Given an arbitrary Class instance, including one that's runtime generated (no .class file on disk), is there any way to get the class bytes?
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Have you looked at Apache Bytecode Engineering Library?Mick Mnemonic– Mick Mnemonic2016-01-28 20:28:26 +00:00Commented Jan 28, 2016 at 20:28
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2 Answers
In general, this is not possible. While loading a class, JVM parses its bytecode and converts it to the internal representation. After that JVM is free to forget the original bytecode, and that's what really happens with HotSpot JVM.
However, with the certain hacks it is possible to inspect the internal class representation and convert it back to a valid class file (though it will differ from the original bytecode). Such process is used in HotSpot JVM to reconstitute a class file for Instrumentation purposes.
As a proof of concept, I've written a program that extracts processed bytecode of a loaded class from JVM memory using Unsafe. This is only for a demo; don't ever try this in production.
EDIT
You can also get class file data (possibly modified by JVM) using Instrumentation API. This requires a Java agent loaded either at VM bootstrap (using -javaagent VM option) or at runtime via Dynamic Attach mechanism.
Here is an example of such agent: GetBytecode.java.
Finally, if you don't mind using native code, there are JVMTI functions GetConstantPool and GetBytecodes to obtain the bytecodes of the particular Java method.
Here is a JVMTI example.
3 Comments
Peter Lawrey
Note: Instrumentation will even allow you to see the byte code of dynamically created classes such as Lambdas. +1
kumarD
@apangin is there a way to generate the method definition using its byte codes and its constant pool, using jvmti?
apangin
@kumarD Not sure what you mean. Constant Pool is a property of a class, while bytecode is an attribute of a method. In what form would you like to get a method definition?
String classFile = "/" + Object.class.getName().replace('.', '/') + ".class";
System.out.println(classFile);
URL url = Test.class.getResource(classFile);
System.out.println(url);
produces this output.
/java/lang/Object.class
jar:file:/usr/java/jdk1.8.0_60/jre/lib/rt.jar!/java/lang/Object.class
so you may use url to read in bytes. (not applicable for runtime generated classes)
2 Comments
apangin
BTW, this will be invalid starting from JDK 9, because there won't be rt.jar with the original bytecode any more.
Holger
or simple:
Object.class.getResource("Object.class"), or for an arbitrary Class object: classObj.getResource(classObj.getSimpleName()+".class"). @apangin: this will continue to work with Java 9 as that’s the point of abstraction. It’s just that the URL will no longer have a jar: but a jrt: scheme.