Why some resource files are put under META-INF directory

Question

I am wondering why some resources files are put under the META-INF directory in the JAR? I am always put the resources like test.properties under the root diretcory. Any advantage to put them in the META-INF?

Stuart

possible duplicate of What's the purpose of META-INF? – Wersh

similar question: https://mcmap.net/q/65359/-what-39-s-the-purpose-of-meta-inf – Ligurian

Answer 1

Lot of Java (EE) APIs have a contract that when you put a specific configuration/metadata file in the META-INF folder of your (or a 3rd party) JAR, then the API will automatically do the API-specific job, such as scanning classes, preloading specific classes and/or executing specific code based on the meta information.

An example provided by the standard Java SE API is the ServiceLoader. Among others, the JDBC 4.0 compatible drivers implement this. This way just dropping the JDBC driver JAR file folder will automatically load the driver class during Java application's startup/initialization without the need for any manual Class.forName("com.example.Driver") line in your code.

Further there is also the Java EE 6 provided JSF 2.0 API which scans during application's startup all JAR files for a faces-config.xml file in the META-INF folder. If present, it then will then take it as a hint to scan the entire JAR file for classes implementing the JSF specific annotations like @ManagedBean so that they get auto-instantiated and auto-configured. This saves time in potentially expensive job of scanning thousands of classes in all JARs in the entire classpath. In older versions of those API's the configuration was usually done by (verbose) XML files.

All with all, the major goal is to save the developer from code and/or configuration boilerplate. The JAR's META-INF folder is used for configuration files/hints. Some API's indeed also put static files/resources in there for own use. The META-INF folder is also part of the classpath, so the loading of those files by the classloader is easy done.

Mccallion

Answer 2

In servlet 3.0, certain static resources are available through the web context, such as .css, java script, and .png files, so you no longer need to use ServletContext getResource() and getResourceAsStream(). For more information, check out web-fragment.xml (https://blogs.oracle.com/swchan/entry/servlet_3_0_web_fragment) which is one resource that covers this subject.

Personally, I prefer to structure my projects the way Maven likes them, with a src/main/resources directory which is part of the application's classpath.

Evangelicalism

Answer 3

It's just a convention that some (most?) third party jars use to look for files that you provide. For your own classes and files, you can choose to put them where you like.

Parish