Unsupported major.minor version 51.0 but everything is set to JDK 1.6

Question

I have a Maven Eclipse application which I'm trying to run through the command prompt using the following command:

java -cp target/classes;"target/dependency/*" com.example.Main

Unfortunately, it throws an error:

UnsupportedClassVersionError : unsupported major.minor version 51.0

I did some digging and found out it has something to do with the application being compiled on a JDK of a newer version, than the JRE it runs on. Apparently the 51 means it was built for 1.7, but I don't have a JDK 7 on my computer...

So I made sure that everything is set to version 1.6 :

system.properties file in the root contains

java.runtime.version=1.6

Added the maven property

<java.version>1.6</java.version>

Specified the maven compiler plugin in my pom.xml

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>2.1</version>
    <configuration>
        <source>1.6</source>
        <target>1.6</target>
    </configuration>
</plugin>

I checked the version of my java using the command java -version which returned

Java version "1.6.0_38"
Java(TM) SE Runtime Environment (build 1.6.0_38-b05)
Java HotSpot(TM) 64-bit Server VM (build 20.13-b02, mixed mode)

And an update of my project after setting the maven-compiler-plugin resulted in Eclipse automatically setting the compiler level to 1.6, the project facet level to 1.6 and adding the JavaSE-1.6 jre system library to my project.

So I have absolutely no idea where this JDK 1.7 is coming from. I never installed JDK 1.7 on this computer to begin with...

Also, originally, the maven-compiler-plugin wasn't present so it was build with 1.5 I guess. But I did a mvn clean install, then a mvn package, and then tried the command again after all the settings were adjusted and it still throws the same error.

Possum

You are definitely missing something. Recheck your java version. Check on command prompt : java -version and javac -version – Debutante

Maybe a dependency needs Java 1.7 – Talcahuano

java -version and javac -version both return 1.6.0_38, but I don't actually compile using the command prompt. I only use it to run my app. I'm compiling using Maven, and there I use the maven plugn. – Possum

Adrian, you're a life saver. My pom specified the wrong version of Jetty. Jetty 9 requires JDK 1.7, please post your response as an answer so I can give you proper credit. – Possum

I use IntelliJ 14, and I don't include a compiler plugin. I fixed this issue by removing the target/ directory. After that, worked just fine! – Territus

Answer 1

Probably you have a dependency that needs Java 1.7

Talcahuano

Answer 2

You're probably using a dependency which requires Java SE 7.

If this is the case either upgrade to Java SE 7 (recommended if you're using the JRE from Oracle without support contract, as Java SE 6 reached End of Life in February) or replace this dependency.

Ginnygino

Yep it was Jetty 9. I can't upgrade to JDK 7 but I downgraded to Jetty 8 which does the job as well. – Possum

How did you downgrade the Jetty version? – Commandant

@Commandant by using earlier versions of jetty – Natelson