首先，我们需要了解什么是数据库连接池。数据库连接池是数据库连接的缓存池，它的主要目的是减少数据库连接的创建和销毁次数，提升系统性能。使用数据库连接池可以减少每次连接数据库所需的时间和资源。Java实现MongoDB的数据库连接池有两种方式，分别是通过第三方连接池库和手动实现连接池。

通过第三方连接池库实现MongoDB的数据库连接池

1. 首先，需要导入MongoDB的Java驱动jar包及连接池jar包

<dependency>
    <groupId>org.mongodb</groupId>
    <artifactId>mongodb-driver-sync</artifactId>
    <version>4.4.1</version>
</dependency>
<dependency>
    <groupId>com.mongodb</groupId>
    <artifactId>mongodb-driver-sync</artifactId>
    <version>4.4.1</version>
</dependency>
<dependency>
    <groupId>org.mongodb</groupId>
    <artifactId>mongodb-driver-core</artifactId>
    <version>4.4.1</version>
</dependency>
<dependency>
    <groupId>com.zaxxer</groupId>
    <artifactId>HikariCP</artifactId>
    <version>3.4.1</version>
</dependency>

2. 接下来，在代码中配置HikariCP连接池

import com.mongodb.client.MongoDatabase;
import com.mongodb.client.MongoCollection;
import com.mongodb.MongoClientSettings;
import com.mongodb.ServerAddress;
import com.mongodb.MongoCredential;
import com.mongodb.client.MongoClients;

import com.zaxxer.hikari.HikariDataSource;

public class MongoDBConfig {

    private static final String DATABASE_NAME = "test";
    private static final String USERNAME = "username";
    private static final String PASSWORD = "password";
    private static final String HOST = "localhost";
    private static final int PORT = 27017;

    public static HikariDataSource getDataSource() {
        MongoClientSettings settings = MongoClientSettings.builder()
                .applyToClusterSettings(builder -> builder.hosts(Arrays.asList(new ServerAddress(HOST, PORT))))
                .credential(MongoCredential.createCredential(USERNAME, DATABASE_NAME, PASSWORD.toCharArray()))
                .build();
        MongoDatabase database = MongoClients.create(settings).getDatabase(DATABASE_NAME);
        HikariDataSource hikariDataSource = new HikariDataSource();
        hikariDataSource.setJdbcUrl("jdbc:mongodb://" + HOST + ":" + PORT + "/" + DATABASE_NAME);
        hikariDataSource.setDataSourceClassName("org.mongodb.jdbc.MongoDataSource");
        hikariDataSource.setUsername(USERNAME);
        hikariDataSource.setPassword(PASSWORD);
        return hikariDataSource;
    }

    public static MongoDatabase getMongoDatabase() {
        HikariDataSource dataSource = getDataSource();
        return dataSource.unwrap(MongoDatabase.class);
    }

    public static MongoCollection<Document> getMongoCollection(String collectionName) {
        MongoDatabase database = getMongoDatabase();
        return database.getCollection(collectionName);
    }
}

3. 最后，在代码中调用连接池获取MongoDB的数据库连接

MongoCollection<Document> collection = null;
try (Connection connection = MongoDBConfig.getDataSource().getConnection()) {
    collection = MongoDBConfig.getMongoCollection("collectionName");
    // do something with the collection
} catch (SQLException e) {
    e.printStackTrace();
} finally {
    if (collection != null) collection.close();
}

手动实现MongoDB的数据库连接池

1. 首先，需要导入MongoDB的Java驱动jar包

<dependency>
    <groupId>org.mongodb</groupId>
    <artifactId>mongodb-driver-sync</artifactId>
    <version>4.4.1</version>
</dependency>
<dependency>
    <groupId>com.mongodb</groupId>
    <artifactId>mongodb-driver-sync</artifactId>
    <version>4.4.1</version>
</dependency>
<dependency>
    <groupId>org.mongodb</groupId>
    <artifactId>mongodb-driver-core</artifactId>
    <version>4.4.1</version>
</dependency>

2. 接下来，在代码中手动实现连接池

import com.mongodb.MongoClientSettings;
import com.mongodb.MongoCredential;
import com.mongodb.ServerAddress;
import com.mongodb.client.MongoClients;
import com.mongodb.client.MongoDatabase;
import org.apache.commons.pool2.PooledObjectFactory;
import org.apache.commons.pool2.impl.GenericObjectPool;
import org.apache.commons.pool2.impl.GenericObjectPoolConfig;

import java.util.Arrays;

public class MongoDBPool {

    private static final String DATABASE_NAME = "test";
    private static final String USERNAME = "username";
    private static final String PASSWORD = "password";
    private static final String HOST = "localhost";
    private static final int PORT = 27017;
    private static final int MAX_TOTAL = 8;
    private static final int MAX_IDLE = 4;

    private static final GenericObjectPool<MongoDatabase> mongoPool;

    static {
        MongoClientSettings settings = MongoClientSettings.builder()
                .applyToClusterSettings(builder -> builder.hosts(Arrays.asList(new ServerAddress(HOST, PORT))))
                .credential(MongoCredential.createCredential(USERNAME, DATABASE_NAME, PASSWORD.toCharArray()))
                .build();
        PooledObjectFactory<MongoDatabase> pooledObjectFactory = new MongoDBPoolFactory(settings);
        GenericObjectPoolConfig<MongoDatabase> genericObjectPoolConfig = new GenericObjectPoolConfig<>();
        genericObjectPoolConfig.setMaxTotal(MAX_TOTAL);
        genericObjectPoolConfig.setMaxIdle(MAX_IDLE);
        mongoPool = new GenericObjectPool<>(pooledObjectFactory, genericObjectPoolConfig);
    }

    public static MongoDatabase borrowObject() {
        try {
            return mongoPool.borrowObject();
        } catch (Exception e) {
            throw new RuntimeException("Error: Could not borrow connection from MongoDB connection pool");
        }
    }

    public static void returnObject(MongoDatabase database) {
        mongoPool.returnObject(database);
    }
}

3. 最后，在代码中调用连接池获取MongoDB的数据库连接

MongoDatabase database = null;
try {
    database = MongoDBPool.borrowObject();
    MongoCollection<Document> collection = database.getCollection("collectionName");
    // do something with the collection
} finally {
    if (database != null) {
        MongoDBPool.returnObject(database);
    }
}

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