Is this code:
Completable.create(emitter -> {
myDao().insert(data); // can throw exceptions
emitter.onComplete();
});
The same as this code:
Completable.create(emitter -> {
try {
myDao().insert(data); // can throw exceptions
emitter.onComplete();
} catch (Exception e) {
emitter.onError(e);
}
});
In other words: Do I have to surround my code with try/catch in order to propagate errors or will it be done automatically for me?
Related
I keep getting the following error message in VS Code when I try to run this dart package:
**The method 'errorMessage' isn't defined for the type 'Object'.
Try correcting the name to the name of an existing method, or defining a method named 'errorMessage'.
**
Anytime I write this code:
void main(){
try {
depositMoney(-200);
} catch (e) {
print(e.errorMessage());
}
}
class DepositException implements Exception {
String errorMessage() {
print("You cannot enter amount less than 0! ");
return errorMessage();
}
}
void depositMoney(int amount) {
if (amount < 0) {
throw new DepositException();
}
}
What should I do??
Change your try catch block like this,
try {
depositMoney(-200);
} on DepositException catch (e) {
print(e.errorMessage());
}
This is because, DepositException is a custom exception class.
If you want catch all errors, you can do it this way,
try {
depositMoney(-200);
} catch (e) {
if(e is DepositException){
print(e.errorMessage());
}else{
print(e);
}
}
My test method looks like this:
private static System.Timers.Timer _myTimer;
static void Main(string[] args)
{
using (_myTimer = new System.Timers.Timer(1000))
{
_myTimer.Elapsed += (o, e) => Console.WriteLine($"timer elapsed");
_myTimer.AutoReset = true;
_myTimer.Enabled = true;
Thread.Sleep(4000); // let the timer fire a couple of times
} // dispose timer?
Thread.Sleep(2000); // timer won't fire here
try
{
Console.WriteLine($"no problem accessing _myTimer: {_myTimer.Interval}"); // this won't throw an ObjectDisposedException on _myTimer
}
catch (Exception ex)
{
Console.WriteLine($"Exception: {ex}");
}
GC.Collect();
GC.WaitForPendingFinalizers();
GC.Collect();
try
{
Console.WriteLine($"no problem accessing _myTimer: {_myTimer.Interval}"); // still no ObjectDisposedException
}
catch (Exception ex)
{
Console.WriteLine($"Exception: {ex}");
}
try
{
//_myTimer.Start(); // throws the ObjectDisposedException
_myTimer.Dispose(); // does not throw the ObjectDisposedException
}
catch (Exception ex)
{
Console.WriteLine($"Exception: {ex}");
}
GC.Collect();
GC.WaitForPendingFinalizers();
GC.Collect();
try
{
Console.WriteLine($"no problem accessing _myTimer: {_myTimer.Interval}"); // still no ObjectDisposedException
}
catch (Exception ex)
{
Console.WriteLine($"Exception: {ex}");
}
Console.WriteLine("Press any key to continue...");
Console.ReadLine();
}
I would expect to get the ObjectDisposedException after leaving the using block.
Accessing the _myTimer.Interval works all the way to the end of the program. Also, I can call _myTimer.Dispose() anytime. Even waiting for the GarbageCollector does not help to get the ObjectDisposedException.
However, I do get the ObjectDisposedException if I call _myTimer.Start() after leaving the using block.
How can _myTimer be around for the entire lifetime of my program?
Calling Dispose doesn't remove the object or references to it. It will not be GCed as long as there are references to it. Dispose releases unmanaged resources within the object, which is likely but by no means guaranteed to cause at least some of its methods to stop working and start throwing ObjectDisposedException.
I'm using WebSocket variable in my MyWebSocket class. For listen i give a callback function as parameter.
If this callback function throws an exception in calling class MyChat, then i can not catch that exception anywhere.
My simplified code is:
class MyWebSocket {
WebSocket _ws;
...
// initialized in controller: _ws = WebSocket.connect(_some_url_);
// everything works (connect, listen)
...
void listen({void Function(dynamic) myOnListen}) {
try {
_ws.listen(myOnListen)
.onError((e) => print("MyWebSocket in MyChat.onError: $e"));
} catch (e) {
print("Catched in MyWebSocket: $e");
}
}
}
class MyChat {
MyWebSocket _chatWs = MyWebSocket();
...
void initWS() {
try {
_chatWs.listen(myOnListen: processMsg);
} catch (e) {
print("Catched in MyChat: $e");
}
}
void processMsg(dynamic msg) {
if(_some_stuff_like_invalid_msg_or_not_logged_in_...)
throw Exception("There is my Exception");
}
}
I have built try-catch in every possible place to catch exceptions - no success, i got only unhandled exception:
E/flutter: [ERROR:flutter/lib/ui/ui_dart_state.cc(148)] Unhandled Exception: Exception: There is my Exception
E/flutter: #0 MyChat.processMsg
Beware that you cannot use the passed listener as a key for later removal. For that you can pass the new listener created in MyWebSocket class when listen() is called, then use this key to remove the listener.
