只下载json-lib的jar还不行，JSON-LIB的依赖库如下

EZMorph
下载地址http://ezmorph.sourceforge.net
Commons
Collections http://jakarta.apache.org/commons/collections/
Commons Lang
http://jakarta.apache.org/commons/lang/
Commons BeanUtils
http://commons.apache.org/beanutils/
The Logging Component
http://commons.apache.org/logging/

eclise 导出的libraries如下

<?xml version="1.0" encoding="UTF-8"?>
<eclipse-userlibraries version="2">
    <library name="mylib" systemlibrary="false">
        <archive path="D:/jdk/mylib/commons-lang-2.3.jar"/>
        <archive path="D:/jdk/mylib/ezmorph-1.0.4.zip"/>
        <archive path="D:/jdk/mylib/json-lib-2.1-jdk13.jar"/>
        <archive path="D:/jdk/mylib/commons-logging-1.1.jar"/>
        <archive path="D:/jdk/mylib/commons-beanutils.jar"/>
        <archive path="D:/jdk/mylib/commons-collections-3.2.jar"/>
    </library>
</eclipse-userlibraries>