Exception handling techniques

Status
Draft

Exceptions Overview
Exceptions are occurrences that alter the normal program flow. They can have various causes, such as hardware failures, resource exhaustion, programming bugs etc. When an exceptional event occurs, an exception is said to be "thrown." The code responsible for treating the exception is called an "exception handler," and it "catches" the thrown exception. Exception handling refers to passing the execution of a program to an appropriate exception handler when an exception occurs. For example, if a method that opens a file is called, and the file cannot be opened, the execution of that method will stop, and the code from the appropriate exception handler will be run.

In Java, exception-handling code is cleanly separated from the exception-generating code. The try block (also called "guarded region") is the code in which exceptions may occur, and it must be immediately followed by either a finally block or at least one catch block. In Java 7, there is a new syntactic construction, called "try-with-resources", which is the only exception to this rule, as it is not forced to be followed by a finally or catch block.

A finally block contains code that is always executed at some point after the try block, whether an exception was thrown or not. Even if there is a return statement in the try block, the finally block executes right after the return statement is encountered, and before the return executes. If the try block executes without exception, the finally block is executed immediately after the try block completes. If an exception was thrown, the finally block executes immediately after the corresponding catch block completes. However, if the JVM exits while the try or catch code is being executed, then the finally block may not execute. Likewise, if the thread executing the try or catch code is interrupted or killed, the finally block may not execute even though the application keeps running.

Swallowing Exceptions
Swallowing exceptions means to continue processing as if nothing had gone wrong even though an exception has been thrown. You catch the exception, but do nothing meaningful with it. The classical example is an empty catch block:

catch(Exception e) { }

Swallowing exceptions is considered bad practice, because the ignored exception may lead the application to an unexpected failure, at a point in the code that bears no apparent relation to the source of the problem. Finding the cause of the failure in such cases can be very challenging and time-consuming. Merely letting an exception propagate outward can at least cause the program to fail swiftly, preserving information to aid in debugging the failure.

In addition, in case an exception is caught, it is strongly recommended to log information about it, as it is very useful for debugging purposes.

Resource cleanup and finally block
One of the situations when exceptions may occur is when we work with resources, such as files, databases, network sockets etc.

Opening/closing, connecting/disconnecting, read/write statements are examples of operations that may throw exceptions in particular cases. If our code needs to work with resources, it is important to release them as soon as we are finished using them.

The following example is about writing a text in a file. The constructor, the write, the flush of the file operation can throw IOException, hence they are being placed inside the try block. We are aware that we must close the file after writing in it. A naive (and wrong) approach would be the following:

File file = new File("file.txt"); Writer writer = null; try { writer = new PrintWriter(file); writer.write("Hello world!"); writer.flush; writer.close; }   catch(FileNotFoundException e) { System.err.println("Caught FileNotFoundException: " + e.getMessage); }   catch (IOException e) { System.err.println("Caught IOException: " + e.getMessage); }

As you can see in the example, we close a resource inside the try block. However this is not a good practice, because if an exception is thrown before our close statement is reached, then it will not be executed and resources might thus leak. We could add the close statement in the catch block, which leads to code duplication. If we have several catch blocks, this means we have to add the close statement there. Whenever we need to add a new catch block, we must also remember to put the close statement there. This way of handling exceptions is error-prone and unnecessarily complicated.

So where should we place the close statement? We would need a block that always gets executed, no matter if we have exceptions or not, like... finally.

Indeed, the ideal place to release resources is the finally block. As we have seen before, finally always gets executed. This means the closing of resources will get executed even if exceptions occur, and in addition we do not have code duplication, which is mainly what we want. This approach is the best way to correctly handle the resources our code used.

To fix the above example, a finally block was added and the file was closed there, as shown in the code below.

File file = new File("file.txt"); Writer writer = null; try { writer = new PrintWriter(file); writer.write("Hello world!"); writer.flush; }   catch(FileNotFoundException e) { System.err.println("Caught FileNotFoundException: " + e.getMessage); }   catch (IOException e) { System.err.println("Caught IOException: " + e.getMessage); } finally { if (writer != null) try { writer.close; } catch (IOException e) { System.err.println("Caught IOException: " + e.getMessage); }   }

Try-with-resources
In Java 7, resource cleanup was automated by means of the try-with-resources block. In practice, this new syntax allows you to declare resources that are part of the try block. You define the resources ahead of time and the runtime automatically closes those resources (if they are not already closed) after the execution of the try block.

A resource can be any object that implements the interface java.lang.AutoCloseable.

Although the try-with-resources block can have a finally or catch block, it is not mandatory.

In our example, since Writer implements AutoCloseable, we can place it in a try-with-resources block as follows:

File file = new File("file.txt"); try(Writer writer = new PrintWriter(file)) { writer.write("Hello world!"); writer.flush; }

The code looks far more readable this way, and the developer is no longer supposed to close the resources, it is automatically done as long as the classes implement the AutoCloseable interface. Also note that the function that contains this code must throw IOException in order to compile, since there are no catch blocks.