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Maven installation issues: JAVA_HOME should point to a JDK not JRE?

I'm super new to CS and very unfamiliar with UNIX/Bash vocab.
I'm currently trying to install Maven, and I made a few mistakes by copy pasting old installation instructions (with incorrect version numbers) and now the whole process is so messed up. I first tried to follow the given instructions on the website, that didn't work, then I tried a few other ones, and now I'm worried that I'm left with a mess by following different sets of instructions.
I've reached a point now where if I type in mvn -version I get the following:
The JAVA_HOME environment variable is not defined correctly
This environment variable is needed to run this program
NB: JAVA_HOME should point to a JDK not a JRE
Originally, my JAVA_HOME was set to jdk-install-dir, which still gave me the above error message. I tried redownloading the jdk (version 13.0.1) from Oracle, dragged that file to my home directory, and unzipped it. Then I set my JAVA_HOME to that unzipped file, jdk-13.0.1.jdk, and updated my PATH variable. After all this, I'm still getting this same error message, and I'm not sure what to do.
For the reference, here are what some relevant environment variables are set to (I didn't include irrelevant info from PATH):
~ echo $JAVA_HOME
jdk-13.0.1.jdk
~ echo $PATH
/usr/local/apache-maven/apache-maven-3.6.3/bin:/opt/apache-maven-3.6.3/bin:jdk-install-dir/bin:/usr/local/apache-maven/apache-maven-3.3.9/bin:/opt/apache-maven-3.6.3/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/Users/allyson/apache-maven-3.6.3/bin:/Users/allyson/apache-maven-3.6.3/bin:jdk-12.0.1.jdk/bin:/Users/allyson/apache-maven-3.6.3/bin:JDK-13.0.1.jdk/bin
~ echo $M2
/usr/local/apache-maven/apache-maven-3.6.3/bin
~ echo $M2_HOME
/Users/allyson/apache-maven-3.6.3
One thing I'm curious about: for the M2 and M2_HOME variables, is it supposed to be that one is in my home directory (/Users/allyson) and one is for /usr/local?

OK, take a deep breath, and we'll walk through this. Each of these environment variables has a purpose, and once you understand what those purposes are, this makes a lot more sense. Mixing tutorials is not necessarily a problem, but you'll want to understand what you're doing, rather than just blindly copy values from the internet.
JAVA_HOME is intended to identify to the system environment where to find a java runtime environment. It needs to be set to the full path of where your JDK has been installed. On windows, this might be C:\Program Files\Java\jdk-13.0.1. On a Linux system, you have a bit more flexibility. Common locations might be /opt/java/jdk-13.0.1 or /usr/local/java/jdk-13.0.1. If you installed your JDK somewhere else, then you need to use that path instead. The message NB: JAVA_HOME should point to a JDK not a JRE refers to a common mistake when installing maven -- maven requires a JDK, not a plain JRE. This error is so common that any time JAVA_HOME points to a folder that isn't a JDK, it prints this warning (even if the folder in question isn't actually a JRE).
M2_HOME is supposed to be set to the full path where maven is installed (i.e. the place where you unzipped it). This more or less helps maven "find itself" if it should need to for whatever reason. Strictly speaking, this one isn't necessary. (It's not set on my system, and maven works fine for me). It's mostly a convenience for setting the next environment variable.
M2 is the full path to the folder where the maven executable is. This will almost always be $M2_HOME/bin, but it's certainly possible to do weird things, and this will let you work around those situations. Obviously, this won't work if you didn't specify $M2_HOME. This one isn't strictly necessary, either, and is mostly a convenient way of setting up the next one.
PATH is where your Linux system looks to find programs to run when you type their name on the command line. For ease of use, you'll want to make sure that the maven and java executables are included somewhere in the : delimited list. Most Linux distributions already have a default PATH set up for you in a shell resource file of some kind. You'll want to refer to their documentation for how to add another entry to the path, but a common idiom would be PATH=$PATH:$M2 (which would append the value of $M2 to the value of $PATH and then store the result back into PATH. If you didn't set up $M2 or $M2_HOME, you'll need to do something else.
So, TL;DR, if you installed your JDK in /opt/java/jdk-13.0.1 and unzipped maven into /opt/maven/apache-maven-3.6.3, your bear minimum working values are:
export JAVA_HOME=/opt/java/jdk-13.0.1
export PATH=$PATH:$JAVA_HOME/bin:/opt/maven/apache-maven-3.6.3/bin
And if you wanted a complete set
export JAVA_HOME=/opt/java/jdk-13.0.1
export M2_HOME=/opt/maven/apache-maven-3.6.3
export M2=$M2_HOME/bin
export PATH=$PATH:$JAVA_HOME/bin:$M2
It's worth noting that most Java IDEs will include a GUI for setting up maven and Java within the IDE (the settings will typically only work within that IDE). It's often much easier for beginners to get up and running that way.

