Why do JVM arguments start with “-D”?

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.

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