javac -version
java -version
They will be different and javac will have a higher version number.
To get around this, run using java from the JDK version or if you have a newer JRE/JDK that will work as well.
which javac will tell you the location, for example, /usr/bin/javac. Just run directly using /usr/bin/java <program>.
OR you can set the environment variable as a permanent solution.
This error means that the JRE that is being used to execute your class code does not recognise the version of Java used. Usually because the version of Java that generated your class file (i.e. compiled it) is newer.
To fix it, you can either
a) Compile your Java sources with the same, or older, version of the Java compiler as will be used to run it. i.e. install the appropriate JDK.
b) Compile your Java sources with the newer version of the Java compiler but in compatibility mode. i.e. use the -target parameter.
c) Run your compiled classes in a JRE that is the same, or newer, version as the JDK used to compile the classes.
You can check the versions you are currently using with
javac -version for the compiler, and java -version for the runtime.
Should I install the JDK, and setup my PATH variable to the JDK
instead of JRE?
For compilation, certainly, install and configure the specific JDK that you want.
For runtime, you can use the one that comes with the JDK or a standalone JRE, but regardless, make sure that you have installed the right versions and that you have configured your PATH such that there are no surprises.
What is the difference between the PATH variable in JRE or JDK?
The PATH environment variable tells the command shell where to look for the command you type. When you type java, the command shell interpreter will look through all the locations specified in the PATH variable, from left to right, to find the appropriate java runtime executable to run. If you have multiple versions of Java installed - i.e. you have the java executable in multiple locations specified in the PATH variable, then the first one encountered when going from left to right will be the one that is executed.
The compiler command is javac and only comes with the JDK. The runtime command is java and comes with the JDK and is in the JRE.
It is likely that you have one version (51.0 = Java 7) of javac installed, and you also have the same version of java installed, but that another previous version of java is appearing earlier in the PATH and so is being invoked instead of the one you expect.
Today, this error message appeared in our Tomcat 7 on Ubuntu 12.04.2 LTS (Precise Pangolin):
/var/log/tomcat7/localhost.2014-04-08.log:
Apr 8, 2014 9:00:55 AM org.apache.catalina.core.StandardContext filterStart
SEVERE: Exception starting filter struts2
java.lang.UnsupportedClassVersionError: controller/ReqAccept : Unsupported major.minor version 51.0 (unable to load class controller.ReqAccept)
The Struts application is compiled with Java 7.
It turned out, someone uses "service tomcat [stop/start]" to restart Tomcat 7,
$ ps -ef | grep java
tomcat7 31783 1 32 20:13 ? 00:00:03 /usr/lib/jvm/default-java/bin/java...
$ /usr/lib/jvm/default-java/bin/java -version
java version "1.6.0_27"
Which causes the "Unsupported major.minor version 51.0" error.
When we used "/etc/init.d/tomcat7 [stop/start]" to restart Tomcat 7, the problem was solved.
$ ps -ef | grep java
tomcat7 31886 1 80 20:24 ? 00:00:10 /usr/local/java/jdk1.7.0_15/bin/java
$ /usr/local/java/jdk1.7.0_15/bin/java -version
java version "1.7.0_15"
The error is misleading, Unsupported major.minor version 51.0. This gives the impression that version 51 (Java 7) is not supported. And we should use Java 6.
The error should have been:
The current Java version, 50, is unsupported. Use Java version 7 (51:0 and greater) instead.`
Your Java file is compiled with a different version (higher compiler version) than the version (lower runtime version) you are trying to run it with.
It is basic understanding that classes compiled with lower versions are expected to run in the later higher versions. But the opposite (compiled with higher compiler version and trying to run it with lower runtime version) is quite not possible sometimes.
Hence you are shown this error, when trying to execute your program. Unsupported major.minor version x.x
Q: I have created an application in Java 7, but when my users try to
run it they get an Unsupported major.minor version 51.0 error. What
does this mean and what can I do about it?
A: If you compile an application using javac in Java 7, the resulting classfiles will have the 51.0 version number. Versions of
Java prior to 7 do not recognize this number, so your users will have
to upgrade to Java 7 prior to running your application. If you are not
using any Java 7 APIs you can try to compile your application using
javac -target 1.6 to create a 1.6-compatible classfile. If your
application is deployed using webstart you can specify the minimum
version required. For more information, see the docs on Java Web Start
and JNLP here. This issue will go away once we trigger autoupdate to
Java 7 for end-users currently having Java 6 on their desktops. The
timeline for this is not yet determined, we want to give developers
time to work out any issues between their code and JDK 7 first.
(Source: oracle.com.)
First let's get some basics right...
JRE is a component in NetBeans/Eclipse/standalone that is going to provide you with libraries, JVM, Java plugins & Java web start. Note that it does not provide compliers or debuggers.
JDK is the superset of JRE along with compliers and debuggers.
So when you have your default library as a JRE instead of JDK, you are going to have a nice time importing stuff, but it won't compile.
Instead, set your path to JDK (I use NetBeans, and I set them using netbeans.conf in netbeans/etc/netbeans.conf and change the path).
–
I had the problem whereby I was having to run a Maven compilation on my project from the command line in order to run my unit tests; if I made a change to the test class and let Eclipse automatically recompile it, then I got the "Unsupported major.minor version 51.0" error.
I do have both JDK6 and JDK7 installed, but all my JRE settings were pointing at 1.6, both in the pom and from the project properties page in Eclipse. No amount of Maven Update Project and/or refreshing solved this.
Finally I tried closing the project and re-opening it, and this seemed to fix it! HTH
Install JDK 7.0.55 and set the Java for Eclipse for JDK 7.0.55.
Build the project with JDK 7.0.55 by configuring on build path JDK 7.0.55.
Set the compiler in Eclipse for JDK 7.0.55 by menu Windows -> Preferences -> Java -> Compiler - choose 1.7.
How to resolve Java Runtime (class file version 55.0), this version of the Java Runtime only recognizes class file versions up to 52.0 error?
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