Error handling library for Android and Java

Download the latest JAR or grab via Maven:

<dependency>
  <groupId>com.workable</groupId>
  <artifactId>error-handler</artifactId>
  <version>0.9</version>
  <type>pom</type>
</dependency>

or Gradle:

compile 'com.workable:error-handler:0.9'

Let's say we're building a messaging Android app that uses both the network and a local database.

// Configure the default ErrorHandler to define your global/default error handling logic
// i.e. somewhere inside MessagingApp.java

ErrorHandler
  .defaultErrorHandler()
  // bind an error matcher to a code
  .bindErrorCode(400, errorCode -> throwable -> {
      return ((HttpException) throwable).code() == 400;
  })
  .bindErrorCode(404, errorCode -> throwable -> {
      return ((HttpException) throwable).code() == 404;
  })
  .bindErrorCode(500, errorCode -> throwable -> {
      return ((HttpException) throwable).code() == 500;
  })
  .bindErrorCodeClass(DBError.class, errorCode -> throwable -> {
      return DBError.from(throwable) == DBError.READ_ONLY;
  })
  .on(500, (throwable, errorHandler) -> {
    //Handle HTTP 500 errors
    Toast.makeText(context, "Operation not available!", Toast.LENGTH_SHORT).show();
  })
  .on(404, (throwable, errorHandler) -> {
    //Handle 404 in general  
    Toast.makeText(context, "Failed to find the requested resource!", Toast.LENGTH_SHORT).show();
  })
  .otherwise((throwable, errorHandler) -> {
    //Handle unknown errors
    Toast.makeText(context, "Something went wrong. We are fixing it ASAP!", Toast.LENGTH_LONG).show();
  })
  .always((throwable, errorHandler) -> {
    Crashlytics.log(throwable);
  });

// 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(404, (throwable, errorHandler) -> {
        //We handle 404 specifically on this screen and we override the default action
        Toast.makeText(context, "Failed to find some messages!", Toast.LENGTH_SHORT).show();
        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 we expected it and knew how to handle it
        errorHandler.skipAlways();
    })
    .handle(ex);
}

ErrorHandler is thread-safe

Create a new Instance of ErrorHandler with no default ErrorHandler.

createIsolated()

Create a new Instance of ErrorHandler that will use the default ErrorHandler (if set).

create()

Get the default ErrorHandler instance. If there is not one available, it creates a new Instance.

defaultErrorHandler()

Action will be executed if a given Throwable matches with Matcher.

on(Matcher, Action)

Action will be executed if a given Throwable is an instance of <? extends Exception>.

on(Class<? extends Exception>, Action)

Action will be executed if a given Throwable is bound to T, through bindErrorCode().

on(T, Action)

Action will be executed if no previous Action has been executed.

otherwise(Action)

Action will be executed for every given Throwable.

always(Action)

Instruct the current ErrorHandler instance to skip the execution of subsequent registered Actions, through on().

skipFollowing()

Instruct the current ErrorHandler instance to skip the execution of subsequent registered Actions through always().

skipAlways()

Instruct the current ErrorHandler instance to skip the execution of registered Actions on the defaultErrorHandler instance.

skipDefaults()

The Throwable that we should act upon.

handle(Throwable)

Bind a MatcherFactory to T.

bindErrorCode(T, MatcherFactory)

Bind a MatcherFactory to Class.

bindErrorCodeClass(Class<T>, MatcherFactory)

Clear current ErrorHandler instance from all Actions and Matchers.

clear()

A common problem in software, specially in UI software is that of error handling.

One way to classify errors can be by how they relate to the problem domain. Some errors, like network or database errors are orthogonal to the problem domain and designate truly exceptional conditions, while others are core parts of the domain like validation and authentication errors.

Another way to classify errors is by their scope. Are they common throughout the application or specific to a single screen, object or even method? Think of UnauthorizedException versus InvalidPasswordException.

And let's not forget another very simple distinction between errors. Those that are known at authoring time and thus expected (despite of how probable is that they occur), and those that are unknown until runtime.

With that in mind, we usually want to:

1. have a default handler for every expected (exceptional, common or not) error
2. handle specific errors as appropriate based on where and when they occur
3. have a default catch-all handler for unknown errors
4. override any default handler if needed
5. keep our code DRY

Java, as a language, provides you with a way to do the above. By mapping exceptional or very common 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.

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.