I am writing data to a file, when I write this data I want to do it so that if the file does not open it will give the user a message saying that something whent wrong. The way I do this is by calling the method to write, if it fails it returns false. That way I can prompt the user to do something to check what has happened.
However when I create the object I cant return anything from the constructor so I am a bit stumped about what I should do.
public class Writetofile {
BufferedWriter writer = null;
public Writetofile(String[]details) throws IOException{
String machine= details[0];
String date=details[1];
String start_time = details[2];
try{
File new_cal= new File("C:\\Activity_Calibrator\\log\\"+machine+"\\"+machine+date+".txt");
new_cal.getParentFile().mkdir();
FileWriter fwriter = new FileWriter(new_cal);
writer = new BufferedWriter(fwriter);
writer.write("Linear Calibratiton for " + machine + " carried out " + date+" ./n");
writer.close();
}
catch(Exception e){ in here I would like to be able to send a message back to m
code so that it can tell the user to check the folder etc}
}
when I call the record data if something goes wrong it will return a false to the calling class. and I can put a message.
public boolean recordData(String record) throws IOException{
try{
writer.append(record);
writer.close();
return true;
}
catch(Exception e){
return false;
}
}
}
}
A constructor should not DO anything. A constructor is an initialization phase closely tied to the allocation of an object.
Throwing exceptions, or doing anything in a constructor that might throw an exception is to be avoided.
Java does not separate the phases of allocation and initialization, no code, especially IO code should be in a constructor.
Related
Below is the sample code I'm using to understand exception handling in completablefuture in java8.
If we make use of exceptionally method as per doc,
exceptionally method catches even runtime exception as well and proceeds to last block in the pipeline.
if we don't use exceptionally method then, it justs prints running and exits.
Correct me if my understanding isn't correct.
Question is Lets say if i want to throw runtime exception and want application to stop. Basically if i throw Runtime exception , it shouldn't proceed to next block in pipeline. How should i do that. Any pointers are helpful.
public static void main(String[] args) {
final CompletableFuture<String> retrieveName = CompletableFuture.supplyAsync(() -> {
System.out.println("running");
int i = 0;
if(i == 0) {
throw new RuntimeException("ding");
}
return "test";
}).exceptionally(it -> {
System.out.println(it.getMessage());
return "empty";
}).thenApply(it -> {
System.out.println("last block" + it);
return "dummy";
});
}
Try this:
public static void main(String[] args) {
try {
final CompletableFuture<String> retrieveName = CompletableFuture.supplyAsync(() -> {
System.out.println("running");
int i = 0;
if (i == 0) {
throw new RuntimeException("ding");
}
return "test";
}).exceptionally(it -> {
if (it.getMessage().contains("ding")) {
throw (RuntimeException) it;
}
System.out.println(it.getMessage());
return "empty";
}).thenApply(it -> {
System.out.println("last block" + it);
return "dummy";
});
retrieveName.join();
} catch (Exception e) {
System.out.println("main() exception, cause=" + e.getCause());
}
}
This is the output:
running
main() exception, cause=java.lang.RuntimeException: ding
I made 3 small changes to your code:
Wrapped it all in a try-catch
Threw a RuntimeException in exceptionally() for the "ding" exception.
Added a call to retrieveName.join(). From the Javadoc for CompletableFuture.join():
public T join​()
Returns the result value when complete, or throws an (unchecked) exception if completed exceptionally.
Update based on OP feedback ------->
Lets say if i want to throw runtime exception and want application to
stop. Basically if i throw Runtime exception , it shouldn't proceed to
next block in pipeline. How should i do that.
You can achieve what you want with just 2 changes to your code:
[1] Completely remove the exceptionally() callback so the CompletableFuture (CF) terminates with an exception. In exceptionally() in the OP code the exception was being swallowed rather than rethrown, and returning a CF, so the thenApply() method was still performed.
[2] Add a call to retrieveName.join() at the end of main(). This is a blocking call, but since the thread had terminated with an exception that 's not really relevant for the sample code. The join() method will extract the thrown RunTimeException and re-throw it, wrapped in a CompletionException.
Here's your modified code:
public static void main(String[] args) {
final CompletableFuture<String> retrieveName = CompletableFuture.supplyAsync(() -> {
System.out.println("running");
int i = 0;
if(i == 0) {
throw new RuntimeException("ding");
}
return "test";
}).thenApply(it -> {
System.out.println("last block" + it);
return "dummy";
});
retrieveName.join();
}
Notes:
This is not how to do things in Production. The blocking call from join() was not a problem here, but could be for a long running CF. But you obviously can't extract the exception from the CF until it is complete, so it makes sense that the join() call blocks.
