Douglas Rezende Freak's Blog: abril 2021

quinta-feira, 29 de abril de 2021

In this tutorial, we show how to work with MongoDB in Java. There is a concise Java tutorial on ZetCode.

MongoDB is a NoSQL cross-platform document-oriented database. It is one of the most popular databases available. MongoDB is developed by MongoDB Inc. and is published as free and open-source software.

A record in MongoDB is a document, which is a data structure composed of field and value pairs. MongoDB documents are similar to JSON objects. The values of fields may include other documents, arrays, and arrays of documents. MongoDB stores documents in collections. Collections are analogous to tables in relational databases and documents to rows.

MongoDB represents JSON documents in binary-encoded format called BSON behind the scenes. BSON extends the JSON model to provide additional data types, ordered fields, and to be efficient for encoding and decoding within different languages. The .NET driver uses BsonDocument to represent BSON.

Installing MongoDB

The following command can be used to install MongoDB on a Debian-based Linux.

$ sudo apt-get install mongodb

The command installs the necessary packages that come with MongoDB.

$ sudo service mongodb status
mongodb start/running, process 975

With the sudo service mongodb status command we check the status of the mongodb server.

$ sudo service mongodb start
mongodb start/running, process 6448

The mongodb server is started with the sudo service mongodb start command.

Creating a database

The mongo tool is an interactive JavaScript shell interface to MongoDB, which provides an interface for systems administrators as well as a way for developers to test queries and operations directly with the database.

$ mongo testdb
MongoDB shell version v4.0.7
connecting to: mongodb://127.0.0.1:27017/testdb?gssapiServiceName=mongodb
...
> db
testdb
> db.cars.insert({name: "Audi", price: 52642})
> db.cars.insert({name: "Mercedes", price: 57127})
> db.cars.insert({name: "Skoda", price: 9000})
> db.cars.insert({name: "Volvo", price: 29000})
> db.cars.insert({name: "Bentley", price: 350000})
> db.cars.insert({name: "Citroen", price: 21000})
> db.cars.insert({name: "Hummer", price: 41400})
> db.cars.insert({name: "Volkswagen", price: 21600})

We create a testdb database and insert eight documents in the cars collection.

Java MongoDB driver

We use the following Maven declaration to include the MongoDB Java driver in our project.

<dependency>
    <groupId>org.mongodb</groupId>
    <artifactId>mongo-java-driver</artifactId>
    <version>x.y.z</version>
</dependency>

It is an all-in-one JAR, which embeds the core driver and BSON. BSON, short for Binary JSON, is a binary-encoded serialization of JSON-like documents.

Java MongoDB list database collections

The first example connects to the testdb database and retrieves its collections.

pom.xml

<?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.zetcode</groupId>
    <artifactId>mongocommand</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>jar</packaging>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.source>11</maven.compiler.source>
        <maven.compiler.target>11</maven.compiler.target>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.mongodb</groupId>
            <artifactId>mongo-java-driver</artifactId>
            <version>3.10.2</version>
        </dependency>
    </dependencies>

</project>

This is our pom.xml file.

com/zetcode/MongoListCollections.java

package com.zetcode;

import com.mongodb.client.MongoClients;
import com.mongodb.client.MongoDatabase;

import java.util.logging.Level;
import java.util.logging.Logger;

public class MongoListCollections {

    public static void main(String[] args) {

        Logger mongoLogger = Logger.getLogger("org.mongodb.driver");
        mongoLogger.setLevel(Level.SEVERE);

        try (var mongoClient = MongoClients.create("mongodb://localhost:27017")) {

            MongoDatabase database = mongoClient.getDatabase("testdb");

            for (String name : database.listCollectionNames()) {

                System.out.println(name);
            }
        }
    }
}

The example connects to the testdb database and retrieves all its collections.

Logger mongoLogger = Logger.getLogger("org.mongodb.driver");
mongoLogger.setLevel(Level.SEVERE);

We set the logging level for MongoDB. We only show SEVERE error messages.

try (var mongoClient = MongoClients.create("mongodb://localhost:27017")) {

A MongoClient class is used to connect to the MongoDB server. It is created with the MongoClients.create() method call. The 27017 is the default port on which the MongoDB server listens.

