What s the purpose of META-INF

Question

In Java  you often see a META-INF folder containing some meta files  What is the purpose of this folder and what can I put there

User · Answer

You can also place static resources in there.

In example:

META-INF/resources/button.jpg

and get them in web3.0-container via

http://localhost/myapp/button.jpg

> Read more

The /META-INF/MANIFEST.MF has a special meaning:

If you run a jar using java -jar myjar.jar org.myserver.MyMainClass you can move the main class definition into the jar so you can shrink the call into java -jar myjar.jar.
You can define Metainformations to packages if you use java.lang.Package.getPackage("org.myserver").getImplementationTitle().
You can reference digital certificates you like to use in Applet/Webstart mode.

User · Answer

From the official JAR File Specification  link goes to the Java 7 version  but the text hasn t changed since at least v1 3       The META-INF directory      The following files directories in the META-INF directory are recognized and interpreted by the Java 2 Platform to configure applications  extensions  class loaders and services          MANIFEST MF         The manifest file that is used to define extension and package related data          INDEX LIST         This file is generated by the new  -i  option of the jar tool  which contains location information for packages defined in an application or extension   It is part of the JarIndex implementation and used by class loaders to speed up their class loading process          x SF         The signature file for the JAR file   x  stands for the base file name          x DSA         The signature block file associated with the signature file with the same base file name  This file stores the digital signature of the corresponding signature file          services          This directory stores all the service provider configuration files

User · Answer

Adding to the information here  the META-INF is a special folder which the ClassLoader treats differently from other folders in the jar  Elements nested inside the META-INF folder are not mixed with the elements outside of it    Think of it like another root  From the Enumerator lt URL gt  ClassLoader getSystemResources String path  method et al perspective   When the given path starts with  META-INF   the method searches for resources that are nested inside the META-INF folders of all the jars in the class path   When the given path doesn t start with  META-INF   the method searches for resources in all the other folders  outside the META-INF  of all the jars and directories in the class path   If you know about another folder name that the getSystemResources method treats specially  please comment about it

User · Answer

I ve noticed that some Java libraries have started using META-INF as a directory in which to include configuration files that should be packaged and included in the CLASSPATH along with JARs   For example  Spring allows you to import XML Files that are on the classpath using    lt import resource  classpath  META-INF cxf cxf xml    gt   lt import resource  classpath  META-INF cxf cxf-extensions-  xml    gt    In this example  I m quoting straight out of the Apache CXF User Guide   On a project I worked on in which we had to allow multiple levels of configuration via Spring  we followed this convention and put our configuration files in META-INF   When I reflect on this decision  I don t know what exactly would be wrong with simply including the configuration files in a specific Java package  rather than in META-INF   But it seems to be an emerging de facto standard  either that  or an emerging anti-pattern  -

User · Answer

From the official JAR File Specification  link goes to the Java 7 version  but the text hasn t changed since at least v1 3       The META-INF directory      The following files directories in the META-INF directory are recognized and interpreted by the Java 2 Platform to configure applications  extensions  class loaders and services          MANIFEST MF         The manifest file that is used to define extension and package related data          INDEX LIST         This file is generated by the new  -i  option of the jar tool  which contains location information for packages defined in an application or extension   It is part of the JarIndex implementation and used by class loaders to speed up their class loading process          x SF         The signature file for the JAR file   x  stands for the base file name          x DSA         The signature block file associated with the signature file with the same base file name  This file stores the digital signature of the corresponding signature file          services          This directory stores all the service provider configuration files

User · Answer

All answers are correct  Meta-inf has many purposes  In addition  here is an example about using tomcat container   Go to  Tomcat Doc and check   Standard Implementation   copyXML   attribute   Description is below      Set to true if you want a context XML descriptor embedded inside the application  located at  META-INF context xml  to be copied to the owning Host s xmlBase when the application is deployed  On subsequent starts  the copied context XML descriptor will be used in preference to any context XML descriptor embedded inside the application even if the descriptor embedded inside the application is more recent  The flag s value defaults to false  Note if the deployXML attribute of the owning Host is false or if the copyXML attribute of the owning Host is true  this attribute will have no effect

