Month: May 2006
Some notes
- One can argue that a step from one VM into another should not, from debugger’s point of view, be a step within one thread, but a new thread should be open. But this involves too much interfacing with the debugger (thinking about how to tell it that it needs to register a
ThreadStartEventto open a new thread when it didn’t request it, e.g.), so I scrap the idea, but feel it worthy of noting. - %#@#$%@#$%@^@@^%
java.lang.ClassCastException: org.hrum.dbdb.DelegatingThreadReference cannot be cast to com.sun.tools.jdi.ThreadReferenceImpl
Did I mention how this pisses me off? - Ok, even though Dbdb needs Java 6, the proof-of-concept is not supporting
new JDBC driver
loading… Just a note to self…
ReflectiveVisitor
Inigo Surguy presents< an implementation of Visitor pattern based on reflection. I found useful a slight modification of it, that allows for visiting not only the narrowest type, but every type possible , based on the visitAll flag: ReflectiveVisitor.java. This functionality comes in quite handy for tasks such as, for instance, validation. For example, if one has a complex class hierarchy (especially with multiple< inheritance) and different types have different requirements for it to be valid, and a concrete instance may implement a number of those, these validations can be well separated out.
See also:
What’s going on here?
Consider the following code:
When running, this code prints the following:
MethodEntryEvent evt;
ObjectReference con;
...Class evtClass = evt.getClass();
try {
System.out.println("Class of evt: " + evtClass);
System.out.println("Methods of evt: " +
Arrays.asList(evtClass.getMethods()));
try {
Value v = evt.returnValue();
System.out.println(v);
} catch (Throwable ex) {
ex.printStackTrace();
}
java.lang.reflect.Method retvalMethod =
evtClass.getMethod("returnValue", null);
retvalMethod.setAccessible(true);
con = (ObjectReference)retvalMethod.invoke(evt, (Object[])null);
} catch (Throwable t) {
t.printStackTrace();
}
System.out.println("Returned: " + con);
Class of evt: class com.sun.tools.jdi.EventSetImpl$MethodExitEventImpl Methods of evt: [ public com.sun.jdi.Value com.sun.tools.jdi.EventSetImpl$MethodExitEventImpl.returnValue(), public java.lang.String com.sun.tools.jdi.EventSetImpl$LocatableEventImpl.toString(), public com.sun.jdi.Method com.sun.tools.jdi.EventSetImpl$LocatableEventImpl.method(), public com.sun.jdi.Location com.sun.tools.jdi.EventSetImpl$LocatableEventImpl.location(), public com.sun.jdi.ThreadReference com.sun.tools.jdi.EventSetImpl$ThreadedEventImpl.thread(), public int com.sun.tools.jdi.EventSetImpl$EventImpl.hashCode(), public boolean com.sun.tools.jdi.EventSetImpl$EventImpl.equals(java.lang.Object), public com.sun.jdi.request.EventRequest com.sun.tools.jdi.EventSetImpl$EventImpl.request(), public com.sun.jdi.VirtualMachine com.sun.tools.jdi.MirrorImpl.virtualMachine(), public final native java.lang.Class java.lang.Object.getClass(), public final void java.lang.Object.wait(long,int) throws java.lang.InterruptedException, public final void java.lang.Object.wait() throws java.lang.InterruptedException, public final native void java.lang.Object.wait(long) throws java.lang.InterruptedException, public final native void java.lang.Object.notify(), public final native void java.lang.Object.notifyAll()] java.lang.NoSuchMethodError: com.sun.jdi.event.MethodExitEvent.returnValue()Lcom/sun/jdi/Value; at org.hrum.dbdb.DriverManagerMethodExitEventListener.process(DriverManagerMethodExitEventListener.java:99) at org.hrum.dbdb.DbdbEventQueue.removeDebug(DbdbEventQueue.java:168) at org.hrum.dbdb.DbdbEventQueue.remove(DbdbEventQueue.java:47) at org.eclipse.jdt.internal.debug.core.EventDispatcher.run(EventDispatcher.java:226) at java.lang.Thread.run(Thread.java:619) Returned: instance of oracle.jdbc.driver.T4CConnection(id=435)(class com.sun.tools.jdi.ObjectReferenceImpl)
Now, I will run this in debug mode and set a breakpoint at the red line above. When the breakpoint is hit, evaluation of
evt.returnValue() returns an instance of com.sun.tools.jdi.ObjectReferenceImpl. However, when the execution is resumed, the result is as above (that is, evt.returnValue() results in a NoSuchMethodError).
Further, if we remove the green line (retvalMethod.setAccessible(true);), we will get an IllegalAccessException on the invocation:
What is going on? I’d say it’s left as an exercise for the reader, but honestly, at the moment, I don’t feel like looking for an answer at all. I will perhaps let Bob and Dr. Heinz Max Kabutz (did I mention how much I enjoy referring to Dr.Heinz Max Kabutz?) to do the detective work…Class org.hrum.dbdb.DriverManagerMethodExitEventListener can not access a member of class com.sun.tools.jdi.EventSetImpl$MethodExitEventImpl with modifiers “public”
ENVIRONMENT: This code is part of a plug-in project I am running in Eclipse 3.2RC3, with Mustang.
BOOK REVIEW: “Eclipse: Building Commercial-Quality Plug-ins”
I suppose this is more of a praise of Eclipse plug-in architecture and available documentation than a review of the book per se, but I did not get from Eclipse: Building Commercial-Quality Plug-ins anything I could not by scanning online docs and playing with Eclipse myself. I was up and running with my plug-in project in a very short time without opening this book, and once I did, I did not find anything I have not already learned or known where to turn for more info…
It may be easy to say that many such books are just a rehash of the wealth of online information already freely available, but sometimes the books do have added value, say, by presenting the material for faster learning and/or reference. In this case, there can be no such added advantage – again, because the Eclipse project’s own design and documentation is very clear and thorough… I realized all that before getting the book; in buying it, I was looking for another advantage – hidden tips and tricks, kind of like Covert Java. For example, how do I debug a plug-in project that depends on a non-plugin one? No such luck. I’ll be returning this book to the store now, and maybe trying to see if Contributing to Eclipse: Principles, Patterns, and Plugins is closer to what I want…Ten years since…
After 10 years, I finally finished an old incomplete (from Spring ’96!) in 6.863J.
