Anyone that uses the M2E(maven-to-eclipse) plugin in eclipse knows the issue where you have run a build, then update maven on your project only to have it reset the JRE and throw up a wall of project errors! I noticed the issue in the post I made on running Java EE 8 with Java 9 using Open Liberty

The solution is to ensure the compiler is running the correct version you require, so

<properties>
 <maven.compiler.source>1.8</maven.compiler.source>
 <maven.compiler.target>1.8</maven.compiler.target>
 <failOnMissingWebXml>false</failOnMissingWebXml>
</properties>

Or –

 <build>
 <plugins>
 <plugin>
 <groupId>org.apache.maven.plugins</groupId>
 <artifactId>maven-compiler-plugin</artifactId>
 <version>3.7.0</version>
 <configuration>
 <source>1.8</source>
 <target>1.8</target>
 </configuration>
 </plugin>
 </plugins>
 </build>

There is a further point worth noting if you are using Java 9 or later, as the version number format is different. So while Java 8 has a version of 1.8, Java 9 has a version of 9

This means you need to state the version in your pom as

<properties>
 <maven.compiler.source>9</maven.compiler.source>
 <maven.compiler.target>9</maven.compiler.target>
 <failOnMissingWebXml>false</failOnMissingWebXml>
 </properties>

Or using the full version in my JavaEE8 project(https://github.com/farrelmr/javaee8/blob/master/pom.xml) this would be stated as –

<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.javabullets.javaee8</groupId>
 <artifactId>javaee8</artifactId>
 <version>1.0-SNAPSHOT</version>
 <packaging>war</packaging>
 <dependencies>
 <dependency>
 <groupId>javax</groupId>
 <artifactId>javaee-api</artifactId>
 <version>8.0</version>
 <scope>provided</scope>
 </dependency>
 </dependencies>
 <build>
 <finalName>javaee8</finalName>
 </build>
 <properties>
 <maven.compiler.source>9</maven.compiler.source>
 <maven.compiler.target>9</maven.compiler.target>
 <failOnMissingWebXml>false</failOnMissingWebXml>
 </properties>
</project>

This will prevent the problem in eclipse

This post may seem obvious – but it is worth stating that the change in version number for Java 9 means you must check you are using the correct version number in maven