版本：
net.sf.json-lib:json-lib:2.4
org.json:json:2.2

import net.sf.json.JSONObject;
import org.json.JSONException;

public class CompareJsonJar {
    public static void main(String[] args) throws JSONException {
        String text= "{\"keywords\": \"好\",\"mainFeedIdStr\":123}";
        String key = "mainFeedIdStr";
        System.out.println("---------------------------net.sf.json------------------------");
        JSONObject netJson = JSONObject.fromObject(text);
        String mainFeedIdStr = netJson.getString(key);
        System.out.println("mainFeedIdStr = " + mainFeedIdStr);
        Long mainFeedId = netJson.getLong(key);
        System.out.println("mainFeedId = " + mainFeedId);

        System.out.println("---------------------------org.json------------------------");
        org.json.JSONObject orgJson = new org.json.JSONObject(text);
        mainFeedIdStr = orgJson.getString(key);
        System.out.println("mainFeedIdStr = " + mainFeedIdStr);
        mainFeedId = orgJson.getLong(key);
        System.out.println("mainFeedId = " + mainFeedId);
    }
}

输出：

---------------------------net.sf.json------------------------
mainFeedIdStr = 123
mainFeedId = 123
---------------------------org.json------------------------
Exception in thread "main" java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at com.intellij.rt.execution.CommandLineWrapper.main(CommandLineWrapper.java:121)
Caused by: org.json.JSONException: JSONObject["mainFeedIdStr"] not a string.
    at org.json.JSONObject.getString(JSONObject.java:639)
    at CompareJsonJar.main(CompareJsonJar.java:17)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)
    ... 5 more

由此可以看出，org.json.JSONObject.getString(key)不能获取数值；而net.sf.json.JSONObject.getString(key)可以；