MongoDatabase database = mongoClient.getDatabase("testdb");

With the getDatabase() method, we retrieve the testdb database.

for (String name : database.listCollectionNames()) {

    System.out.println(name);
}

The listCollectionNames() method finds all the collections in the testdb database.

cars
cities

In our database, we have these two collections.

Java MongoDB database statistics

The next example connects to the testdb database and gets its statistics.

com/zetcode/MongoCommand.java

package com.zetcode;

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

import java.util.Map;

public class MongoCommand {

    public static void main(String[] args) {

        try (var mongoClient = MongoClients.create("mongodb://localhost:27017")) {

            var database = mongoClient.getDatabase("testdb");

            var stats = database.runCommand(new Document("dbstats", 1));

            for (Map.Entry<String, Object> set : stats.entrySet()) {

                System.out.format("%s: %s%n", set.getKey(), set.getValue());
            }
        }
    }
}

The example connects to the testdb database and executes the dbstats command. It shows some database statistics.

var stats = database.runCommand(new Document("dbstats", 1));

With the runCommand() method, we execute the dbstats command. The command returns a Document, which is a representation of a MongoDB document as a map.

for (Map.Entry<String, Object> set : stats.entrySet()) {

    System.out.format("%s: %s%n", set.getKey(), set.getValue());
}

We iterate through the entries of the document.

db: testdb
collections: 2
views: 0
objects: 9
avgObjSize: 48.111111111111114
dataSize: 433.0
storageSize: 57344.0
numExtents: 0
indexes: 2
indexSize: 57344.0
fsUsedSize: 1.4818904064E11
fsTotalSize: 2.547211264E11
ok: 1.0

This is a sample output.

Java MongoDB read data

MongoCollection is used to store mongo documents returned from a collection. MongoCursor is a cursor to iterate through the results of a database query. It makes sure that it is closed in case of an exception.

com/zetcode/MongoReadAll.java

package com.zetcode;

import com.mongodb.client.MongoClients;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoCursor;
import org.bson.Document;

import java.util.ArrayList;

public class MongoReadAll {

    public static void main(String[] args) {

        try (var mongoClient = MongoClients.create("mongodb://localhost:27017")) {

            var database = mongoClient.getDatabase("testdb");

            MongoCollection<Document> collection = database.getCollection("cars");

            try (MongoCursor<Document> cur = collection.find().iterator()) {

                while (cur.hasNext()) {

                    var doc = cur.next();
                    var cars = new ArrayList<>(doc.values());

                    System.out.printf("%s: %s%n", cars.get(1), cars.get(2));
                }
            }
        }
    }
}

In the example, we iterate over all data of the cars collection.

MongoCollection<Document> collection = database.getCollection("cars");

We retrieve the cars collection with the getCollection() method.

try (MongoCursor<Document> cur = collection.find().iterator()) {

    while (cur.hasNext()) {

        var doc = cur.next();
        var cars = new ArrayList<>(doc.values());

        System.out.printf("%s: %s%n", cars.get(1), cars.get(2));
    }
}

We iterate through the documents of the collection. The find() method finds all documents in the collection.

Audi: 52642.0
Mercedes: 57127.0
Skoda: 9000.0
Volvo: 29000.0
Bentley: 350000.0
Citroen: 21000.0
Hummer: 41400.0
Volkswagen: 21600.0

This is the output of the example.

