Docs Menu
Docs Home
/
Languages
/
Java
/
Java Reactive Streams Driver
/
Read Data From MongoDB

Count Documents

On this page

  • Overview
  • Sample Data
  • Retrieve an Exact Count
  • Count All Documents
  • Count Specific Documents
  • Customize Count Behavior
  • Modify Count Example
  • Retrieve an Estimated Count
  • Customize Estimated Count Behavior
  • Modify Estimated Count Example
  • API Documentation

Overview

In this guide, you can learn how to retrieve an exact and an estimated count of the number of documents in a collection.

Sample Data

The examples in this guide use the sample_restaurants.restaurants collection from the Atlas sample datasets. To learn how to create a free MongoDB Atlas cluster and load the sample datasets, see Get Started.

Important

Project Reactor Library

This guide uses the Project Reactor library to consume Publisher instances returned by the Java Reactive Streams driver methods. To learn more about the Project Reactor library and how to use it, see Getting Started in the Reactor documentation. To learn more about how we use Project Reactor library methods in this guide, see the Write Data to MongoDB guide.

Retrieve an Exact Count

Use the countDocuments() method to count the number of documents in a collection. To count the number of documents that match specific search criteria, pass a query filter to the countDocuments() method.

To learn more about specifying a query, see Specify a Query.

Count All Documents

To return a count of all documents in the collection, call the countDocuments() method and pass in no parameters, as shown in the following example:

Publisher<Long> countPublisher = restaurants.countDocuments();
Mono.from(countPublisher).doOnNext(System.out::println).blockLast();

Count Specific Documents

To return a count of documents that match specific search criteria, specify your query in the countDocuments() method, as shown in the following example. To learn more about how to specify a query, see the Specify a Query guide.

Publisher<Long> countPublisher = restaurants.countDocuments(
        eq("cuisine", "Italian"));
Mono.from(countPublisher)
        .doOnNext(System.out::println)
        .blockLast();

Customize Count Behavior

You can modify the behavior of the countDocuments() method by passing optional parameters to the method. The CountOptions class provides methods that modify the behavior of the countDocuments() method. To use the CountOptions class, construct a new instance of the class, then call one or more of its methods to modify the count operation. You can chain these method calls together.

To modify the behavior of the count operation, pass the class instance and chained method calls as the last argument to the countDocuments() method.

The following table describes the methods in the CountOptions class:

Method
Description
collation(Collation collation)
Specifies the kind of language collation to use when sorting results. For more information, see Collation in the MongoDB Server manual.
comment(BsonValue comment)
Attaches a BsonValue comment to the operation. For more information, see the insert command fields guide in the MongoDB Server manual.
comment(String comment)
Attaches a String comment to the operation. For more information, see the insert command fields guide in the MongoDB Server manual.
hint(Bson hint)
Sets the index for the operation as a Bson value. For more information, see the hint statement in the MongoDB Server manual.
hintString(String hint)
Sets the index for the operation as a String value. For more information, see the hint statement in the MongoDB Server manual.
limit(int limit)
Sets a limit for the maximum number of documents the cursor returns. For more information, see cursor in the MongoDB Server documentation.
MaxTime(long maxTime, TimeUnit timeUnit)
Sets the maximum execution time on the server for the operation. If the operation does not complete before the time limit, the driver terminates the operation.
skip(int skip)
Sets the number of documents the query skips before returning results. For more information, see skip in the MongoDB Server manual.

Modify Count Example

The following code uses the countDocuments() method to count all the documents in the restaurants collection with a cuisine value of "Italian". It also attaches the comment "Count all Italian restaurants" to the operation as a String.

Publisher<Long> countPublisher = restaurants.countDocuments(
        eq("cuisine", "Italian"),
       new CountOptions().comment("Count all Italian restaurants"));
Mono.from(countPublisher)
        .doOnNext(System.out::println)
        .blockLast();

Retrieve an Estimated Count

You can get an estimate of the number of documents in a collection by calling the estimatedDocumentCount() method. The method estimates the number of documents based on collection metadata, which might be faster than performing an accurate count.

The following example estimates the number of documents in a collection:

Publisher<Long> countPublisher = restaurants.estimatedDocumentCount();
Mono.from(countPublisher)
        .doOnNext(System.out::println)
        .blockLast();

Customize Estimated Count Behavior

You can modify the behavior of the estimatedDocumentCount() method by passing optional parameters to the method. The EstimatedDocumentCountOptions class provides methods that modify the behavior of the estimatedDocumentCount() method. To use the EstimatedDocumentCountOptions class, construct a new instance of the class, then call one or more of its methods to modify the count operation. You can chain these method calls together.

To modify the behavior of the count operation, pass the class instance and chained method calls as the only argument to the estimatedDocumentCount() method.

The following table describes the options you can set to customize estimatedDocumentCount():

Property
Description
comment(BsonValue comment)
Attaches a BsonValue comment to the operation. For more information, see the insert command fields guide in the MongoDB Server manual.
comment(String comment)
Attaches a String comment to the operation. For more information, see the insert command fields guide in the MongoDB Server manual.
MaxTime(long maxTime, TimeUnit timeUnit)
Sets the maximum execution time on the server for the operation. If the operation does not complete before the time limit, the driver terminates the operation.

Modify Estimated Count Example

The following code uses the estimatedDocumentCount() method to estimate the count of documents in the restaurants collection. It also attaches "Estimated count of all documents" to the operation as a String.

Publisher<Long> countPublisher = restaurants.estimatedDocumentCount(
        new EstimatedDocumentCountOptions()
                .comment("Estimated count of all documents"));
Mono.from(countPublisher)
        .doOnNext(System.out::println)
        .blockLast();

API Documentation

To learn more about any of the methods or types discussed in this guide, see the following API documentation:

Back

Specify Documents to Return

Next

Retrieve Distinct Field Values