Always bear in mind that main() is not running in the same thread(s) as the CF.
An alternative approach (if viable) might be to handle all the necessary post-exception actions (logging, etc,) within exceptionally() and then terminate normally with a suitable return value (e.g. "Exception handled!") rather than propagating the exception.
You can check whether the CF is still running by calling the non-blocking isDone() method. You can also check whether the CF ended with an exception (isCompletedExceptionally()) or was cancelled(isCancelled​()).
How can I change/update the following REST call from Spring MVC to return a error if the user did not enter of the the two names I was coding for.. something like a not found?
#RequestMapping(value = "/{name}", method = RequestMethod.GET)
#ResponseBody
public User getName(#PathVariable String name, ModelMap model)
{
logger.debug("I am in the controller and got user name: " + name);
/*
Simulate a successful lookup for two users. This
is where your real lookup code would go.
*/
if ("name2".equals(name))
{
return new User("real name 2", name);
}
if ("name1".equals(name))
{
return new User("real name 1", name);
}
return null;
}
Define a new exception class, e.g. ResourceNotFoundException and throw an instance of this from your annotated controller method getName.
Then also define an annotated exception handler method in your Controller class to handle that exception, and return a 404 Not Found status code, potentially logging it.
#ExceptionHandler(ResourceNotFoundException.class)
#ResponseStatus(value = HttpStatus.NOT_FOUND)
public void handleResourceNotFoundException(ResourceNotFoundException ex)
{
LOG.warn("user requested a resource which didn't exist", ex);
}
Or even returning some error message, using #ResponseBody annotation:
#ExceptionHandler(ResourceNotFoundException.class)
#ResponseStatus(value = HttpStatus.NOT_FOUND)
#ResponseBody
public String handleResourceNotFoundException(ResourceNotFoundException ex)
{
return ex.getMessage();
}
You could create an object specifically for returning error responses. That way, you can say whatever you want. For example:
#ExceptionHandler(HttpMessageNotReadableException.class)
public ResponseEntity<ResponseStatus> handleHttpMessageNotReadableException(HttpMessageNotReadableException ex){
ResponseStatus responseStatus = new ResponseStatus("400", "Bad Request. " + ex);
responseStatus.setResponseStatusTime(timestampService.createTimestamp());
HttpStatus status = HttpStatus.BAD_REQUEST;
ResponseEntity<ResponseStatus> response = new ResponseEntity<ResponseStatus>(responseStatus, status);
return response;
}
In this example you can see that there is a ResponseStatus object. This object contains a field for a status code, status message, date and time. You may not need date and time but I find it useful for when someone sends me an error they have seen, because then it is easy to track down exactly where it happened in our server logs.
In my ServiceStack service, I throw an exception that has an inner exception. When I caught a WebServiceRequest on the client side, the ErrorCode was the inner exception type name.
This is bad for me because it doesn't allow me to respond to the specific exception type that was thrown on the server.
I'm failing to see why ServiceStack was designed this way. It's pretty typical to catch lower level exceptions and wrap them with more informative and sometimes end-user friendly exceptions.
How can I change the default behavior so it uses the surface level exception and not the inner-most?
After looking at the first example at https://github.com/ServiceStack/ServiceStack/wiki/Error-Handling, I decided to check out at DtoUtils.HandleException, which looks like this:
public static object HandleException(IResolver iocResolver, object request, Exception ex)
{
if (ex.InnerException != null && !(ex is IHttpError))
ex = ex.InnerException;
var responseStatus = ex.ToResponseStatus();
if (EndpointHost.DebugMode)
{
// View stack trace in tests and on the client
responseStatus.StackTrace = GetRequestErrorBody(request) + ex;
}
Log.Error("ServiceBase<TRequest>::Service Exception", ex);
if (iocResolver != null)
LogErrorInRedisIfExists(iocResolver.TryResolve<IRedisClientsManager>(), request.GetType().Name, responseStatus);
var errorResponse = CreateErrorResponse(request, ex, responseStatus);
return errorResponse;
}
The very first instruction replaces the exception with it's inner exception. I'm not sure what the the thinking was with that. It seems counter intuitive to me and so I just re-implemented the method in my AppHost class, removing that first if statement block:
public override void Configure(Container container)
{
ServiceExceptionHandler += (request, exception) => HandleException(this, request, exception);
}
/// <remarks>
/// Verbatim implementation of DtoUtils.HandleException, without the innerexception replacement.