Java MongoDB query operators

It is possible to filter the data using MongoDB query operators such as $gt, $lt, or $ne. The query operators can be specified in the BasicDBObject class.

com/zetcode/MongoReadGreaterThan.java

package com.zetcode;

import com.mongodb.BasicDBObject;
import com.mongodb.client.MongoClients;
import com.mongodb.client.MongoCollection;
import org.bson.Document;

import java.util.function.Consumer;

public class MongoReadGreaterThan {

    public static void main(String[] args) {

        try (var mongoClient = MongoClients.create("mongodb://localhost:27017")) {

            var database = mongoClient.getDatabase("testdb");

            MongoCollection<Document> collection = database.getCollection("cars");

            var query = new BasicDBObject("price",
                    new BasicDBObject("$gt", 30000));

            collection.find(query).forEach((Consumer<Document>) doc ->
                     System.out.println(doc.toJson()));
        }
    }
}

The example prints all documents whose car prices' are greater than 30,000.

sbygoogle = window.adsbygoogle || []).push({});

var query = new BasicDBObject("price",
    new BasicDBObject("$gt", 30000));

We use the $gt query operator.

collection.find(query).forEach((Consumer<Document>) doc ->
    System.out.println(doc.toJson()));

The forEach() method is a syntactic sugar to avoid the need for the application code to worry about having to close the cursor manually. The data is printed in the JSON format using the toJson() method.

{"_id": {"$oid": "5d4d13d6463315268eb7376b"}, "name": "Audi", "price": 52642.0}
{"_id": {"$oid": "5d4d13f5463315268eb7376c"}, "name": "Mercedes", "price": 57127.0}
{"_id": {"$oid": "5d4d140d463315268eb7376f"}, "name": "Bentley", "price": 350000.0}
{"_id": {"$oid": "5d4d1415463315268eb73771"}, "name": "Hummer", "price": 41400.0}

This is the output of the example in the JSON format. Only cars more expensive than 30,000 are included.

Java MongoDB factory filter query methods

The Java MongoDB driver contains factory methods for query filters.

com/zetcode/MongoFilter.java

package com.zetcode;

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

import java.util.ArrayList;

import static com.mongodb.client.model.Filters.and;
import static com.mongodb.client.model.Filters.gt;
import static com.mongodb.client.model.Filters.lt;

public class MongoFilter {

    public static void main(String[] args) {

        try (var mongoClient = MongoClients.create("mongodb://localhost:27017")) {

            var database = mongoClient.getDatabase("testdb");

            MongoCollection<Document> collection = database.getCollection("cars");

            FindIterable fit = collection.find(and(lt("price", 50000),
                    gt("price", 20000))).sort(new Document("price", -1));

            var docs = new ArrayList<Document>();

            fit.into(docs);

            for (Document doc : docs) {

                System.out.println(doc);
            }
        }
    }
}

In the example, we retrieve cars whose prices fall between 20,000 and 50,000.

FindIterable fit = collection.find(and(lt("price", 50000),
    gt("price", 20000))).sort(new Document("price", -1));

The and(), gt(), and lt() are factory filter methods. In addition, the data is sorted with the sort() method.

Document{{_id=5d4d1415463315268eb73771, name=Hummer, price=41400.0}}
Document{{_id=5d4d1408463315268eb7376e, name=Volvo, price=29000.0}}
Document{{_id=5d4d1419463315268eb73772, name=Volkswagen, price=21600.0}}
Document{{_id=5d4d1411463315268eb73770, name=Citroen, price=21000.0}}

This is the output of the example.

Java MongoDB projections

The Projections class provides static factory methods for all the MongoDB projection operators. By default, all fields of each document are projected. We can use the include and exclude() methods to determine which fields should be projected into our output.

com/zetcode/MongoProjection.java

package com.zetcode;

import com.mongodb.client.FindIterable;
import com.mongodb.client.MongoClients;
import com.mongodb.client.MongoCollection;
import org.bson.Document;

import java.util.ArrayList;

import static com.mongodb.client.model.Projections.excludeId;

public class MongoProjection {

    public static void main(String[] args) {

        try (var mongoClient = MongoClients.create("mongodb://localhost:27017")) {

            var database = mongoClient.getDatabase("testdb");

            MongoCollection<Document> collection = database.getCollection("cars");

            FindIterable it = collection.find().projection(excludeId());

            var docs = new ArrayList<Document>();

            it.into(docs);

            for (Document doc : docs) {

                System.out.println(doc);
            }
        }
    }
}

The example excludes the _id field from the output.

