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Java Connect and Query Quickstart
Java Connect and Query Quickstart

How to set up your Java project and perform CRUD (create, read, update, and delete) operations with Java.

Hannah Geiser avatar
Written by Hannah Geiser
Updated over a week ago

Introduction

This article will demonstrate how to set up your Java project with Maven and execute a MongoDB command in Java. Then we will explore the most common operations — such as create, read, update, and delete — using the MongoDB Java driver. We'll walk through of the more powerful options and features available as part of the MongoDB Java driver for each of these operations.

Connect to Java Driver
Run queries in Atlas collections

Table of contents

Prerequisites

To follow along, you can use any environment you like and the integrated development environment of your choice. I'll use Maven 3.6.2 and the Java OpenJDK 13, but all the code will be compatible with Java versions 8 to 13, so feel free to use the JDK of your choice and update the Java version accordingly in the pom.xml file we are about to set up.

Connect to Java Driver

Set up

To begin with, we will need to set up a new Maven project. You have two options at this point. You can either clone this series' git repository or you can create and set up the Maven project.

Using the git repository

If you choose to use git, you will get all the code immediately. I still recommend you read through the manual set-up. You can clone the repository if you like with the following command.

git clone https://github.com/mongodb-developer/java-quick-start

Or you can download the repository as a zip file.

Setting up manually

You can either use your favorite IDE to create a new Maven project for you or you can create the Maven project manually. Either way, you should get the following folder architecture:

java-quick-start/
├── pom.xml
└── src
    └── main
        └── java
            └── com
                └── mongodb
                    └── quickstart

The pom.xml file should contain the following code:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.mongodb</groupId>
    <artifactId>java-quick-start</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven-compiler-plugin.source>8</maven-compiler-plugin.source>
        <maven-compiler-plugin.target>8</maven-compiler-plugin.target>
        <maven-compiler-plugin.version>3.10.1</maven-compiler-plugin.version>
        <mongodb-driver-sync.version>4.6.1</mongodb-driver-sync.version>
        <mongodb-crypt.version>1.4.1</mongodb-crypt.version>
        <logback-classic.version>1.2.11</logback-classic.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.mongodb</groupId>
            <artifactId>mongodb-driver-sync</artifactId>
            <version>${mongodb-driver-sync.version}</version>
        </dependency>
        <dependency>
            <groupId>org.mongodb</groupId>
            <artifactId>mongodb-crypt</artifactId>
            <version>${mongodb-crypt.version}</version>
        </dependency>
        <dependency>
            <groupId>ch.qos.logback</groupId>
            <artifactId>logback-classic</artifactId>
            <version>${logback-classic.version}</version>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>${maven-compiler-plugin.version}</version>
                <configuration>
                    <source>${maven-compiler-plugin.source}</source>
                    <target>${maven-compiler-plugin.target}</target>
                </configuration>
            </plugin>
        </plugins>
    </build>

</project>

To verify that everything works correctly, you should be able to create and run a simple "Hello MongoDB!" program. In src/main/java/com/mongodb/quickstart, create the HelloMongoDB.java file:

package com.mongodb.quickstart;

public class HelloMongoDB {

    public static void main(String[] args) {
        System.out.println("Hello MongoDB!");
    }
}

Then compile and execute it with your IDE or use the command line in the root directory (where the src folder is):

mvn compile exec:java -Dexec.mainClass="com.mongodb.quickstart.HelloMongoDB"

The result should look like this:

[...]
[INFO] Scanning for projects...
[INFO]
[INFO] --------------------< com.mongodb:java-quick-start >--------------------
[INFO] Building java-quick-start 1.0-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO]
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ java-quick-start ---
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] Copying 1 resource
[INFO]
[INFO] --- maven-compiler-plugin:3.8.1:compile (default-compile) @ java-quick-start ---
[INFO] Nothing to compile - all classes are up to date
[INFO]
[INFO] --- exec-maven-plugin:1.4.0:java (default-cli) @ java-quick-start ---
Hello MongoDB!
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  0.786 s
[INFO] Finished at: 2019-10-02T20:19:36+02:00
[INFO] ------------------------------------------------------------------------

Note: If you see some warnings about an illegal reflective access from guice.java, it's safe to ignore them. Guice is used by Maven and needs an update. You can read more about it in this GitHub issue. These warnings will disappear in a future release of Guice and Maven.

Connecting with Java

Now that our Maven project works and we have resolved our dependencies, we can start using MongoDB Atlas with Java.

Connect to Java Driver

If you have imported the sample dataset as suggested in the Quick Start Atlas blog post, then with the Java code we are about to create, you will be able to see a list of the databases in the sample dataset.

The first step is to instantiate a MongoClient by passing a MongoDB Atlas connection string into the MongoClients.create() static method. This will establish a connection to MongoDB Atlas using the connection string. Then we can retrieve the list of databases on this cluster and print them out to test the connection with MongoDB.

In src/main/java/com/mongodb, create the Connection.java file:

package com.mongodb.quickstart;

import com.mongodb.client.MongoClient;
import com.mongodb.client.MongoClients;
import org.bson.Document;

import java.util.ArrayList;
import java.util.List;

public class Connection {

    public static void main(String[] args) {
        String connectionString = System.getProperty("mongodb.uri");
        try (MongoClient mongoClient = MongoClients.create(connectionString)) {
            List<Document> databases = mongoClient.listDatabases().into(new ArrayList<>());
            databases.forEach(db -> System.out.println(db.toJson()));
        }
    }
}

As you can see, the MongoDB connection string is retrieved from the System Properties, so we need to set this up. Once you have retrieved your MongoDB Atlas connection string, you can add the mongodb.uri system property into your IDE.

