can i make .jar file compatible with older versions of java

Question

I'm getting this error:

enter image description here

due to my Java version. I would like to make a .jar file compatible with older versions of Java as well as newer versions. My PC isn't a problem but my audience uses older versions of Java. How could I make my program compatible with older versions as well as newer versions and how far should i go with it?

possible duplicate of [Unsupported major.minor version 51.0](http://stackoverflow.com/questions/10382929/unsupported-major-minor-version-51-0) — Mat, May 06 '14 at 16:52
His versions are going the opposite direction. I'd say close but no cigar. — Qix - MONICA WAS MISTREATED, May 06 '14 at 17:14
So, did you find a way to recompile/convert a jar without recompiling it from the source code? — Igor Kanshyn, Sep 20 '18 at 19:16

score 6 · Accepted Answer · answered May 06 '14 at 16:51

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You could go back and compile your sources and generate a new jar file with an older javac thats compatible with the particular version of java.

answered May 06 '14 at 16:51

Amir Afghani

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Hello downvoter. Can you please tell me what is wrong with this answer ? – Amir Afghani May 06 '14 at 16:54
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Why would he go back and compile with an older javac when Java is forward compatible, especially when the sources are already compiled with an older javac? Your answer doesn't make sense. – Qix - MONICA WAS MISTREATED May 06 '14 at 16:56

score 0 · Answer 2 · edited May 23 '17 at 12:13

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Either compile you jar using an old compiler or set the java target to the version your clients support.

To set the version, follow this thread: Setting the target version of Java in ant javac

edited May 23 '17 at 12:13

Community

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answered May 06 '14 at 16:52

Alexandre Santos

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Note that if you change the `source` and `target` for the compiler then you must also get hold of a copy of the class library from the corresponding version of Java and set the `bootclasspath`, otherwise even if the bytecode you produce is compatible with the Java 7 JVM it might still call methods that only exist in Java 8. I generally find it easier to just compile using the correct target version of Java in the first place. – Ian Roberts May 06 '14 at 16:58

Qix - MONICA WAS MISTREATED · Answer 3 · 2014-05-06T17:13:25.843

UnsupportedClassVersionError isn't about the JAR (a JAR is simply a zip archive), but instead the classes inside the JAR.

.class files are just compiled Java sources and are in a format called ByteCode that the JVM (Java Virtual Machine) uses to perform operations on the underlying platform. Bytecode's format changes every once in a while (Java has been around since 1991) and thus the version numbers that are hard-coded into compiled classes formats usually change between JRE releases.

tl;dr: What you'll need to do is acquire the source and re-compile against the JRE you want. There is no way to just tell the class files to change or be compatible with older versions. Archived JRE versions are available but you'll have to register for an account to download versions other than the latest.

EDIT

I wasn't paying attention to the version number. Your .class files are compiled with J2SE 1.8 and you're running the JRE7. Install the JRE 1.8.

score 0 · Answer 4 · answered May 06 '14 at 17:21

The only way to be sure your code is actually compatible with an older major java version is to compile under a JDK of that version. While its possible to compile for an older JVM version, you can never be sure you don't use methods/classes that do not exist in an older JRE if you use a newer JDK to compile against.

To ensure your code actually runs fine with a newer JRE, you will need to test it with the newer JRE. No way around that, while newer JRE's retain great backward compatibility there are some slight differences that can trip your code up.

score 0 · Answer 5 · answered Jul 28 '15 at 10:15

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You can use NetBeans IDE for this purpose.

Run>> Set Project Configuration >> Customize >> In the left panel, choose Sources. Finally, select your minimum JDK version in Source/Binary Format.

answered Jul 28 '15 at 10:15

Nabin Bhandari

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can i make .jar file compatible with older versions of java

5 Answers5