Will Java code built and compiled against a 32-bit JDK into 32-bit byte code work in a 64-bit JVM? Or does a 64-bit JVM require 64-bit byte code?
To give a little more detail, I have code that was working in a Solaris environment running a 32-bit JVM, but now I'm getting issues after upgrading the JDK and Weblogic Server to 64-bit.
Yes to the first question and no to the second question; it's a virtual machine. Your problems are probably related to unspecified changes in library implementation between versions. Although it could be, say, a race condition.
There are some hoops the VM has to go through. Notably references are treated in class files as if they took the same space as ints on the stack. double and long take up two reference slots. For instance fields, there's some rearrangement the VM usually goes through anyway. This is all done (relatively) transparently.
Also some 64-bit JVMs use "compressed oops". Because data is aligned to around every 8 or 16 bytes, three or four bits of the address are useless (although a "mark" bit may be stolen for some algorithms). This allows 32-bit address data (therefore using half as much bandwidth, and therefore faster) to use heap sizes of 35- or 36-bits on a 64-bit platform.