FindIterable it = collection.find().projection(excludeId());

The projection() method sets a document describing the fields to return for all matching documents. The excludeId() is a synonym for exclude("_id").

Document{{name=Audi, price=52642.0}}
Document{{name=Mercedes, price=57127.0}}
Document{{name=Skoda, price=9000.0}}
Document{{name=Volvo, price=29000.0}}
Document{{name=Bentley, price=350000.0}}
Document{{name=Citroen, price=21000.0}}
Document{{name=Hummer, price=41400.0}}
Document{{name=Volkswagen, price=21600.0}}

This is the output for the example.

Java MongoDB limit data output

The limit query option specifies the number of documents to be returned and the skip() option skips the specified number of documents.

com/zetcode/MongoSkipLimit.java

package com.zetcode;

import com.mongodb.client.FindIterable;
import com.mongodb.client.MongoClients;
import com.mongodb.client.MongoCollection;
import org.bson.Document;

import java.util.function.Consumer;

public class MongoSkipLimit {

    public static void main(String[] args) {

        try (var mongoClient = MongoClients.create("mongodb://localhost:27017")) {

            var database = mongoClient.getDatabase("testdb");

            MongoCollection<Document> collection = database.getCollection("cars");
            FindIterable<Document> fit = collection.find().skip(2).limit(5);

            fit.forEach((Consumer<Document>) System.out::println);
        }
    }
}

The example reads from the testdb.cars collection, skips the first two documents, and limits the output to five documents.

FindIterable<Document> fit = collection.find().skip(2).limit(5);

The FindIterable's skip() method skips the first two documents and the limit() method limits the output to five documents.

fit.forEach((Consumer<Document>) System.out::println);

Here we use Java 8 construct to print the documents.

Document{{_id=5d4d13fb463315268eb7376d, name=Skoda, price=9000.0}}
Document{{_id=5d4d1408463315268eb7376e, name=Volvo, price=29000.0}}
Document{{_id=5d4d140d463315268eb7376f, name=Bentley, price=350000.0}}
Document{{_id=5d4d1411463315268eb73770, name=Citroen, price=21000.0}}
Document{{_id=5d4d1415463315268eb73771, name=Hummer, price=41400.0}}

This is the output of the example.

Java MongoDB create collection

The MongoDatabase's createCollection() method creates a new collection in the database. The MongoCollection's insertMany() method inserts one or more documents into the collection.

com/zetcode/MongoCreateCollection.java

package com.zetcode;

import com.mongodb.MongoCommandException;
import com.mongodb.client.MongoClients;
import com.mongodb.client.MongoCollection;
import org.bson.Document;

import java.util.ArrayList;

public class MongoCreateCollection {

    public static void main(String[] args) {

        try (var mongoClient = MongoClients.create("mongodb://localhost:27017")) {

            var database = mongoClient.getDatabase("testdb");

            try {

                database.createCollection("cars");
            } catch (MongoCommandException e) {

                database.getCollection("cars").drop();
            }

            var docs = new ArrayList<Document>();

            MongoCollection<Document> collection = database.getCollection("cars");

            var d1 = new Document("_id", 1);
            d1.append("name", "Audi");
            d1.append("price", 52642);
            docs.add(d1);

            var d2 = new Document("_id", 2);
            d2.append("name", "Mercedes");
            d2.append("price", 57127);
            docs.add(d2);

            var d3 = new Document("_id", 3);
            d3.append("name", "Skoda");
            d3.append("price", 9000);
            docs.add(d3);

            var d4 = new Document("_id", 4);
            d4.append("name", "Volvo");
            d4.append("price", 29000);
            docs.add(d4);

            var d5 = new Document("_id", 5);
            d5.append("name", "Bentley");
            d5.append("price", 350000);
            docs.add(d5);

            var d6 = new Document("_id", 6);
            d6.append("name", "Citroen");
            d6.append("price", 21000);
            docs.add(d6);

            var d7 = new Document("_id", 7);
            d7.append("name", "Hummer");
            d7.append("price", 41400);
            docs.add(d7);

            var d8 = new Document("_id", 8);
            d8.append("name", "Volkswagen");
            d8.append("price", 21600);
            docs.add(d8);

            collection.insertMany(docs);
        }
    }
}

The example creates a cars collection and inserts nine documents into it.

try {

    database.createCollection("cars");
} catch (MongoCommandException e) {

    database.getCollection("cars").drop();
}

A new collection is created with the createCollection() method. If the collection already exists, it is dropped.

