ErrorHandler
Error handling library for Android and Java
Encapsulate error handling logic into objects that adhere to configurable defaults. Then pass them around as parameters or inject them via DI.
Download
Download the latest JAR or grab via Maven:
<dependency>
<groupId>com.workable</groupId>
<artifactId>error-handler</artifactId>
<version>1.1.0</version>
<type>pom</type> </dependency>or Gradle:
compile 'com.workable:error-handler:1.1.0'Usage
Let's say we're building a messaging Android app that uses both the network and a local database.
We need to:
Setup a default ErrorHandler once
- Configure the default ErrorHandler
- Alias errors to codes that are easier to use like Integer, String and Enum values
- Map errors to actions to take when those errors occur (exceptions thrown)
// somewhere inside MessagingApp.java ErrorHandler
.defaultErrorHandler()
// Bind certain exceptions to "offline"
.bind("offline", errorCode -> throwable -> {
return throwable instanceof UnknownHostException || throwable instanceof ConnectException;
}
)
// Bind HTTP 404 status to 404
.bind(404, errorCode -> throwable -> {
return throwable instanceof HttpException && ((HttpException) throwable).code() == 404;
}
)
// Bind HTTP 500 status to 500
.bind(500, errorCode -> throwable -> {
return throwable instanceof HttpException && ((HttpException) throwable).code() == 500;
}
)
// Bind all DB errors to a custom enumeration
.bindClass(DBError.class, errorCode -> throwable -> {
return DBError.from(throwable) == errorCode;
}
)
// Handle HTTP 500 errors
.on(500, (throwable, errorHandler) -> {
displayAlert("Kaboom!");
}
)
// Handle HTTP 404 errors
.on(404, (throwable, errorHandler) -> {
displayAlert("Not found!");
}
)
// Handle "offline" errors
.on("offline", (throwable, errorHandler) -> {
displayAlert("Network dead!");
}
)
// Handle unknown errors
.otherwise((throwable, errorHandler) -> {
displayAlert("Oooops?!");
}
)
// Always log to a crash/error reporting service
.always((throwable, errorHandler) -> {
Logger.log(throwable);
}
);
Use ErrorHandler inside catch blocks
// ErrorHandler instances created using ErrorHandler#create(), delegate to the default ErrorHandler // So it's actually a "handle the error using only defaults" // i.e. somewhere inside MessageListActivity.java try {
fetchNewMessages();
}
catch (Exception ex) {
ErrorHandler.create().handle(ex);
}
Run blocks of code using ErrorHandler.run
ErrorHandler.run(() -> fetchNewMessages());
Override defaults when needed
// Configure a new ErrorHandler instance that delegates to the default one, for a specific method call // i.e. somewhere inside MessageListActivity.java try {
fetchNewMessages();
}
catch (Exception ex) {
ErrorHandler
.create()
.on(StaleDataException.class, (throwable, errorHandler) -> {
reloadList();
errorHandler.skipDefaults();
}
)
.on(404, (throwable, errorHandler) -> {
// We handle 404 specifically on this screen by overriding the default action
displayAlert("Could not load new messages");
errorHandler.skipDefaults();
}
)
.on(DBError.READ_ONLY, (throwable, errorHandler) -> {
// We could not open our database to write the new messages
ScheduledJob.saveMessages(someMessages).execute();
// We also don't want to log this error because ...
errorHandler.skipAlways();
}
)
.handle(ex);
}
Things to know
ErrorHandler is thread-safe.
API
Initialize
-
defaultErrorHandler()Get the default ErrorHandler. -
create()Create a new ErrorHandler that is linked to the default one. -
createIsolated()Create a new empty ErrorHandler that is not linked to the default one.
Configure
-
on(Matcher, Action)Register an Action to be executed if Matcher matches the error. -
on(Class<? extends Exception>, Action)Register an Action to be executed if error is an instance ofException. -
on(T, Action)Register an Action to be executed if error is bound to T, throughbind()orbindClass(). -
otherwise(Action)Register an Action to be executed only if no other Action gets executed. -
always(Action)Register an Action to be executed always and after all other actions. Works like afinallyclause. -
skipFollowing()Skip the execution of any subsequent Actions except those registered viaalways(). -
skipAlways()Skip all Actions registered viaalways(). -
skipDefaults()Skip any default actions. Meaning any actions registered on thedefaultErrorHandlerinstance. -
bind(T, MatcherFactory<T>)Bind instances of T to match errors through a matcher provided by MatcherFactory. -
bindClass(Class<T>, MatcherFactory<T>)Bind class T to match errors through a matcher provided by MatcherFactory. -
clear()Clear all registered Actions.
Execute
handle(Throwable)Handle the given error.
About
When designing for errors, we usually need to:
- have a default handler for every expected error // i.e. network, subscription errors
- handle specific errors as appropriate based on where and when they occur // i.e. network error while uploading a file, invalid login
- have a catch-all handler for unknown errors // i.e. system libraries runtime errors we don't anticipate
- keep our code DRY
Java, as a language, provides you with a way to do the above. By mapping cross-cutting errors to runtime exceptions and catching them lower in the call stack, while having specific expected errors mapped to checked exceptions and handle them near where the error occurred. Still, countless are the projects where this simple strategy has gone astray with lots of errors being either swallowed or left for the catch-all Thread.UncaughtExceptionHandler. Moreover, it usually comes with significant boilerplate code. ErrorHandler however eases this practice through its fluent API, error aliases and defaults mechanism.
This library doesn't try to solve Java specific problems, although it does help with the log and shallow anti-pattern as it provides an opinionated and straightforward way to act inside every catch block. It was created for the needs of an Android app and proved itself useful very quickly. So it may work for you as well. If you like the concept and you're developing in Swift or Javascript, we're baking 'em and will be available really soon.
License
The MIT License Copyright (c) 2013-2016 Workable SA Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.