Proper Usage of Java -D Command-Line Parameters
When Passing a -D Parameter in Java, What Is the Proper Way of Writing the Command-Line and Then Accessing It from Code? for Example, I Have Tried Writing...
When passing a -D parameter in Java, what is the proper way of writing the command-line and then accessing it from code?
For example, I have tried writing something like this...
if (System.getProperty("test").equalsIgnoreCase("true"))
{
//Do something
}
And then calling it like this...
java -jar myApplication.jar -Dtest="true"
But I receive a NullPointerException. What am I doing wrong?
3 Answers
I suspect the problem is that you've put the "-D" after the -jar. Try this:
java -Dtest="true" -jar myApplication.jar
From the command line help:
java [-options] -jar jarfile [args...]
In other words, the way you've got it at the moment will treat -Dtest="true" as one of the arguments to pass to main instead of as a JVM argument.
(You should probably also drop the quotes, but it may well work anyway - it probably depends on your shell.)
That should be:
java -Dtest="true" -jar myApplication.jar
Then the following will return the value:
System.getProperty("test");
The value could be null, though, so guard against an exception using a Boolean:
boolean b = Boolean.parseBoolean( System.getProperty( "test" ) );
Note that the getBoolean method delegates the system property value, simplifying the code to:
if( Boolean.getBoolean( "test" ) ) {
// ...
}
You're giving parameters to your program instead to Java. Use
java -Dtest="true" -jar myApplication.jar
instead.
Consider using
"true".equalsIgnoreCase(System.getProperty("test"))
to avoid the NPE. But do not use "Yoda conditions" always without thinking, sometimes throwing the NPE is the right behavior and sometimes something like
System.getProperty("test") == null || System.getProperty("test").equalsIgnoreCase("true")
is right (providing default true). A shorter possibility is
!"false".equalsIgnoreCase(System.getProperty("test"))
but not using double negation doesn't make it less hard to misunderstand.
David Miller brings 15 years of experience in global economics, personal finance strategy, and market dynamics. He specializes in turning complex economic trends into actionable insights for everyday readers.