$JAVA_HOME should be a full path, not the dir name.
scream#kafka1:~$ echo $JAVA_HOME
/opt/java/jdk-11.0.5/
scream#kafka1:~$

I'm working on Ubuntu 20, and my own issue was about the jdk version located in the JAVA_HOME variable, which wasn't the right version. This was my trip around that issue:
First, What is the JAVA_HOME value?
$ echo $JAVA_HOME
$ /usr/lib/jvm/jdk-11.0.10
Next question/step, where is the Java app pointing to?
(Source: https://stackoverflow.com/a/42706056/2373421)
$ java -XshowSettings:properties -version 2>&1 > /dev/null | grep 'java.home'
java.home = /usr/lib/jvm/jdk-11.0.12
So, the JAVA_HOME variable wasn't keeping the right value. Let's change that:
$ export JAVA_HOME=/usr/lib/jvm/jdk-11.0.12
The test outcome is right now:
$ mvn -v
Apache Maven 3.8.1 (05c21c65bdfed0f71a2f2ada8b84da59348c4c5d)
Maven home: /opt/apache-maven-3.8.1
Java version: 11.0.12, vendor: Oracle Corporation, runtime: /usr/lib/jvm/jdk-11.0.12

I'm doing this for future me's bc after all the links in StackOverflow, the solution was to check mvn.cmd file on C:\Program Files\Maven\apache-maven-3.6.3\bin...
On line 52 it sets the JDK, but it didn't had the "/bin" where my OpenJDK 15 stored the java.exe
This fixed the issue on Windows 11 but it could serve as an idea for other OS. Hope that it helps!

On Windows 8.1 "javac -version" works but Cordova doesn't recognize it

I'm trying to set up Cordova. When I run cordova build android I receive the following error:
(node:6816) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): Failed to run "javac -version", make sure that you have a JDK installed.
You can get it from: http://www.oracle.com/technetwork/java/javase/downloads.
Your JAVA_HOME is invalid: C:\Program Files\Java\jdk1.8.0_161;C:\Program Files\Java\jdk1.8.0_161\bin
However I can run javac -version just fine.
C:\>javac -version
javac 1.8.0_161
I've tried to set JAVA_HOME in various ways: pointing to the root; pointing to the bin folder; set it as a User variable and also a System one; add %JAVA_HOME%\bin to Path (both for User and System) and so on.
I've checked several articles about this issue, and they say that if I'm able to use javac -version then that's supposed to indicate that my environmental variables are set correctly. Even if that's the case, Cordova still doesn't seem to recognize it.
Update
Following Stephen C's instructions, I've reset my variables, yet my error still persists. As you can see in the image I can call javac just fine, and both JAVA_HOME and Path seem to be set in the right way. (Note that actually it is %JAVA_HOME%\bin and not C:\Program Files\Java\jdk1.8.0_161\bin in the Path.)

