault = true,
          defaultValue = "true"
      )
      boolean enabled
  ) { }
  

Lookup context injection

• Use @MaxMindDbIpAddress to inject the IP address being decoded. Supported parameter types are InetAddress and String.
• Use @MaxMindDbNetwork to inject the network of the resulting record. Supported parameter types are Network and String.
• Context annotations cannot be combined with @MaxMindDbParameter on the same constructor argument. Values are populated for every lookup without being cached between different IPs.

Custom deserialization

• Use @MaxMindDbCreator to mark a static factory method or constructor that should be used for custom deserialization of a type from a MaxMind DB file.

• This annotation is similar to Jackson’s @JsonCreator and is useful for types that need custom deserialization logic, such as enums with non-standard string representations or types that require special initialization.

• The annotation can be applied to both constructors and static factory methods.

• Example with an enum:

  public enum ConnectionType {
      DIALUP("Dialup"),
      CABLE_DSL("Cable/DSL");
  
      private final String name;
  
      ConnectionType(String name) {
          this.name = name;
      }
  
      @MaxMindDbCreator
      public static ConnectionType fromString(String s) {
          return switch (s) {
              case "Dialup" -> DIALUP;
              case "Cable/DSL" -> CABLE_DSL;
              default -> null;
          };
      }
  }
  

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 = networks.next();

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

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

    // ...
}

Caching#

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.

Usage:

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_MAPPED mode, which memory maps the file. On Windows, a live memory mapping may prevent the file from being renamed, replaced, or deleted. This is not a Java FileLock, but it can have similar effects when updating a database file in place.

Closing the Reader releases this library’s reference to the mapped buffer, but Java does not provide a supported way to unmap the underlying MappedByteBuffer immediately. The mapping remains valid until the buffer becomes unreachable and is garbage collected. Any outstanding lookup or Networks iterator may also keep a duplicate buffer reachable.

To avoid this behavior, use the MEMORY mode. If you must use MEMORY_MAPPED, close and dereference the Reader and any iterators that were created from it before replacing the file. You may call System.gc() to encourage earlier cleanup, but garbage collection is not guaranteed to run immediately.

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.

Format#

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.

Requirements#

This API requires Java 17 or greater.

Contributing#

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

Versioning#

The MaxMind DB Reader API uses Semantic Versioning.

Copyright and License#

This software is Copyright (c) 2014-2026 by MaxMind, Inc.

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