META-INF文件夹是干啥的，META-INF文件夹的作用， META-INF文件夹能删吗

今天有人问到 META-INF文件夹是干啥的，META-INF文件夹的作用， META-INF文件夹能删吗，还有项目的META-INF下面一般会有个MANIFEST.MF 文件，都是干啥的。

百度搜了一下，基本找不到答案。于是上stakoverflow搜，得到如下答案：

If you remove META-INF from a jar then there is no MANIFEST.MF and so java -jar can't find the main class.

You can create jars without META-INF but when you are going to execute a jar a META-INF/MANIFEST.MF is required.

See http://docs.oracle.com/javase/7/docs/technotes/guides/jar/jar.html

这句英文很简单，简单翻译如下：

如果你将Jar中的META-INF文件夹删除，那么jar文件里边就没有MANIFEST.MF文件。那么，java -jar就找不到main class.

没有META-INF你仍然可以创建一个Jar文件。但是，当你想要执行jar文件的时候，这个jar是需要具备 META-INF/MANIFEST.MF的。

我们在使用eclipse或者myeclipse新建项目之后，在项目中可以找到META-INF文件夹，下面有个MANIFEST.MF。

打开之后发现里边只有：

1 2	`Manifest-Version:` `1.0` `Class-Path:`

瞬间变有疑问：这么少点信息，有啥作用。为啥上面那段英文说的这个文件这么的不可或缺。

于是我随便在项目的lib中找了一个jar文件commons-io-2.3.jar，打开，把里边的MANIFEST.MF文件拿出来，打开如下：

Manifest-Version: 1.0
Export-Package: org.apache.commons.io.output;version="2.3",org.apache.
 commons.io.monitor;version="2.3",org.apache.commons.io.filefilter;ver
 sion="2.3",org.apache.commons.io.comparator;version="2.3",org.apache.
 commons.io.input;version="2.3",org.apache.commons.io;version="2.3"
Implementation-Title: Commons IO
Built-By: ggregory
Tool: Bnd-1.50.0
Implementation-Vendor: The Apache Software Foundation
Implementation-Vendor-Id: org.apache
Specification-Title: Commons IO
Bundle-License: http://www.apache.org/licenses/LICENSE-2.0.txt
Bundle-SymbolicName: org.apache.commons.io
X-Compile-Target-JDK: 1.6
Implementation-Version: 2.3
Specification-Vendor: The Apache Software Foundation
Bundle-Name: Commons IO
Created-By: Apache Maven Bundle Plugin
X-Compile-Source-JDK: 1.6
Bundle-Vendor: The Apache Software Foundation
Build-Jdk: 1.6.0_31
Bundle-Version: 2.3.0
Bnd-LastModified: 1334069963279
Bundle-ManifestVersion: 2
Bundle-Description: The Commons IO library contains utility classes, s
 tream implementations, file filters, file comparators, endian transfo
 rmation classes, and much more.
Bundle-DocURL: http://commons.apache.org/io/
Implementation-Build: tags/2.3-RC1@r1311772; 2012-04-10 10:59:15-0400
Specification-Version: 2.3
Include-Resource: META-INF/LICENSE.txt=LICENSE.txt,META-INF/NOTICE.txt
 =NOTICE.txt
Archiver-Version: Plexus Archiver