Assembling jar with dependent classes

Question

For efficient Applet use I'd like to copy selectively into a jar my applet classes plus all classes they are dependent on from a set of libraries, thus producing the minimum jar library with which the applets can run (obviously classes which are part of the standard JRE are not to be included.
Now I can see, in outline, how to write a program to achieve this, but I have this feeling I've read about such a facility somewhere. I thought it was in ANT but I can't find it in the ANT documentation now. If someone knows of a utility (preferably free) that can do this please can you save me the trouble.

Answer 1

Well, I think I've found one on sourceforge: http://genjar.sourceforge.net/
I've also found out how to get dependant classes from a .class file relatively simply. Class references are listed in the Constants Table which is near the start of the .class file format. I've written a piece of code which seems to fetch them OK.
package org.igis.assjar;
* <p>Read a class file stream and return all the referenced classes from the
* class.</p>
* @author  malcolmm
public class GetClassRefs {
    private static final byte CONSTANT_Class = 7;
    private static final byte CONSTANT_FieldRef = 9;
    private static final byte CONSTANT_MethodRef = 10;
    private static final byte CONSTANT_InterfaceMethodRef = 11;
    private static final byte CONSTANT_String = 8;
    private static final byte CONSTANT_Integer = 3;
    private static final byte CONSTANT_Float = 4;
    private static final byte CONSTANT_Long = 5;
    private static final byte CONSTANT_Double = 6;
    private static final byte CONSTANT_NameAndType = 12;
    private static final byte CONSTANT_Utfs = 1;
    private static final int [] lengths = new int[13];
    static {
        lengths[CONSTANT_Class] = 3;
        lengths[CONSTANT_FieldRef] = 5;
        lengths[CONSTANT_MethodRef] = 5;
        lengths[CONSTANT_InterfaceMethodRef] = 5;
        lengths[CONSTANT_String] = 3;
        lengths[CONSTANT_Integer] = 5;
        lengths[CONSTANT_Float] = 5;
        lengths[CONSTANT_Long] = 9;
        lengths[CONSTANT_Double] = 9;
        lengths[CONSTANT_NameAndType] = 5;
        lengths[CONSTANT_Utfs] = 1;
    private final static int magic = 0xcafebabe;
    String mainClass;
    String [] refs;
    /** Creates a new instance of GetClassRefs */
    public GetClassRefs(java.io.InputStream ins) throws java.io.IOException,java.lang.ClassFormatError {
        java.io.DataInputStream in = new java.io.DataInputStream(ins);
        Object[] constTable;
        int nRefs;
        if(in.readInt() != magic)
            throw new ClassFormatError("Bad class magic");
        in.skipBytes(4);
        int cpCount = in.readUnsignedShort();
        constTable = new Object[cpCount];
        nRefs = 0;
        for(int i = 1; i < cpCount; i++) {
            int tag = in.readUnsignedByte();
            switch(tag) {
                case CONSTANT_Utfs:
                    String clsName = in.readUTF();
                    if(clsName.length() > 0) {
                        if(clsName.charAt(0) == '[') {
                            do
                                clsName = clsName.substring(1);
                            while(clsName.charAt(0) == '[');
                            if(clsName.charAt(0) == 'L' && clsName.endsWith(";"))
                                clsName = clsName.substring(1, clsName.length() - 1);
                        constTable[i] = clsName;
                    break;
                case CONSTANT_Class:
                    nRefs++;
                    constTable[i] = new Integer(in.readUnsignedShort());
                    break;
                case CONSTANT_Long:
                case CONSTANT_Double:
                    i++;
                    in.skipBytes(8);
                    break;
                default:
                    if(tag < 0 || tag >= lengths.length || lengths[tag] == 0)
                        throw new ClassFormatError("Invalid constants tag " + tag);
                    in.skipBytes(lengths[tag] - 1);
                    break;
        in.skipBytes(2);
        int thisClass = in.readUnsignedShort();
        if(!(constTable[thisClass] instanceof Integer))
            throw new ClassFormatError("Invalid main class index");
        int cpp = ((Integer)constTable[thisClass]).intValue();
        if(!(constTable[cpp] instanceof String))
            throw new ClassFormatError("Invalid main class pointer");
        mainClass = (String)constTable[cpp];
        refs = new String[nRefs - 1];
        int j = 0;
        for(int i = 1; i < constTable.length; i++) {
            Object entry = constTable;
if(entry instanceof Integer) {
int idx = ((Integer)entry).intValue();
if(idx < 1 || idx >= constTable.length)
throw new ClassFormatError("Out of range class reference");
if(constTable[idx] instanceof String) {
if(i == thisClass)
mainClass = (String)constTable[idx];
else
refs[j++] = (String)constTable[idx];
else
throw new ClassFormatError("Class reference not string");
public String getMainClass() {
return mainClass;
public String[] getRefs() throws ClassFormatError {
return refs;