Find java class dependencies at runtime

Question

What's the most effective way to get a list of dependencies for a Java class at runtime?

Using this (based on ASM ByteCode Manipulator 3.3.1), I can do the following:

final Collection<Class<?>> classes = 
  getClassesUsedBy(MyService.class.getName(), "com");

This returns references to BasicService and IService, but misses ContainerValue, and that's the issue. I played around with the ASM code but could not figure out how to pick up ContainerValue.

package com.foo.bar;

    public class MyService extends BasicService implements IService {
         public String invoke(){
            return new ContainerValue("bar").toString();
    }

As a side note, if I make ContainerValue the return type on invoke, it works.

Is there any alternative to using ASM to get a list of dependencies for a class? Why the heck is that so difficult?

Answer 1

“Is there any alternative to using ASM to get a list of dependencies for a class?” Well there are several alternatives. One is to implement the operation without additional libraries.

“Why the heck is that so difficult?” It’s not that difficult. But you should not judge by looking at a mighty library intended for lots of different use cases when you need it for a rather tiny task.

Here is a piece of code performing the entire dependency scan in a straightforward manner. It’s quite efficient but will become a nightmare once you want it to do other things than that. So once you need other byte code operations I recommend turning back to use a library for it.

public static Set<Class<?>> getDependencies(Class<?> from)
  throws IOException, ClassNotFoundException {

  while(from.isArray()) from=from.getComponentType();
  if(from.isPrimitive()) return Collections.emptySet();
  byte[] buf=null;
  int read=0;
  try (InputStream is = from.getResourceAsStream( '/' + from.getName().replace('.', '/') + ".class")) {
    for(int r; ;read+=r) {
      int num=Math.max(is.available()+100, 100);
      if(buf==null) buf=new byte[num];
      else if(buf.length-read<num)
        System.arraycopy(buf, 0, buf=new byte[read+num], 0, read);
      r=is.read(buf, read, buf.length-read);
      if(r<=0) break;
    }
  }
  Set<String> names=getDependencies(ByteBuffer.wrap(buf, 0, read));
  Set<Class<?>> classes=new HashSet<>(names.size());
  ClassLoader cl=from.getClassLoader();
  for(String name:names) classes.add(Class.forName(name, false, cl));
  classes.remove(from);// remove self-reference
  return classes;
}

public static Set<String> getDependencies(ByteBuffer bb) {

  if(bb.getInt()!=0xcafebabe)
    throw new IllegalArgumentException("Not a class file");
  bb.position(8);
  final int numC=bb.getChar();
  BitSet clazz=new BitSet(numC), sign=new BitSet(numC);
  for(int c=1; c<numC; c++) {
    switch(bb.get()) {
      case CONSTANT_Utf8: bb.position(bb.getChar()+bb.position()); break;
      case CONSTANT_Integer: case CONSTANT_Float:
      case CONSTANT_FieldRef: case CONSTANT_MethodRef:
      case CONSTANT_InterfaceMethodRef: case CONSTANT_InvokeDynamic:
        bb.position(bb.position()+4); break;
      case CONSTANT_Long: case CONSTANT_Double:
        bb.position(bb.position()+8); c++; break;
      case CONSTANT_String: bb.position(bb.position()+2); break;
      case CONSTANT_NameAndType:
        bb.position(bb.position()+2);// skip name, fall through:
      case CONSTANT_MethodType: sign.set(bb.getChar()); break;
      case CONSTANT_Class: clazz.set(bb.getChar()); break;
      case CONSTANT_MethodHandle: bb.position(bb.position()+3); break;
      default: throw new IllegalArgumentException(
        "constant pool item type "+(bb.get(bb.position()-1)&0xff));
    }
  }
  bb.position(bb.position()+6);
  bb.position(bb.getChar()*2+bb.position());
  for(int type=0; type<2; type++) { // fields and methods
    int numMember=bb.getChar();
    for(int member=0; member<numMember; member++) {
      bb.position(bb.position()+4);
      sign.set(bb.getChar());
      int numAttr=bb.getChar();
      for(int attr=0; attr<numAttr; attr++) {
        bb.position(bb.position()+2);
        bb.position(bb.getInt()+bb.position());
      }
    }
  }
  bb.position(10);
  HashSet<String> names=new HashSet<>();
  for(int c=1; c<numC; c++) {
    switch(bb.get()) {
      case CONSTANT_Utf8:
        int strSize=bb.getChar(), strStart=bb.position();
        boolean s = sign.get(c);
        if(clazz.get(c))
          if(bb.get(bb.position())=='[') s=true;
          else addName(names, bb, strStart, strSize);
        if(s) addNames(names, bb, strStart, strSize);
        bb.position(strStart+strSize);
        break;
      case CONSTANT_Integer: case CONSTANT_Float:
      case CONSTANT_FieldRef: case CONSTANT_MethodRef:
      case CONSTANT_InterfaceMethodRef: case CONSTANT_NameAndType:
      case CONSTANT_InvokeDynamic:
        bb.position(bb.position()+4); break;
      case CONSTANT_Long: case CONSTANT_Double:
        bb.position(bb.position()+8); c++; break;
      case CONSTANT_String: case CONSTANT_Class:case CONSTANT_MethodType:
        bb.position(bb.position()+2); break;
      case CONSTANT_MethodHandle: bb.position(bb.position()+3); break;
      default: throw new AssertionError();
    }
  }
  return names;
}

private static void addName(HashSet<String> names,
  ByteBuffer src, int s, int strSize) {
  final int e=s+strSize;
  StringBuilder dst=new StringBuilder(strSize);
  ascii: {
    for(;s<e; s++) {
      byte b=src.get(s);
      if(b<0) break ascii;
      dst.append((char)(b=='/'? '.': b));
    }
    names.add(dst.toString());
    return;
  }
  final int oldLimit=src.limit(), oldPos=dst.length();
  src.limit(e).position(s);
  dst.append(StandardCharsets.UTF_8.decode(src));
  src.limit(oldLimit);
  for(int pos=oldPos, len=dst.length(); pos<len; pos++)
    if(dst.charAt(pos)=='/') dst.setCharAt(pos, '.');
  names.add(dst.toString());
  return;
}

private static void addNames(HashSet<String> names,
  ByteBuffer bb, int s, int l) {
  final int e=s+l;
  for(;s<e; s++) {
    if(bb.get(s)=='L') {
      int p=s+1; while(bb.get(p)!=';') p++;
      addName(names, bb, s+1, p-s-1);
      s=p;
    }
  }
}
private static final byte CONSTANT_Utf8 = 1, CONSTANT_Integer = 3,
  CONSTANT_Float = 4, CONSTANT_Long = 5, CONSTANT_Double = 6,
  CONSTANT_Class = 7, CONSTANT_String = 8, CONSTANT_FieldRef = 9,
  CONSTANT_MethodRef = 10, CONSTANT_InterfaceMethodRef = 11,
  CONSTANT_NameAndType = 12, CONSTANT_MethodHandle = 15,
  CONSTANT_MethodType = 16, CONSTANT_InvokeDynamic = 18;