Or if you prefer to use Maven in command line, here is the equivalent command line you can run in the root directory:

mvn compile exec:java -Dexec.mainClass="com.mongodb.quickstart.Connection" -Dmongodb.uri="mongodb+srv://username:password@cluster0-abcde.mongodb.net/test?w=majority"

Note: Don't forget the double quotes around the MongoDB URI to avoid surprises from your shell.

The standard output should look like this:

{"name": "admin", "sizeOnDisk": 303104.0, "empty": false}
{"name": "config", "sizeOnDisk": 147456.0, "empty": false}
{"name": "local", "sizeOnDisk": 5.44731136E8, "empty": false}
{"name": "sample_airbnb", "sizeOnDisk": 5.761024E7, "empty": false}
{"name": "sample_geospatial", "sizeOnDisk": 1384448.0, "empty": false}
{"name": "sample_mflix", "sizeOnDisk": 4.583424E7, "empty": false}
{"name": "sample_supplies", "sizeOnDisk": 1339392.0, "empty": false}
{"name": "sample_training", "sizeOnDisk": 7.4801152E7, "empty": false}
{"name": "sample_weatherdata", "sizeOnDisk": 5103616.0, "empty": false}

Insert Operations

Set up

In the Connecting with Java section, we created the classes HelloMongoDB and Connection. Now we will work on the Create class.

If you didn't set up your free cluster on MongoDB Atlas, now is great time to do so. Get the directions for creating your cluster.

Checking the collection and data model

In the sample dataset, you can find the database sample_training, which contains a collection grades. Each document in this collection represents a student's grades for a particular class.

Here is the JSON representation of a document in the mongo shell.

MongoDB Enterprise Cluster0-shard-0:PRIMARY> db.grades.findOne({student_id: 0, class_id: 339})
{
    "_id" : ObjectId("56d5f7eb604eb380b0d8d8ce"),
    "student_id" : 0,
    "scores" : [
        {
            "type" : "exam",
            "score" : 78.40446309504266
        },
        {
            "type" : "quiz",
            "score" : 73.36224783231339
        },
        {
            "type" : "homework",
            "score" : 46.980982486720535
        },
        {
            "type" : "homework",
            "score" : 76.67556138656222
        }
    ],
    "class_id" : 339
}

And here is the extended JSON representation of the same student. You can retrieve it in MongoDB Compass, our free GUI tool, if you want.

Extended JSON is the human readable version of a BSON document without loss of type information. You can read more about the Java driver and BSON in the MongoDB Java driver documentation.

{
    "_id": {
        "$oid": "56d5f7eb604eb380b0d8d8ce"
    },
    "student_id": {
        "$numberDouble": "0"
    },
    "scores": [{
        "type": "exam",
        "score": {
            "$numberDouble": "78.40446309504266"
        }
    }, {
        "type": "quiz",
        "score": {
            "$numberDouble": "73.36224783231339"
        }
    }, {
        "type": "homework",
        "score": {
            "$numberDouble": "46.980982486720535"
        }
    }, {
        "type": "homework",
        "score": {
            "$numberDouble": "76.67556138656222"
        }
    }],
    "class_id": {
        "$numberDouble": "339"
    }
}

As you can see, MongoDB stores BSON documents and for each key-value pair, the BSON contains the key and the value along with its type. This is how MongoDB knows that class_id is actually a double and not an integer, which is not explicit in the mongo shell representation of this document.

We have 10,000 students (student_id from 0 to 9999) already in this collection and each of them took 10 different classes, which adds up to 100,000 documents in this collection. Let's say a new student (student_id 10,000) just arrived in this university and received a bunch of (random) grades in his first class. Let's insert this new student document using Java and the MongoDB Java driver.

In this university, the class_id varies from 0 to 500, so I can use any random value between 0 and 500.

Selecting databases and collections

Firstly, we need to set up our Create class and access this sample_training.grades collection.

package com.mongodb.quickstart;

import com.mongodb.client.MongoClient;
import com.mongodb.client.MongoClients;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoDatabase;
import org.bson.Document;

public class Create {

    public static void main(String[] args) {
        try (MongoClient mongoClient = MongoClients.create(System.getProperty("mongodb.uri"))) {

            MongoDatabase sampleTrainingDB = mongoClient.getDatabase("sample_training");
            MongoCollection<Document> gradesCollection = sampleTrainingDB.getCollection("grades");

        }
    }
}

Secondly, we need to represent this new student in Java using the Document class.

Random rand = new Random();
Document student = new Document("_id", new ObjectId());
student.append("student_id", 10000d)
       .append("class_id", 1d)
       .append("scores", asList(new Document("type", "exam").append("score", rand.nextDouble() * 100),
                                new Document("type", "quiz").append("score", rand.nextDouble() * 100),
                                new Document("type", "homework").append("score", rand.nextDouble() * 100),
                                new Document("type", "homework").append("score", rand.nextDouble() * 100)));

As you can see, we reproduced the same data model from the existing documents in this collection as we made sure that student_id, class_id, and score are all doubles.

Also, the Java driver would have generated the _id field with an ObjectId for us if we didn't explicitly create one here, but it's good practice to set the _id ourselves. This won't change our life right now, but it makes more sense when we directly manipulate POJOs and we want to create a clean REST API. We do this in our mapping POJOs post.

Note as well that we are inserting a document into an existing collection and database, but if these didn’t already exist, MongoDB would automatically create them the first time you to go insert a document into the collection.

