Other issues

One reason that my packed format is more compact is that multiple class files are combined into a single packed format that shares information. If each class file were packed separately, the total amount of data that needs to be communicated increases. Another question is how much of the compression in my packed format is due to gzip, and how much is because of the more compact encoding. On normal classfiles, gzip provides a compression factor of about 2. These effects of combining classfiles and using gzip are broken out in Table 5. Not using gzip may be appropriate on very lightweight clients where running zip is impossible or too expensive.

 

Table 5: Effects of separate packing and not gzipping

	% of size of jar file
	of gzip'd classfiles
Option	javac	mpegaudio
Standard	22%	37%
Packed Separately	52%	56%
Not gzip'd	49%	99%
Packed Separately and not gzip'd	87%	118%

 

There is one issue we must be careful about when decompressing an archive. Normally, when we need to create a reference to a constant pool entry in a reconstructed classfile, we can just assign the element referenced to any free slot in the constant pool. However, the bytecode LDC instruction can only encode an index in the range 1-255. These instructions can only reference integer, float and string constants.

The first fix is to assign integer, float and string constant pool entries the smallest available index. Other constant pool entries are assigned in the largest available index; we transmit the total number of constant pool entries required as part of are encoding.

This almost fixes the problem. However, if there are more than 255 integer, float and string constants referenced in a classfile, which ones are assigned small indices? We would like to ensure that the same set of constants is assigned small indices as in the original classfile; otherwise, we would have to change some LDC instructions to LDC_W instructions, which are of different sizes. This would then require patching all jump offsets that traversed the changed instruction.

Instead, if a integer, float or string constant is referenced with a LDC_W instruction, then it is assigned a high constant pool index; if it is referenced with a LDC instruction, it is assigned a low constant pool index. This assumes that a classfile doesn't reference the same constant pool entry with both a LDC and a LDC_W instruction. It would be inefficient to do so, and can be fixed (and made more efficient) when the classfile is encoded if necessary.

This almost fixes the problem, except that a integer, float or string constant can also be referenced as a constant value for a field. We use an additional bit in the access flags for a field to encode whether a constant value int/float/string should be assigned a high index.

 

Table 6: Compression ratios

	Size in KBytes				Size as % of jar format			Size as % of packed format
Benchmark	jar	j0r.gz	Jazz	Packed	j0r.gz	Jazz	Packed	Strings	Opcodes	Ints	Refs	Misc
209_db	6	5	4	3	84%	66%	49%	34%	28%	9%	17%	13%
201_compress	10	6	4	3	59%	41%	29%	29%	32%	14%	17%	8%
Hanoi_jax	21	16	12	7	74%	58%	32%	21%	30%	13%	27%	9%
205_raytrace	24	15	12	7	64%	50%	30%	20%	33%	9%	22%	16%
Hanoi_big	30	20	15	9	67%	52%	29%	25%	27%	14%	26%	8%
Hanoi	46	31	23	13	67%	49%	29%	22%	29%	12%	29%	8%
228_jack	55	36	30	17	65%	55%	30%	32%	21%	14%	21%	11%
222_mpegaudio	62	45	34	23	73%	54%	37%	9%	24%	37%	12%	18%
icebrowserbean	116	88	80	39	76%	69%	34%	21%	31%	11%	26%	12%
javafig_dashO	131	113	102	53	86%	78%	41%	23%	28%	8%	29%	12%
202_jess	136	64	42	23	47%	31%	17%	23%	28%	12%	26%	11%
javafig	170	143	122	64	84%	71%	38%	28%	26%	8%	27%	11%
jmark20	173	91	86	35	53%	50%	20%	22%	25%	13%	28%	12%
213_javac	226	143	90	50	63%	40%	22%	18%	29%	15%	27%	11%
ImageEditor	257	162	123	64	63%	48%	25%	22%	28%	16%	24%	10%
tools	737	513	477	204	70%	65%	28%	26%	27%	10%	27%	11%
visaj	1,157	703	691	238	61%	60%	21%	23%	26%	12%	31%	8%
swingall	1,657	998	887	338	60%	54%	20%	19%	28%	13%	31%	9%
rt	4,652	2,820	8,435	1,069	61%	181%	23%	22%	28%	13%	27%	10%

jar	Size of jar file with individual class files stripped of debugging information and compressed
j0r.gz	Size of gzip of jar file with class files stripped of debugging information and but not compressed
Jazz	Size of Jazz archive [BHV98] (See Section 13.1)
Packed	Size of archive produced by techniques in this paper

 

 

Figure 2: Graph of compression ratios