正确使用Java -D命令行参数

Question

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?

Answer 1

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.)

Answer 2

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" ) ) {
   // ...
}

Answer 3

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.