java.lang.instrument

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Francesco Devittori - 08 Dec 2005 09:28 GMT

Hi everybody,
this is my first post in this newsgroup, I hope the question is appropriate.

I am working with java.lang.instrument and I'd like to discuss some
aspects, is there anybody that works with this api?

Francesco

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Chris Smith - 08 Dec 2005 15:27 GMT

> this is my first post in this newsgroup, I hope the question is appropriate.
>
> I am working with java.lang.instrument and I'd like to discuss some
> aspects, is there anybody that works with this api?

This is a fine place to ask. What aspects of the package do you want to
discuss?

Don't worry about finding experienced people in advance. If your
question sparks someone's interest, they may even go learn the package
to answer your questions. I don't know much about it, for example...
but I've always thought the concept was intriguing.

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Francesco Devittori - 08 Dec 2005 17:17 GMT

>>this is my first post in this newsgroup, I hope the question is appropriate.
>>
[quoted text clipped - 8 lines]
> to answer your questions. I don't know much about it, for example...
> but I've always thought the concept was intriguing.

Ok, thanks.
Sorry for the long message, but maybe someone is interested.
I'm working on a project that uses this api and here are the first
things I noticed:

1) exceptions thrown in the agent (or in objects created from the agent)
simply make the agent die silently. No output, nothing.
This makes debugging a nightmare. I can't understand why things are that
way (maybe this happens because of something I do?).

2) exactly which classes go through the instrumentation?
I.e. classes called by the agent are not rewritten (I guess).
Then what happens if a "normal" class calls a class that was already
loaded by the agent? It gets instrumented or not?

3) a very strange problem: if you run your class with a classpath that
includes the .class of your agent, then the application gets stuck at
some random point (you have to Ctrl-Z/pkill -9 java).

For example: java -cp someDir -javaagent:myAgent.jar HelloWorld
if myAgent.class is also inside someDir/ then you have this problem.

In my case the application gets stuck inside a Bcel method
(JavaClass.parse()).

This same problem occurs if the agent tries to load a class that is not
inside a jar file.
I.e. in the agent you say "MyClass obj = new MyClass();" then you cannot
have MyClass.class in your classpath, but you have to put it inside a
jar (and put the jar in the classpath of course).

Maybe these things are due to something I do wrong, I'm curious if
someone has some experience (I cannot find a lot of documentation online)

Francesco

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Thomas Hawtin - 08 Dec 2005 19:44 GMT

> 1) exceptions thrown in the agent (or in objects created from the agent)
> simply make the agent die silently. No output, nothing.
> This makes debugging a nightmare. I can't understand why things are that
> way (maybe this happens because of something I do?).

Looking at the source
j2se/src/share/classes/sun/instrument/TransformationManager.java, I spy:

catch (Throwable t) {
// don't let any one transformer mess it up for the others.
// This is where we need to put some logging. What should go
here? FIXME
}

You can grab the source from http://tiger.dev.java.net/ (although not
the latest version, yet). You can then just edit that file to dump the
exception and run with -Xbootclass/p:. Perhaps even supply a fix to
Mustang. Alternatively, compile with debugging information and run it
through your debugger (if it supports that sort of thing).

> 3) a very strange problem: if you run your class with a classpath that
> includes the .class of your agent, then the application gets stuck at
> some random point (you have to Ctrl-Z/pkill -9 java).

Can you find the stack with Ctrl-\ (or ctrl-break(?) for Windows)?

It may be worth replacing/prefixing the package name of third party
libraries. For instance, BCEL is include with the JRE, but has been renamed.

Tom Hawtin

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Francesco Devittori - 09 Dec 2005 07:27 GMT

>> 1) exceptions thrown in the agent (or in objects created from the
>> agent) simply make the agent die silently. No output, nothing.
[quoted text clipped - 15 lines]
> Mustang. Alternatively, compile with debugging information and run it
> through your debugger (if it supports that sort of thing).

Thanks, the idea is fine. I put some t.printStackTrace() there and load
it with -Xbootclass/p:
How can I provide a fix to Mustang? Is there a standard way of doing
this? (bugs.sun.com?)

>> 3) a very strange problem: if you run your class with a classpath that
>> includes the .class of your agent, then the application gets stuck at
[quoted text clipped - 5 lines]
> libraries. For instance, BCEL is include with the JRE, but has been
> renamed.

Now I try to reproduce the problem and get the stack.

> Tom Hawtin

Thanks for these interesting suggestions!

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Thomas Fritsch - 09 Dec 2005 09:12 GMT

> How can I provide a fix to Mustang? Is there a standard way of doing
> this? (bugs.sun.com?)

Exactly. On that page click "Report a bug or request a feature". In the
report page you select "Request for enhancement (RFE)".

