JDK, JRE an JARs compatibility

Question

I know a bit about JDK and JRE source and binary compatibility (e.g. this and this), but not sure about the following situation:

Consider I have an application which is compiled using JDK5 and runs on JRE6. It uses some libraries (jars) which are also compiled using JDK5.

Now I want to compile my application using JDK6. What new problems could arise in runtime in such a case (particularly, in compatibility with the "old" jars)? Should I fully retest the application (touch every library) or can rely on promised JDK/JRE compatibility?

Answer 1

Normally no problems should arise if you set the compiler option of JDK6 to use 1.5 source compatibility. However sometimes this is not always true.

I remember once when compiling 1.4 code with 1.5 compiler (using 1.4 compatibility). The jars where ok (1.4 binary level) but the application crashed due to a funny conversion.

We used a BigDecimal number passing an integer as argument to the constructor. The 1.4 version had only a constructor from double but the 1.5 version had both, the int and the double constructors. So when compiling with 1.4 compiler made the automatic conversion from int to double, but with the 1.5 compiler it checked that the int constructor existed and did not realize that conversion. Then when using the perfect binary compatible code on 1.4 JRE the program crashed with a NoSuchMethodException.

I have to admit that it was a strange case, but it is one of those cases where logic does not work. So my advice is if you plan to compile for older versions of JRE try to use the target version JDK whenever possible.