/// </remarks>
public static object HandleException(IResolver iocResolver, object request, Exception ex)
{
var responseStatus = ex.ToResponseStatus();
if (EndpointHost.DebugMode)
{
// View stack trace in tests and on the client
responseStatus.StackTrace = DtoUtils.GetRequestErrorBody(request) + ex;
}
var log = LogManager.GetLogger(typeof(DtoUtils));
log.Error("ServiceBase<TRequest>::Service Exception", ex);
if (iocResolver != null)
DtoUtils.LogErrorInRedisIfExists(iocResolver.TryResolve<IRedisClientsManager>(), request.GetType().Name, responseStatus);
var errorResponse = DtoUtils.CreateErrorResponse(request, ex, responseStatus);
return errorResponse;
}
This is obviously not ideal, since I had to copy a bunch of code that is totally unrelated to the problem that I had with the original implementation. It makes me feel like I have to maintain this method whenever I update ServiceStack. I would love to here of a better way to accomplish this.
Anyway, I have the exception handling that I like in my client code:
catch (WebServiceException ex)
{
if (ex.ErrorCode == typeof (SomeKindOfException).Name)
{
// do something useful here
}
else throw;
}
It doesn't seem like you'll have to maintain a bunch of code. You're writing one method to implement your own error handling. You could try calling DtoUtils.HandleException(this, request, exception) in your own method and modify the HttpError object returned. Not sure you have access to change all properties/values you're looking for.
public static object HandleException(IResolver iocResolver, object request, Exception ex)
{
HttpError err = (HttpError)DtoUtils.HandleException(this, request, ex);
err.Reponse = ex.InnerException;
}
Using Play Framework 2.1 with OpenID, if I cancel my authentication from the OpenID Provider, I get this exception :
[RuntimeException: play.api.libs.openid.Errors$AUTH_CANCEL$]
Here's my code :
Promise<UserInfo> userInfoPromise = OpenID.verifiedId();
UserInfo userInfo = userInfoPromise.get(); // Exception thrown here
But since it's a Runtime exception, I can't catch it with a try/catch so I'm stuck on how to avoid exception and returns something nicer than a server error to the client.
How can I do that?
A Promise is success biased, for all its operations, it assumes it actually contains a value and not an error.
You get the exception because you try to call get on a promise which contains an untransformed error.
What you want is to determine if the Promise is a success or an error, you can do that with pattern matching for instance.
try this code:
AsyncResult(
OpenID.verifiedId.extend1( _ match {
case Redeemed(info) => Ok(info.attributes.get("email").getOrElse("no email in valid response"))
case Thrown(throwable) => {
Logger.error("openid callback error",throwable)
Unauthorized
}
}
)
)
You may want to read more on future and promises, I recommend this excellent article :
http://danielwestheide.com/blog/2013/01/09/the-neophytes-guide-to-scala-part-8-welcome-to-the-future.html
edit :
checking the documentation (http://www.playframework.com/documentation/2.1.0/JavaOpenID) in java it seems you are supposed to catch and handle exceptions yourself.
In any case, you should catch exceptions and if one is thrown redirect
back the user to the login page with relevant information.
something like this should work :
public class Application extends Controller {
public static Result index() {
return ok("welcome");
}
public static Result auth() {
Map<String, String> attributes = new HashMap<String, String>();
attributes.put("email", "http://schema.openid.net/contact/email");
final Promise<String> stringPromise = OpenID.redirectURL("https://www.google.com/accounts/o8/id", "http://localhost:9000/auth/callback",attributes);
return redirect(stringPromise.get());
}
public static Result callback() {
try{
Promise<UserInfo> userInfoPromise = OpenID.verifiedId();
final UserInfo userInfo = userInfoPromise.get();
System.out.println("id:"+userInfo.id);
System.out.println("email:"+userInfo.attributes.get("email"));
return ok(userInfo.attributes.toString());
} catch (Throwable e) {
System.out.println(e.getMessage());
e.printStackTrace();
return unauthorized();
}
}
}
I spend most of my time in C# and am trying to figure out which is the best practice for handling an exception and cleanly return an error message from a called method back to the calling method.
