java - 我将如何解析 Java 类文件常量池？

Question

根据 https://en.wikipedia.org/wiki/Java_class_file#General_layout - 类文件的 Java 常量池从文件的 10 个字节开始。

到目前为止，我已经能够解析之前的所有内容(检查它是否是类文件、主要/次要版本、常量池大小的魔法)但我仍然不明白如何解析常量池。例如，是否有用于指定方法引用和其他内容的操作码？

在以十六进制表示文本之前，有什么方法可以引用每个十六进制值，以找出以下值是什么？

我是否应该通过 NOP (0x00) 拆分每组条目，然后解析不是文本值的每个字节？

例如，我如何才能准确计算出每个值代表什么？

Answer 1

您需要的类文件的唯一相关文档是 The Java® Virtual Machine Specification , 特别是 Chapter 4. The class File Format而且，如果你要解析的不仅仅是常量池，Chapter 6. The Java Virtual Machine Instruction Set .

常量池由可变长度的项目组成，其第一个字节决定了它的类型，而类型又决定了大小。大多数项目由一个或两个指向其他项目的索引组成。不需要任何 3rd 方库的简单解析代码可能如下所示:

public static final int HEAD=0xcafebabe;
// Constant pool types
public static final byte CONSTANT_Utf8               = 1;
public static final byte CONSTANT_Integer            = 3;
public static final byte CONSTANT_Float              = 4;
public static final byte CONSTANT_Long               = 5;
public static final byte CONSTANT_Double             = 6;
public static final byte CONSTANT_Class              = 7;
public static final byte CONSTANT_String             = 8;
public static final byte CONSTANT_FieldRef           = 9;
public static final byte CONSTANT_MethodRef          =10;
public static final byte CONSTANT_InterfaceMethodRef =11;
public static final byte CONSTANT_NameAndType        =12;
public static final byte CONSTANT_MethodHandle       =15;
public static final byte CONSTANT_MethodType         =16;
public static final byte CONSTANT_InvokeDynamic      =18;
public static final byte CONSTANT_Module             =19;
public static final byte CONSTANT_Package            =20;

static void parseRtClass(Class<?> clazz) throws IOException, URISyntaxException {
    URL url = clazz.getResource(clazz.getSimpleName()+".class");
    if(url==null) throw new IOException("can't access bytecode of "+clazz);
    Path p = Paths.get(url.toURI());
    if(!Files.exists(p))
        p = p.resolve("/modules").resolve(p.getRoot().relativize(p));
    parse(ByteBuffer.wrap(Files.readAllBytes(p)));
}
static void parseClassFile(Path path) throws IOException {
    ByteBuffer bb;
    try(FileChannel ch=FileChannel.open(path, StandardOpenOption.READ)) {
        bb=ch.map(FileChannel.MapMode.READ_ONLY, 0, ch.size());
    }
    parse(bb);
}
static void parse(ByteBuffer buf) {
    if(buf.order(ByteOrder.BIG_ENDIAN).getInt()!=HEAD) {
        System.out.println("not a valid class file");
        return;
    }
    int minor=buf.getChar(), ver=buf.getChar();
    System.out.println("version "+ver+'.'+minor);
    for(int ix=1, num=buf.getChar(); ix<num; ix++) {
        String s; int index1=-1, index2=-1;
        byte tag = buf.get();
        switch(tag) {
            default:
                System.out.println("unknown pool item type "+buf.get(buf.position()-1));
                return;
            case CONSTANT_Utf8: decodeString(ix, buf); continue;
            case CONSTANT_Class: case CONSTANT_String: case CONSTANT_MethodType:
            case CONSTANT_Module: case CONSTANT_Package:
                s="%d:\t%s ref=%d%n"; index1=buf.getChar();
                break;
            case CONSTANT_FieldRef: case CONSTANT_MethodRef:
            case CONSTANT_InterfaceMethodRef: case CONSTANT_NameAndType:
                s="%d:\t%s ref1=%d, ref2=%d%n";
                index1=buf.getChar(); index2=buf.getChar();
                break;
            case CONSTANT_Integer: s="%d:\t%s value="+buf.getInt()+"%n"; break;
            case CONSTANT_Float: s="%d:\t%s value="+buf.getFloat()+"%n"; break;
            case CONSTANT_Double: s="%d:\t%s value="+buf.getDouble()+"%n"; ix++; break;
            case CONSTANT_Long: s="%d:\t%s value="+buf.getLong()+"%n"; ix++; break;
            case CONSTANT_MethodHandle:
                s="%d:\t%s kind=%d, ref=%d%n"; index1=buf.get(); index2=buf.getChar();
                break;
             case CONSTANT_InvokeDynamic:
                s="%d:\t%s bootstrap_method_attr_index=%d, ref=%d%n";
                index1=buf.getChar(); index2=buf.getChar();
                break;
        }
        System.out.printf(s, ix, FMT[tag], index1, index2);
    }
}
private static String[] FMT= {
    null, "Utf8", null, "Integer", "Float", "Long", "Double", "Class",
    "String", "Field", "Method", "Interface Method", "Name and Type",
    null, null, "MethodHandle", "MethodType", null, "InvokeDynamic",
    "Module", "Package"
};

private static void decodeString(int poolIndex, ByteBuffer buf) {
    int size=buf.getChar(), oldLimit=buf.limit();
    buf.limit(buf.position()+size);
    StringBuilder sb=new StringBuilder(size+(size>>1)+16)
        .append(poolIndex).append(":\tUtf8 ");
    while(buf.hasRemaining()) {
        byte b=buf.get();
        if(b>0) sb.append((char)b);
        else
        {
            int b2 = buf.get();
            if((b&0xf0)!=0xe0)
                sb.append((char)((b&0x1F)<<6 | b2&0x3F));
            else
            {
                int b3 = buf.get();
                sb.append((char)((b&0x0F)<<12 | (b2&0x3F)<<6 | b3&0x3F));
            }
        }
    }
    buf.limit(oldLimit);
    System.out.println(sb);
}

不要对 getChar() 调用感到困惑，我将它们用作获取无符号短整型的便捷方式，而不是 getShort()&0xffff。

上面的代码简单地打印了对其他池项的引用索引。为了解码项目，您可以首先将所有项目的数据存储到随机访问数据结构中，即数组或 List 因为项目可能引用具有更高索引号的项目。请注意从索引 1 开始......