User · Answer

I have been thinking about this issue recently   There really doesn t seem to be any restriction on use of META-INF   There are certain strictures  of course  about the necessity of putting the manifest there  but there don t appear to be any prohibitions about putting other stuff there   Why is this the case   The cxf case may be legit   Here s another place where this non-standard is recommended to get around a nasty bug in JBoss-ws that prevents server-side validation against the schema of a wsdl   http   community jboss org message 570377 570377  But there really don t seem to be any standards  any thou-shalt-nots   Usually these things are very rigorously defined  but for some reason  it seems there are no standards here   Odd   It seems like META-INF has become a catchall place for any needed configuration that can t easily be handled some other way

User · Answer

I ve noticed that some Java libraries have started using META-INF as a directory in which to include configuration files that should be packaged and included in the CLASSPATH along with JARs   For example  Spring allows you to import XML Files that are on the classpath using    lt import resource  classpath  META-INF cxf cxf xml    gt   lt import resource  classpath  META-INF cxf cxf-extensions-  xml    gt    In this example  I m quoting straight out of the Apache CXF User Guide   On a project I worked on in which we had to allow multiple levels of configuration via Spring  we followed this convention and put our configuration files in META-INF   When I reflect on this decision  I don t know what exactly would be wrong with simply including the configuration files in a specific Java package  rather than in META-INF   But it seems to be an emerging de facto standard  either that  or an emerging anti-pattern  -

User · Answer

Adding to the information here  the META-INF is a special folder which the ClassLoader treats differently from other folders in the jar  Elements nested inside the META-INF folder are not mixed with the elements outside of it    Think of it like another root  From the Enumerator lt URL gt  ClassLoader getSystemResources String path  method et al perspective   When the given path starts with  META-INF   the method searches for resources that are nested inside the META-INF folders of all the jars in the class path   When the given path doesn t start with  META-INF   the method searches for resources in all the other folders  outside the META-INF  of all the jars and directories in the class path   If you know about another folder name that the getSystemResources method treats specially  please comment about it

User · Answer

You have MANIFEST MF file inside your META-INF folder  You can define optional or external dependencies that you must have access to   Example   Consider you have deployed your app and your container at run time  found out that your app requires a newer version of a library which is not  inside lib folder  in that case if you have defined the optional newer version in MANIFEST MF then your app will refer to dependency from there  and will not crash    Source  Head First Jsp  amp  Servlet

User · Answer

META-INF in Maven  In Maven the META-INF folder is understood because of the Standard Directory Layout  which by name convention package your project resources within JARs  any directories or files placed within the   basedir  src main resources directory are packaged into your JAR with the exact same structure starting at the base of the JAR  The Folder   basedir  src main resources META-INF usually contains  properties files while in the jar contains a generated MANIFEST MF  pom properties  the  pom xml  among other files  Also frameworks like Spring use classpath  META-INF resources  to serve web resources  For more information see How do I add resources to my Maven Project

User · Answer

From the official JAR File Specification  link goes to the Java 7 version  but the text hasn t changed since at least v1 3       The META-INF directory      The following files directories in the META-INF directory are recognized and interpreted by the Java 2 Platform to configure applications  extensions  class loaders and services          MANIFEST MF         The manifest file that is used to define extension and package related data          INDEX LIST         This file is generated by the new  -i  option of the jar tool  which contains location information for packages defined in an application or extension   It is part of the JarIndex implementation and used by class loaders to speed up their class loading process          x SF         The signature file for the JAR file   x  stands for the base file name          x DSA         The signature block file associated with the signature file with the same base file name  This file stores the digital signature of the corresponding signature file          services          This directory stores all the service provider configuration files

User · Answer

I have been thinking about this issue recently   There really doesn t seem to be any restriction on use of META-INF   There are certain strictures  of course  about the necessity of putting the manifest there  but there don t appear to be any prohibitions about putting other stuff there   Why is this the case   The cxf case may be legit   Here s another place where this non-standard is recommended to get around a nasty bug in JBoss-ws that prevents server-side validation against the schema of a wsdl   http   community jboss org message 570377 570377  But there really don t seem to be any standards  any thou-shalt-nots   Usually these things are very rigorously defined  but for some reason  it seems there are no standards here   Odd   It seems like META-INF has become a catchall place for any needed configuration that can t easily be handled some other way