Insert document

Finally, we can insert this document.

gradesCollection.insertOne(student);

Here is the final Create class to insert one document in MongoDB with all the details I mentioned above.

package com.mongodb.quickstart;

import com.mongodb.client.MongoClient;
import com.mongodb.client.MongoClients;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoDatabase;
import org.bson.Document;
import org.bson.types.ObjectId;

import java.util.Random;

import static java.util.Arrays.asList;

public class Create {

    public static void main(String[] args) {
        try (MongoClient mongoClient = MongoClients.create(System.getProperty("mongodb.uri"))) {

            MongoDatabase sampleTrainingDB = mongoClient.getDatabase("sample_training");
            MongoCollection<Document> gradesCollection = sampleTrainingDB.getCollection("grades");

            Random rand = new Random();
            Document student = new Document("_id", new ObjectId());
            student.append("student_id", 10000d)
                   .append("class_id", 1d)
                   .append("scores", asList(new Document("type", "exam").append("score", rand.nextDouble() * 100),
                                            new Document("type", "quiz").append("score", rand.nextDouble() * 100),
                                            new Document("type", "homework").append("score", rand.nextDouble() * 100),
                                            new Document("type", "homework").append("score", rand.nextDouble() * 100)));

            gradesCollection.insertOne(student);
        }
    }
}

You can execute this class with the following Maven command line in the root directory or using your IDE (see above for more details). Don't forget the double quotes around the MongoDB URI to avoid surprises.

mvn compile exec:java -Dexec.mainClass="com.mongodb.quickstart.Create" -Dmongodb.uri="mongodb+srv://USERNAME:PASSWORD@cluster0-abcde.mongodb.net/test?w=majority"

And here is the document extracted from MongoDB Compass.

{
    "_id": {
        "$oid": "5d97c375ded5651ea3462d0f"
    },
    "student_id": {
        "$numberDouble": "10000"
    },
    "class_id": {
        "$numberDouble": "1"
    },
    "scores": [{
        "type": "exam",
        "score": {
            "$numberDouble": "4.615256396625178"
        }
    }, {
        "type": "quiz",
        "score": {
            "$numberDouble": "73.06173415145801"
        }
    }, {
        "type": "homework",
        "score": {
            "$numberDouble": "19.378205578990727"
        }
    }, {
        "type": "homework",
        "score": {
            "$numberDouble": "82.3089189278531"
        }
    }]
}

Note that the order of the fields is different from the initial document with "student_id": 0.

We could get exactly the same order if we wanted to by creating the document like this.

Random rand = new Random();
Document student = new Document("_id", new ObjectId());
student.append("student_id", 10000d)
       .append("scores", asList(new Document("type", "exam").append("score", rand.nextDouble() * 100),
                                new Document("type", "quiz").append("score", rand.nextDouble() * 100),
                                new Document("type", "homework").append("score", rand.nextDouble() * 100),
                                new Document("type", "homework").append("score", rand.nextDouble() * 100)))
       .append("class_id", 1d);

But if you do things correctly, this should not have any impact on your code and logic as fields in JSON documents are not ordered.

Insert multiple documents

Now that we know how to create one document, let's learn how to insert many documents.

Of course, we could just wrap the previous insert operation into a for loop. Indeed, if we loop 10 times on this method, we would send 10 insert commands to the cluster and expect 10 insert acknowledgments. As you can imagine, this would not be very efficient as it would generate a lot more TCP communications than necessary.

Instead, we want to wrap our 10 documents and send them in one call to the cluster and we want to receive only one insert acknowledgement for the entire list.

Let's refactor the code. First, let's make the random generator a private static final field.

private static final Random rand = new Random();

Let's make a grade factory method.

private static Document generateNewGrade(double studentId, double classId) {
    List<Document> scores = asList(new Document("type", "exam").append("score", rand.nextDouble() * 100),
                                   new Document("type", "quiz").append("score", rand.nextDouble() * 100),
                                   new Document("type", "homework").append("score", rand.nextDouble() * 100),
                                   new Document("type", "homework").append("score", rand.nextDouble() * 100));
    return new Document("_id", new ObjectId()).append("student_id", studentId)
                                              .append("class_id", classId)
                                              .append("scores", scores);
}

And now we can use this to insert 10 documents all at once.

List<Document> grades = new ArrayList<>();
for (double classId = 1d; classId <= 10d; classId++) {
    grades.add(generateNewGrade(10001d, classId));
}

gradesCollection.insertMany(grades, new InsertManyOptions().ordered(false));

As you can see, we are now wrapping our grade documents into a list and we are sending this list in a single call with the insertMany method.

By default, the insertMany method will insert the documents in order and stop if an error occurs during the process. For example, if you try to insert a new document with the same _id as an existing document, you would get a DuplicateKeyException.

Therefore, with an ordered insertMany, the last documents of the list would not be inserted and the insertion process would stop and return the appropriate exception as soon as the error occurs.

As you can see here, this is not the behavior we want because all the grades are completely independent from one to another. So, if one of them fails, we want to process all the grades and then eventually fall back to an exception for the ones that failed.

This is why you see the second parameter new InsertManyOptions().ordered(false) which is true by default.

