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How to get source code from .jar file

I have my project in .jar file.
But i lost the source code of it.
How to get source code from .jar file.

First of all I would like to tell you that if your project is big and complex, you are in trouble. Generated source code via external tools(no matter whatever the tool is) is never same as the real code. Code like comments, constants, inner classes, etc gets messy.
For simpler code and projects you can use -
Java Decompiler (JD-GUI) http://jd.benow.ca/
DJ Java Decompiler http://www.neshkov.com/dj.html
But always know this that its not what was written originally in the source code.

You must decompile your code. You can use Java decompiler http://jd.benow.ca/

How can I see the source code of packages in Eclipse?

i started to learn android program ,and i curious to see the code of the classes/
for example the class of import android.app.Activity.
how can i see the source code of the packages in eclipse?

You can attach source code to libraries. yourProject / Properties / Java Build Path / Libraries / yourLibrary / Source attachment, and then enter the folder or zip file containing the source code (which you need to download separately).
If you only have the object code (.class files), then you need to learn to read ByteCode (which Eclipse shows pretty nicely) or use a Java ByteCode disassembler.

As in android sdk what we get is a compiled jar file which contains all the classes.
So in eclipse you can't read the source, but you may use, The official online version to read the source code of classes:
http://developer.android.com/reference/packages.html
[In eclipse although by CTRL+Click on import may show you a bit about the class, although that won't be easy to understand :)]

How to decompile a Java .class file from rt.jar

I have extracted a .class file from the rt.jar file in jre1.7, and have attempted to open it using notepad++, however it is unreadable. How can I decompile the .class file into a .java file so I can view the code?
Please note that I am interested in the decompilation of the code, not viewing the source code provided in the JDK downloads.

As #Jon Skeet said, don't use JRE but JDK, which contains the entire source code. If you for some reason insist on your approach, then I recommend JD-GUI for decompilation.

.class files are the compiled binary files. You can't read the Java code from them, but you can run them. If you want to see the source you usually need .java files. These are either packed in the same jar but then the src folder, or they can be downloaded in a seperate package.

You can use any decompiler tool like cavaj for decompiling a class file to source code (java file). I'm using it, and it works perfectly.

Try any of the decompilers listed here:
Procyon - https://bitbucket.org/mstrobel/procyon/wiki/Java%20Decompiler
CFR - http://www.benf.org/other/cfr/
JD - http://jd.benow.ca/
Fernflower - https://github.com/fesh0r/fernflower
Or use online decompiler - http://www.javadecompilers.com/

Decompiling java class files and comparing with svn

We have normal java files residing in the SVN. We have made some changes in those files , but it happens that , those files are lost (they are not in SVN). But we have the class files that are generated using the newly changed files.
Can we use the class files , decompile them and compare it against SVN. What is the easiest way to do it?
There are about 400 changed files. So comparing one by one is not feasible.
I am looking for any tool or scripts.
Also is there any decompiler , that would decompile a whole folder at one go?
Thanks

For decompiling use JAD, most commonly used tool. Comparing is a bit tricky though. I would suggest the following scenario:
Grab the latest source code from SVN
Build it
Decompile it (!)
Take your compiled classes that include some modifications but you don't have sources
Decompile them as well
Compare both decompiled sources directories
Why compiling and decompiling the original source codes? Because JAD produces pretty good results, but it will never generate the exact same sources that were used. So if you want to avoid headaches when comparing original sources and decompiled ones (and pinpoint the actual differences quickly), you have to compare two JAD outputs rather than original source and synthetic JAD output.
I hope having two directory structures to compare won't be a problem for you. You can use Total Commander on Windows or various utilities/scripts on Linux, like:
$ diff -r dir1 dir2

jad can decompile a whole folder but the result depends on a couple of factors. First, JAD only supports Java 4 well. Java 5 and up will contain odd byte code chunks that JAD didn't understand.
If the code is compiled with debug symbols, you can realign line numbers (the jadclipse plugin can do that) but JAD itself can't do it.
If you compiled the code with -g:source, then the class files contain the complete source code. At first glance, I don't know how to get at this but tools like javap (comes with the JDK) or ASM should allow you to get it.

It is possible to compare a decompiled source file with the orginal, provided you did not use obfuscation when generating class files. However, automated comparison will be difficult because the decompiled source is often slightly different than the original source due to compiler optimization for example.
Personally I use jad as a decompiler, but I'm not sure you can provide it with a whole folder at one go.

I used the below command(jad decompiler) to compile all the class files in a folder in one go.
jad -o -r -sjava -dsrc tree/**/*.class

Use jd-gui, very good decompiler, but the compering ... its going to be painful.

Right now im doing exactly that, using http://java.decompiler.free.fr/ to decompile and beyond compare (http://www.scootersoftware.com/) to compare packages and files. It looks like a great idea to make a fast compare against the actual version (svn) compiled and decompiled, to check which files (and which sections) are up to date.

Byte code to Java source code

Is it possible to convert a .class file to .java file?
How can this be done?
What about the correctness of the code extracted from this option?

It is possible. You need a Java Decompiler to do this.
You'll find mostly it'll do a surprisingly good job. What you'll get is a valid .java file which will compile to the .class file but this .java file won't necessarily be the same as the original source code. Things like looping constructs might come out differently, and anything that's compile time only such as generics and annotations won't be re-created.
You might have a problem if the code has been obfuscated. This is a process which alters the class files to make them hard to decompile. For example, class and variable names are changed to all be similar so you'll end up with code like aa.a(ab) instead of employee.setName(name) and it's very hard to work out what's going on.
I remember using JAD to do this but I don't think this is actively maintained so it may not work with never versions of Java. A Google search for java decompiler will give you plenty of options.

This is possible using one of the available Java decompilers. Since you are working from byte-code which may have been optimised by the compiler (inlining static variables, restructing control flow etc) what you get out may not be exactly the same as the code that was originally compiled but it will be functionally equivalent.

Adding to the previous answers: recently, a new wave of decompilers has been coming, namely Procyon, CFR, JD, Fernflower
Here's a list of modern decompilers as of March, 2015:
Procyon
CFR
JD
Fernflower
You may test above mention decompilers online, no installation required and make your own educated choice.
Java decompilers in the cloud: http://www.javadecompilers.com/

It is always possible. Search for "java disassembler".
But source code comments and temporary variables will not be available.
If you decompile a class and see the code is too complex with variable names and method names are like a,b,c... that means that the project is obfuscated.

Not exactly a decompiler, but the JDK contains javap, a disassembler:
javap -c org.example.MyClass
Depending on your usecase, it might still be interesting to know or use.
Note that results of class file decompilation depend on the included information within a class file. If I remember correctly, included debug information (see -g flag of javac) is important, especially for naming of variables and the like.

DJ is the easy to use java decompiler . Just open any .class file and it will show you its java code.
Also, you can use jadClipse plugin for eclipse to directly decompile the .class into .java
What about the correctness of the code extracted from this option?
In any case, the code which will be generated by any java decompiler will not be the same as it was written in orginal java class. As it just decodes the bytecode into java code. The only thing you can be sure is, that the output will be same as the output of orginal java code.