Why do JVM arguments start with "-D"?

Solution 1:

Why couldn't the architects of Java let us simply do:

java -jar -myProp="Hello World" myProgram.jar

It could work today but suppose that in next Java versions a -myProp argument is introduced as a JVM option.
How to distinguish your -myProp from the -myProp JVM option ? No way.
So it exists an obvious reason to use -D to define system properties.

As other example, instead of -myProp suppose you program relies on a -client system property.
It will not run :

java -jar -client="davidxxx" myProgram.jar

You would have a JVM error such as :

Unrecognized option: -client=davidxxx

as -client is a JVM standard option that expects no value.

But if you use -D-client, it is now fine as here -Dclient is defined as a system property that is distinct from the -client standard JVM option :

java -jar -D-client="davidxxx" myProgram.jar

Or by using both :

java -jar -client -D-client="davidxxx" myProgram.jar

To go further, not all JVM arguments start with -D. but most of them have a prefix (-D, -X, -XX) that allows in a someway to define namespaces.

You have distinct categories of JVM arguments :

1. Standard Options (-D but not only).

These are the most commonly used options that are supported by all implementations of the JVM.

You use -D to specify System properties but most of them don't have any prefix :-verbose, -showversion, and so for...

2. Non-Standard Options (prefixed with -X)

These options are general purpose options that are specific to the Java HotSpot Virtual Machine.
For example : -Xmssize, -Xmxsize

3. Advanced Runtime Options (prefixed with -XX)

These options control the runtime behavior of the Java HotSpot VM.

4. Advanced JIT Compiler Options (prefixed with -XX)

These options control the dynamic just-in-time (JIT) compilation performed by the Java HotSpot VM.

5. Advanced Serviceability Options (prefixed with -XX)

These options provide the ability to gather system information and perform extensive debugging.

6. Advanced Garbage Collection Options (prefixed with -XX)

These options control how garbage collection (GC) is performed by the Java HotSpot VM.

Solution 2:

"Define". The meaning is similar to a preprocessor definition in C. The -D signifies that the definition is in the context of the application, and not in the Java interpreter context like any other option before the executable name.

The usage of the letter "D" isn't specifically explained in the documentation, but the only use is to "define" a key in the system properties map - except for this reference:

The System class maintains a Properties object that defines the configuration of the current working environment. For more about these properties, see System Properties. The remainder of this section explains how to use properties to manage application configuration.

Solution 3:

If you do not specify anything like -myProp="XYZ" it means it is passed as an argument to main method of the program.

-D means you can use this value using System.getProperty

-X is used for extension arguments like -Xdebug -Xnoagent -Djava.compiler=NONE -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=8000

Yes, they could have interchanged.. the characters; but these characters are used to specify what type of parameter is passed and who is the consumer.