Let's refactor the code a bit and here is the final Create class.

package com.mongodb.quickstart;

import com.mongodb.client.MongoClient;
import com.mongodb.client.MongoClients;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoDatabase;
import com.mongodb.client.model.InsertManyOptions;
import org.bson.Document;
import org.bson.types.ObjectId;

import java.util.ArrayList;
import java.util.List;
import java.util.Random;

import static java.util.Arrays.asList;

public class Create {

    private static final Random rand = new Random();

    public static void main(String[] args) {
        try (MongoClient mongoClient = MongoClients.create(System.getProperty("mongodb.uri"))) {

            MongoDatabase sampleTrainingDB = mongoClient.getDatabase("sample_training");
            MongoCollection<Document> gradesCollection = sampleTrainingDB.getCollection("grades");

            insertOneDocument(gradesCollection);
            insertManyDocuments(gradesCollection);
        }
    }

    private static void insertOneDocument(MongoCollection<Document> gradesCollection) {
        gradesCollection.insertOne(generateNewGrade(10000d, 1d));
        System.out.println("One grade inserted for studentId 10000.");
    }

    private static void insertManyDocuments(MongoCollection<Document> gradesCollection) {
        List<Document> grades = new ArrayList<>();
        for (double classId = 1d; classId <= 10d; classId++) {
            grades.add(generateNewGrade(10001d, classId));
        }

        gradesCollection.insertMany(grades, new InsertManyOptions().ordered(false));
        System.out.println("Ten grades inserted for studentId 10001.");
    }

    private static Document generateNewGrade(double studentId, double classId) {
        List<Document> scores = asList(new Document("type", "exam").append("score", rand.nextDouble() * 100),
                                       new Document("type", "quiz").append("score", rand.nextDouble() * 100),
                                       new Document("type", "homework").append("score", rand.nextDouble() * 100),
                                       new Document("type", "homework").append("score", rand.nextDouble() * 100));
        return new Document("_id", new ObjectId()).append("student_id", studentId)
                                                  .append("class_id", classId)
                                                  .append("scores", scores);
    }
}

As a reminder, every write operation (create, replace, update, delete) performed on a single document is ACID in MongoDB. Which means insertMany is not ACID by default but, good news, since MongoDB 4.0, we can wrap this call in a multi-document ACID transaction to make it fully ACID. I explain this in more detail in my blog about multi-document ACID transactions.

Read documents

Create data

We created the class Create. Now we will work in the Read class.

We wrote 11 new grades, one for the student with {"student_id": 10000} and 10 for the student with {"student_id": 10001} in the sample_training.grades collection.

As a reminder, here are the grades of the {"student_id": 10000}.

MongoDB Enterprise Cluster0-shard-0:PRIMARY> db.grades.findOne({"student_id":10000})
{
    "_id" : ObjectId("5daa0e274f52b44cfea94652"),
    "student_id" : 10000,
    "class_id" : 1,
    "scores" : [
        {
            "type" : "exam",
            "score" : 39.25175977753478
        },
        {
            "type" : "quiz",
            "score" : 80.2908713167313
        },
        {
            "type" : "homework",
            "score" : 63.5444978481843
        },
        {
            "type" : "homework",
            "score" : 82.35202261582563
        }
    ]
}

We also discussed BSON types and we noted that student_id and class_id are doubles.

MongoDB treats some types as equivalent for comparison purposes. For instance, numeric types undergo conversion before comparison.

So, don't be surprised if I filter with an integer number and match a document which contains a double number for example. If you want to filter documents by value types, you can use the $type operator.

You can read more about type bracketing and comparison and sort order in our documentation.

Read a specific document

Let's read the document above. To achieve this, we will use the method find, passing it a filter to help identify the document we want to find.

Please create a class Read in the com.mongodb.quickstart package with this code:

package com.mongodb.quickstart;

import com.mongodb.client.*;
import org.bson.Document;

import java.util.ArrayList;
import java.util.List;

import static com.mongodb.client.model.Filters.*;
import static com.mongodb.client.model.Projections.*;
import static com.mongodb.client.model.Sorts.descending;

public class Read {

    public static void main(String[] args) {
        try (MongoClient mongoClient = MongoClients.create(System.getProperty("mongodb.uri"))) {
            MongoDatabase sampleTrainingDB = mongoClient.getDatabase("sample_training");
            MongoCollection<Document> gradesCollection = sampleTrainingDB.getCollection("grades");

            // find one document with new Document
            Document student1 = gradesCollection.find(new Document("student_id", 10000)).first();
            System.out.println("Student 1: " + student1.toJson());
        }
    }
}

Also, make sure you set up your mongodb.uri in your system properties using your IDE if you want to run this code in your favorite IDE.

Alternatively, you can use this Maven command line in your root project (where the src folder is):

mvn compile exec:java -Dexec.mainClass="com.mongodb.quickstart.Read" -Dmongodb.uri="mongodb+srv://USERNAME:PASSWORD@cluster0-abcde.mongodb.net/test?w=majority"

The standard output should be:

Student 1: {"_id": {"$oid": "5daa0e274f52b44cfea94652"},
    "student_id": 10000.0,
    "class_id": 1.0,
    "scores": [
        {"type": "exam", "score": 39.25175977753478},
        {"type": "quiz", "score": 80.2908713167313},
        {"type": "homework", "score": 63.5444978481843},
        {"type": "homework", "score": 82.35202261582563}
    ]
}

The MongoDB driver comes with a few helpers to ease the writing of these queries. Here's an equivalent query using the Filters.eq() method.

gradesCollection.find(eq("student_id", 10000)).first();

Of course, I used a static import to make the code as compact and easy to read as possible.

import static com.mongodb.client.model.Filters.eq;

Read a range of documents

In the previous example, the benefit of these helpers is not obvious, but let me show you another example where I'm searching all the grades with a student_id greater than or equal to 10,000.

// without helpers
gradesCollection.find(new Document("student_id", new Document("$gte", 10000)));
// with the Filters.gte() helper
gradesCollection.find(gte("student_id", 10000));

As you can see, I'm using the $gte operator to write this query. You can learn about all the different query operators in the MongoDB documentation.

