If I have a boolean field like:
private static final boolean DEBUG = false;
and within my code I have statements like:
if(DEBUG) System.err.println("err1");
does the Java preprocessor just get rid of the if statement and the unreachable code?
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2"The Java language has no preprocessor," (java.sun.com/developer/JDCTechTips/2003/tt0408.html) Are you talking about the Java Compiler?S.Lott– S.Lott2009-08-27 23:55:20 +00:00Commented Aug 27, 2009 at 23:55
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Thanks for the article, I didn't know Java doesn't have a preprocessor. So I am just talking about the compiler.twolfe18– twolfe182009-08-28 19:12:38 +00:00Commented Aug 28, 2009 at 19:12
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It's true that Java doesn't have a preprocessor with the same capabilities as that of C/C++. However it does have annotation processors which offer compile-time processing. See Oracle's Annotations TutorialKomodoDave– KomodoDave2013-02-09 12:24:01 +00:00Commented Feb 9, 2013 at 12:24
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The link above is broken ... it's now docs.oracle.com/javase/tutorial/java/annotationsPerry– Perry2016-12-22 00:50:34 +00:00Commented Dec 22, 2016 at 0:50
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2 Answers
Most compilers will eliminate the statement. For example:
public class Test {
private static final boolean DEBUG = false;
public static void main(String... args) {
if (DEBUG) {
System.out.println("Here I am");
}
}
}
After compiling this class, I then print a listing of the produced instructions via the javap command:
javap -c Test
Compiled from "Test.java"
public class Test extends java.lang.Object{
public Test();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."":()V
4: return
public static void main(java.lang.String[]);
Code:
0: return
}
As you can see, no System.out.println! :)
1 Comment
twolfe18
also, I checked when you have a statement that is something like
if(DEBUG && condition_that_may_be_true) ..., and if DEBUG is always false it cuts it out.Yes, the Java compiler will eliminate the compiled code within if blocks that are controlled by constants. This is an acceptable way to conditionally compile "debug" code that you don't want to include in a production build.
3 Comments
Ralph
Can you give the Java Language Specification page that states this?
Greg Hewgill
@Ralph: See 14.21 Unreachable Statements for the discussion in the JLS. The bit about the
if statement is right near the end of that section.Page now here for jse7: docs.oracle.com/javase/specs/jls/se7/html/jls-14.html#jls-14.21 and here for jse5: docs.oracle.com/javase/specs/jls/se5.0/html/…