Java version automatically change to java 1.5 after maven update

Had the same issue when I installed Java 9. My project would default to J2SE-1.5 Execution Environment. Strangely, Java 9 compliance level is not referenced like previous versions, i.e. "1.8", but as "9". So I had to provide my properties and Maven compiler plugin config accordingly:

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

and

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <configuration>
        <source>9</source>
        <target>9</target>
    </configuration>
</plugin>

This seems to have solved the problem. Works for versions 9 and above.

The root-cause of this issue is that if for any reason Eclipse's cannot resolve a valid value for the maven.compiler.source property when generating/updating the .classpath file from the pom, it will simply default to using org.eclipse.jdt.launching.JRE_CONTAINER/org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/J2SE-1.5.

As expertly answered by @jorge-campos, there are multiple ways to set that property.

However, Jorge's answer didn't appear to work for me. Here were my settings:

<properties>
    <javaVersion>1.8</javaVersion>
    <maven.compiler.source>${java.version}</maven.compiler.source>
    <maven.compiler.target>${java.version}</maven.compiler.target>
</properties>

...

Exactly. ${java.version} is never going to resolve to the (completely different) property javaVersion and Eclipse ignored the property and used the default.

Which brings me back to the "for any reason" part I opened with; developer stupidity can be one of those reasons.

Open your pom.xml file and add the following lines on it:

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

Where 1.8 is the Java version of your current JDK/JRE. Another way of doing this is adding a <build> with the maven-compile-plugin as:

<build>
<plugins>
  <plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.2</version> <!-- or whatever current version -->
    <configuration>
      <source>1.8</source>
      <target>1.8</target>
    </configuration>
  </plugin>
</plugins>
</build>

If you are looking for a way to make it work with Java versions 9+ please take a look at @JDelorean's answer.