Minecraft, a Java game, is free this weekend. The Windows version downloads as an exe file. I was curious what the EXE file is doing and where it's unpacking and running the actual game JAR from. So using a command, I found the command-line arguments to the running javaw.exe process; and oddly enough, it was launched with a classpath pointing to the executable! (meaning, the .exe file was acting as a jar). Indeed, after renaming Minecraft.exe to Minecraft.jar, I was able to open it and see the loader class files and such, as if it were a normal JAR file and not an EXE at all.
How is this possible? And how can I do it with my own JAR files?
This used previously to be very common - especially in the days of floppy disks where space was precious and it was tedious for the unzip program to be on a different disk than the zip file.
The reason why it can be done is because the zip-file inventory structure is located at the end of the zip-file, not the front, so a zip file can contain a large number of initial irrelevant bytes as long as the inventory structure does not point to them (and by extension jar-files too). A very frequent use for this has been to enclose a small unzip-only program which could then unpack the zip file.
One utility to prepend such a program is the unzipsfx. Here is a manual page for it:
http://linuxcommand.org/man_pages/unzipsfx1.html
It appears that Minecraft uses another prepended program which invokes Java on itself.
EDIT: Looked inside with an hex editor. Minecraft.exe is wrapped with Launch4j.
after renaming Minecraft.exe to Minecraft.jar, I was able to open it and see the loader class files and such
Some EXE files are in fact self-extracting ZIP files. JAR files are in turn normal ZIP files with a special file structure. I bet that you was just opening it using a ZIP tool after renaming it. Note that some ZIP tools will auto-integrate in Windows explorer (or the other way round) so that it happens seemingly transparently.
The ZIP (and by extension, JAR) file format is flexible in that it allows the archive to be embedded inside another file format. This is what makes self-extracting ZIP archives possible (some small code is embedded in areas that the ZIP file parameters ensure are ignored by unzip utilities). It has also been used for some particularly sneaky exploits as well.
My guess is that Minecraft similarly exploited the ability to make the archive a valid Windows executable and added code to launch the JVM with itself in the classpath.
See also: Wikipedia: Combining ZIP with other file formats
Launch4J does this. It's really pretty impressive.
If you want a quick solution without delving too much and using a wrapper, Jsmooth does its job well.
Related
I am familiar with the jar structure and it will have a .class files in the classes directory as well as META-INF directory containing the information to main().
But where does the actual java code resides in a jar?
Does it resides a compiled byte code? But don't different machines have different compilers?
I know that I can extract the java code using a decompiler which might be illegal. But I am not interested in doing so. I am more interested in understanding how code is stored?
Is it encrypted? If so, what is the encryption algorithm? what is the location inside the jar ?
Unless you specify otherwise, the source code is not included in the JAR file. The JAR file normally only contain class files (compiled JVM instructions), not source code.
A JAR file is just a ZIP file, renamed to mean Java ARchive.
You can check what's inside, unzipping it. If you're on a OS that doesn't allow you to decompress that archive because doesn't appear to be a compressed file, just change the extension to ZIP.
JAR files are not encrypted.
Java Sources are compiled in a platform-neutral Java bytecode, that's a kind of intermediate binary.
Once JVM load the classes it either interprets the bytecode or just-in-time compiles it to the underlying machine. JARs usually only contain that bytecode
Usually sources are not included in the JARs, especially for distribution. Some projects deliver sources as well in a separate archive. You should check with the provider of the JAR you're dealing with to get sources.
If decompiling is illegal or not depends on the terms of the license applied to JAR. You should check those.
Decompiling a class object is not a very easy task, but a guy used to do a very good job with his JAD.
Unfortunately it's no more maintained, but there are some websites where you can still download it.
Decompiled classes will not look exactly as the real sources, and you could have to make some changes, but you'll definitely get an idea about the source.
I have been using apache ant to compile my java programs for quite a while, the problem now is that we have to learn to program in the .Net Framework.
I have been struggling with the building process of my csproj in ms build. The requirement we have was to create a build file of a program using only the VisualStudio Command Prompt and notepad++ to create the proj file.
Is there any task in msbuild where i can package my files similar to java jar? I searched within the MsBuild task reference but haven't found anything yet. Any help would be appreciated, thanks!
A JAR file allows Java runtimes to efficiently deploy a set of classes and their associated resources. The elements in a JAR file can be compressed, which, together with the ability to download an entire application in a single request, makes downloading a JAR file much more convenient than separately downloading the many uncompressed files which would form a single Java Application.
When you build a Dotnet application, you usually get a set of files (one .exe and multiple dlls as the simple scenario).
These files being "zipped into one common file" ... that concept does not exist in DotNet.
(except for Silverlight, but that's a different story...that's a "xap" file I believe).
Most people use a Msbuild Task to package their files together.
I use something like this:
MSBuild and creating ZIP files
to zip up my "binaries" on 1 zip file, and zip up my config files in a separate zip file.
That's a "poor man's" method...but it works.
So there is not a direct apples to apples comparison.
..........
That's the simple explanation.
When .dlls reside in the GAC, that's a different ball game.
EDIT::::::::
Here is an example of an extender library to zip files using MSBuild.
How do I zip a folder in MSBuild?
Here is 2 of the most common extension libraries
http://msbuildtasks.tigris.org/
and
http://msbuildextensionpack.codeplex.com/
This is the directory structure I want to create when I finally deploy my software. It is a Java chat client with a webcam feature and for the webcam I am using LTI-CIVIL.
