Get InputStream of a class that is dynamically created (using bytecode)?

Question

I create a class dynamically - by constructing the bytecode and loading it with class loader. Later, I need to get InputStream of this class. With common class, I get this by getting resource URL for this class and then opening the stream (url.openStream()). Obviously, here I can't do that, as dynamically loaded class does not have the URL.

How can I get InputStream of dynamically created class?

The only solution I can think of atm is to save the bytecodes to some temp file/memory and then to provide InputStream from it.

EDIT

I need InputStream to make another bytecode change on top of existing.

If you have the bytecode as a byte[] then you could construct a ByteArrayInputStream. — Elliott Frisch
– Elliott Frisch, Commented Oct 16, 2016 at 19:02
I do have it, but it's generated on different place of the app and in different time. If I start to collect all the byte[] for each proxy class it is generated, it's going to be too much. Anyway, I am exploring this option. — igr
– igr, Commented Oct 16, 2016 at 19:04
Since you don't explicitly state what you need to do with the InputStream, I can only guess, but ObjectInputStream may be helpful? ref: javadoc for ObjectInputStream — StvnBrkdll
– StvnBrkdll, Commented Oct 16, 2016 at 19:12
@mangotang thanx - but no, I need it for further bytecode change (question updated) — igr
– igr, Commented Oct 16, 2016 at 19:14
First of all, you should get rid of the intermediate URL step, as that requires defining a custom protocol and implementing a protocol handler for in-memory representations. Just use, e.g. Foo.class.getResourceAsStream("Foo.class"). That way, the ClassLoader can return a ByteArrayInputStream without the need to support URLs. If you change it to use temporary files to save memory in-between, the application logic doesn’t change. — Holger
– Holger, Commented Oct 17, 2016 at 15:26

Rafael Winterhalter · Accepted Answer · 2016-10-18 22:37:51Z

1

If the class loader does not provide a class's byte code via either getResource or getResourceAsStream (this is legal, the class loader contract does not specify such a thing), you can still use a Java agent for asking the VM to create the byte code for you.

For example, you can register the following transformer:

instrumentation.addTransformer(new ClassFileTransformer() {
  @Override
  public byte[] transform(ClassLoader loader,
                          String name,
                          Class<?> classBeingRedefined,
                          ProtectionDomain pd,
                          byte[] classFileBuffer) {
    doSomethingWith(classFileBuffer);
    return null;
  }
}, true);

After attaching a Java agent, you can call: instrumentation.retransform(someGeneratedClass) to trigger the above class file transformer which will contain the generated type's class file even if it is not available from its class loader.

answered Oct 18, 2016 at 22:37

Rafael Winterhalter

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4 Comments

igr Over a year ago

Heya Rafael! Thanx for the answer; just agent is not always available as an option. I solved my problem in other way, but still, i might collect generated bytecode in some temp files (unless they are created every time on usage)

Bohdan Mart Over a year ago

does anybody created agent for this?

Bohdan Mart Over a year ago

seems like this github.com/rmannibucau/dumping-javaagent if it works

Bohdan Mart Over a year ago

confirm. It works on Java16!

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