What's the difference between system properties System.getProperties() and environment variables System.getenv() in a JVM?
System properties are set on the Java command line using the -Dpropertyname=value syntax. They can also be added at runtime
using System.setProperty(String key, String value) or via the various
System.getProperties().load() methods.
To get a specific system property you can use System.getProperty(String key) or System.getProperty(String key, String def).
Environment variables are set in the OS, e.g. in Linux export HOME=/Users/myusername or on Windows SET WINDIR=C:\Windows etc,
and, unlike properties, may not be set at runtime.
To get a specific environment variable you can use System.getenv(String name).
I think the difference between the two boils down to access. Environment variables are accessible by any process and Java system properties are only accessible by the process they are added to.
Also as Bohemian stated, env variables are set in the OS (however they 'can' be set through Java) and system properties are passed as command line options or set via setProperty().
Related
iam using the below link to understand environment variables and system properties.
https://docs.oracle.com/javase/tutorial/essential/environment/env.html
The link says environment variables are set by OS and passed to applications.
When i fetch environment variables using System.getenv() it shows me lot of properties which i never set.
So it must be OS (im using macOS) which had set these properties.
Some of the properties in System.getenv() are MAVEN_CMD_LINE_ARGS, JAVA_MAIN_CLASS_1420, JAVA_MAIN_CLASS_1430.
My question is why would OS would like to set the java specific properties in environment variables? Ideally these should be set by JVM (in System.properties()).
P.S.: From whatever i have read on net i understand that environment variables are set by OS and System.properties() are set by JVM
Also if someone can point me to a good link on environment variable and System.properties it will be very helpful. Iam very confused between the two.
Environment variables is an OS concept, and are passed by the program that starts your Java program.
That is usually the OS, e.g. double-click in an explorer window or running command in a command prompt, so you get the OS-managed list of environment variables.
If another program starts your Java program1, e.g. an IDE (Eclipse, IntelliJ, NetBeans, ...) or a build tool (Maven, Groovy, ...), it can modify the list of environment variables, usually by adding more. E.g. the environment variable named MAVEN_CMD_LINE_ARGS would tend to indicate that you might be running your program with Maven.
In a running Java program, the list of environment variables cannot be modified.
System properties is a Java concept. The JVM will automatically assign a lot of
system properties on startup.
You can add/override the values on startup by using the -D command-line argument.
In a running Java program, the list of system properties can be modified by the program itself, though that is generally a bad idea.
1) For reference, if a Java program wants to start another Java program, it will generally use a ProcessBuilder to set that up. The environment variables of the new Java process will by default be the same as the current Java program, but can be modified for the new Java program by calling the environment() method of the builder.
I've set the environment variable under Application servers > server1 > Process definition > Environment Entries
My Property name is region and value in dev.
I'm reading this property value like below:
String environment= System.getProperty("region");
But I'm getting null, what I'm doing wrong here? I tried to set the variable under this path as well: This also returning null - how can I read the environment variable in Java?
Environment > WebSphere Variables
I'm using WebSphere version 8.0.0.3
Environment entries on the server's process definition are set as OS-level environment variables, not Java system properties. It's the equivalent of calling "set region=dev" on the command line before starting your JVM. Two potential approaches:
1) Use System.getenv() instead of System.getProperty() - getenv() is the method used to retrieve environment variables.
2) Instead of defining the property as an environment variable on the process, define it as a system property in the Process Definition -> Java Virtual Machine panel. You can either create a new System Property or add it to your generic JVM arguments as a -D option ("-Dregion=dev").
I'm running in an environment where I can pass parameters but not set environment variables using a normal commandline.
I would like to set environment variables nevertheless. Is there an alternative way to "fool" java that an environment variable is set, e.g. using -D parameters?
(I'm running spark in oozie through hue; all in the Cloudera stack).
The java command itself doesn't seem to allow that. It has a -D parameter, but that sets Java 'system properties':
$ java -help 2>&1 | grep -A1 '\-D'
-D<name>=<value>
set a system property
Java system properties are a sort of Java properties. Like environment variables, java properties are key-value pairs, but aren't the same thing as environment variables: If your Java application reacts to a specific environment variable, setting a system property of the same name won't have any effect unless the application explicitly reacts to that property, too.
If your environment allows you to run arbitrary Java applications and if it allows your Java applications to execute other processes, you can write a little wrapper that sets the environment variables on a ProcessBuilder (see the question Arnon linked in his comment: How do I set environment variables from Java?) to then invoke java with your actual JAR from it. You could either hard-code the environment variables to set, or set them according to system properties the wrapper receives. (Or you could even implement your own shell in Java and pass a script to it.)
Though, if you can modify the source of that actual JAR, a much more idiomatic solution would be to make it itself react to properties instead of (or additionally to) environment variables. (Unless you have to control environment variables that the java command / the JVM reacts to rather than the JAR you'd like to run. Then this approach would not be applicable, of course.)
If you can run arbitrary Java code, you can create and run a Process using ProcessBuilder, including an environment of your choice.
Hence, write a java program that parses a command-line like
java -cp .... your.Prog FOO=BAR BAZ=BOOM command arguments ...
and starts command with the environment extended by FOO and BAZ
(Note that command could be java ...)
I'm working on Java.
is there any option in the "java" to set the user defined environmental variable to the executable that we will execute using "java" command.
e.g. we have one option -XX:+AllowUserSignalHandlers.
You can set tuning parameter to JVM at the start of the program itself.
ex -
java -XMS50m -XX:+AllowUserSignalHandlers Test.class
Once jvm is started you can not set environment variables but you can view them System.genEnv it retuns an unmodifiable string map view of the current system environment.
If you are creating a new process then you can set environment variables using ProcessBuilder.environment.
There is no environment variable that java.exe parses and used to set command line arguments.
You have to write a program/script that would process your custom environment variable and then pass to actual java program.
If I had a java application that needed specific environment variables to be set, but I cannot place them inside the bash.rc or bash_profile, nor can I place them in /etc/profile.d is it possible to have them set within the Java process? Or do these need to be set before the java process is run? Also manually setting them each time is not an option because I want them to be transparent to the user.
Start the java process from a shell script. The script would first define and export the environment variables.
I suspect this is not possible. System.getenv() is an unmodifiable map, meaning you can't modify elements in it.
Environment Variables
And for setting environment variables, ProcessBuilder
Have you try ?
System.setProperty("KEY", "VALUE");