我的代码如下

+ (void) get:(NSString *)url parameters:(NSDictionary *)params success:(void (^)(id responseObj))success failure:(void (^)(NSError *error))failure;
{
    //获得请求管理者
    AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
    manager.requestSerializer = [AFJSONRequestSerializer serializer];//请求
    manager.responseSerializer = [AFJSONResponseSerializer serializer];
    manager.responseSerializer = [AFJSONResponseSerializer serializerWithReadingOptions:NSJSONReadingAllowFragments];
    manager.responseSerializer.acceptableContentTypes = [NSSet setWithArray:@[@"text/html",@"text/plain"]];

    [manager GET:url parameters:params success:^(AFHTTPRequestOperation *operation, id responseObject) {
        NSLog(@"%@", operation.request.URL);
        success(responseObject);

    } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
//        NSLog(@"operation: %@" , operation.responseString );
        failure(error);
    }];
}

返回的错误信息是：
Error Domain=NSCocoaErrorDomain Code=3840 "The operation couldn’t be completed. (Cocoa error 3840.)" (Invalid value around character 0.) UserInfo=0x7fcad2a51c30 {NSDebugDescription=Invalid value around character 0., NSUnderlyingError=0x7fcad2aadca0 "Request failed: internal server error (500)"}

我用operation.responseString打印的结果如下：

operation:  
HTTP Status 500 - java.lang.NoSuchMethodException: com.kdtech.action.BusinessAction.myTask(org.apache.struts.action.ActionMapping, org.apache.struts.action.ActionForm, javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse)
type Exception report
message java.lang.NoSuchMethodException: com.kdtech.action.BusinessAction.myTask(org.apache.struts.action.ActionMapping, org.apache.struts.action.ActionForm, javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse)
description The server encountered an internal error that prevented it from fulfilling this request.
exception 
javax.servlet.ServletException: java.lang.NoSuchMethodException: com.kdtech.action.BusinessAction.myTask(org.apache.struts.action.ActionMapping, org.apache.struts.action.ActionForm, javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse)
    org.apache.struts.action.RequestProcessor.processException(RequestProcessor.java:523)
    org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:421)
    org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:224)
    org.apache.struts.action.ActionServlet.process(ActionServlet.java:1194)
    org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:414)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:723)
root cause 
java.lang.NoSuchMethodException: com.kdtech.action.BusinessAction.myTask(org.apache.struts.action.ActionMapping, org.apache.struts.action.ActionForm, javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse)
    java.lang.Class.getMethod(Class.java:1665)
    org.apache.struts.actions.DispatchAction.getMethod(DispatchAction.java:322)
    org.apache.struts.actions.DispatchAction.dispatchMethod(DispatchAction.java:262)
    org.apache.struts.actions.DispatchAction.execute(DispatchAction.java:194)
    org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:419)
    org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:224)
    org.apache.struts.action.ActionServlet.process(ActionServlet.java:1194)
    org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:414)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:723)
note The full stack trace of the root cause is available in the Apache Tomcat/6.0.37 logs.
Apache Tomcat/6.0.37

实在是找不到原因了，求各位大大答疑解惑！！