在开发过程使用了AFNetworking库，版本2.x，先运行第一个官方例子（替换GET 后面的url即可）：

AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];

[manager GET:@"http://example.com/resources.json" parameters:nil success:^(AFHTTPRequestOperation *operation, id responseObject) {

    NSLog(@"JSON: %@", responseObject);

} failure:^(AFHTTPRequestOperation *operation, NSError *error) {

    NSLog(@"Error: %@", error);

}];
提示Error，但其实返回的是200 OK。
Error: Error Domain=com.alamofire.error.serialization.response Code=-1016 "Request failed: unacceptable content-type: text/plain" 
UserInfo={com.alamofire.serialization.response.error.response=<NSHTTPURLResponse: 0x100438a90> 
{ URL: http://testadmin.test.com/webservice/test } { status code: 200, headers {
    Connection = "Keep-Alive";
    "Content-Encoding" = gzip;
    "Content-Length" = 61;
    "Content-Type" = "text/plain";
    Date = "Mon, 22 Feb 2016 09:19:32 GMT";
    "Keep-Alive" = "timeout=10, max=10000";
    Server = "SNMW1.0";
    Vary = "Accept-Encoding";
} }, NSErrorFailingURLKey=http://testadmin.test.com/webservice/test,

经过查资料，是默认的AFNetworking库，不支持解析 text/plain 类型，需要简单修改源代码就可以。
打开AFURLResponseSerialization.m，找到类 AFJSONResponseSerializer，修改它的init函数：
原内容：
- (instancetype)init {
    self = [super init];
    if (!self) {
        return nil;
    }

    self.acceptableContentTypes = [NSSet setWithObjects:@"application/json", @"text/json", @"text/javascript", nil];
    return self;
}

修改为：
- (instancetype)init {
    self = [super init];
    if (!self) {
        return nil;
    }

    self.acceptableContentTypes = [NSSet setWithObjects:@"application/json", @"text/html", @"text/json", @"text/javascript",@"text/plain", nil];
    return self;
}

其实就是增加了text/plain,就可以了。