User · Answer

All answers are correct  Meta-inf has many purposes  In addition  here is an example about using tomcat container   Go to  Tomcat Doc and check   Standard Implementation   copyXML   attribute   Description is below      Set to true if you want a context XML descriptor embedded inside the application  located at  META-INF context xml  to be copied to the owning Host s xmlBase when the application is deployed  On subsequent starts  the copied context XML descriptor will be used in preference to any context XML descriptor embedded inside the application even if the descriptor embedded inside the application is more recent  The flag s value defaults to false  Note if the deployXML attribute of the owning Host is false or if the copyXML attribute of the owning Host is true  this attribute will have no effect

User · Answer

The META-INF folder is the home for the MANIFEST MF file  This file contains meta data about the contents of the JAR  For example  there is an entry called Main-Class that specifies the name of the Java class with the static main   for executable JAR files

User · Answer

Generally speaking  you should not put anything into META-INF yourself   Instead  you should rely upon whatever you use to package up your JAR   This is one of the areas where I think Ant really excels  specifying JAR file manifest attributes   It s very easy to say something like    lt jar     gt       lt manifest gt           lt attribute name  Main-Class  value  MyApplication   gt       lt  manifest gt   lt  jar gt    At least  I think that s easy      -   The point is that META-INF should be considered an internal Java meta directory   Don t mess with it   Any files you want to include with your JAR should be placed in some other sub-directory or at the root of the JAR itself

User · Answer

Generally speaking  you should not put anything into META-INF yourself   Instead  you should rely upon whatever you use to package up your JAR   This is one of the areas where I think Ant really excels  specifying JAR file manifest attributes   It s very easy to say something like    lt jar     gt       lt manifest gt           lt attribute name  Main-Class  value  MyApplication   gt       lt  manifest gt   lt  jar gt    At least  I think that s easy      -   The point is that META-INF should be considered an internal Java meta directory   Don t mess with it   Any files you want to include with your JAR should be placed in some other sub-directory or at the root of the JAR itself

User · Answer

If you re using JPA1  you might have to drop a persistence xml file in there which specifies the name of a persistence-unit you might want to use   A persistence-unit provides a convenient way of specifying a set of metadata files  and classes  and jars that contain all classes to be persisted in a grouping   import javax persistence EntityManagerFactory  import javax persistence Persistence           EntityManagerFactory emf         Persistence createEntityManagerFactory persistenceUnitName     See more here  http   www datanucleus org products datanucleus jpa emf html

User · Answer

You have MANIFEST MF file inside your META-INF folder  You can define optional or external dependencies that you must have access to   Example   Consider you have deployed your app and your container at run time  found out that your app requires a newer version of a library which is not  inside lib folder  in that case if you have defined the optional newer version in MANIFEST MF then your app will refer to dependency from there  and will not crash    Source  Head First Jsp  amp  Servlet

User · Answer

You can also place static resources in there.

In example:

META-INF/resources/button.jpg

and get them in web3.0-container via

http://localhost/myapp/button.jpg

> Read more

The /META-INF/MANIFEST.MF has a special meaning:

If you run a jar using java -jar myjar.jar org.myserver.MyMainClass you can move the main class definition into the jar so you can shrink the call into java -jar myjar.jar.
You can define Metainformations to packages if you use java.lang.Package.getPackage("org.myserver").getImplementationTitle().
You can reference digital certificates you like to use in Applet/Webstart mode.

User · Answer

If you re using JPA1  you might have to drop a persistence xml file in there which specifies the name of a persistence-unit you might want to use   A persistence-unit provides a convenient way of specifying a set of metadata files  and classes  and jars that contain all classes to be persisted in a grouping   import javax persistence EntityManagerFactory  import javax persistence Persistence           EntityManagerFactory emf         Persistence createEntityManagerFactory persistenceUnitName     See more here  http   www datanucleus org products datanucleus jpa emf html

User · Answer

Generally speaking  you should not put anything into META-INF yourself   Instead  you should rely upon whatever you use to package up your JAR   This is one of the areas where I think Ant really excels  specifying JAR file manifest attributes   It s very easy to say something like    lt jar     gt       lt manifest gt           lt attribute name  Main-Class  value  MyApplication   gt       lt  manifest gt   lt  jar gt    At least  I think that s easy      -   The point is that META-INF should be considered an internal Java meta directory   Don t mess with it   Any files you want to include with your JAR should be placed in some other sub-directory or at the root of the JAR itself