MongoCollection<Document> collection = database.getCollection("cars");

A MongoCollection of documents is created with the getCollection() method.

var d1 = new Document("_id", 1);
d1.append("name", "Audi");
d1.append("price", 52642);
docs.add(d1);

A new Document is created. It contains the information about the car—its ID, name, and price.

collection.insertMany(docs);

The documents are written to the collection with the insertMany() method.

Java MongoDB create collection from JSON

The JSON class has methods for parsing JSON documents. JSON (JavaScript Object Notation) is a lightweight data-interchange format. It is easy for humans to read and write.

com/zetcode/MongoCollectionFromJSON.java

package com.zetcode;

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

public class MongoCollectionFromJSON {

    public static void main(String[] args) {

        try (var mongoClient = MongoClients.create("mongodb://localhost:27017")) {

            var database = mongoClient.getDatabase("testdb");

            MongoCollection<Document> collection = database.getCollection("continents");

            var africa = BasicDBObject.parse("{_id : '" + ObjectId.get() + "', name : 'Africa'}");
            var asia = BasicDBObject.parse("{_id : '" + ObjectId.get() + "', name : 'Asia'}");
            var europe = BasicDBObject.parse("{_id : '" + ObjectId.get() + "', name : 'Europe'}");
            var america = BasicDBObject.parse("{_id : '" + ObjectId.get() + "', name : 'America'}");
            var australia = BasicDBObject.parse("{_id : '" + ObjectId.get() + "', name : 'Australia'}");
            var antarctica = BasicDBObject.parse("{_id : '" + ObjectId.get() + "', name : 'Antarctica'}");

            collection.insertOne(new Document(africa));
            collection.insertOne(new Document(asia));
            collection.insertOne(new Document(europe));
            collection.insertOne(new Document(america));
            collection.insertOne(new Document(australia));
            collection.insertOne(new Document(antarctica));
        }
    }
}

The example creates a continents collection from JSON data.

var africa = BasicDBObject.parse("{_id : '" + ObjectId.get() + "', name : 'Africa'}");

JSON data is parsed with the BasicDBObject.parse method.

collection.insertOne(new Document(africa));

The BasicDBObject is passed to Document and inserted into the collection with the insertOne() method.

> db.continents.find()
{ "_id" : "5d4af89645ffb636567b6448", "name" : "Africa" }
{ "_id" : "5d4af89645ffb636567b6449", "name" : "Asia" }
{ "_id" : "5d4af89645ffb636567b644a", "name" : "Europe" }
{ "_id" : "5d4af89645ffb636567b644b", "name" : "America" }
{ "_id" : "5d4af89645ffb636567b644c", "name" : "Australia" }
{ "_id" : "5d4af89645ffb636567b644d", "name" : "Antarctica" }

We show the created collection with mongo.

Java MongoDB modify documents

The MongoCollection's deleteOne() method is used to delete a document and updateOne() to update a document.

com/zetcode/MongoModify.java

package com.zetcode;

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

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

public class MongoModify {

    public static void main(String[] args) {

        try (var mongoClient = new MongoClient("localhost", 27017)) {

            var database = mongoClient.getDatabase("testdb");

            MongoCollection<Document> collection = database.getCollection("cars");

            collection.deleteOne(eq("name", "Skoda"));
            collection.updateOne(new Document("name", "Audi"),
                    new Document("$set", new Document("price", 52000)));

        }
    }
}

The example deletes a document containing Skoda and updates the price of Audi.

collection.deleteOne(eq("name", "Skoda"));

The deleteOne() deletes the document of Skoda. The eq() creates a filter that matches all documents where the value of the field name equals the specified value.

collection.updateOne(new Document("name", "Audi"),
    new Document("$set", new Document("price", 52000)));

The price of Audi is changed to 52,000 with the updateOne() method.

