Obtaining the bytecode for a class without reading it from .class file

[Bogdan Tudor]

Hello everybody,

I was wondering if anyone tried to obtain the bytecode of a class (
obtain it as a byte[] for example), but without having to read it from
the .class file found in the classpath.
I am asking this because sometimes the classpath could contain some
non-disk locations, so reading from disk the .class won't always work.

Thanks,
Bogdan Tudor

 

[Steve W. Jackson]

Hello everybody,

I was wondering if anyone tried to obtain the bytecode of a class (
obtain it as a byte[] for example), but without having to read it from
the .class file found in the classpath.
I am asking this because sometimes the classpath could contain some
non-disk locations, so reading from disk the .class won't always work.

Thanks,
Bogdan Tudor

I haven't done anything of the sort myself, but I recall seeing some
sample code when I researched how to use a ClassLoader. I can't say
where, but it was "out there" on the Internet someplace. The idea I
remember was that the bytes were received from some source or another
and written locally so that a custom ClassLoader could then load them
and proceed. But if you know enough about how a ClassLoader works you
could just as easily do the same with a non-disk (in-memory) source of
bytes, I'm sure.

Lotsa luck.

= Steve =

 

[Oliver Wong]

Hello everybody,

I was wondering if anyone tried to obtain the bytecode of a class (
obtain it as a byte[] for example), but without having to read it from
the .class file found in the classpath.
I am asking this because sometimes the classpath could contain some
non-disk locations, so reading from disk the .class won't always work.

I haven't done anything of the sort myself, but I recall seeing some
sample code when I researched how to use a ClassLoader. I can't say
where, but it was "out there" on the Internet someplace. The idea I
remember was that the bytes were received from some source or another
and written locally so that a custom ClassLoader could then load them
and proceed. But if you know enough about how a ClassLoader works you
could just as easily do the same with a non-disk (in-memory) source of
bytes, I'm sure.

Might not be that easy. There are a bunch of methods in ClassLoader
where you can pass in a byte[], and out pops a Class<?>, but I don't see any
methods that provide the reverse (i.e. you provide a Class<?> and it returns
a byte[]).

- Oliver

 

[Mark Rafn]

I was wondering if anyone tried to obtain the bytecode of a class (
obtain it as a byte[] for example), but without having to read it from
the .class file found in the classpath.

Where else would you get it? If it's a class that doesn't exist as static
bytecode on the classpath (say, an on-demand class that your ClassLoader
generates for you), it'll depend on the implementation whether you can get it
or not. This is pretty rare, though.

I am asking this because sometimes the classpath could contain some
non-disk locations, so reading from disk the .class won't always work.

Oh, you mean you don't want to reimpliment what the classloader already does
to read classfiles from various places. Good, just use the classloader.
getClass().getClassLoader().getResourceAsStream()
is probably what you're looking for. You'll need to munge the classname to
get the path (translate '.' to '/', only look for outer classes, etc.).

 

[Daniel Pitts]

Hello everybody,

I was wondering if anyone tried to obtain the bytecode of a class (
obtain it as a byte[] for example), but without having to read it from
the .class file found in the classpath.
I am asking this because sometimes the classpath could contain some
non-disk locations, so reading from disk the .class won't always work.

Thanks,
Bogdan Tudor

You can use a ClassLoader to get any resource in the classpath (local
or not, and class or not). Usually you get the resource as a Stream.
You can read the bytes from the stream.

 

[Hemal Pandya]

Hello everybody,

I was wondering if anyone tried to obtain the bytecode of a class (
obtain it as a byte[] for example), but without having to read it from
the .class file found in the classpath.
I am asking this because sometimes the classpath could contain some
non-disk locations, so reading from disk the .class won't always work.

I haven't done anything of the sort myself, but I recall seeing some
sample code when I researched how to use a ClassLoader. I can't say
where, but it was "out there" on the Internet someplace. The idea I
remember was that the bytes were received from some source or another
and written locally so that a custom ClassLoader could then load them
and proceed. But if you know enough about how a ClassLoader works you
could just as easily do the same with a non-disk (in-memory) source of
bytes, I'm sure.

Might not be that easy. There are a bunch of methods in ClassLoader
where you can pass in a byte[], and out pops a Class<?>, but I don't see any
methods that provide the reverse (i.e. you provide a Class<?> and it returns
a byte[]).

I assume the OP is looking to get to the byte array of previously
loaded classes.

It seems the ClassLoader eventually defines a class from a byte array
or ByteBuffer. Maybe you can have a custom class loader, that saves the
byte array in a map somewhere before using the default class loader.

 

[M.J. Dance]

Hello everybody,

I was wondering if anyone tried to obtain the bytecode of a class (
obtain it as a byte[] for example), but without having to read it from
the .class file found in the classpath.
I am asking this because sometimes the classpath could contain some
non-disk locations, so reading from disk the .class won't always work.

One example is Instrumentation. This way you get to meddle with the bytes even
before ClassLoader gets them. Is that what you had in mind?

 

[Tom Hawtin]

It seems the ClassLoader eventually defines a class from a byte array
or ByteBuffer. Maybe you can have a custom class loader, that saves the
byte array in a map somewhere before using the default class loader.

For common class loaders, can't you just load mypackage/MyClass.class as
a resource. That probably isn't going to work with dynamically generated
classes (for instance from java.lang.reflect.Proxy) but I believe should
for java.net.URLClassLoader derived class loaders.

Tom Hawtin