Iterators

The find method returns an object that implements the interface FindIterable, which ultimately extends the Iterable interface so we can use an iterator to go through the list of documents we are receiving from MongoDB:

FindIterable<Document> iterable = gradesCollection.find(gte("student_id", 10000));
MongoCursor<Document> cursor = iterable.iterator();
System.out.println("Student list with cursor: ");
while (cursor.hasNext()) {
    System.out.println(cursor.next().toJson());
}

Lists

Lists are usually easier to manipulate than iterators, so we can also do this to retrieve directly an ArrayList<Document>:

List<Document> studentList = gradesCollection.find(gte("student_id", 10000)).into(new ArrayList<>());
System.out.println("Student list with an ArrayList:");
for (Document student : studentList) {
    System.out.println(student.toJson());
}

Consumers

We could also use a Consumer which is a functional interface:

Consumer<Document> printConsumer = document -> System.out.println(document.toJson());
gradesCollection.find(gte("student_id", 10000)).forEach(printConsumer);

Cursors, sort, skip, limit, and projections

As we saw above with the Iterator example, MongoDB leverages cursors to iterate through your result set.

If you are already familiar with the cursors in the mongo shell, you know that transformations can be applied to it. A cursor can be sorted and the documents it contains can be transformed using a projection. Also, once the cursor is sorted, we can choose to skip a few documents and limit the number of documents in the output. This is very useful to implement pagination in your frontend for example.

Let's combine everything we have learnt in one query:

List<Document> docs = gradesCollection.find(and(eq("student_id", 10001), lte("class_id", 5)))
                                                  .projection(fields(excludeId(),
                                                                     include("class_id",
                                                                             "student_id")))
                                                  .sort(descending("class_id"))
                                                  .skip(2)
                                                  .limit(2)
                                                  .into(new ArrayList<>());

System.out.println("Student sorted, skipped, limited and projected: ");
for (Document student : docs) {
    System.out.println(student.toJson());
}

Here is the output we get:

{"student_id": 10001.0, "class_id": 3.0}
{"student_id": 10001.0, "class_id": 2.0}

Remember that documents are returned in the natural order, so if you want your output ordered, you need to sort your cursors to make sure there is no randomness in your algorithm.

Indexes

If you want to make these queries (with or without sort) efficient, you need indexes.

To make my the query efficient, create this index:

db.grades.createIndex({"student_id": 1, "class_id": -1})

When I run an explain on this query, this is the winning plan I get:

"winningPlan" : {
            "stage" : "LIMIT",
            "limitAmount" : 2,
            "inputStage" : {
                "stage" : "PROJECTION_COVERED",
                "transformBy" : {
                    "_id" : 0,
                    "class_id" : 1,
                    "student_id" : 1
                },
                "inputStage" : {
                    "stage" : "SKIP",
                    "skipAmount" : 2,
                    "inputStage" : {
                        "stage" : "IXSCAN",
                        "keyPattern" : {
                            "student_id" : 1,
                            "class_id" : -1
                        },
                        "indexName" : "student_id_1_class_id_-1",
                        "isMultiKey" : false,
                        "multiKeyPaths" : {
                            "student_id" : [ ],
                            "class_id" : [ ]
                        },
                        "isUnique" : false,
                        "isSparse" : false,
                        "isPartial" : false,
                        "indexVersion" : 2,
                        "direction" : "forward",
                        "indexBounds" : {
                            "student_id" : [
                                "[10001.0, 10001.0]"
                            ],
                            "class_id" : [
                                "[5.0, -inf.0]"
                            ]
                        }
                    }
                }
            }
        }

With this index, we can see that we have no SORT stage, so we are not doing a sort in memory as the documents are already sorted "for free" and returned in the order of the index.

Also, we can see that we don't have any FETCH stage, so this is a covered query, the most efficient type of query you can run in MongoDB. Indeed, all the information we are returning at the end is already in the index, so the index itself contains everything we need to answer this query.

Final code to read documents

package com.mongodb.quickstart;

import com.mongodb.client.*;
import org.bson.Document;

import java.util.ArrayList;
import java.util.List;
import java.util.function.Consumer;

import static com.mongodb.client.model.Filters.*;
import static com.mongodb.client.model.Projections.*;
import static com.mongodb.client.model.Sorts.descending;

public class Read {

    public static void main(String[] args) {
        try (MongoClient mongoClient = MongoClients.create(System.getProperty("mongodb.uri"))) {
            MongoDatabase sampleTrainingDB = mongoClient.getDatabase("sample_training");
            MongoCollection<Document> gradesCollection = sampleTrainingDB.getCollection("grades");

            // find one document with new Document
            Document student1 = gradesCollection.find(new Document("student_id", 10000)).first();
            System.out.println("Student 1: " + student1.toJson());

            // find one document with Filters.eq()
            Document student2 = gradesCollection.find(eq("student_id", 10000)).first();
            System.out.println("Student 2: " + student2.toJson());

            // find a list of documents and iterate throw it using an iterator.
            FindIterable<Document> iterable = gradesCollection.find(gte("student_id", 10000));
            MongoCursor<Document> cursor = iterable.iterator();
            System.out.println("Student list with a cursor: ");
            while (cursor.hasNext()) {
                System.out.println(cursor.next().toJson());
            }

            // find a list of documents and use a List object instead of an iterator
            List<Document> studentList = gradesCollection.find(gte("student_id", 10000)).into(new ArrayList<>());
            System.out.println("Student list with an ArrayList:");
            for (Document student : studentList) {
                System.out.println(student.toJson());
            }

            // find a list of documents and print using a consumer
            System.out.println("Student list using a Consumer:");
            Consumer<Document> printConsumer = document -> System.out.println(document.toJson());
            gradesCollection.find(gte("student_id", 10000)).forEach(printConsumer);

            // find a list of documents with sort, skip, limit and projection
            List<Document> docs = gradesCollection.find(and(eq("student_id", 10001), lte("class_id", 5)))
                                                  .projection(fields(excludeId(), include("class_id", "student_id")))
                                                  .sort(descending("class_id"))
                                                  .skip(2)
                                                  .limit(2)
                                                  .into(new ArrayList<>());

            System.out.println("Student sorted, skipped, limited and projected: ");
            for (Document student : docs) {
                System.out.println(student.toJson());
            }
        }
    }
}

Update documents

Update one document

Let's edit the document with {student_id: 10000}. To achieve this, we will use the method updateOne.

