通知客户异常

Question

I have a Spring MVC based application server which communicated with client through JSON. one of the queries a client can perform is "do I have pending messages ?" to do that, the clients create an authentication object and post it to the server. In return, the server sends back a list of pending-messages, for ex. List<PendingMessage> for ex. List<PendingMessage>

Now, If the aren't pending messages I'm returning null. but the question is, how to I notify the client If I have an error, for instance, the DB connection has lost, the authentication was wrong, or any other exception I'm willing to share with the client ?

Answer 1

You can catch the different types of exceptions the code can throw. Based on that you can define your own custom message & throw that to the client. On the client side you have to just do a getMessage() on the Exception. This should solve the problem.

Answer 2

You can do this purely by throwing exceptions. SpringMVC maps exceptions to standard http status codes(200s, 400s, 500s, etc.). Just like anytime you are coding in Java you can create custom exceptions to fit your specific needs.

Answer 3

If you're designing both the client and server, then you probably want to define some kind of protocol, which can be fun. You you can think of your web server as a rpc server which returns data in a defined format for given requests.

For example, using HTTP status codes, a successful response might return the following data:

{
    status: 200,
    message: "OK",
    data: {
        name: John Doe,
        age: 31
    },
    error: null
}

A request that results in an error:

{
    status: 400,
    message: "Bad Request",
    data: null,
    error: "Database request timed out"
}

You'd pretty much have a try catch block catching any expected exceptions which can then be transformed into an appropriate JSON response outlining the problem which can then be handled by the client.

If the request simply has an error before even reaching the servlet, the API used to make the request probably exposes an onError callback of some sort which can then be used to notify the client of the error.

Answer 4

Start with HTTP response status codes. For example, if you have DB connectivity issues, then return 500 Internal Server Error.

This approach might not provide your client with all the information they need. So, in addition, you can provide details as JSON in the response body. Structure this JSON however you want. If you want the client app to go to a specific URL after receiving your response, then consider providing that URL in the response.

One final point: it would be bad form to combine 200 OK with an error in the response body. I've seen this done before, and it's annoying.