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Thomas Hawtin - 10 Dec 2005 01:00 GMT

>> How can I provide a fix to Mustang? Is there a standard way of doing
>> this? (bugs.sun.com?)
>
> Exactly. On that page click "Report a bug or request a feature". In the
> report page you select "Request for enhancement (RFE)".

The submitting fixes part, rather than submitting a bug or RFE report,
is in the paragraph below, labeled "Submitting bug fixes".

Tom Hawtin

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Francesco Devittori - 09 Dec 2005 07:48 GMT

> Francesco Devittori wrote:
>> 3) a very strange problem: if you run your class with a classpath that
>> includes the .class of your agent, then the application gets stuck at
>> some random point (you have to Ctrl-Z/pkill -9 java).
>
> Can you find the stack with Ctrl-\ (or ctrl-break(?) for Windows)?

I have the stack. My little test includes these files:

MyAgent.class
MyAgent.jar
MyAgent.java
bcel-5.1.jar
hello/HelloWorld.java
hello/HelloWorld.class

When I run

java -cp hello:bcel-5.1.jar -javaagent:MyAgent.jar HelloWorld

no problems. But if I run

java -cp hello:bcel-5.1.jar:. -javaagent:MyAgent.jar HelloWorld

then it deadlocks. (The difference is that there is "." in the cp, which
includes MyAgent.class - It happens the same if I put MyAgent.class in
another dir and append that dir to the cp)

So here the stack trace:
---------------------------------------------------------------

java -cp hello:bcel-5.1.jar:. -javaagent:MyAgent.jar HelloWorld
parsing HelloWorld
parsing java/lang/Enum
Full thread dump Java HotSpot(TM) Client VM (1.5.0_05-b05 mixed mode):

"Low Memory Detector" daemon prio=1 tid=0x080eedb8 nid=0xb77 runnable
[0x00000000..0x00000000]

"CompilerThread0" daemon prio=1 tid=0x080eb128 nid=0xb76 waiting on
condition [0x00000000..0xaa1ab438]

"Signal Dispatcher" daemon prio=1 tid=0x080ea0f0 nid=0xb75 runnable
[0x00000000..0x00000000]

"Finalizer" daemon prio=1 tid=0x080be428 nid=0xb74 waiting for monitor
entry [0xaa43d000..0xaa43eb50]
at
org.apache.bcel.classfile.ConstantPool.<init>(ConstantPool.java:97)
at
org.apache.bcel.classfile.ClassParser.readConstantPool(ClassParser.java:254)
at
org.apache.bcel.classfile.ClassParser.parse(ClassParser.java:162)
at MyAgent.transform(MyAgent.java:28)
at
sun.instrument.TransformerManager.transform(TransformerManager.java:122)
at
sun.instrument.InstrumentationImpl.transform(InstrumentationImpl.java:155)
at java.lang.ref.Finalizer.runFinalizer(Finalizer.java:82)
at java.lang.ref.Finalizer.access$100(Finalizer.java:14)
at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:160)

"Reference Handler" daemon prio=1 tid=0x080bcca0 nid=0xb73 in
Object.wait() [0xaa4be000..0xaa4bebd0]
at java.lang.Object.wait(Native Method)
- waiting on <0xaa6502a8> (a java.lang.ref.Reference$Lock)
at java.lang.Object.wait(Object.java:474)
at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:116)
- locked <0xaa6502a8> (a java.lang.ref.Reference$Lock)

"main" prio=1 tid=0x0805cb58 nid=0xb70 waiting for monitor entry
[0xbfd14000..0xbfd175c8]
at java.lang.ClassLoader.findBootstrapClass(Native Method)
at java.lang.ClassLoader.findBootstrapClass0(ClassLoader.java:891)
at java.lang.ClassLoader.loadClass(ClassLoader.java:301)
- locked <0xaa653038> (a sun.misc.Launcher$ExtClassLoader)
at java.lang.ClassLoader.loadClass(ClassLoader.java:299)
- locked <0xaa6502b0> (a sun.misc.Launcher$AppClassLoader)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:268)
- locked <0xaa6502b0> (a sun.misc.Launcher$AppClassLoader)
at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)
- locked <0xaa6502b0> (a sun.misc.Launcher$AppClassLoader)
at
org.apache.bcel.classfile.ConstantPool.<init>(ConstantPool.java:103)
at
org.apache.bcel.classfile.ClassParser.readConstantPool(ClassParser.java:254)
at
org.apache.bcel.classfile.ClassParser.parse(ClassParser.java:162)
at MyAgent.transform(MyAgent.java:28)
at
sun.instrument.TransformerManager.transform(TransformerManager.java:122)
at
sun.instrument.InstrumentationImpl.transform(InstrumentationImpl.java:155)
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:620)
at
java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:260)
at java.net.URLClassLoader.access$100(URLClassLoader.java:56)
at java.net.URLClassLoader$1.run(URLClassLoader.java:195)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
- locked <0xaa6502b0> (a sun.misc.Launcher$AppClassLoader)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:268)
- locked <0xaa6502b0> (a sun.misc.Launcher$AppClassLoader)
at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)
- locked <0xaa6502b0> (a sun.misc.Launcher$AppClassLoader)