Please create a class Update in the com.mongodb.quickstart package with this code:

package com.mongodb.quickstart;

import com.mongodb.client.MongoClient;
import com.mongodb.client.MongoClients;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoDatabase;
import com.mongodb.client.model.FindOneAndUpdateOptions;
import com.mongodb.client.model.ReturnDocument;
import com.mongodb.client.model.UpdateOptions;
import com.mongodb.client.result.UpdateResult;
import org.bson.Document;
import org.bson.conversions.Bson;
import org.bson.json.JsonWriterSettings;

import static com.mongodb.client.model.Filters.and;
import static com.mongodb.client.model.Filters.eq;
import static com.mongodb.client.model.Updates.*;

public class Update {

    public static void main(String[] args) {
        JsonWriterSettings prettyPrint = JsonWriterSettings.builder().indent(true).build();

        try (MongoClient mongoClient = MongoClients.create(System.getProperty("mongodb.uri"))) {
            MongoDatabase sampleTrainingDB = mongoClient.getDatabase("sample_training");
            MongoCollection<Document> gradesCollection = sampleTrainingDB.getCollection("grades");

            // update one document
            Bson filter = eq("student_id", 10000);
            Bson updateOperation = set("comment", "You should learn MongoDB!");
            UpdateResult updateResult = gradesCollection.updateOne(filter, updateOperation);
            System.out.println("=> Updating the doc with {\"student_id\":10000}. Adding comment.");
            System.out.println(gradesCollection.find(filter).first().toJson(prettyPrint));
            System.out.println(updateResult);
        }
    }

}

As you can see in this example, the method updateOne takes two parameters:

  • The first one is the filter that identifies the document we want to update.

  • The second one is the update operation. Here, we are setting a new field comment with the value "You should learn MongoDB!".

In order to run this program, make sure you set up your mongodb.uri in your system properties using your IDE if you want to run this code in your favorite IDE (see above for more details).

Alternatively, you can use this Maven command line in your root project (where the src folder is):

mvn compile exec:java -Dexec.mainClass="com.mongodb.quickstart.Update" -Dmongodb.uri="mongodb+srv://USERNAME:PASSWORD@cluster0-abcde.mongodb.net/test?w=majority"

The standard output should look like this:

=> Updating the doc with {"student_id":10000}. Adding comment.
{
  "_id": {
    "$oid": "5dd5c1f351f97d4a034109ed"
  },
  "student_id": 10000.0,
  "class_id": 1.0,
  "scores": [
    {
      "type": "exam",
      "score": 21.580800815091415
    },
    {
      "type": "quiz",
      "score": 87.66967927111044
    },
    {
      "type": "homework",
      "score": 96.4060480668003
    },
    {
      "type": "homework",
      "score": 75.44966835508427
    }
  ],
  "comment": "You should learn MongoDB!"
}
AcknowledgedUpdateResult{matchedCount=1, modifiedCount=1, upsertedId=null}

Upsert a document

An upsert is a mix between an insert operation and an update one. It happens when you want to update a document, assuming it exists, but it actually doesn't exist yet in your database.

In MongoDB, you can set an option to create this document on the fly and carry on with your update operation. This is an upsert operation.

In this example, I want to add a comment to the grades of my student 10002 for the class 10 but this document doesn't exist yet.

filter = and(eq("student_id", 10002d), eq("class_id", 10d));
updateOperation = push("comments", "You will learn a lot if you read the MongoDB blog!");
UpdateOptions options = new UpdateOptions().upsert(true);
updateResult = gradesCollection.updateOne(filter, updateOperation, options);
System.out.println("\n=> Upsert document with {\"student_id\":10002.0, \"class_id\": 10.0} because it doesn't exist yet.");
System.out.println(updateResult);
System.out.println(gradesCollection.find(filter).first().toJson(prettyPrint));

As you can see, I'm using the third parameter of the update operation to set the option upsert to true.

I'm also using the static method Updates.push() to push a new value in my array comments which does not exist yet, so I'm creating an array of one element in this case.

This is the output we get:

=> Upsert document with {"student_id":10002.0, "class_id": 10.0} because it doesn't exist yet.
AcknowledgedUpdateResult{matchedCount=0, modifiedCount=0, upsertedId=BsonObjectId{value=5ddeb7b7224ad1d5cfab3733}}
{
  "_id": {
    "$oid": "5ddeb7b7224ad1d5cfab3733"
  },
  "class_id": 10.0,
  "student_id": 10002.0,
  "comments": [
    "You will learn a lot if you read the MongoDB blog!"
  ]
}

Update my documents

The same way we were able to update one document with updateOne(), we can update multiple documents with updateMany().

filter = eq("student_id", 10001);
updateResult = gradesCollection.updateMany(filter, updateOperation);
System.out.println("\n=> Updating all the documents with {\"student_id\":10001}.");
System.out.println(updateResult);

In this example, I'm using the same updateOperation as earlier, so I'm creating a new one element array comments in these 10 documents.

Here is the output:

=> Updating all the documents with {"student_id":10001}.
AcknowledgedUpdateResult{matchedCount=10, modifiedCount=10, upsertedId=null}

The findOneAndUpdate method

Finally, we have one last very useful method available in the MongoDB Java Driver: findOneAndUpdate().

