Backport Java 5/6 features to Java 1.4?

Question

We are stuck with Java2SE v1.4 till the end of 2010. That's really nasty, but we can't help it. What options do we have to use some of the new features already now? I can think of several ways like

• changing the bytecode, e.g. using Retrotranslator or Retroweaver.

• backport of libraries, e.g. Concurrent Backport, but this does not help for generics.

• emulation of Java 5 features, e.g. checked Collections, Varargs with helper methods, etc.

• changing source code by precompilation, stripping all 1.5 stuff before final compilation, e.g. using Declawer can do this.

I am most interested in very positive experience with it in production environments using Weblogic and "real" stuff.

Answer 1

Thanks for your answers. Here is the summary of all relevant answers and my own research.

Changing the bytecode: The Retros
This is done by the "retro"-tools: Retrotranslator, Retroweaver and JBossRetro. Retrotranslator seems to be the most mature and active of them tool. These tools scan all classes and change the bytecode to remove Java 5 and 6 features. Many Java5 features are supported, some by using 3rd party backport libraries. This option is most popular and there is some positive feedback from users. Experiments showed that it's working as expected. See a short overview on developerworks.

Pro: You can develop entirely in Java 5, build modules and all kind of JARs. In the end you just transform all classes to Java 1.4 and package your EAR. This is easily done with Retrotranslator's Maven integration (org.codehaus.mojo:retrotranslator-maven-plugin).

Con: Conservative environments do not allow changed bytecode to be deployed. The result of the retro-step is not visible to any coder and can't be approved. The second problem is fear: There might be some cryptic production problem and the retro-code is another step that might be blamed for that. App-server vendors might refuse help due to changed bytecode. So nobody wants to take responsibility to use it in production. As this is rather a policital than a technical problem, so I see no solution. It has happened to us, so I was looking for further options :-(

Compiling Java5 to Java 1.4: jsr14
There is an unsupported option, javac -source 1.5 and -target jsr14 which compiles the Java5 source to valid Java 1.4 bytecode. Most features like varargs or extended for loop are translated by the compiler anyway. Generics and annotations are stripped. Enums are not supported and I don't know about autoboxing, as the valueOf methods were mostly introduced in Java5.

Con: Only byte code is translated, library usage is not changed. So you have to be careful not to use Java5 specific APIs (but could use Backports). Further you have to build all modules at the same time, because for development time you propably want Java5 code with generic and annotation information. So you have to build the entire project from scratch for Java 1.4 production.

Changing Source back to Java 1.4: Declawer
As answered in a related question, there is Declawer, a compiler extension, that works for generics and varargs, but not for enhanced for loop or autoboxing. The generated source "is a little funky, but not too bad".

Pro: The generated source is available and can be reviewed. In worst case fixes can be made in this source. There is no "magic", because the source is valid Java. Some people even use JAD (Java decompiler) to get the Java 1.4 source again. The output of Jad readable is readable if you compile with debug information and don't use inner classes.

Con: Similar to -target jsr14, you need an extra step in the deployment. Same problems with libraries, too.

Changing Source back to Java 1.4: by hand
Several answers suggested doing it by hand. For an automatic, repeating build process this is of course not useful, but for one-time changes it's reasonable. Just automate what is possible. Maybe look at Antlr for creating a home-grown conversion tool.

Backported Libraries:
The problem is, that Java5 also ships new libraries, that are not available in older JREs, see related question. Fortunately there are several backported libraries that give you some functionality of Java5, but can't simulate language features, like generics.

• Annotations, discussed at TSS
• Concurrent
• com.sun.net.httpserver (Java 6 to 5)
• Gif writing (Java 6 to 5)
• Start your own backport project ;-)
• You might copy classes you need from the JDK or other libraries, but most likely they are related to other classes.

Emulating Java5 features in Java 1.4 code:
I was thinking about some things you might do to make your life easier and still staying with Java 1.4. The most important features are typesafe collections, here are some ideas:

• Instead of using generics you can create your own typesafe containers with some template.
• Add a typesafe Iterator (which is no Iterator any more).
• Add asList methods that allows 1,2,...,n arguments and an array of them (to simulate varargs).
• Methods for varargs (converting 1,...,n arguments to arrays) and valueOf can be put in some helper class.

Answer 2

sourcecode precompilation, stripping all 1.5 stuff before final compilation and deployment. Are there any tools which can do this?

Yes. They're called Retrotranslator or Retroweaver. Apart from Generics (which only exist for the compiler's sake anyway), you cannot simply "strip 1.5 stuff". Enums (and maybe also some other features) have to be replaced with functionally equivalent code. Which is exactly what those tools do.