Embedded MongoDB when running integration tests

I have found Embedded MongoDB library which looks quite promising and does what you have asked for.

Currently supports MongoDB versions: 1.6.5 to 3.1.6, provided the binaries are still available from the configured mirror.

Here is short example of use, which I have just tried and it works perfectly:

public class EmbeddedMongoTest {
    private static final String DATABASE_NAME = "embedded";

    private MongodExecutable mongodExe;
    private MongodProcess mongod;
    private Mongo mongo;

    @Before
    public void beforeEach() throws Exception {
        MongoDBRuntime runtime = MongoDBRuntime.getDefaultInstance();
        mongodExe = runtime.prepare(new MongodConfig(Version.V2_3_0, 12345, Network.localhostIsIPv6()));
        mongod = mongodExe.start();
        mongo = new Mongo("localhost", 12345);
    }

    @After
    public void afterEach() throws Exception {
        if (this.mongod != null) {
            this.mongod.stop();
            this.mongodExe.stop();
        }
    }

    @Test
    public void shouldCreateNewObjectInEmbeddedMongoDb() {
        // given
        DB db = mongo.getDB(DATABASE_NAME);
        DBCollection col = db.createCollection("testCollection", new BasicDBObject());

        // when
        col.save(new BasicDBObject("testDoc", new Date()));

        // then
        assertThat(col.getCount(), Matchers.is(1L));
    }
}

Here's an updated (for 2022) version of the accepted answer from @rozky (a lot has been changed in both the Mongo and Embedded MongoDB libraries).

package com.example.mongo;

import com.mongodb.BasicDBObject;
import com.mongodb.MongoClient;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoDatabase;
import de.flapdoodle.embed.mongo.MongodExecutable;
import de.flapdoodle.embed.mongo.MongodProcess;
import de.flapdoodle.embed.mongo.MongodStarter;
import de.flapdoodle.embed.mongo.config.MongodConfig;
import de.flapdoodle.embed.mongo.config.Net;
import de.flapdoodle.embed.mongo.distribution.Version;
import de.flapdoodle.embed.process.runtime.Network;
import java.util.Date;
import org.junit.After;
import static org.junit.Assert.*;
import org.junit.Before;
import org.junit.Test;

public class EmbeddedMongoTest {

    private static final String DATABASE_NAME = "embedded";

    private MongodExecutable mongodExe;
    private MongodProcess mongod;
    private MongoClient mongo;

    @Before
    public void beforeEach() throws Exception {
        MongodStarter starter = MongodStarter.getDefaultInstance();
        String bindIp = "localhost";
        int port = 12345;
        MongodConfig mongodConfig = MongodConfig.builder()
            .version(Version.Main.PRODUCTION)
            .net(new Net(bindIp, port, Network.localhostIsIPv6()))
            .build();
        this.mongodExe = starter.prepare(mongodConfig);
        this.mongod = mongodExe.start();
        this.mongo = new MongoClient(bindIp, port);
    }

    @After
    public void afterEach() throws Exception {
        if (this.mongod != null) {
            this.mongod.stop();
            this.mongodExe.stop();
        }
    }

    @Test
    public void shouldCreateNewObjectInEmbeddedMongoDb() {
        // given
        MongoDatabase db = mongo.getDatabase(DATABASE_NAME);
        db.createCollection("testCollection");
        MongoCollection<BasicDBObject> col = db.getCollection("testCollection", BasicDBObject.class);

        // when
        col.insertOne(new BasicDBObject("testDoc", new Date()));

        // then
        assertEquals(1L, col.countDocuments());
    }

}

There is Foursquare product Fongo. Fongo is an in-memory java implementation of mongo. It intercepts calls to the standard mongo-java-driver for finds, updates, inserts, removes and other methods. The primary use is for lightweight unit testing where you don't want to spin up a mongo process.

If you're using Maven you may be interested in a plugin I've created that wraps the flapdoodle.de 'embedded mongo' API:

embedmongo-maven-plugin

It provides a start goal that you can use to start any version of MongoDB you want (e.g. during pre-integration-test), and a stop goal that will stop MongoDB (e.g. during post-integration-test).