In most web applications, when a user updates something, they want to see this update reflected on their web page. Without the findOneAndUpdate() method, you would have to run an update operation and then fetch the document with a find operation to make sure you are printing the latest version of this object in the web page.

The findOneAndUpdate() method allows you to combine these two operations in one.

// findOneAndUpdate
filter = eq("student_id", 10000);
Bson update1 = inc("x", 10); // increment x by 10. As x doesn't exist yet, x=10.
Bson update2 = rename("class_id", "new_class_id"); // rename variable "class_id" in "new_class_id".
Bson update3 = mul("scores.0.score", 2); // multiply the first score in the array by 2.
Bson update4 = addToSet("comments", "This comment is uniq"); // creating an array with a comment.
Bson update5 = addToSet("comments", "This comment is uniq"); // using addToSet so no effect.
Bson updates = combine(update1, update2, update3, update4, update5);
// returns the old version of the document before the update.
Document oldVersion = gradesCollection.findOneAndUpdate(filter, updates);
System.out.println("\n=> FindOneAndUpdate operation. Printing the old version by default:");
System.out.println(oldVersion.toJson(prettyPrint));

// but I can also request the new version
filter = eq("student_id", 10001);
FindOneAndUpdateOptions optionAfter = new FindOneAndUpdateOptions().returnDocument(ReturnDocument.AFTER);
Document newVersion = gradesCollection.findOneAndUpdate(filter, updates, optionAfter);
System.out.println("\n=> FindOneAndUpdate operation. But we can also ask for the new version of the doc:");
System.out.println(newVersion.toJson(prettyPrint));

Here is the output:

=> FindOneAndUpdate operation. Printing the old version by default:
{
  "_id": {
    "$oid": "5dd5d46544fdc35505a8271b"
  },
  "student_id": 10000.0,
  "class_id": 1.0,
  "scores": [
    {
      "type": "exam",
      "score": 69.52994626959251
    },
    {
      "type": "quiz",
      "score": 87.27457417188077
    },
    {
      "type": "homework",
      "score": 83.40970667948744
    },
    {
      "type": "homework",
      "score": 40.43663797673247
    }
  ],
  "comment": "You should learn MongoDB!"
}

=> FindOneAndUpdate operation. But we can also ask for the new version of the doc:
{
  "_id": {
    "$oid": "5dd5d46544fdc35505a82725"
  },
  "student_id": 10001.0,
  "scores": [
    {
      "type": "exam",
      "score": 138.42535412437857
    },
    {
      "type": "quiz",
      "score": 84.66740178906916
    },
    {
      "type": "homework",
      "score": 36.773091359279675
    },
    {
      "type": "homework",
      "score": 14.90842128691825
    }
  ],
  "comments": [
    "You will learn a lot if you read the MongoDB blog!",
    "This comment is uniq"
  ],
  "new_class_id": 10.0,
  "x": 10
}

As you can see in this example, you can choose which version of the document you want to return using the appropriate option.

I also used this example to show you a bunch of update operators:

  • set will set a value.

  • inc will increment a value.

  • rename will rename a field.

  • mul will multiply the value by the given number.

  • addToSet is similar to push but will only push the value in the array if the value doesn't exist already.

There are a few other update operators. You can consult the entire list in our documentation.

Final code for updates

package com.mongodb.quickstart;

import com.mongodb.client.MongoClient;
import com.mongodb.client.MongoClients;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoDatabase;
import com.mongodb.client.model.FindOneAndUpdateOptions;
import com.mongodb.client.model.ReturnDocument;
import com.mongodb.client.model.UpdateOptions;
import com.mongodb.client.result.UpdateResult;
import org.bson.Document;
import org.bson.conversions.Bson;
import org.bson.json.JsonWriterSettings;

import static com.mongodb.client.model.Filters.and;
import static com.mongodb.client.model.Filters.eq;
import static com.mongodb.client.model.Updates.*;

public class Update {

    public static void main(String[] args) {
        JsonWriterSettings prettyPrint = JsonWriterSettings.builder().indent(true).build();

        try (MongoClient mongoClient = MongoClients.create(System.getProperty("mongodb.uri"))) {
            MongoDatabase sampleTrainingDB = mongoClient.getDatabase("sample_training");
            MongoCollection<Document> gradesCollection = sampleTrainingDB.getCollection("grades");

            // update one document
            Bson filter = eq("student_id", 10000);
            Bson updateOperation = set("comment", "You should learn MongoDB!");
            UpdateResult updateResult = gradesCollection.updateOne(filter, updateOperation);
            System.out.println("=> Updating the doc with {\"student_id\":10000}. Adding comment.");
            System.out.println(gradesCollection.find(filter).first().toJson(prettyPrint));
            System.out.println(updateResult);

            // upsert
            filter = and(eq("student_id", 10002d), eq("class_id", 10d));
            updateOperation = push("comments", "You will learn a lot if you read the MongoDB blog!");
            UpdateOptions options = new UpdateOptions().upsert(true);
            updateResult = gradesCollection.updateOne(filter, updateOperation, options);
            System.out.println("\n=> Upsert document with {\"student_id\":10002.0, \"class_id\": 10.0} because it doesn't exist yet.");
            System.out.println(updateResult);
            System.out.println(gradesCollection.find(filter).first().toJson(prettyPrint));

            // update many documents
            filter = eq("student_id", 10001);
            updateResult = gradesCollection.updateMany(filter, updateOperation);
            System.out.println("\n=> Updating all the documents with {\"student_id\":10001}.");
            System.out.println(updateResult);

