==========error======={"readyState":4,"responseText":"<html><head><title>Apache Tomcat/6.0.35 - Error report</title><style><!--H1 {font-family:Tahoma,Arial,sans-serif;color:white;background-color:#525D76;font-size:22px;} H2 {font-family:Tahoma,Arial,sans-serif;color:white;background-color:#525D76;font-size:16px;} H3 {font-family:Tahoma,Arial,sans-serif;color:white;background-color:#525D76;font-size:14px;} BODY {font-family:Tahoma,Arial,sans-serif;color:black;background-color:white;} B {font-family:Tahoma,Arial,sans-serif;color:white;background-color:#525D76;} P {font-family:Tahoma,Arial,sans-serif;background:white;color:black;font-size:12px;}A {color : black;}A.name {color : black;}HR {color : #525D76;}--></style> </head><body><h1>HTTP Status 500 - </h1><HR size="1" noshade="noshade"><p><b>type</b> Exception report</p><p><b>message</b> <u></u></p><p><b>description</b> <u>The server encountered an internal error () that prevented it from fulfilling this request.</u></p><p><b>exception</b> <pre>org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: Object is null (through reference chain: java.util.HashMap["data"]->java.util.ArrayList[0]->net.sf.json.JSONObject["applyExcuse"]->net.sf.json.JSONNull["empty"]); nested exception is org.codehaus.jackson.map.JsonMappingException: Object is null (through reference chain: java.util.HashMap["data"]->java.util.ArrayList[0]->net.sf.json.JSONObject["applyExcuse"]->net.sf.json.JSONNull["empty"])
org.springframework.http.converter.json.MappingJacksonHttpMessageConverter.writeInternal(MappingJacksonHttpMessageConverter.java:194)
org.springframework.http.converter.AbstractHttpMessageConverter.write(AbstractHttpMessageConverter.java:179)
org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:148)
org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(……
网上各种解释说明，然并卵。
最后发现报错中有 “org.codehaus.jackson.map.JsonMappingException: Object is null ” 意思是map值中有null。
传回前端时，null值没法转换成JSON。
ajax定义数据传输为json。
解决方案：
1、自己循环或者找工具类处理null为“”。
2、传回前端String。前端转换为JSON。