查看jar包编译的jdk版本

在jar包中，用winrar解压一个类文件，然后在命令行下面输入
javap -verbose classname
会输出一些信息，大致如下:
Compiled from "HtmlCrawer.java"
public class org.eagleeye.html.HtmlCrawer extends java.lang.Object
  SourceFile: "HtmlCrawer.java"
  minor version: 0
  major version: 50
  Constant pool:
const #1 = class #2; //  org/eagleeye/html/HtmlCrawer
const #2 = Asciz org/eagleeye/html/HtmlCrawer;
const #3 = class #4; //  java/lang/Object
const #4 = Asciz java/lang/Object;
const #5 = Asciz client;
....
后面省略了，可以看到前面有两行:
minor version: 0
major version: 50
表示了类文件的版本，具体内容可以参考:

http://bloodcode8.appspot.com/2011/01/13/%E6%9F%A5%E7%9C%8Bjar%E5%8C%85%E7%BC%96%E8%AF%91%E7%9A%84jdk%E7%89%88%E6%9C%AC.html