The real benefit of using this plugin over others is that there is no requirement for MongoDB to be installed beforehand. MongoDB binaries are downloaded and stored in ~/.embedmongo for future builds.

If you are using sbt and specs2, I wrote the same kind of wrapper for embedmongo

https://github.com/athieriot/specs2-embedmongo

with spring-boot 1.3 you can use EmbeddedMongoAutoConfiguration

pom.xml

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>1.3.2.RELEASE</version>
</parent>
 ...
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-data-mongodb</artifactId>
    </dependency>
    <dependency>
        <groupId>de.flapdoodle.embed</groupId>
        <artifactId>de.flapdoodle.embed.mongo</artifactId>
        <version>${embedded-mongo.version}</version>
    </dependency>

MongoConfig

@Configuration
@EnableAutoConfiguration(exclude = { EmbeddedMongoAutoConfiguration.class })
public class MongoConfig{
}

You can run MongoDB in memory as of version 3.2.6. From the site:

Starting in MongoDB Enterprise version 3.2.6, the in-memory storage engine is part of general availability (GA) in the 64-bit builds. Other than some metadata and diagnostic data, the in-memory storage engine does not maintain any on-disk data, including configuration data, indexes, user credentials, etc.

Not just for unit testing, but also explained how to use inmemory mongodb with rest api.

maven dependency:

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-mongodb</artifactId>
        </dependency>

        <dependency>
            <groupId>de.flapdoodle.embed</groupId>
            <artifactId>de.flapdoodle.embed.mongo</artifactId>
        </dependency>

=============================================================================

application.properties

server.port = 8080
spring.data.mongodb.database=user_db
spring.data.mongodb.port=27017
spring.data.mongodb.host=localhost

=============================================================================

UserRepository.java

public interface UserRepository extends MongoRepository{

}

for reference and all java code use below link:(step by step explanation)

https://www.youtube.com/watch?v=2Tq2Q7EzhSA&t=7s

Performances are better when executing mongod with storageEngine='ephemeralForTest'

new MongodConfigBuilder()
    .version(Version.Main.PRODUCTION)
    .cmdOptions(new MongoCmdOptionsBuilder()
         .useStorageEngine("ephemeralForTest")
         .build())
    .net(new Net("localhost", port, Network.localhostIsIPv6()))
    .build()

To run Embedded mongodb for integration test following are the maven dependency needed:

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-mongodb</artifactId>
            <version>2.5.2</version>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-autoconfigure</artifactId>
            <version>2.5.2</version>
        </dependency>

        <dependency>
            <groupId>de.flapdoodle.embed</groupId>
            <artifactId>de.flapdoodle.embed.mongo</artifactId>
            <version>3.0.0</version>
            <scope>test</scope>
        </dependency>

Try using below code snipped for EmbeddedMongoAutoConfiguration:

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.autoconfigure.mongo.MongoProperties;
import org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration;
import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.Bean;

@SpringBootApplication
public class EmbeddedMongoApplication {

    public static void main(String[] args) {
         System.setProperty("os.arch", "x86_64");
         SpringApplication.run(EmbeddedMongoApplication.class, args);
    }
    
    @Bean
    public EmbeddedMongoAutoConfiguration embeddedMongoAutoConfiguration(MongoProperties mongoProperties) {
        return new EmbeddedMongoAutoConfiguration(mongoProperties);
    }
}

Note:

Embedded mongodb will be downloaded to below Path. So take into consideration that path have proper permission.

Linux : $HOME/.embedmongo/linux/mongodb-linux-x86_64-3.2.2.tgz
Windows : C:\Users\<username>\.embedmongo\win32\mongodb-win32-x86_64-3.x.x.zip

In production, you will be using a real database.

If you want your tests to reflect how your product behaves in production, use a real instance of Mongo.

A fake implementation may not behave exactly the same as a real one. When testing, you should strive for correctness. Speed of execution comes second.