What Java Version Are You Running? Let’s Take a Look Under the Hood of the JDK!

From time to time, you need to check which Java version is installed on your computer or server, for instance when starting on a new project or configuring an application to run on a server.

But did you know there are multiple ways you can do this and even get much more information than you might think, very quickly?

Let's find out...

Reading the Java Version in the Terminal

Probably the easiest way to find the installed version is by using the java -version terminal command:

$ java -version
openjdk version "19" 2022-09-20
OpenJDK Runtime Environment Zulu19.28+81-CA (build 19+36)
OpenJDK 64-Bit Server VM Zulu19.28+81-CA (build 19+36, mixed mode, sharing)

Checking Version Files in the Installation Directory

The above output results from info read by the java executable from a file inside its installation directory.

Let's explore what we can find there.

On my machine, as I use SDKMAN to switch between different Java versions, all my versions are stored here:

$ ls -l /Users/frankdelporte/.sdkman/candidates/java/
total 0
drwxr-xr-x  15 frankdelporte  staff  480 Apr 17  2022 11.0.15-zulu
drwxr-xr-x  16 frankdelporte  staff  512 Apr 17  2022 17.0.3.fx-zulu
drwxr-xr-x  15 frankdelporte  staff  480 Mar 29  2022 18.0.1-zulu
drwxr-xr-x  15 frankdelporte  staff  480 Sep  7 18:36 19-zulu
drwxr-xr-x  18 frankdelporte  staff  576 Apr 18  2022 8.0.332-zulu
lrwxr-xr-x   1 frankdelporte  staff    7 Nov 21 21:09 current -> 19-zulu

And in each of these directories a release file can be found which also shows us the version information, including some extra information.

$ cat /Users/frankdelporte/.sdkman/candidates/java/19-zulu/release
IMPLEMENTOR="Azul Systems, Inc."
IMPLEMENTOR_VERSION="Zulu19.28+81-CA"
JAVA_VERSION="19"
JAVA_VERSION_DATE="2022-09-20"
LIBC="default"
MODULES="java.base java.compiler ... jdk.unsupported jdk.unsupported.desktop jdk.xml.dom"
OS_ARCH="aarch64"
OS_NAME="Darwin"
SOURCE=".:git:3d665268e905"

$ cat /Users/frankdelporte/.sdkman/candidates/java/8.0.332-zulu//release
JAVA_VERSION="1.8.0_332"
OS_NAME="Darwin"
OS_VERSION="11.2"
OS_ARCH="aarch64"
SOURCE="git:f4b2b4c5882e"

Getting More Information With showSettings

In 2010, an experimental flag (indicated with the X) was added to OpenJDK to provide more configuration information: -XshowSettings.

This flag can be called with different arguments, each producing an other information output.

The cleanest way to call this flag, is by adding -version, otherwise you will get the long Java manual output as no application code was found to be executed.

Reading the System Properties

By using the -XshowSettings:properties flag, a long list of various properties is shown.