class MyWebSocket {
WebSocket _ws;
void listen({void Function(dynamic) myOnListen, void Function(Error) onError}) {
try {
_ws.listen((){
try {
myOnListen({"label": "DATA"});
} catch (e) {
if(onError is Function)
onError(e)
}
})
.onError(onError);
} catch (e) {
print("Catched in MyWebSocket: $e");
}
}
}
class MyChat {
MyWebSocket _chatWs = MyWebSocket();
void initWS() {
try {
_chatWs.listen(myOnListen: processMsg, onError: (Error error){
print("ERROR: "+error.toString());
});
} catch (e) {
print("Catched in MyChat: $e");
}
}
void processMsg(dynamic msg) {
if(_some_stuff_like_invalid_msg_or_not_logged_in_...)
throw Exception("There is my Exception");
}
}
You need to handle it inside your processMsg
If you analyze carefully the execution of your code the _ws.listent register a listener for when you receive a message and that will happen in the FUTURE, but you don't get an error WHILE you are REGISTERING your listener that's why that doesn't work the way you expect.
processMsg will do something in the future, it will throw an error and it's at the end of the pipeline.
You are simulating an exception and nothing is handling it, it happens in a different stack frame, in the future.
The void listen({void Function(dynamic) myOnListen}) function execution is long gone when you receive and you precessMsg.
Alternatively you could do:
Function tryCatchHOF(Function cb) {
decorated(dynamic param) {
try {
cb(param);
} catch (e) {
print("$e");
}
}
;
return decorated;
}
void processMsg(dynamic msg) {
if (true) throw Exception("There is my Exception");
}
var processMsgWithTryCatch = tryCatchHOF(processMsg);
processMsgWithTryCatch('');
and then pass processMsgWithTryCatch down to your listener if you don't want to handle inside processMsg
I hope that make sense
I used ListeningExecutorService of guava in my project, got confused about the exception handling.
I used a thread pool, submit a task to it, and set a timeout to the listenableFuture, and add a callback to it.
final ListeningExecutorService threadPool = MoreExecutors.listeningDecorator(Executors.newCachedThreadPool());
Futures.addCallback(listenableFuture, new FutureCallback<**>() {
#Override
public void onSuccess(#Nullable ** data) {
xxxxxxx;
}
public void onFailure(Throwable t) {
xxxxxxxxxx;
if (t instanceof CancellationException) {
throw new QueryException("yyyy");
} else {
throw new QueryException("zzzzz");
}
}
});
I can't catch the exception inside the callback. So I use another ListenableFuture to get the exception
ListenableFuture allFutures = Futures.allAsList(allFuturesList);
try {
allFutures.get();
} catch (CancellationException ce) {
throw new QueryException("");
} catch (InterruptedException t) {
throw new QueryException("");
} catch (ExecutionException e) {
Throwable t = e.getCause();
if (t instanceof QueryException)
throw (QueryException) t;
else
throw new QueryException();
} catch (QueryException qe) {
throw qe;
} catch (Exception e) {
throw new QueryException();
} finally {
}
But I when the callback throw a QueryException, the allFutures can't catch it, allFutures can only catch a CancellationException without the detail error message.
How can I get my detail error message?
Futures.allAsList doesn't do what you expect it to do
From the Javadoc: (emphasis is from me)
Canceling this future will attempt to cancel all the component futures, and if any of the provided futures fails or is canceled, this one is, too.
What you should probably do is create your own aggregating future. You can base your code on Guava's own internal mechanism. See the source code for more info.
Anyways, do not throw exceptions in your FutureCallback::onFailure method.
I have a snippet to download a file from jersey 2.22,
StreamingOutput stream = (OutputStream os) -> {
try {
os.write( get a byte array here);
} catch (Exception e) {
LOGGER.error("Failed to fetch file, id:" + fileName, e);
throw new Exception());
}
};
the above code is working, but i'm wondering if the output stream I'm writing in needs to close.
I searched jersey's source code, in OutboundMessageContext
public void close() {
if(this.hasEntity()) {
try {
OutputStream e = this.getEntityStream();
e.flush();
e.close();
} catch (IOException var10) {
Logger.getLogger(OutboundMessageContext.class.getName()).log(Level.FINE, var10.getMessage(), var10);
} finally {
if(!this.committingOutputStream.isClosed()) {
try {
this.committingOutputStream.close();
} catch (IOException var9) {
Logger.getLogger(OutboundMessageContext.class.getName()).log(Level.FINE, var9.getMessage(), var9);
}
}
}
}
}
this method is to close entity stream. I'm not sure if this is the right close method working with StreamingOutput, can anyone help?