Javassist is a Java library to generate, modify and inspect bytecode (i.e. the machine language of Java).

Quick example

Let's assume that you want to know which call paths lead to a given method (e.g. foo()) in a given class (e.g. org.soft.micro.Narf).

// get the class pool
ClassPool pool = ClassPool.getDefault();
// access the class without loading it just yet
CtClass clazz = pool.get("org.soft.micro.Narf");
// get the method; you can find the signature (2nd parameter) like this:
// fiji --javap -s org.soft.micro.Narf | grep -A1 foo
CtMethod method = clazz.getMethod("foo", "()V");
// now let's output a whole lot of stuff whenever this method is entered
method.insertBefore("new Exception(\"Here I am!\").printStackTrace();");

Of course, you are not limited to outputting a stack trace. You could also generate Strings of those traces and put them into a Map, possibly counting the occurrences.

Other useful actions include inspecting the parameters passed to the method which you can access by the special names $1, $2, ...

Further reading: Javassist's online tutorial.

Code editing

While Javassist allows to edit bytecode directly, its most powerful feature is that it allows you to modify methods using snippets of Java code. There are two slightly different methods to do so: the CodeConverter and the ExprEditor (it depends which one to use, CodeConverter is easier to use but ExprEditor is more powerful.

Wrapping existing code blocks

A common reason for Javassist'ing code is to handle previously unhandled exceptions, or to make certain parts of the code conditional. As an example, we demonstrate here how, say, a hypothetical OpenAccess' toString() method could be protected against a NullPointerException:

// get the class pool
ClassPool pool = ClassPool.getDefault();
// access the class without loading it just yet
CtClass clazz = pool.get("OpenAccess");
CtMethod method = clazz.getMethod("toString", "()Ljava/lang/String;");
// now let's handle NullPointerExceptions
method.setBody("try {"
  + "  $_ = $proceed($$);"
  + "} catch (java.lang.NullPointerException e) {"
  + "  return \"(null)\";"
  + "}");

Note that the special $_ = $proceed($$) is a Javassist-specific syntax referring to the original code. See the Javassist tutorial for details.

Debugging

Javassist has very useful tools for disassembly and introspection. For example, you could inspect the contents of a class using some code like this:

// get the class pool
ClassPool pool = ClassPool.getDefault();
// access the class without loading it just yet
CtClass clazz = pool.get("org.soft.micro.Narf");
// output the contents
ClassFilePrinter.print(clazz.getClassFile());

Or you could disassemble a method like this:

// get the class pool
ClassPool pool = ClassPool.getDefault();
// access the class without loading it just yet
CtClass clazz = pool.get("org.soft.micro.Narf");
// get the method
CtMethod method = clazz.getMethod("foo", "()V");
// disassemble the method
InstructionPrinter.print(method, System.err);

If you absolutely do not want the output to go to System.err, you could substitute it by new PrintStream(new IJLogOutputStream()) (the class IJLogOutputStream is defined in Fiji Updater's fiji.updater.util class for you to reuse).

VerifyError

For security reasons, HotSpot does not allow any malformed bytecode to execute. To verify that the bytecode is reasonably sane, the code is verified before being executed.

Unfortunately, the error messages of said verifier are not exactly stellar.

If you want to avoid being misled by the error message (the method mentioned in the VerifyError is most likely to be not the offending one), and moreover want to know where in the bytecode the error happened, probably the easiest way to go forward is to use BCEL (you can get it directly from the Maven repository), Apache's Byte Code Engineering Library. For that, you have to write out .class files first:

CtClass clazz = ...;
...;

File directory = ...;
File file = new File(directory, clazz.getName().replace('.', '/') + ".class");
file.getParentFile().makeDirectories();
DataOutputStream out = new DataOutputStream(new FileOutputStream(file));
clazz.getClassFile().write(out);
out.close();

Then you can use the verifier:

fiji --classpath=/path/to/bcel.jar:directory/ \
        --main-class=org.apache.bcel.verifier.Verifier \
        my.class.Name

(Side note: Apache calls the verifier JustIce, but the name is not reflected in the program name, only in its output.)

ImageJ

Javassist

Contents

Quick example

Code editing

Wrapping existing code blocks

Debugging

VerifyError