jvm-visitor

jvm-visitor

This module extends kylestev/jvm.js by exposing classes used for traversing the contents of a ClassInfo object from jvm.js (a JavaScript representation of the JVM class file format) by utilizing the Visitor Pattern.

Examples

Field Logging Visitor

import { Jar } from 'jvm';
import { ClassVisitor } from 'jvm-visitor';

// instantiate a new ClassVisitor object
let visitor = new ClassVisitor;
// bind an event listener to the `visit-field` event whose callback is passed
// a `ClassInfo` object as well as a `FieldInfo` object.
visitor.on('visit-field', (cls, field) => {
  console.log('  %s %s.%s', field.desc, cls.name, field.name);
});

// parse the jar contents
Jar.unpack('/path/to/your.jar')
  // called when the Promise returned from `Jar#unpack(string)` succeeds
  .then((jar) => {
    // iterate all `ClassInfo` objects (`cls`) in the jar
    for (let [name, cls] of jar) {
      // pass the `ClassInfo` object to `ClassVisitor#accept(ClassInfo)`
      // in order to visit `cls` and its members
      visitor.accept(cls);
    }
  })
  // catch any uncaught errors during this Promise chain and log them to the console
  .catch(console.error.bind(console));

VerboseClassVisitor

jvm-visitor ships with a verbose version of the default ClassVisitor which has the same interface for interacting with it and behaves the same way with one caveat: it binds event listeners for each type of even that ClassVisitor emits and prints out basic information about the class file when visited.

This can aid debugging efforts without hampering development time as you can simply swap new ClassVisitor references with new VerboseClassVisitor in your code -- even if you're binding your own event listeners on top of those used by VerboseClassVisitor.

import { Jar } from 'jvm';
import { VerboseClassVisitor } from 'jvm-visitor';

let visitor = new VerboseClassVisitor;

Jar.unpack('/path/to/your.jar')
  .then((jar) => {
    for (let [name, cls] of jar) {
      visitor.accept(cls);
      // output will look similar to the following:
      // >>> [visit-start] ClassName
      // >>> [visit-field] FieldDesc ClassName.FieldName
      // >>> [visit-field] ...
      // >>> [visit-method] ClassName#MethodName+MethodDesc
      // >>> [visit-method] ...
      // >>> [visit-end] ClassName completed in 0.004365655s
      // >>> [visit-start] ClassName1
      // >>> ...
      // >>> [visit-end] ClassName1 completed in 0.004233272s
    }
  })
  .catch(console.error.bind(console));