What exactly does a jar file contain

Question

As an intern  I use company code in my projects and they usually send me a jar file to work with  I add it to the build path in Eclipse and usually all is fine and dandy   However  I got curious to know  what each class contained and when I try to open one of the classes in the jar file  it tells me that I need a source file   What does this mean  I come from a C C   background so is a jar similar to an already compiled  o file and all I can see is the  h stuff  Or is there actual code in the jar file that I m using that s encrypted so I can t read it   Thanks for all the answers   Edit  Thanks  guys  I knew it was a sort of like an archive but I was confused to why when I tried to open the  class files  I got a bunch of random characters  The output was similar when I tried to open a  o file in C so I just wanted to make sure  Thanks

User · Accepted Answer

A JAR file is actually just a ZIP file  It can contain anything - usually it contains compiled Java code    class   but sometimes also Java sourcecode    java    However  Java can be decompiled - in case the developer obfuscated his code you won t get any useful class function variable names though

User · Answer

Just check if the aopalliance jar file has  java files instead of  class files  if so  just extract the jar file  import it in eclipse  amp  create a jar though eclipse  It worked for me

User · Answer

Jar  Java Archive  contains group of  class files   1 To create Jar File  Zip File    if one  class  say  Demo class  then use command jar -cvf NameOfJarFile jar Demo class  usually it   s not feasible for only one  class file    if more than one  class  say  Demo class   DemoOne class  then use command jar -cvf NameOfJarFile jar Demo class DemoOne class   if all  class is to be group  say  Demo class   DemoOne class etc  then use command jar -cvf NameOfJarFile jar   class   2 To extract Jar File  Unzip File       jar -xvf NameOfJarFile jar   3 To display table of content      jar -tvf NameOfJarFile jar

User · Answer

A jar file is a zip file with some additional files containing metadata   Despite the  jar extension  it is in zip format  and any utilities that deal with  zip files are also able to deal with  jar files    http   docs oracle com javase 8 docs technotes guides jar index html  Jar files can contain any kind of files  but they usually contain class files and supporting configuration files  properties   graphics and other data files needed by the application   Class files contain compiled Java code  which is executable by the Java Virtual Machine   http   en wikipedia org wiki Java class file

User · Answer

A  jar file contains compiled code    class files  and other data resources related to that code  It enables you to bundle multiple files into a single archive file  It also contains metadata  Since it is a zip file it is capable of compressing the data that you put into it    Couple of things i found useful    http   www skylit com javamethods faqs createjar html  http   docs oracle com javase tutorial deployment jar basicsindex html  The book OSGi in practice defines JAR files as   JARs are archive files based on the ZIP file format  allowing many files to be aggregated into a single file  Typically the files contained in the archive are a mixture of compiled Java class files and resource files such as images and documents  Additionally the specification defines a standard location within a JAR archive for metadata     the META-INF folder     and several standard file names and formats within that directly  most important of which is the MANIFEST MF file

User · Answer

However  I got curious to what each class contained and when I try to open one of the classes in the jar file  it tells me that I need a source file    A jar file is basically a zip file containing  class files and potentially other resources  and metadata about the jar itself   It s hard to compare C to Java really  as Java byte code maintains a lot more metadata than most binary formats - but the class file is compiled code instead of source code   If you either open the jar file with a zip utility or run jar xf foo jar you can extract the files from it  and have a look at them  Note that you don t need a jar file to run Java code - classloaders can load class data directly from the file system  or from URLs  as well as from jar files

User · Answer

The best way to understand what the jar file contains is by executing this    Go to command line and execute jar tvf jarfilename jar

User · Answer

A  jar file is akin to a  exe file  In essence  they are both executable zip files  different zip algorithms    In a jar file  you will see folders and class files  Each class file is similar to your  o file  and is a compiled java archive   If you wanted to see the code in a jar file  download a java decompiler  located here  http   java decompiler free fr  q jdgui  and a  jar extractor  7zip works fine

User · Answer

JAR stands for Java ARchive  It s a file format based on the popular ZIP file format and is used for aggregating many files into one  Although JAR can be used as a general archiving tool  the primary motivation for its development was so that Java applets and their requisite components   class files  images and sounds  can be downloaded to a browser in a single HTTP transaction  rather than opening a new connection for each piece  This greatly improves the speed with which an applet can be loaded onto a web page and begin functioning  The JAR format also supports compression  which reduces the size of the file and improves download time still further  Additionally  individual entries in a JAR file may be digitally signed by the applet author to authenticate their origin

User · Answer

While learning about JAR  I came across this thread  but couldn t get enough information for people like me  who have  NET background  so I m gonna add few points which can help persons like myself with  NET background   First we need to define similar concept to JAR in  NET which is Assembly and assembly  shares a lot in common with Java JAR files    So  an assembly is the fundamental unit of code packaging in the  NET environment  Assemblies are self contained and typically contain the intermediate code from compiling classes  metadata about the classes  and any other files needed by the packaged code to perform its task  Since assemblies are the fundamental unit of code packaging  several actions related to interacting with types must be done at the assembly level  For instance  granting of security permissions  code deployment  and versioning are done at the assembly level   Java JAR files perform a similar task in Java with most differences being in the implementation  Assemblies are usually stored as EXEs or DLLs while JAR files are stored in the ZIP file format   Source of Information -  5- Assemblies

User · Answer

JD-GUI is a very handy tool for browsing and decompiling JARs

User · Answer

Jar file contains compiled Java binary classes in the form of   class which can be converted to readable  java class by decompiling it using some open source decompiler  The jar also has an optional META-INF MANIFEST MF which tells us how to use the jar file - specifies other jar files for loading with the jar

[java] What exactly does a jar file contain?

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