Conditional Java compilation

I'm a longtime C++ programmer, new to Java. I'm developing a Java Blackberry project in Eclipse. Question - is there a way to introduce different configuration sets within the project and then compile slightly different code based on those?

In Visual Stud开发者_C百科io, we have project configurations and #ifdef; I know there's no #ifdef in Java, but maybe something on file level?

You can set up 'final' fields and ifs to get the compiler to optimize the compiled byte-codes.

...
public static final boolean myFinalVar=false;
...
if (myFinalVar) { 
 do something ....
 ....
}

If 'myFinalVar' is false when the code is compiled the 'do something....' bit will be missed out of the compiled class. If you have more than one condition - this can be tidied up a bit: shift them all to another class (say 'Config.myFinalVar') and then the conditions can all be kept in one neat place.

This mechanism is described in 'Hardcore Java'.

[Actually I think this is the same mechanism as the "poor man's ifdef" posted earlier.]

you can manage different classpath, for example, implement each 'Action' in a set of distinct directories:

dir1/Main.java
dir2/Action.java
dir3/Action.java

then use a different classpath for each version

javac -sourcepath dir1 -cp dir2 dir1/Main.java

or

javac -sourcepath dir1 -cp dir3 dir1/Main.java

In JDK6, you can do it by using Java's ServiceLoader interface. Check it here.

If you want this specifically for BlackBerry, the BlackBerry JDE has a pre-processor:

You can enable preprocessing for your applications by updating the Eclipse™ configuration file.

In C:\Program Files\Eclipse\configuration\config.ini, add the following line: osgi.framework.extensions=net.rim.eide.preprocessing.hook If you enable preprocessing after you have had a build, you must clean the project from the Project menu before you build the project again.

Then you can do things in the code like:

//#ifdef SOMETHING
// do something here
//#else
// do something else
//#endif

For details see Specifying preprocessor defines

Can one call that a poor mans ifdef: http://www.javapractices.com/topic/TopicAction.do?Id=64?

No, Java doesn't have an exact match for that functionality. You could use aspects, or use an IOC container to inject different implementation classes.

You can integrate m4 into your build process to effectively strap an analogue to the C preprocessor in front of the Java compiler. Much hand-waving lies in the "integrate" step, but m4 is the right technology for the text processing job.

Besides Maven, Ant and other build tools that provide similar functionality, one would rather build interfaces in Java and switch the implementations at Runtime.
See the Strategy Pattern for more details

In opposite to C/C++ this will not come with a big performance penality, as Javas JIT-compiler optimizes at runtime and is able to inline this patterns in most cases.
The big pro of this pattern is the flexibility - you can change the underlying Implementation without touching the core classes.

You should also check IoC and the Observer Pattern for more details.

You could use maven's resource filtering in combination mit public static final fields, which will be indeed get compiled conditionally.

private static final int MODE = ${mode};

...

if (MODE == ANDROID) {
    //android specific code here
} else {

}

Now you need to add a property to your maven pom called "mode", which should be of the same value as your ANDROID constant.

The java compiler should (!) remove the if and the else block, thus leaving your android code.

Not testet, so there is no guarantee and i would prefer configuration instead of conditional compilation.

There are a couple of projects that bring support for comment-based conditional compilation to Java:

• java-comment-preprocessor
• JPSG

Example in JPSG:

/* with Android|Iphone platform */
class AndroidFoo {
  void bar() {
    /* if Android platform */
    doSomething();
    /* elif Iphone platform */
    doSomethingElse();
    /* endif */
  }
}

In eclipse you could use multiple projects

• Main (contains common code)
• Version1 (contains version1 code)

• Version2 (contains version2 code)

  1. Main -> Select Project->Properties->Java Build Path->Projects tab
  2. Select Add...
  3. Add "Version1" xor "Version2" and OK back to the workspace.

Version1 and Version two contain the same files but different implementations. In Main you normally write e.g.

import org.mycustom.Version;

And if you included Version1/Version2 project as reference it will compile with the Version.java file from Version1/Version2 project.