$ java -XshowSettings:properties -version
Property settings:
    file.encoding = UTF-8
    file.separator = /
    ftp.nonProxyHosts = local|*.local|169.254/16|*.169.254/16
    http.nonProxyHosts = local|*.local|169.254/16|*.169.254/16
    java.class.path =
    java.class.version = 63.0
    java.home = /Users/frankdelporte/.sdkman/candidates/java/19-zulu/zulu-19.jdk/Contents/Home
    java.io.tmpdir = /var/folders/np/6j1kls013kn2gpg_k6tz2lkr0000gn/T/
    java.library.path = /Users/frankdelporte/Library/Java/Extensions
        /Library/Java/Extensions
        /Network/Library/Java/Extensions
        /System/Library/Java/Extensions
        /usr/lib/java
        .
    java.runtime.name = OpenJDK Runtime Environment
    java.runtime.version = 19+36
    java.specification.name = Java Platform API Specification
    java.specification.vendor = Oracle Corporation
    java.specification.version = 19
    java.vendor = Azul Systems, Inc.
    java.vendor.url = http://www.azul.com/
    java.vendor.url.bug = http://www.azul.com/support/
    java.vendor.version = Zulu19.28+81-CA
    java.version = 19
    java.version.date = 2022-09-20
    java.vm.compressedOopsMode = Zero based
    java.vm.info = mixed mode, sharing
    java.vm.name = OpenJDK 64-Bit Server VM
    java.vm.specification.name = Java Virtual Machine Specification
    java.vm.specification.vendor = Oracle Corporation
    java.vm.specification.version = 19
    java.vm.vendor = Azul Systems, Inc.
    java.vm.version = 19+36
    jdk.debug = release
    line.separator = \n
    native.encoding = UTF-8
    os.arch = aarch64
    os.name = Mac OS X
    os.version = 13.0.1
    path.separator = :
    socksNonProxyHosts = local|*.local|169.254/16|*.169.254/16
    stderr.encoding = UTF-8
    stdout.encoding = UTF-8
    sun.arch.data.model = 64
    sun.boot.library.path = /Users/frankdelporte/.sdkman/candidates/java/19-zulu/zulu-19.jdk/Contents/Home/lib
    sun.cpu.endian = little
    sun.io.unicode.encoding = UnicodeBig
    sun.java.launcher = SUN_STANDARD
    sun.jnu.encoding = UTF-8
    sun.management.compiler = HotSpot 64-Bit Tiered Compilers
    user.country = BE
    user.dir = /Users/frankdelporte
    user.home = /Users/frankdelporte
    user.language = en
    user.name = frankdelporte

openjdk version "19" 2022-09-20
OpenJDK Runtime Environment Zulu19.28+81-CA (build 19+36)
OpenJDK 64-Bit Server VM Zulu19.28+81-CA (build 19+36, mixed mode, sharing)

If you ever faced the problem of an unsupported Java version 59 (are similar), you'll now also understand where this value is defined, it's right here in this list as java.class.version.

It's an internal number used by Java to define the version.

Java release
8   9   10  11  12  13  14  15  16  17  18  19
Class version
52  53  54  55  56  57  58  59  60  61  62  63

Reading the Locale Information

In case you didn't know yet, I live in Belgium and use English as my computer language, as you can see when using the -XshowSettings:locale flag:

$ java -XshowSettings:locale -version
Locale settings:
    default locale = English (Belgium)
    default display locale = English (Belgium)
    default format locale = English (Belgium)
    available locales = , af, af_NA, af_ZA, af_ZA_#Latn, agq, agq_CM, agq_CM_#Latn,
        ak, ak_GH, ak_GH_#Latn, am, am_ET, am_ET_#Ethi, ar, ar_001,
        ar_AE, ar_BH, ar_DJ, ar_DZ, ar_EG, ar_EG_#Arab, ar_EH, ar_ER,
        ...
        zh_MO_#Hant, zh_SG, zh_SG_#Hans, zh_TW, zh_TW_#Hant, zh__#Hans, zh__#Hant, zu,
        zu_ZA, zu_ZA_#Latn

openjdk version "19" 2022-09-20
OpenJDK Runtime Environment Zulu19.28+81-CA (build 19+36)
OpenJDK 64-Bit Server VM Zulu19.28+81-CA (build 19+36, mixed mode, sharing)

Reading the VM Settings

With the -XshowSettings:vm flag, some info is shown about the Java Virtual Machine.

As you can see in the second example, the amount of maximum heap memory size can be defined with the -Xmx flag.

$ java -XshowSettings:vm -version
VM settings:
    Max. Heap Size (Estimated): 8.00G
    Using VM: OpenJDK 64-Bit Server VM

openjdk version "19" 2022-09-20
OpenJDK Runtime Environment Zulu19.28+81-CA (build 19+36)
OpenJDK 64-Bit Server VM Zulu19.28+81-CA (build 19+36, mixed mode, sharing)

$ java -XshowSettings:vm -Xmx512M -version
VM settings:
    Max. Heap Size: 512.00M
    Using VM: OpenJDK 64-Bit Server VM

openjdk version "19" 2022-09-20
OpenJDK Runtime Environment Zulu19.28+81-CA (build 19+36)
OpenJDK 64-Bit Server VM Zulu19.28+81-CA (build 19+36, mixed mode, sharing)