I was told that I can not use DLLs right from the JAR and I will have to extract them somewhere. All cool. However, what I cannot get my head around is how can I make it work ?
LTI comes with a large number of files in the zip that they provide on their site. If you are using Eclipse, you need to set the path to appropriate folder for the native library. However, this limits me to Eclipse and prevents me from distributing the JAR to my friends. Apparently, I will now have to point to that folder, and maybe load the files, programatiaclly
I am a beginner so if someone can download LTI-CIVIL, have a look at the directory structure and let me know how to achieve what I am trying to do then that would be highly appreciated.
AFAIK, for my 32 bit Windows, I need to point to native/win32-x86 folder.
What I am trying to do is to load the appropriate files in memory so that I can provide webcam facility. I want to avoid installers and simply give a zip file with a directory structure mentioned above so that people can extract, run the jar file from the folder and start chatting.
Clarification: I am trying to send a library with jar file and not in jar. I know extracting and using dlls from jar is tough
I'm assuming that it is not your own code which loads the native libraries (System.load), and they are loaded by a third-party jar (lti-civil).
In this case you have to set the enviroment variable LD_LIBRARY_PATH appropiately before lti-civil attempts to load the native libraries.
Either:
With a launcher script (e.g .bat), set the variable before running java, or set the system property, something like:
java -jar your.jar -Djava.library.path=/path/to/native/folder
At runtime. In the entry point of your program.
This is a bit "hackish", but it works.
Check this link for example:
http://nicklothian.com/blog/2008/11/19/modify-javalibrarypath-at-runtime/
Since you do not know the exact path beforehand, in both cases you will have to also find the correct path where the native libraries are located.
If the path to the libraries is relative to the path of the jar/launcher, then find the current path of the executable:
in a .bat launcher:
Get Directory Path of an executing Batch file
in java:
How to get the path of a running JAR file?
And then that, you can assume the libraries are located in path relative to this (../native), just calculate the path (and maybe expand it to an absolute path).
After you have calculated the absolute path, set the enviroment/system property as described in the first part of the answer.
I have a jar file which is used in html file as applet. I want to modify the content of the jar file and to rebuild the jar file so that the html will work fine with the new jar file. How can i do this??
I already tried unzipping using 7zip nad modified the source and created the new jar. But when i use it in html it shows some java.lang.Classnotfound error
You can unjar or rejar the classes and source files as you wish.
unjar
jar -xvf abc.jar
jar
jar cf abc.jar input-files
http://docs.oracle.com/javase/tutorial/deployment/jar/build.html
Make the changes in the code (.java files), recompile to get the .class files. Then simply replace the old .class files in the jar with the new ones. I usually use WinZip, but you can use whatever app that can handle .Zip files. It should just work.
I've faced cases where the launcher of the app uses some sort of verification and checks for this kind of changes. I had to use a new launch script. This doesn't seem to be your case though.
This is surely possible from the command line. Use the u option for jar
From the Java Tutorials:
jar uf jar-file input-file(s)
"Any files already in the archive having the same pathname as a file being added will be overwritten."
See Updating a JAR File
A brief test shows this quickly updates changes apart from trying to delete the file.
I haven't seen this answer on other threads about modifying jar files, and many, marked as duplicates, suggest there is no alternative but to remake the jar completely. Please correct if wrong.
JARs are just ZIP files, use whatever utility you like and edit away!
Disclaimer: When reverse engineering any code be sure that you are staying within the limits of the law and adhering to the license of that code.
Follow the instructions above to unpack the JAR.
Find the original source of the JAR (perhaps its on SourceForge) and download the source, modify the source, and rebuild your own JAR.
You can also decompile the class files in the JAR. This is a rather advanced process and has a lot of "gotchas".
I have an exe that I know was written in java. I understand that java programs can be made into an exe and there are tools to convert jar files to exe but is it possible to convert back? AFAIK jar files can be run on any platform that can run java and I would like to use a windows compiled java program on mac without using any extra virtualisation (wine,vmware)
It depends how the exe has been built:
If it has simply wrapped, with a tool like JSmooth, the same tool can extract the jar
If it it has been compiled, with for instance gcj (as illustrated by this question),... I am not sure.
If it has been compiled by a static compiler (or AOT - Ahead-Of-Time -), I believe it is not possible to extract the orignial jars.
If your application was wrapped using JSmooth, you can look in your default temp directory (C:\Documents and Settings\Username\Local Settings\Temp) while the application is running.
Open a windows explorer window to the temp dir, then start up your application. You should see a jar file show up (Temp#.jar). Just make a copy of this and you should be on your way.
i suggest just rename your file extension from .exe to .jar and try to extract them.
to view the code use java Decompiler LIKE:
http://java-decompiler.github.io
It is possible to convert file.exe back to file.jar , To go with this reversing process to will require you to know how the file was beeen encrypted ,for example if is the first level encryption class name and files are not hidden , for the second level file classes are hidden , for the 3rd level file and classes are hidden . to get this well download jar to exe software and try how they convert jar to exe then you will be able to know the reverse. to know how you proceed with reversing the process there is a single blog which helped me and all tools you may use are listed there like : Resource Hacker
Winhex
Ollydbg 1.10+ MemoryDump 0.9 and Olly Advanced or StrongOD Plugin(for advanced ctrl+g).
DJ Java Decompiler
7-Zip or Winrar
The link:
https://reverseengineeringtips.blogspot.com/2014/12/unpacking-jar2exe-21-extracting-jar.html?showComment=1480364662658#c447064983483780468
thanks