I think you are confusing JAVA_HOME, PATH and CLASSPATH
The JAVA_HOME variable should be set to the name of a single directory. Not multiple directories with separators. The JAVA_HOME directory is the top directory of your JDK or JRE installation. Not the "bin" directory.
The PATH and CLASSPATH variables consist of a list of paths. For PATH the paths are pathnames for directories. For CLASSPATH the paths are pathnames for directories or JAR / ZIP files (or a particular kind of wildcard).
The PATH is where the shell searches for commands if you use a command name that is a simple name; e.g. java or javac.
`
The CLASSPATH is one of the ways that you can tell Java tools to look cor compiled classes to load, compile against, etcetera.
Your error message says this:
Your JAVA_HOME is invalid:
C:\Program Files\Java\jdk1.8.0_161;C:\Program Files\Java\jdk1.8.0_161\bin
This is wrong for two reasons:
JAVA_HOME should not be a path
C:\Program Files\Java\jdk1.8.0_161\bin is not a Java home. The Java home is C:\Program Files\Java\jdk1.8.0_161 .... probably.
The other thing you may have gotten wrong is that changes to environment variables do not necessarily propagate. For example, if you start a shell and then change an environment variable via the Windows GUI, the change may not propagate to the shell. You may need to exit the shell and restart it to pick up the new value.
If you are unsure, you can run (for example) echo %PATH% to see the current value of PATH in the current shell or script.
UPDATE
According to http://cordova.apache.org/docs/en/7.x/guide/platforms/android/index.html, you need to:
install Android Studio
set the ANDROID_HOME environment variable to the location of your Android SDK installation.
It is also recommended that you add the Android SDK's tools, tools/bin, and platform-tools directories to your PATH.

Remove java.dll from C:\Windows\System32 (if any).
I'm not sure but It may work correctly.

Multiple versions of "JDK", which "java.exe" file to execute?

I have installed multiple versions of JDK, including 1.6,1.7,1.8.
I want to know which version of java.exe gets executed when I run software that needs JRE. I did some testing, see below:
Tests:
cmd.exe command: echo %java_home%
prints: java version "1.8.0_31"
cmd.exe command: java -version
prints: java version "1.8.0_31"
Then I checked the register table in HKEY_LOCAL_MACHINE\SOFTWARE\JavaSoft\Java Development Kit\1.8 and got the javaHome key value
result: C:\Program Files\Java\jdk1.8.0_31.
java code: System.out.println(System.getProperty("java.home"));
prints: C:\Program Files (x86)\Java\jdk1.7.0_15\jre
Finally, I opened the task manager, searched the java.exe process and located the java.exe directory at:
result: C:\Program Files (x86)\Java\jdk1.7.0_15\bin
Question:
I'm confused which jave.exe file the computer chooses. Can you help?

Passive approach
You may want what the where windows command line utility does; assuming you have only one java.exe in your %PATH%
Try:
where java.exe
If you have multiple version of java pin your %PATH% this will return multiple values. (unfortunately)
Active approach
Set your environment this way:
Have your PATH environment variable to point ONLY to c:\tools\java\default
Store all your jre in one folder like
C:\tools\Java\JRE_1.6
C:\tools\Java\JRE_1.7
C:\tools\Java\JRE_1.8
Then open a comman propt and inside C:\tools\Java\ execute
junction default JRE_1.6
This will create a junction (wich is more or less like a symbolic lync).
In this way you will always have your default java in c:\tools\java\default.
If you then need to change your default java to the 1.8 version you just neet to execute
junction -d default
junction default JRE_1.8

From information you provided your default java is set to java 8 and the program you run is like to be run in some ide that use java 7.
Java -version command gives the default set java in your system.

Apache Maven installation in window 8

Actually I tried to install Apache Maven to my system.
I followed these steps:
Downloaded apache-maven-3.3.3 in my Window 8 machine and copied to Program Files.
Then added maven path C:\Program Files\apache-maven-3.3.3 as M2_HOME in system variable and added C:\Program Files\Java\jdk1.8.0_31 as JAVA_HOME.
Then added ;%M2_HOME%\bin to default Path.
Then tried to run as mvn -version in CMD.
But I got the result like:
mvn is not recognized as an internal or external command...

I remember I had similar issue some time ago on Windows 7. On Windows 8 it may behave similarly. Your solution should work correctly, but for some reason it's not working.
Add path to bin/ directory inside your Maven directory to the Path system variable after semicolon. In your case it will be:
C:\Program Files\apache-maven-3.3.3\bin
Save Path variable, close terminal, run it again and type: mvn -version.