> db.cars.find()
{ "_id" : ObjectId("5d4d13d6463315268eb7376b"), "name" : "Audi", "price" : 52000 }
{ "_id" : ObjectId("5d4d13f5463315268eb7376c"), "name" : "Mercedes", "price" : 57127 }
{ "_id" : ObjectId("5d4d1408463315268eb7376e"), "name" : "Volvo", "price" : 29000 }
{ "_id" : ObjectId("5d4d140d463315268eb7376f"), "name" : "Bentley", "price" : 350000 }
{ "_id" : ObjectId("5d4d1411463315268eb73770"), "name" : "Citroen", "price" : 21000 }
{ "_id" : ObjectId("5d4d1415463315268eb73771"), "name" : "Hummer", "price" : 41400 }
{ "_id" : ObjectId("5d4d1419463315268eb73772"), "name" : "Volkswagen", "price" : 21600 }

We confirm the changes with the mongo tool.

In this tutorial, we have worked with MongoDB and Java.

quarta-feira, 28 de abril de 2021

Black Sabbath - The Wizard

Weezer - Island In The Sun (Official Music Video)

Dr. Eneas Carneiro

A Guide to MongoDB with Java

A Guide to MongoDB with Java

by Shubham Aggarwal

Persistence

MongoDB

1. Overview

In this article, we'll have a look at integrating MongoDB, a very popular NoSQL open source database with a standalone Java client.

MongoDB is written in C++ and has quite a number of solid features such as map-reduce, auto-sharding, replication, high availability etc.

2. MongoDB

Let's start with a few key points about MongoDB itself:
freestar

    stores data in JSON-like documents that can have various structures
    uses dynamic schemas, which means that we can create records without predefining anything
    the structure of a record can be changed simply by adding new fields or deleting existing ones

The above-mentioned data model gives us the ability to represent hierarchical relationships, to store arrays and other more complex structures easily.

3. Terminologies

Understanding concepts in MongoDB becomes easier if we can compare them to relational database structures.

Let's see the analogies between Mongo and a traditional MySQL system:

    Table in MySQL becomes a Collection in Mongo
    Row becomes a Document
    Column becomes a Field
    Joins are defined as linking and embedded documents

This is a simplistic way to look at the MongoDB core concepts of course, but nevertheless useful.

Now, let's dive into implementation to understand this powerful database.
freestar

4. Maven Dependencies

We need to start by defining the dependency of a Java Driver for MongoDB:

<dependency>
    <groupId>org.mongodb</groupId>
    <artifactId>mongo-java-driver</artifactId>
    <version>3.4.1</version>
</dependency>

5. Using MongoDB

Now, let's start implementing Mongo queries with Java. We will follow with the basic CRUD operations as they are the best to start with.
5.1. Make a Connection With MongoClient

First, let's make a connection to a MongoDB server. With version >= 2.10.0, we'll use the MongoClient:

MongoClient mongoClient = new MongoClient("localhost", 27017);

And for older versions use Mongo class:
freestar

Mongo mongo = new Mongo("localhost", 27017);

5.2. Connecting to a Database

Now, let's connect to our database. It is interesting to note that we don't need to create one. When Mongo sees that database doesn't exist, it will create it for us:

DB database = mongoClient.getDB("myMongoDb");

Sometimes, by default, MongoDB runs in authenticated mode. In that case, we need to authenticate while connecting to a database.

