java - 在运行时查找 java 类依赖项

Question

在运行时获取 Java 类的依赖项列表的最有效方法是什么？

使用 this (基于 ASM ByteCode Manipulator 3.3.1)，我可以执行以下操作:

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

这将返回对 BasicService 的引用和 IService ，但未命中 ContainerValue ，这就是问题所在。我尝试了 ASM 代码，但不知道如何获取 ContainerValue。

package com.foo.bar;

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

作为旁注，如果我做 ContainerValue invoke 上的返回类型， 有用。

除了使用 ASM 获取类的依赖项列表之外，还有其他选择吗？为什么这么难？

Answer 1

“除了使用 ASM 获取类的依赖项列表之外，还有其他选择吗？”
那么有几种选择。一种是在没有额外库的情况下实现操作。
“怎么这么难？”
这并不难。但是，当你需要一个相当小的任务时，你不应该通过查看一个用于许多不同用例的强大库来判断。
这是一段以直接方式执行整个依赖项扫描的代码。它非常有效，但一旦你想让它做其他事情，就会变成一场噩梦。因此，一旦您需要其他字节码操作，我建议您转回去使用库。

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;