"java -version" not working in command prompt

I downloaded this Java JDK and installed it.
But when I go to the command prompt to check the version, it says it's not recognized.
Is anyone else experiencing this issue with the latest Java?
I might not have installed the right version. I need the java that works with grails
C:\>java
'java' is not recognized as an internal or external command,
operable program or batch file.
C:\>java -version
'java' is not recognized as an internal or external command,
operable program or batch file.
C:\>
when i do a search on my computer for java, it does not find anything

Windows 2k8 R2 server-
For both java [-option] or %JAVA_HOME% to work in the command line you need the following:
In Control Panel->System and Security->System->Advanced system setting->Advanced->Environment Variables->System Variables
Edit the Path variable and add a ";" after the last value and add the the path to the Java bin directory:
e.g.- C:\Progra~2\Java\jre6\bin\
Add the JAVA_HOME Variable with the value set to the path for the java executable:
e.g.- C:\Progra~2\Java\jre6\bin\java.exe

Press simultaneously the "windows" and "pause" buttons on your keyboard, this will bring up the System Preferences dialog.
In the Advanced tab, find Environment Variables.
Then, in the User (upper) section, create or update the following two variables :
JAVA_HOME = where you put your JDK, eg. C:/Java/SDK
PATH = %JAVA_HOME%/bin
Close the dialogs.
Then, in a new command-line console, try "javac -version" and see if it's detected.
It's important that you use a new console, because environment variables are read only when the console is launched.

Java is typically installed (on Windows) as C:\Program Files\Java\jdk<version>
That installation directory has a subdirectory bin which you need to append to your PATH environment variable via the control panel. Then, the commands like java, javac etc. will be available on the command line.
BTW, the same is true for Grails.

Is the -version flag not recognized, or is the "java" command not recognized? One way to test this is just to type 'java' by itself and run it and see what happens.
If the command is not recognized, make sure that the JDK's install path is in your windows PATH. If not, you won't be able to use any of the java executables from the command prompt. Here's another link that may help out.

You need to manually add the path to javac.exe and java.exe to your operating system path. The Java installation program doesn't do that for you.

You most likely don't have java.exe in your system's PATH variable.

For Linux:
check $PATH and $JAVA_HOME. You can configure it in /etc/environment
From console you can check it like:
$ echo $PATH
For Windows:
My Computer -> Properties -> Advanced -> Environment Variables
Check there PATH.
From console you can check it like:
echo %PATH%

You should have a Java icon in Windows Control Panel. Locate the Java tab and click the View button. That will show you the path to the Java executables.
Last but not least, make sure you have restarted the computer so changes in the PATH variable can take effect.

You installed the JDK. Isn't java.exe part of the JRE? Do you have that installed?

Maybe your system variables in the environment variables are not set properly. Follow the steps in the link below. Finally, make sure the path component in system variables has only one JDK path. Delete other JDK paths that you won't use.
https://docs.oracle.com/javase/tutorial/essential/environment/paths.html

You might have installed a previous or older version of Java so you can just uninstall it and directly download JDK from Java JDK
I was also facing the same issue but when I downloaded and installed the latest version the issue was resolved.
Also uninstall any other Java versions which may be present in the program files.
To get to know other versions which may be present in your computer use Windows File Explorer to go to:
Files > Windows C > Program Files > Java
All the JDK and JRE you have will be listed there. Uninstall any other Java versions other than what you installed. And then make sure you have set the path variable of the JDK.

For me, it was incorrect line in the PATH
(1) Check PATH: Type "Edit System Environment Variables" on the search -> System Properties -> Advance -> Environment Variables -> System variables -> Path
(2) On the list, mine was C:\Program Files (x86)\Common Files\Oracle\Java\javapath so I just add \ at the end (the exe files in the javapath folder)
OS: Windows 10 64 bit

IF you set the PATH and it's not showing up in cmd when you run %PATH%, try restarting your computer.