"VM Thread" prio=1 tid=0x080b6b48 nid=0xb72 runnable

"VM Periodic Task Thread" prio=1 tid=0x080f0e08 nid=0xb78 waiting on
condition

Found one Java-level deadlock:
=============================
"Finalizer":
waiting to lock monitor 0x080bfddc (object 0xaa6502b0, a
sun.misc.Launcher$AppClassLoader),
which is held by "main"
"main":
waiting to lock monitor 0x080bfe1c (object 0xae5c4860, a [[I),
which is held by "Finalizer"

Java stack information for the threads listed above:
===================================================
"Finalizer":
at
org.apache.bcel.classfile.ConstantPool.<init>(ConstantPool.java:97)
at
org.apache.bcel.classfile.ClassParser.readConstantPool(ClassParser.java:254)
at
org.apache.bcel.classfile.ClassParser.parse(ClassParser.java:162)
at MyAgent.transform(MyAgent.java:28)
at
sun.instrument.TransformerManager.transform(TransformerManager.java:122)
at
sun.instrument.InstrumentationImpl.transform(InstrumentationImpl.java:155)
at java.lang.ref.Finalizer.runFinalizer(Finalizer.java:82)
at java.lang.ref.Finalizer.access$100(Finalizer.java:14)
at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:160)
"main":
at java.lang.ClassLoader.findBootstrapClass(Native Method)
at java.lang.ClassLoader.findBootstrapClass0(ClassLoader.java:891)
at java.lang.ClassLoader.loadClass(ClassLoader.java:301)
- locked <0xaa653038> (a sun.misc.Launcher$ExtClassLoader)
at java.lang.ClassLoader.loadClass(ClassLoader.java:299)
- locked <0xaa6502b0> (a sun.misc.Launcher$AppClassLoader)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:268)
- locked <0xaa6502b0> (a sun.misc.Launcher$AppClassLoader)
at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)
- locked <0xaa6502b0> (a sun.misc.Launcher$AppClassLoader)
at
org.apache.bcel.classfile.ConstantPool.<init>(ConstantPool.java:103)
at
org.apache.bcel.classfile.ClassParser.readConstantPool(ClassParser.java:254)
at
org.apache.bcel.classfile.ClassParser.parse(ClassParser.java:162)
at MyAgent.transform(MyAgent.java:28)
at
sun.instrument.TransformerManager.transform(TransformerManager.java:122)
at
sun.instrument.InstrumentationImpl.transform(InstrumentationImpl.java:155)
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:620)
at
java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:260)
at java.net.URLClassLoader.access$100(URLClassLoader.java:56)
at java.net.URLClassLoader$1.run(URLClassLoader.java:195)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
- locked <0xaa6502b0> (a sun.misc.Launcher$AppClassLoader)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:268)
- locked <0xaa6502b0> (a sun.misc.Launcher$AppClassLoader)
at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)
- locked <0xaa6502b0> (a sun.misc.Launcher$AppClassLoader)

Found 1 deadlock.

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Roedy Green - 09 Dec 2005 17:22 GMT

On Fri, 09 Dec 2005 08:48:34 +0100, Francesco Devittori
<frenkatfrenkdtcm> wrote, quoted or indirectly quoted someone who said

> java -cp hello:bcel-5.1.jar -javaagent:MyAgent.jar HelloWorld

-javaagent:MyAgent.jar
what kind of syntax is that? Do you mean that to be appended to the
classpath?

don't you mean?

java -cp hello:bcel-5.1.jar:javaagent:MyAgent.jar HelloWorld

you can use the where utility or -verbose to see where Java is finding
its classes. Keep in mind that . is the root of packages. It will
find classes all over the tree starting there.

see http://mindprod.com/jgloss/where.html

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Thomas Fritsch - 09 Dec 2005 17:46 GMT

> On Fri, 09 Dec 2005 08:48:34 +0100, Francesco Devittori
> <frenkatfrenkdtcm> wrote, quoted or indirectly quoted someone who said
[quoted text clipped - 4 lines]
> what kind of syntax is that? Do you mean that to be appended to the
> classpath?

