When working with Apache Maven projects in Eclipse, developers might encounter a situation where an imported Maven project defaults to using Java 1.5 instead of the desired Java 1.6 or a later version. This issue can arise due to several reasons related to the project's configuration or the environment setup. Understanding the root causes and paths for resolution can save developers time and frustration.

Causes of Version Mismatch

1. POM File Configuration

Maven projects rely heavily on a pom.xml file for project configuration. This file specifies the project's dependencies, build settings, and important configurations such as the JDK version.

• Cause: The pom.xml contains a maven-compiler-plugin configuration that sets the source and target version to 1.5.

1  <build>
2      <plugins>
3          <plugin>
4              <groupId>org.apache.maven.plugins</groupId>
5              <artifactId>maven-compiler-plugin</artifactId>
6              <version>3.8.1</version>
7              <configuration>
8                  <source>1.5</source>
9                  <target>1.5</target>
10              </configuration>
11          </plugin>
12      </plugins>
13  </build>

2. Inherited POM Settings

Projects often inherit from parent POM files that define standard settings for child projects.

• Cause: The parent POM file specifies the compiler version as 1.5, which the child project inherits unless overridden.

3. Eclipse Workspace JRE

Eclipse may have a default JRE system library for all new projects that is set incorrectly.

• Cause: The default JRE in Eclipse's preferences is set to Java 1.5.

4. Project-Specific Configuration

When importing projects into Eclipse, sometimes .classpath or project-specific settings might reference an outdated JRE version.

Ensuring Correct JDK Version

Modify pom.xml

Update the Maven compiler plugin settings to use Java 1.6 or the desired version.

1<build>
2    <plugins>
3        <plugin>
4            <groupId>org.apache.maven.plugins</groupId>
5            <artifactId>maven-compiler-plugin</artifactId>
6            <version>3.8.1</version>
7            <configuration>
8                <source>1.6</source>
9                <target>1.6</target>
10            </configuration>
11        </plugin>
12    </plugins>
13</build>

Use Parent POM Configuration

Ensure that the parent POM has the correct Java version or override it in your project if it is incorrect.

Update Eclipse Settings

1. Default Workspace JRE

1. Navigate to: Window > Preferences > Java > Installed JREs.
2. Ensure Java 1.6 or the correct JDK is available and select it as the default.

2. Project-Specific Library

1. Right-click the specific project and select Properties.
2. Go to Java Build Path > Libraries > JRE System Library, then set it to a JDK of version 1.6 or higher.
3. Optionally, configure your Eclipse to prompt you when the workspace's JRE is different from the project's. ​

Validate Maven Configuration in Eclipse

• Maven: Go to Run > Run Configurations > Maven Build and specify the JDK or Maven option appropriate for Java 1.6.

Check Parent and Module Relationships

Examine the parent-child relationship in the Maven build lifecycle to ensure no conflicts in JDK configurations exist between the parent and module POMs.

Summary Table

Below is a summary of key points discussed:

By carefully reviewing and updating both your project's configuration files and Eclipse settings, you can prevent your Maven project from defaulting to Java 1.5 and ensure it uses the desired Java version, such as Java 1.6. Understanding the role of Maven's POM inheritance and Eclipse's internal settings are key in managing this aspect of your Java development efficiently.