Excellent stuff, Dain!
On Dec 29, 2006, at 6:08 PM, Dain Sundstrom wrote:
I just committed the code to enable runtime enhancement of JPA
entities. The code makes use of the java instrumentation APIs[1]
to hook all class loading. The gives us direct access to hook all
class loading in the vm and to modify the java byte code for the
class before it is loaded into memory. I have a few notes on the
the instrumentation APIs so hopefully others will be able to
support this code :)
* To use the instrumentation API, our java agent *must* be
specified on the command line. I have added this to the standalone
startup shell scripts. Without the agent we cannot enhance classes
and pre-enhancement must be used [2]. Embedded servers (Tomcat and
JUnit) must load the java agent or pre-enhance
java -javaagent:$OPENEJB_JAVAAGENT_JAR -jar $OPENEJB_CORE_JAR start
Do you think you'll be able to find a naughty way to install an agent
after boot?
* It is critical that application classes not be loaded before the
Assembler. Code that runs before the Assembler, such as the
Verifier, should use the TemporaryClassLoader. The
TemporaryClassLoader loads all locally (even those found in parent
class loaders), which means it doesn't pollute the parent class
loader. (I forked this code from OpenJPA. thanks guys)
That's cool. I suppose this supercedes the patent-pending Alan
Cabrera TempCodebase?
* A javaagent works much like an executable jar in that you specify
a Premain-Class and Boot-Class-Path in the javaagent jar manifest.
One big problem is that the premain class can not be debugged (at
least in IntelliJ). It appears that the premain is executed before
the debugger starts.
Guess that puts a damper on using it to do other naughty things like
use it as a container system boot hook. Though still possible.
* Once you have a javaagent it is very easy to use ASM to modify a
class. There is an example ClassFileTransformer in the JpaTest
that adds a new field to a class at runtime[3].
That is all I can think of now :)
So are we using this in our test suite now or are we still pre-
enhancing?
-David
