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?

Answer 1

http://java.sun.com/j2se/1.3/docs/guide/jar/jar.html

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.

Answer 2

The META-INF folder is the home fort 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.

Answer 3

Working with Manifest Files: The Basics

Answer 4

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:

<jar ...>
    <manifest>
        <attribute name="Main-Class" value="MyApplication"/>
    </manifest>
</jar>

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.

Answer 5

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:

<import resource="classpath:/META-INF/cxf/cxf.xml" />
<import resource="classpath:/META-INF/cxf/cxf-extensions-*.xml" />

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 :-)

Answer 6

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.

Answer 7

If you're using JPA1, you might have to drop a persistance.xml file in there which specifies the name of a persistance-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.jpox.org/docs/1%5F2/jpa/emf.html