what is the general java API compatibility rule

Question

in detail: if we use public API for example, write java program for example , in JDK 1.4, if should run correctly in all version above it. in all update version in 1.4, in 1.5, 1.6 and 1.7?

Also , what is the combability rule between different updater versions , for example 1.6.22 and 1.6.23 what can not be changed, what can be changed? of course, public API definition can not be changed, how about others? javadoc? internal API definition, implementation?

It will be great if someone can point a concrete official document on this topic. thanks,

there is one example in java document bug, that they intended not to change between updater version. see http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6475885

this should be one of its big picture, but we better to have a complete description on this.

need to know the complete story so that we feel safe to upgrade to bigger version.

Answer 1

The general rule is that any code that is written and compiled against the APIs of Java X should run on Java Y where Y >= X.

There are occasional exceptions to this; e.g. where the application's behaviour depends on some undocumented behaviour (typically a bug) in Java X that was corrected in a later version.

AFAIK, there is no single document that lists these incompatibilities. The release notes for all of the Java major releases include a list of changes that could result in breakage of older code.

Having said that, the prudent approach is make sure that you thoroughly test / retest your software when you upgrade to a more recent Java release. And if your software is shipped to customers / clients, let them know if / when it is safe for them to upgrade, and (if necessary) provide them with fixes for any problems that your testing has uncovered.

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need to know the complete story so that we feel safe to upgrade to bigger version.

Feeling safe is beside the point. Thoroughly test your application on the later version. That is the only practical solution. And that would be the case even if each and every incompatibility was exhaustively documented.

Think about it. How can you know for sure that your application won't somehow be affected by change XYZ? Or that some 3rd-party library that you use won't be affected? Answer: you can't.

No manner of complaining here that you think that Oracle should handle this issue differently is going to make any difference. Not that I think that they could handle this better without changing their business model. How much would you be prepared to pay for a Java platform that guaranteed there were no version compatibility issues?

Answer 2

This is not a full answer but I will add that will-it-run and will-it-compile are two different things. Keywords introduced in 1.5 will prevent some 1.4 code from compiling but the byte code will run just fine.