Java -
Java WebStart Cache
As of Java version 6 the Java WebStart Cache can be found in the following locations.
On Linux for user accounts:
/home/$USER/.java/deployment/cache/ 
On MacOS-X for user accounts:
/Users/$USER/Library/Caches/Java/cache/ 
On Windows XP for user accounts:
C:\Documents and Settings\%USERNAME%\Application Data\Sun\Java\Deployment\cache\ 
On Windows Vista and later for user accounts:
C:\Users\%USERNAME%\AppData\LocalLow\Sun\Java\Deployment\cache\ 
On Windows Vista and later for system accounts:
C:\Windows\System32\config\systemprofile\AppData\LocalLow\Sun\Java\Deployment\cache\ 
On Windows Vista and later for system accounts using SysWOW64:
C:\Windows\SysWOW64\config\systemprofile\AppData\LocalLow\Sun\Java\Deployment\cache\ 
Note: there also can be an additional SystemCache directory e.g. on Windows Vista and later for user accounts:
C:\Users\%USERNAME%\AppData\LocalLow\Sun\Java\Deployment\SystemCache\ 
Note that if JAR files are cached it does not imply that they have been executed. The behavior seen with Microsoft Internet Explorer in combination with JRE 1.7 in high security mode is that it will download the JAR file and then prompt the user if he/she want to execute the Java applet.
IDX file format
Caveat: The following information is based on analysis of several dozen *.idx files from different Windows 7 systems. As such, the following information should not be considered to have been exhaustively researched.
Values are in big-endian.
00000000 01 00 00 00 02 5b 00 00 00 00 1d c7 b4 00 00 01 |.....[..........| 00000010 1f 81 29 fe b8 00 00 00 00 00 00 00 00 00 00 01 |..).............| 00000020 2b 24 4a cb dd 01 00 00 00 00 00 00 00 00 00 00 |+$J.............| 00000030 00 00 00 00 00 00 00 00 01 2b 24 4a a4 cd 00 00 |.........+$J....| 00000040 01 2e 45 83 f4 18 00 00 00 00 00 00 00 00 00 01 |..E.............| 00000050 01 00 00 00 00 00 00 00 00 00 00 00 01 2b 24 4a |.............+$J| 00000060 a4 cd 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 00000070 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| 
The header (or section 1) is 128 bytes in size and contains:
Offset Size Value Description 
0	1		Busy (flag byte)	
1	1		Incomplete (flag byte)	
2	4	00 00 02 5b (603)	Format version	
6	1		Force update (flag byte)	
7	1		No-href (flag byte)	
8	1		Is shortcut image (flag byte)	
9	4		Content-Length	
13	8	00 00 01 1f 81 29 fe b8	Last modification date (Number of milli seconds since Jan 1, 1970 00:00:00)	
21	8		expiration date (Number of milli seconds since Jan 1, 1970 00:00:00) 0 if not expires?	
29	8	00 00 01 2b 24 4a cb dd	Validation timestamp (Number of milli seconds since Jan 1, 1970 00:00:00)	
37	1		Known to be signed (flag byte)	
38	4		Size of section 2	
42	4		Size of section 3	
46	4		Size of section 4	
50	4		Size of section 5	
54	8	00 00 01 2b 24 4a a4 cd	Blacklist validation time (Number of milli seconds since Jan 1, 1970 00:00:00)	
62	8	00 00 01 2e 45 83 f4 18	Certificate expiration date (Number of milli seconds since Jan 1, 1970 00:00:00)	
70	1		Class verification status	
71	4		Reduced manifest size	
75	4		section4Pre15Length?	
79	1		Has only signed entries (flag byte)	
80	1		Has single code source (flag byte)	
81	4		section4CertsLength?	
85	4		section4SignersLength?	
89	1		Has missing signed entries (flag byte)	
90	8	00 00 01 2b 24 4a a4 cd	Trusted libraries validation time (Number of milli seconds since Jan 1, 1970 00:00:00)	
98	4		reducedManifest2Length?	
102	26		Unknown, empty values (likely reserved for future expansion of the header)	
The values present in the header are dependent on the version. The definition above is based on version 603 and intended as an example check the Java IDX Format Specification for more current information.