            // findOneAndUpdate
            filter = eq("student_id", 10000);
            Bson update1 = inc("x", 10); // increment x by 10. As x doesn't exist yet, x=10.
            Bson update2 = rename("class_id", "new_class_id"); // rename variable "class_id" in "new_class_id".
            Bson update3 = mul("scores.0.score", 2); // multiply the first score in the array by 2.
            Bson update4 = addToSet("comments", "This comment is uniq"); // creating an array with a comment.
            Bson update5 = addToSet("comments", "This comment is uniq"); // using addToSet so no effect.
            Bson updates = combine(update1, update2, update3, update4, update5);
            // returns the old version of the document before the update.
            Document oldVersion = gradesCollection.findOneAndUpdate(filter, updates);
            System.out.println("\n=> FindOneAndUpdate operation. Printing the old version by default:");
            System.out.println(oldVersion.toJson(prettyPrint));

            // but I can also request the new version
            filter = eq("student_id", 10001);
            FindOneAndUpdateOptions optionAfter = new FindOneAndUpdateOptions().returnDocument(ReturnDocument.AFTER);
            Document newVersion = gradesCollection.findOneAndUpdate(filter, updates, optionAfter);
            System.out.println("\n=> FindOneAndUpdate operation. But we can also ask for the new version of the doc:");
            System.out.println(newVersion.toJson(prettyPrint));
        }
    }
}

Delete Documents

Delete one document

Let's delete the document above.

To achieve this, we will use the method deleteOne.

Please create a class Delete in the com.mongodb.quickstart package with this code:

package com.mongodb.quickstart;

import com.mongodb.client.MongoClient;
import com.mongodb.client.MongoClients;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoDatabase;
import com.mongodb.client.result.DeleteResult;
import org.bson.Document;
import org.bson.conversions.Bson;

import static com.mongodb.client.model.Filters.eq;
import static com.mongodb.client.model.Filters.gte;

public class Delete {

    public static void main(String[] args) {

        try (MongoClient mongoClient = MongoClients.create(System.getProperty("mongodb.uri"))) {
            MongoDatabase sampleTrainingDB = mongoClient.getDatabase("sample_training");
            MongoCollection<Document> gradesCollection = sampleTrainingDB.getCollection("grades");

            // delete one document
            Bson filter = eq("student_id", 10000);
            DeleteResult result = gradesCollection.deleteOne(filter);
            System.out.println(result);
        }
    }
}

As you can see in this example, the method deleteOne only takes one parameter: a filter, just like the find() operation.

In order to run this program, make sure you set up your mongodb.uri in your system properties using your IDE if you want to run this code in your favorite IDE (see above for more details).

Alternatively, you can use this Maven command line in your root project (where the src folder is):

mvn compile exec:java -Dexec.mainClass="com.mongodb.quickstart.Delete" -Dmongodb.uri="mongodb+srv://USERNAME:PASSWORD@cluster0-abcde.mongodb.net/test?w=majority"

The standard output should look like this:

AcknowledgedDeleteResult{deletedCount=1}

FindOneAndDelete()

Are you emotionally attached to your document and want a chance to see it one last time before it's too late? We have what you need.

The method findOneAndDelete() allows you to retrieve a document and delete it in a single atomic operation.

Here is how it works:

Bson filter = eq("student_id", 10002);
Document doc = gradesCollection.findOneAndDelete(filter);
System.out.println(doc.toJson(JsonWriterSettings.builder().indent(true).build()));

Here is the output we get:

{
  "_id": {
    "$oid": "5ddec378224ad1d5cfac02b8"
  },
  "class_id": 10.0,
  "student_id": 10002.0,
  "comments": [
    "You will learn a lot if you read the MongoDB blog!"
  ]
}

Delete many documents

This time we will use deleteMany() instead of deleteOne() and we will use a different filter to match more documents.

Bson filter = gte("student_id", 10000);
DeleteResult result = gradesCollection.deleteMany(filter);
System.out.println(result);

As a reminder, you can learn more about all the query selectors in our documentation.

This is the output we get:

AcknowledgedDeleteResult{deletedCount=10}

Delete a collection

Deleting all the documents from a collection will not delete the collection itself because a collection also contains metadata like the index definitions or the chunk distribution if your collection is sharded for example.

If you want to remove the entire collection and all the metadata associated with it, then you need to use the drop() method.

gradesCollection.drop();

Final code for delete operations

package com.mongodb.quickstart;

import com.mongodb.client.MongoClient;
import com.mongodb.client.MongoClients;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoDatabase;
import com.mongodb.client.result.DeleteResult;
import org.bson.Document;
import org.bson.conversions.Bson;
import org.bson.json.JsonWriterSettings;

import static com.mongodb.client.model.Filters.eq;
import static com.mongodb.client.model.Filters.gte;

public class Delete {

    public static void main(String[] args) {
        try (MongoClient mongoClient = MongoClients.create(System.getProperty("mongodb.uri"))) {
            MongoDatabase sampleTrainingDB = mongoClient.getDatabase("sample_training");
            MongoCollection<Document> gradesCollection = sampleTrainingDB.getCollection("grades");

            // delete one document
            Bson filter = eq("student_id", 10000);
            DeleteResult result = gradesCollection.deleteOne(filter);
            System.out.println(result);

            // findOneAndDelete operation
            filter = eq("student_id", 10002);
            Document doc = gradesCollection.findOneAndDelete(filter);
            System.out.println(doc.toJson(JsonWriterSettings.builder().indent(true).build()));

            // delete many documents
            filter = gte("student_id", 10000);
            result = gradesCollection.deleteMany(filter);
            System.out.println(result);

            // delete the entire collection and its metadata (indexes, chunk metadata, etc).
            gradesCollection.drop();
        }
    }
}

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