Maven -- Why are dependencies versions not overwritten? - spring-boot

Question

My project depends on spring-boot-starter-data-mongodb and I just want to upgrade mongodb driver's version. Here is my pom:
<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.demo</groupId>
<artifactId>demo</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>jar</packaging>
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>2.5.9</version>
<relativePath /> <!-- lookup parent from repository -->
</parent>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
<java.version>11</java.version>
</properties>
<dependencies>
<dependency>
<groupId>org.mongodb</groupId>
<artifactId>mongodb-driver-sync</artifactId>
<version>4.5.0</version>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-data-mongodb</artifactId>
</dependency>
</dependencies>
</project>
Here is final dependency tree:
[INFO] --- maven-dependency-plugin:3.1.2:tree (default-cli) # demo ---
[INFO] com.demo:demo:jar:0.0.1-SNAPSHOT
[INFO] +- org.mongodb:mongodb-driver-sync:jar:4.5.0:compile
[INFO] | +- org.mongodb:bson:jar:4.2.3:compile
[INFO] | \- org.mongodb:mongodb-driver-core:jar:4.2.3:compile
[INFO] \- org.springframework.boot:spring-boot-starter-data-mongodb:jar:2.5.9:compile
[INFO] +- org.springframework.boot:spring-boot-starter:jar:2.5.9:compile
[INFO] | +- org.springframework.boot:spring-boot:jar:2.5.9:compile
[INFO] | +- org.springframework.boot:spring-boot-autoconfigure:jar:2.5.9:compile
[INFO] | +- org.springframework.boot:spring-boot-starter-logging:jar:2.5.9:compile
[INFO] | | +- ch.qos.logback:logback-classic:jar:1.2.10:compile
[INFO] | | | \- ch.qos.logback:logback-core:jar:1.2.10:compile
[INFO] | | +- org.apache.logging.log4j:log4j-to-slf4j:jar:2.17.1:compile
[INFO] | | | \- org.apache.logging.log4j:log4j-api:jar:2.17.1:compile
[INFO] | | \- org.slf4j:jul-to-slf4j:jar:1.7.33:compile
[INFO] | +- jakarta.annotation:jakarta.annotation-api:jar:1.3.5:compile
[INFO] | +- org.springframework:spring-core:jar:5.3.15:compile
[INFO] | | \- org.springframework:spring-jcl:jar:5.3.15:compile
[INFO] | \- org.yaml:snakeyaml:jar:1.28:compile
[INFO] \- org.springframework.data:spring-data-mongodb:jar:3.2.8:compile
[INFO] +- org.springframework:spring-tx:jar:5.3.15:compile
[INFO] +- org.springframework:spring-context:jar:5.3.15:compile
[INFO] | \- org.springframework:spring-aop:jar:5.3.15:compile
[INFO] +- org.springframework:spring-beans:jar:5.3.15:compile
[INFO] +- org.springframework:spring-expression:jar:5.3.15:compile
[INFO] +- org.springframework.data:spring-data-commons:jar:2.5.8:compile
[INFO] \- org.slf4j:slf4j-api:jar:1.7.33:compile
[INFO] ------------------------------------------------------------------------
My question is why bson and mongodb-driver-core's version are still 4.2.3 instead of 4.5.0. mongodb-driver-sync will depends on bson and mongodb-driver-core, and the versions are 4.5.0. Here is the pom of mongodb-driver-sync.
<project>
<groupId>org.mongodb</groupId>
<artifactId>mongodb-driver-sync</artifactId>
<version>4.5.0</version>
<dependencies>
<dependency>
<groupId>org.mongodb</groupId>
<artifactId>bson</artifactId>
<version>4.5.0</version>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>org.mongodb</groupId>
<artifactId>mongodb-driver-core</artifactId>
<version>4.5.0</version>
<scope>compile</scope>
</dependency>
</dependencies>
</project>

Answer 1

Because spring-boot-starter-parent inherits from spring-boot-dependencies, which defines
<mongodb.version>4.2.3</mongodb.version>
<dependencyManagement>
<!-- ... -->
<dependency>
<groupId>org.mongodb</groupId>
<artifactId>mongodb-driver-sync</artifactId>
<version>${mongodb.version}</version>
</dependency>
<!-- ... -->
</dependencyManagement>
You need to override the property in your pom if you want to upgrade it independently:
<mongodb.version>4.5.0</mongodb.version>
Or wait for the Spring Boot 2.7.0 release, which upgrades Mongo to 4.5.0.