User · Answer

I ve noticed that some Java libraries have started using META-INF as a directory in which to include configuration files that should be packaged and included in the CLASSPATH along with JARs   For example  Spring allows you to import XML Files that are on the classpath using    lt import resource  classpath  META-INF cxf cxf xml    gt   lt import resource  classpath  META-INF cxf cxf-extensions-  xml    gt    In this example  I m quoting straight out of the Apache CXF User Guide   On a project I worked on in which we had to allow multiple levels of configuration via Spring  we followed this convention and put our configuration files in META-INF   When I reflect on this decision  I don t know what exactly would be wrong with simply including the configuration files in a specific Java package  rather than in META-INF   But it seems to be an emerging de facto standard  either that  or an emerging anti-pattern  -

User · Answer

From the official JAR File Specification  link goes to the Java 7 version  but the text hasn t changed since at least v1 3       The META-INF directory      The following files directories in the META-INF directory are recognized and interpreted by the Java 2 Platform to configure applications  extensions  class loaders and services          MANIFEST MF         The manifest file that is used to define extension and package related data          INDEX LIST         This file is generated by the new  -i  option of the jar tool  which contains location information for packages defined in an application or extension   It is part of the JarIndex implementation and used by class loaders to speed up their class loading process          x SF         The signature file for the JAR file   x  stands for the base file name          x DSA         The signature block file associated with the signature file with the same base file name  This file stores the digital signature of the corresponding signature file          services          This directory stores all the service provider configuration files

User · Answer

I ve noticed that some Java libraries have started using META-INF as a directory in which to include configuration files that should be packaged and included in the CLASSPATH along with JARs   For example  Spring allows you to import XML Files that are on the classpath using    lt import resource  classpath  META-INF cxf cxf xml    gt   lt import resource  classpath  META-INF cxf cxf-extensions-  xml    gt    In this example  I m quoting straight out of the Apache CXF User Guide   On a project I worked on in which we had to allow multiple levels of configuration via Spring  we followed this convention and put our configuration files in META-INF   When I reflect on this decision  I don t know what exactly would be wrong with simply including the configuration files in a specific Java package  rather than in META-INF   But it seems to be an emerging de facto standard  either that  or an emerging anti-pattern  -

User · Answer

The META-INF folder is the home for the MANIFEST MF file  This file contains meta data about the contents of the JAR  For example  there is an entry called Main-Class that specifies the name of the Java class with the static main   for executable JAR files

User · Answer

Just to add to the information here  in case of a WAR file  the META-INF MANIFEST MF file provides the developer a facility to initiate a deploy time check by the container which ensures that the container can find all the classes your application depends on  This ensures that in case you missed a JAR  you don t have to wait till your application blows at runtime to realize that it s missing

User · Answer

Generally speaking  you should not put anything into META-INF yourself   Instead  you should rely upon whatever you use to package up your JAR   This is one of the areas where I think Ant really excels  specifying JAR file manifest attributes   It s very easy to say something like    lt jar     gt       lt manifest gt           lt attribute name  Main-Class  value  MyApplication   gt       lt  manifest gt   lt  jar gt    At least  I think that s easy      -   The point is that META-INF should be considered an internal Java meta directory   Don t mess with it   Any files you want to include with your JAR should be placed in some other sub-directory or at the root of the JAR itself

User · Answer

The META-INF folder is the home for the MANIFEST MF file  This file contains meta data about the contents of the JAR  For example  there is an entry called Main-Class that specifies the name of the Java class with the static main   for executable JAR files

User · Answer

META-INF in Maven  In Maven the META-INF folder is understood because of the Standard Directory Layout  which by name convention package your project resources within JARs  any directories or files placed within the   basedir  src main resources directory are packaged into your JAR with the exact same structure starting at the base of the JAR  The Folder   basedir  src main resources META-INF usually contains  properties files while in the jar contains a generated MANIFEST MF  pom properties  the  pom xml  among other files  Also frameworks like Spring use classpath  META-INF resources  to serve web resources  For more information see How do I add resources to my Maven Project

User · Answer

The META-INF folder is the home for the MANIFEST MF file  This file contains meta data about the contents of the JAR  For example  there is an entry called Main-Class that specifies the name of the Java class with the static main   for executable JAR files

User · Answer

Just to add to the information here  in case of a WAR file  the META-INF MANIFEST MF file provides the developer a facility to initiate a deploy time check by the container which ensures that the container can find all the classes your application depends on  This ensures that in case you missed a JAR  you don t have to wait till your application blows at runtime to realize that it s missing

[java] What's the purpose of META-INF?

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