What version of javac built my jar?

Question

How can I tell what version of the Java compiler was used to build a jar? I have a jar file, and it could have been built in any one of three JDKs. We need to know exactly

Answer 1

The Java compiler (javac) does not build jars, it translates Java files into class files. The Jar tool (jar) creates the actual jars. If no custom manifest was specified, the default manifest will specify which version of the JDK was used to create the jar.

Answer 2

You can tell the Java binary version by inspecting the first 8 bytes (or using an app that can).

The compiler itself doesn't, to the best of my knowledge, insert any identifying signature. I can't spot such a thing in the file VM spec class format anyway.

Answer 3

Developers and administrators running Bash may find these convenience functions helpful:

jar_jdk_version() {
  [[ -n "$1" && -x "`command -v javap`" ]] && javap -classpath "$1" -verbose $(jar -tf "$1" | grep '.class' | head -n1 | sed -e 's/\.class$//') | grep 'major version' | sed -e 's/[^0-9]\{1,\}//'
}

print_jar_jdk_version() {
  local version
  version=$(jar_jdk_version "$1")
  case $version in 49) version=1.5;; 50) version=1.6;; 51) version=1.7;; 52) version=1.8;; esac
  [[ -n "$version" ]] && echo "`basename "$1"` contains classes compiled with JDK version $version."
}

You can paste them in for one-time usage or add them to ~/.bash_aliases or ~/.bashrc. The results look something like:

$ jar_jdk_version poi-ooxml-3.5-FINAL.jar
49

and

$ print_jar_jdk_version poi-ooxml-3.5-FINAL.jar
poi-ooxml-3.5-FINAL.jar contains classes compiled with JDK version 1.5.

EDIT As jackrabbit points out, you can't 100% rely on the manifest to tell you anything useful. If it was, then you could pull it out in your favorite UNIX shell with unzip:

$ unzip -pa poi-ooxml-3.5-FINAL.jar META-INF/MANIFEST.MF
Manifest-Version: 1.0
Ant-Version: Apache Ant 1.7.1
Created-By: 11.3-b02 (Sun Microsystems Inc.)
Built-By: yegor
Specification-Title: Apache POI
Specification-Version: 3.5-FINAL-20090928
Specification-Vendor: Apache
Implementation-Title: Apache POI
Implementation-Version: 3.5-FINAL-20090928
Implementation-Vendor: Apache

This .jar doesn't have anything useful in the manifest about the contained classes.

Answer 4

There is no need to unpack the JAR (if one of the class names is known or is looked up e.g. using 7zip), so on Windows the following would be sufficient:

javap -cp log4j-core-2.5.jar -verbose org.apache.logging.log4j.core.Logger | findstr major

Answer 5

You check in Manifest file of jar example:

Manifest-Version: 1.0 Created-By: 1.6.0 (IBM Corporation)

Answer 6

Here is Java's way to find this information.

Windows: javap -v <class> | findstr major
Unix: javap -v <class> | grep major

For example:
> javap -v Application | findstr major   major version: 51