Use of the MANIFEST MF file in Java

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I noticed that JAR  WAR and EAR files have a MANIFEST MF file under the META-INF folder   What is the use of the MANIFEST MF file  What all things can be specified in this file

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Manifest MF contains information about the files contained in the JAR file   Whenever a JAR file is created a default manifest mf file is created inside META-INF folder and it contains the default entries like this   Manifest-Version  1 0 Created-By  1 7 0 06  Oracle Corporation    These are entries as    header value    pairs  The first one specifies the manifest version and second one specifies the JDK version with which the JAR file is created   Main-Class header  When a JAR file is used to bundle an application in a package  we need to specify the class serving an entry point of the application  We provide this information using    Main-Class    header of the manifest file   Main-Class   fully qualified classname   The    Main-Class    value here is the class having main method  After specifying this entry we can execute the JAR file to run the application   Class-Path header  Most of the times we need to access the other JAR files from the classes packaged inside application   s JAR file  This can be done by providing their fully qualified paths in the manifest file using    Class-Path    header   Class-Path   jar1-name jar2-name directory-name jar3-name   This header can be used to specify the external JAR files on the same local network and not inside the current JAR   Package version related headers  When the JAR file is used for package versioning the following headers are used as specified by the Java language specification   Headers in a manifest Header                    Definition ------------------------------------------------------------------- Name                      The name of the specification  Specification-Title       The title of the specification  Specification-Version     The version of the specification  Specification-Vendor      The vendor of the specification  Implementation-Title      The title of the implementation  Implementation-Version    The build number of the implementation  Implementation-Vendor     The vendor of the implementation    Package sealing related headers   We can also specify if any particular packages inside a JAR file should be sealed meaning all the classes defined in that package must be archived in the same JAR file  This can be specified with the help of    Sealed    header   Name   package some-package   Sealed true  Here  the package name must end with           Enhancing security with manifest files   We can use manifest files entries to ensure the security of the web application or applet it packages with the different attributes as    Permissions        Codebae        Application-Name        Trusted-Only    and many more   META-INF folder   This folder is where the manifest file resides  Also  it can contain more files containing meta data about the application  For example  in an EJB module JAR file  this folder contains the EJB deployment descriptor for the EJB module along with the manifest file for the JAR  Also  it contains the xml file containing mapping of an abstract EJB references to concrete container resources of the application server on which it will be run   Reference  https   docs oracle com javase tutorial deployment jar manifestindex html

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The content of the Manifest file in a JAR file created with version 1 0 of the Java Development Kit is the following   Manifest-Version  1 0   All the entries are as name-value pairs  The name of a header is separated from its value by a colon  The default manifest shows that it conforms to version 1 0 of the manifest specification  The manifest can also contain information about the other files that are packaged in the archive  Exactly what file information is recorded in the manifest will depend on the intended use for the JAR file  The default manifest file makes no assumptions about what information it should record about other files  so its single line contains data only about itself  Special-Purpose Manifest Headers  Depending on the intended role of the JAR file  the default manifest may have to be modified  If the JAR file is created only for the purpose of archival  then the MANIFEST MF file is of no purpose  Most uses of JAR files go beyond simple archiving and compression and require special information to be in the manifest file  Summarized below are brief descriptions of the headers that are required for some special-purpose JAR-file functions  Applications Bundled as JAR Files  If an application is bundled in a JAR file  the Java Virtual Machine needs to be told what the entry point to the application is  An entry point is any class with a public static void main String   args  method  This information is provided in the Main-Class header  which has the general form   Main-Class  classname   The value classname is to be replaced with the application s entry point   Download Extensions  Download extensions are JAR files that are referenced by the manifest files of other JAR files  In a typical situation  an applet will be bundled in a JAR file whose manifest references a JAR file  or several JAR files  that will serve as an extension for the purposes of that applet  Extensions may reference each other in the same way  Download extensions are specified in the Class-Path header field in the manifest file of an applet  application  or another extension  A Class-Path header might look like this  for example   Class-Path  servlet jar infobus jar acme beans jar   With this header  the classes in the files servlet jar  infobus jar  and acme beans jar will serve as extensions for purposes of the applet or application  The URLs in the Class-Path header are given relative to the URL of the JAR file of the applet or application   Package Sealing  A package within a JAR file can be optionally sealed  which means that all classes defined in that package must be archived in the same JAR file  A package might be sealed to ensure version consistency among the classes in your software or as a security measure  To seal a package  a Name header needs to be added for the package  followed by a Sealed header  similar to this   Name  myCompany myPackage  Sealed  true   The Name header s value is the package s relative pathname  Note that it ends with a     to distinguish it from a filename  Any headers following a Name header  without any intervening blank lines  apply to the file or package specified in the Name header  In the above example  because the Sealed header occurs after the Name  myCompany myPackage header  with no blank lines between  the Sealed header will be interpreted as applying  only  to the package myCompany myPackage   Package Versioning  The Package Versioning specification defines several manifest headers to hold versioning information  One set of such headers can be assigned to each package  The versioning headers should appear directly beneath the Name header for the package  This example shows all the versioning headers   Name  java util  Specification-Title   Java Utility Classes   Specification-Version   1 2  Specification-Vendor   Sun Microsystems  Inc    Implementation-Title   java util   Implementation-Version   build57  Implementation-Vendor   Sun Microsystems  Inc

[java] Use of the MANIFEST.MF file in Java

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