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09.

Throwing an Exception (and a Party)

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Let's talk about something totally different: a powerful part of object-oriented code called exceptions.

In index.php, we create a BrokenShip object. I'm going to do something crazy, guys. I'm going to say, $brokenShip->setStrength() and pass it... banana:

146 lines | index.php
// ... lines 1 - 4
use Model\BrokenShip;
// ... lines 6 - 13
$brokenShip = new BrokenShip('I am so broken');
$brokenShip->setStrength('banana');
// ... lines 16 - 146

That strength makes no sense. And if we try to battle using this ship, we should get some sort of error. But when we refresh... well, it is an error: but not exactly what I expected.

This error is coming from AbstractShip line 65. Open that up. I want you to look at 2 exceptional things here:

125 lines | lib/Model/AbstractShip.php
// ... lines 1 - 4
abstract class AbstractShip
{
// ... lines 7 - 44
public function setStrength($number)
{
if (!is_numeric($number)) {
throw new Exception('Invalid strength passed '.$number);
}
$this->strength = $number;
}
// ... lines 53 - 123
}

First, we planned ahead. When we created the setStrength() method, we said:

You know what? This needs to be a number, so if somebody passes something dumb like "banana," then let's check for that and trigger an error.

And second, in order to trigger an error, we threw an exception:

125 lines | lib/Model/AbstractShip.php
// ... lines 1 - 4
abstract class AbstractShip
{
// ... lines 7 - 44
public function setStrength($number)
{
if (!is_numeric($number)) {
throw new Exception('Invalid strength passed '.$number);
}
// ... lines 50 - 51
}
// ... lines 53 - 123
}

And that's actually what I want to talk about: Exceptions are classes, but they're completely special.

But first, Exception is a core PHP class, and when we added a namespace to this file, we forgot to change it to \Exception:

125 lines | lib/Model/AbstractShip.php
// ... lines 1 - 4
abstract class AbstractShip
{
// ... lines 7 - 44
public function setStrength($number)
{
if (!is_numeric($number)) {
throw new \Exception('Invalid strength passed '.$number);
}
// ... lines 50 - 51
}
// ... lines 53 - 123
}

That's better. Now refresh again. This is a much better error:

Uncaught Exception: Invalid strength passed "banana"

When things go Wrong: Throw an Exception

When things go wrong, we throw exceptions. Why? Well, first: it stops execution of the page and immediately shows us a nice error.

Tip

If you install the XDebug extension, exception messages are more helpful, prettier and will fix your code for you (ok, that last part is a lie).

Catching Exceptions: Much Better than Catching a Cold

Second, exceptions are catchable. Here's what that means.

Suppose that I wanted to kill the page right here with an error. I actually have two options: I can throw an exception, or I could print some error message and use a die statement to stop execution.

But when you use a die statement, your script is truly done: none of your other code executes. But with an exception, you can actually try to recover and keep going!

Let's look at how. Open up PdoShipStorage. Inside fetchAllShipsData(), change the table name to fooooo:

35 lines | lib/Service/PdoShipStorage.php
// ... lines 1 - 4
class PdoShipStorage implements ShipStorageInterface
{
// ... lines 7 - 13
public function fetchAllShipsData()
{
$statement = $this->pdo->prepare('SELECT * FROM FOOOOO');
// ... lines 17 - 19
}
// ... lines 21 - 33
}

That clearly will not work. This method is called by ShipLoader, inside getShips():

67 lines | lib/Service/ShipLoader.php
// ... lines 1 - 8
class ShipLoader
{
// ... lines 11 - 20
public function getShips()
{
// ... lines 23 - 24
$shipsData = $this->queryForShips();
// ... lines 26 - 31
}
// ... lines 33 - 60
private function queryForShips()
{
return $this->shipStorage->fetchAllShipsData();
}
}

When we try to run this, we get an exception:

Base table or view not found

The error is coming from PdoShipStorage on line 18, but we can also see the line that called this: ShipLoader line 23.

Now, what if we knew that sometimes, for some reason, an exception like this might be thrown when we call fetchAllShipsData(). And when that happens, we don't want to kill the page or show an error. Instead, we want to - temporarily - render the page with zero ships.

How can we do this? First, surround the line - or lines - that might fail with a try-catch block. In the catch, add \Exception $e:

72 lines | lib/Service/ShipLoader.php
// ... lines 1 - 8
class ShipLoader
{
// ... lines 11 - 60
private function queryForShips()
{
try {
return $this->shipStorage->fetchAllShipsData();
} catch (\Exception $e) {
// ... lines 66 - 67
}
}
}

Now, if the fetchAllShipsData() method throws an exception, the page will not die. Instead, the code inside catch will be called and then execution will keep going like normal:

72 lines | lib/Service/ShipLoader.php
// ... lines 1 - 8
class ShipLoader
{
// ... lines 11 - 60
private function queryForShips()
{
try {
return $this->shipStorage->fetchAllShipsData();
} catch (\Exception $e) {
// if all else fails, just return an empty array
return [];
}
}
}

That means, we can say $shipData = array().

Using the Exception Object

And just like that, the page works. That's the power of exceptions. When you throw an exception, any code that calls your code has the opportunity to catch the exception and say:

No no no, I don't want the page to die. Instead, let's do something else.

Of course, we probably also don't want this to fail silently without us knowing, so you might trigger an error and print the message for our logs. Notice, in catch, we have access to the Exception object, and every exception has a getMessage() method on it. Use that to trigger an error to our logs:

73 lines | lib/Service/ShipLoader.php
// ... lines 1 - 8
class ShipLoader
{
// ... lines 11 - 60
private function queryForShips()
{
try {
return $this->shipStorage->fetchAllShipsData();
} catch (\Exception $e) {
trigger_error('Exception! '.$e->getMessage());
// if all else fails, just return an empty array
return [];
}
}
}

Ok, refresh! Right now, we see the error on top of the page. But that's just because of our error_reporting settings in php.ini. On production, this wouldn't display, but would write a line to our logs.