We can do it as presented below:

MongoClient mongoClient = new MongoClient();
DB database = mongoClient.getDB("myMongoDb");
boolean auth = database.authenticate("username", "pwd".toCharArray());

5.3. Show Existing Databases

Let's display all existing databases. When we want to use the command line, the syntax to show databases is similar to MySQL:

show databases;

In Java, we display databases using snippet below:

mongoClient.getDatabaseNames().forEach(System.out::println);

The output will be:

local      0.000GB
myMongoDb 0.000GB

Above, local is the default Mongo database.
5.4. Create a Collection

Let's start by creating a Collection (table equivalent for MongoDB) for our database. Once we have connected to our database, we can make a Collection as:
freestar

database.createCollection("customers", null);

Now, let's display all existing collections for current database:

database.getCollectionNames().forEach(System.out::println);

The output will be:

customers

5.5. Save – Insert

The save operation has save-or-update semantics: if an id is present, it performs an update, if not – it does an insert.

When we save a new customer:

DBCollection collection = database.getCollection("customers");
BasicDBObject document = new BasicDBObject();
document.put("name", "Shubham");
document.put("company", "Baeldung");
collection.insert(document);

The entity will be inserted into a database:

{
    "_id" : ObjectId("33a52bb7830b8c9b233b4fe6"),
    "name" : "Shubham",
    "company" : "Baeldung"
}

Next, we’ll look at the same operation – save – with update semantics.
5.6. Save – Update

Let’s now look at save with update semantics, operating on an existing customer:

{
    "_id" : ObjectId("33a52bb7830b8c9b233b4fe6"),
    "name" : "Shubham",
    "company" : "Baeldung"
}

Now, when we save the existing customer – we will update it:
freestar

BasicDBObject query = new BasicDBObject();
query.put("name", "Shubham");

BasicDBObject newDocument = new BasicDBObject();
newDocument.put("name", "John");

BasicDBObject updateObject = new BasicDBObject();
updateObject.put("$set", newDocument);

collection.update(query, updateObject);

The database will look like this:

{
    "_id" : ObjectId("33a52bb7830b8c9b233b4fe6"),
    "name" : "John",
    "company" : "Baeldung"
}

As you can see, in this particular example, save uses the semantics of update, because we use object with given _id.
5.7. Read a Document From a Collection

Let's search for a Document in a Collection by making a query:

BasicDBObject searchQuery = new BasicDBObject();
searchQuery.put("name", "John");
DBCursor cursor = collection.find(searchQuery);

while (cursor.hasNext()) {
    System.out.println(cursor.next());
}

It will show the only Document we have by now in our Collection:

[
    {
      "_id" : ObjectId("33a52bb7830b8c9b233b4fe6"),
      "name" : "John",
      "company" : "Baeldung"
    }
]

5.8. Delete a Document

Let's move forward to our last CRUD operation, deletion:

BasicDBObject searchQuery = new BasicDBObject();
searchQuery.put("name", "John");

collection.remove(searchQuery);

With above command executed, our only Document will be removed from the Collection.

6. Conclusion

This article was a quick introduction to using MongoDB from Java.

This is a Maven based project, so it should be easy to import and run as it is.

Douglas Rezende Freak's Blog

GOW

quinta-feira, 29 de abril de 2021

Sarcofago - Purification Process[The Worst]

Iron Maiden - The Educated Fool

Bruce Zalcer - Live @ Radio Intense / Hybrid DJ Set

Sarcofago - Orgy of Flies[Hate]

MongoDB Java tutorial

MongoDB Java tutorial

Installing MongoDB

Creating a database

Java MongoDB driver

Java MongoDB list database collections

Java MongoDB database statistics

Java MongoDB read data

Java MongoDB query operators

Java MongoDB factory filter query methods

Java MongoDB projections

Java MongoDB limit data output

Java MongoDB create collection

Java MongoDB create collection from JSON

Java MongoDB modify documents

quarta-feira, 28 de abril de 2021

Black Sabbath - The Wizard

Weezer - Island In The Sun (Official Music Video)

Dr. Eneas Carneiro

A Guide to MongoDB with Java

1. Overview

2. MongoDB

3. Terminologies

4. Maven Dependencies

5. Using MongoDB

6. Conclusion