To convert a timestamp in e.g. Python
print datetime.datetime(1970, 1, 1) + datetime.timedelta(milliseconds=0x011f8129feb8) 2009-02-16 22:17:07 00000080 00 00 00 39 68 74 74 70 3a 2f 2f 77 77 77 2e 74 |...9http://www.t| 00000090 6f 70 63 6f 64 65 72 2e 63 6f 6d 2f 63 6f 6e 74 |opcoder.com/cont| 000000a0 65 73 74 2f 63 6c 61 73 73 65 73 2f 43 6f 6e 74 |est/classes/Cont| 000000b0 65 73 74 41 70 70 6c 65 74 2e 6a 61 72 |estApplet.jar | 
Offset Size Value Description 
128	2	00 00	Version string size	
130	2	00 39	Original URL string size	
132	size		Original URL string (UTF-8 without an end-of-string character?)	
000000b0 00 00 00 | ...| 000000c0 0c 36 36 2e 33 37 2e 32 31 30 2e 38 36 00 00 00 |.66.37.210.86 | 
Offset Size Value Description 
...	2	00 00	Namespace string size	
...	2	00 0c	IP string size	
...	size		IP string (UTF-8 without an end-of-string character?)	
000000c0 00 00 00 | ...| 000000d0 07 00 06 3c 6e 75 6c 6c 3e 00 0f 48 54 54 50 2f |...<null>..HTTP/| 000000e0 31 2e 31 20 32 30 30 20 4f 4b 00 0e 63 6f 6e 74 |1.1 200 OK..cont| 000000f0 65 6e 74 2d 6c 65 6e 67 74 68 00 07 31 39 35 31 |ent-length..1951| 00000100 36 36 38 00 0d 6c 61 73 74 2d 6d 6f 64 69 66 69 |668..last-modifi| 00000110 65 64 00 1d 4d 6f 6e 2c 20 31 36 20 46 65 62 20 |ed..Mon, 16 Feb | 00000120 32 30 30 39 20 32 32 3a 31 37 3a 30 37 20 47 4d |2009 22:17:07 GM| 00000130 54 00 0c 63 6f 6e 74 65 6e 74 2d 74 79 70 65 00 |T..content-type.| 00000140 18 61 70 70 6c 69 63 61 74 69 6f 6e 2f 6a 61 76 |.application/jav| 00000150 61 2d 61 72 63 68 69 76 65 00 04 64 61 74 65 00 |a-archive..date.| 00000160 1d 53 61 74 2c 20 31 38 20 53 65 70 20 32 30 31 |.Sat, 18 Sep 201| 00000170 30 20 31 30 3a 30 31 3a 30 36 20 47 4d 54 00 06 |0 10:01:06 GMT..| 00000180 73 65 72 76 65 72 00 06 41 70 61 63 68 65 00 1b |server..Apache..| 00000190 64 65 70 6c 6f 79 2d 72 65 71 75 65 73 74 2d 63 |deploy-request-c| 000001a0 6f 6e 74 65 6e 74 2d 74 79 70 65 00 1a 61 70 70 |ontent-type..app| 000001b0 6c 69 63 61 74 69 6f 6e 2f 78 2d 6a 61 76 61 2d |lication/x-java-| 000001c0 61 72 63 68 69 76 65 1f 8b 08 00 00 00 00 00 00 |archive.........| ... 
Offset Size Value Description 
...	4		Number of header value pairs	
...	...		Array of header value pairs	
A value pair is variable of size and consists of:
Offset Size Value Description 
0	2		Header value identifier string size	
2	size		Header value identifier string	
...	2		Header value string size	
...	size		Header value string	
For the example above the size of the URL string can be found at offset 130 (0x82). The first 4 string values to extract from this data are prefaced with their lengths (or sizes) as 16-bit big-endian values. E.g. to retrieve the original URL string, read the WORD at offset 0x82, and translate it as a big-endian value (e.g. using Perl, unpack("n",\$data)). Beginning at offset 0x84, the string is 57 (0x39) bytes long. At the end of that string, the next WORD is the length of the third string, also in big-endian format.
Once you've completed reading the initial 4 strings, there is a DWORD value which can be interpreted as the number of header values, followed by the individual header value definitions. Each header value definition consists of an identifier and a value string. Both strings are prefaced by a 16-bit big-endian (2-byte) value, containing the length of the string.
In many cases, the first header value contains the HTTP Response code of 302. Other header values (that have been observed so far) include a response of 200, as well as additional data (including time stamps), and the *.idx files themselves appear to contain certificate (and perhaps other) information.
External Links
Tracing and Logging, by Oracle
Almost Cooked UP Some Java
Finding Initial Infection Vector
Java IDX Format Specification, by Mark Woan, January 2013
Java Malware - Identification and Analysis, by Brian Baskin, January 12, 2013
Java Web Vulnerability Mitigation on Windows, by Tim Johnson, May 23, 2013
The State of Java Reversing Tools, by Brian Baskin, September 3, 2013