It is a new command-line syntax introduced with Java 1.5, for adding
so-called agent-libraries to the JVM. For some reason Sun separated it
from the classpath mechanism (maybe to make the agent classes
un-callable by the user's application).
The description is well-hidden in
http://java.sun.com/j2se/1.5.0/docs/api/java/lang/instrument/package-summary.html
in paragraph "Command-Line Interface".

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Roedy Green - 09 Dec 2005 23:37 GMT

On Fri, 09 Dec 2005 17:46:56 GMT, Thomas Fritsch
<i.dont.like.spam@invalid.com> wrote, quoted or indirectly quoted
someone who said :

>It is a new command-line syntax introduced with Java 1.5,

it does not show up when you type plain java.
Note the quirky extra colon.

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Thomas Hawtin - 10 Dec 2005 00:54 GMT

> On Fri, 09 Dec 2005 17:46:56 GMT, Thomas Fritsch
> <i.dont.like.spam@invalid.com> wrote, quoted or indirectly quoted
[quoted text clipped - 3 lines]
>
> it does not show up when you type plain java.

It does for me (1.5.0_06). Right at the bottom.

> Note the quirky extra colon.

It's the same syntax for everything except -classpath/-cp and -D for
java (javac has a mix).

Tom Hawtin

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Francesco Devittori - 11 Dec 2005 09:46 GMT

> On Fri, 09 Dec 2005 17:46:56 GMT, Thomas Fritsch
> <i.dont.like.spam@invalid.com> wrote, quoted or indirectly quoted
[quoted text clipped - 4 lines]
> it does not show up when you type plain java.
> Note the quirky extra colon.

It should, I'm using sun's jdk 1.5.0_05 and it's the last option it shows.

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Thomas Hawtin - 10 Dec 2005 01:07 GMT

>>> 3) a very strange problem: if you run your class with a classpath
>>> that includes the .class of your agent, then the application gets
>>> stuck at some random point (you have to Ctrl-Z/pkill -9 java).
>>
>> Can you find the stack with Ctrl-\ (or ctrl-break(?) for Windows)?

> Found one Java-level deadlock:
> =============================
[quoted text clipped - 5 lines]
> waiting to lock monitor 0x080bfe1c (object 0xae5c4860, a [[I),
> which is held by "Finalizer"

I'm not sure what is locking the array of array of ints, it appears to
be in native code. My guess is that it represents a class-loader type
object for Java Agents. That or something else to do with class loading.

I believe Java Agent classes should be the last resort after all
applicable class-loaders, including the bootstrap class-loader, have not
found the item. sun.misc.Launcher$AppClassLoader is the system
class-loader for the application.

As I understand it: What can happen, is that a code with its
class-loader locked, load a class from a child class-loader. The usual
order of nested synchronisation is from parent class loader to child. So
we have a chance of deadlock.

I think much the same thing is happening here. So, make sure your Java
Agent only appear in the agent path. For libraries like BCEL, you may
need to rename packages.

Tom Hawtin

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Francesco Devittori - 11 Dec 2005 09:48 GMT

>>>> 3) a very strange problem: if you run your class with a classpath
>>>> that includes the .class of your agent, then the application gets
[quoted text clipped - 31 lines]
>
> Tom Hawtin

This makes sense. The only thing I don't understand, is why to rename BCEL?

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Thomas Hawtin - 11 Dec 2005 19:16 GMT

> This makes sense. The only thing I don't understand, is why to rename BCEL?

If the application uses BCEL for something, then you are going to have
the classes in two places at once, and the possibility of the same deadlock.

In any case the application may use a different version of BCEL from the
instrumentation. In that case the instrumentation may fail mysteriously.
Particularly with the way the current instrument API swallows exceptions.

Tom Hawtin

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Andrew Thompson - 09 Dec 2005 09:08 GMT

...

> Sorry for the long message,

What long* message? Is my server dropping posts?

* My take is that a message has to be 100+ lines before I'd
consider it to be long (others have different views, of course,
but I'd be interested to hear the 'limit' expressed by anybody
that disagrees with '100' lines.

OTOH - an interesting or detailed message might breach 200
lines and still not be -long-. (Of course, fewer people
will read longer messages.)

> ...but maybe someone is interested.

I got interested the moment I realised you were talking
about a core Java package I'd never heard of.

In any case, I've not dealt with it, but I will stay tuned in
the hope that the technical question is resolved.

Good luck with it.

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Francesco Devittori - 11 Dec 2005 09:49 GMT

> ...
>
>> Sorry for the long message,
>
> What long* message? Is my server dropping posts?

Sorry, I wrote a long message, edited it and forgot to remove the "sorry
for the long message" :-)

> * My take is that a message has to be 100+ lines before I'd
> consider it to be long (others have different views, of course,
[quoted text clipped - 14 lines]
>
> Good luck with it.

Thanks!

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