For example, here is some ActiveDirectory authentication code. Please imagine this Method as part of a Class (and not just a standalone function.)
bool IsUserAuthenticated(string domain, string user, string pass, out errStr)
{
bool authentic = false;
try
{
// Instantiate Directory Entry object
DirectoryEntry entry = new DirectoryEntry("LDAP://" + domain, user, pass);
// Force connection over network to authenticate
object nativeObject = entry.NativeObject;
// No exception thrown? We must be good, then.
authentic = true;
}
catch (Exception e) { errStr = e.Message().ToString(); }
return authentic;
}
The advantages of doing it this way are a clear YES or NO that you can embed right in your If-Then-Else statement. The downside is that it also requires the person using the method to supply a string to get the Error back (if any.)
I guess I could overload this method with the same parameters minus the "out errStr", but ignoring the error seems like a bad idea since there can be many reasons for such a failure...
Alternatively, I could write a method that returns an Error String (instead of using "out errStr") in which a returned empty string means that the user authenticated fine.
string AuthenticateUser(string domain, string user, string pass)
{
string errStr = "";
try
{
// Instantiate Directory Entry object
DirectoryEntry entry = new DirectoryEntry("LDAP://" + domain, user, pass);
// Force connection over network to authenticate
object nativeObject = entry.NativeObject;
}
catch (Exception e) { errStr = e.Message().ToString(); }
return errStr;
}
But this seems like a "weak" way of doing things.
Or should I just make my method "void" and just not handle the exception so that it gets passed back to the calling function?
void AuthenticateUser(string domain, string user, string pass)
{
// Instantiate Directory Entry object
DirectoryEntry entry = new DirectoryEntry("LDAP://" + domain, user, pass);
// Force connection over network to authenticate
object nativeObject = entry.NativeObject;
}
This seems the most sane to me (for some reason). Yet at the same time, the only real advantage of wrapping those 2 lines over just typing those 2 lines everywhere I need to authenticate is that I don't need to include the "LDAP://" string. The downside with this way of doing it is that the user has to put this method in a try-catch block.
Thoughts?
Is there another way of doing this that I'm not thinking of?
There is no "one size fits all". If you return a flag, that makes it easy to use a method in if() and loops. Exceptions always need a lot of boiler plate code. If you just want a string which you can display to the user (say, in a web UI), returning the error string (or null for "no error") is good, too.
But most of the time, I throw an exception (and in Java a subclass of RuntimeException) because that allows me to return more than a single information about the error (like: Which file caused the error? Which line/column? What was I doing? Which field in a form should be marked as illegal? etc).
In your case, you can't handle the exception in your method, so you shouldn't catch it. Only catch it when you can do something about it.
In this example, I agree, you should let the exception flow through to the consumer. However, as an alternative to the approaches you highlighted, consider this approach.
You can use a response object to hold information coming out of a method run, for example:
public abstract class BaseResponse
{
public bool IsOk { get; protected set;}
public string Message { get; protected set; }
}
public class AuthenticationResponse: BaseResponse
{
public AuthenticationResponse(bool isOk): this(isOk, "") {}
public AuthenticationResponse(bool isOk, string message)
{
IsOk = isOk;
Message = message;
}
}
AuthenticationResponse IsUserAuthenticated(string domain, string user, string pass)
{
bool authentic = false;
string errStr;
try
{
// Instantiate Directory Entry object
DirectoryEntry entry = new DirectoryEntry("LDAP://" + domain, user, pass);
// Force connection over network to authenticate
object nativeObject = entry.NativeObject;
// No exception thrown? We must be good, then.
authentic = true;
}
catch (Exception e) { errStr = e.Message().ToString(); }
return new AuthenticationResponse(authentic, errStr);
}
Then to use it in your if statements:
AuthenticationResponse response;
if((response = IsUserAuthenticated("domain", "user", "pass")).IsOk)
{
// do successful activity
} else {
Console.WriteLine(response.Message)
}
The trick is the return value of an assignment operation is the value that was assigned. So, we can do the assignment and the valid check in the same line. If you didn't need to hold onto the result of the call, you could simply call the method and check the IsOk property.
if(IsUserAuthenticated("domain", "user", "pass").IsOk)
{
// do successful activity
}
Then you can build up your custom response object to return any combination of values from your method as you need.
Don't handle the exception or return a message of any kind. Let the consumer of your method take care of this.