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Detected issues

Issue Method Line number
Use of static methods int 48


Click highlighted lines for details

<?phpnamespace GuzzleHttp;use GuzzleHttp\Promise\PromiseInterface;use Psr\Http\Message\RequestInterface;use Psr\Http\Message\ResponseInterface;/** * Middleware that retries requests based on the boolean result of * invoking the provided "decider" function. */class RetryMiddleware{    /** @var callable */    private $nextHandler;    /** @var callable */    private $decider;    /** @var callable */    private $delay;    /**     * @param callable $decider     Function that accepts the number of retries,     *                              a request, [response], and [exception] and     *                              returns true if the request is to be     *                              retried.     * @param callable $nextHandler Next handler to invoke.     * @param callable $delay       Function that accepts the number of retries     *                              and [response] and returns the number of     *                              milliseconds to delay.     */    public function __construct(        callable $decider,        callable $nextHandler,        callable $delay = null    ) {        $this->decider = $decider;        $this->nextHandler = $nextHandler;        $this->delay = $delay ?: __CLASS__ . '::exponentialDelay';    }    /**     * Default exponential backoff delay function.     *     * @return int milliseconds.     */

Static methods

Summary of issues

  • Tight Coupling
  • Hidden dependencies
  • Global state (if also using static variables)

Tight Coupling

Use of static methods always reduces flexibility by introducing tight coupling[1]. A static method tightly couples the calling code to the specific class the method exists in.

function totalAbs(double valuedouble value2) {
abs(value) + abs(value2);

Here, the method totalAbs has a dependency on the Math class and the .abs() method will always be called. Although for testing purposes this may not be a problem, the coupling reduces flexibility because the total method can only work with doubles/integers, as that's all the Math.abs() function can use. Although type coercion will allow the use of any primitive numeric type, these types have limitations. It's impossible to use another class such as BigInteger or a class for dealing with greater precision decimals or even alternative numbering systems such as Roman numerals.

The totalAbs function takes two doubles and converts them to their absolute values before adding them. This is inflexible because it only works with doubles. It's tied to doubles because that's what the Math.abs() static method requires. If, instead, using OOP an interface was created to handle any number that had this method:

 interface Numeric {
    public function 
abs(): Numeric;

It would then be possible to rewrite the totalAbs method to work with any kind of number:

 function totalAbs(Numeric valueNumeric value): Numeric {
value.abs() + value2.abs();

By removing the static method and using an instance method in its place the totalAbs method is now agnostic about the type of number it is dealing with. It could be called with any of the following (assuming they implement the Numeric interface)

totalAbs(new Integer(4), new Integer(-53));

totalAbs(new Double(34.4), new Integer(-2));

totalAbs(new BigInteger('123445454564765739878989343225778'), new Integer(2343));

totalAbs(new RomanNumeral('VII'), new RomanNumeral('CXV'));

Making the method reusable in a way that it wasn't when static methods were being used. By changing the static methods to instance methods, flexibility has been enhanced as the method can be used with any numeric type, not just numeric types that are supported by the Math.abs() method.

Broken encapsulation

Static methods also break encapsulation. Encapsulation is defined by Rogers[2] as:

the bundling of data with the methods that operate on that data

By passing the numeric value into the abs method, the data being operated on is being separated from the methods that operate on it, breaking encapsulation. Instead using num.abs() the data is encapsulated in the num instance and its type is not visible or relevant to the outside world. abs() will work on the data and work regardless of num's type, providing it implements the abs method.

This is a simple example, but applies to all static methods. Use of polymorphic instance methods that work on encapsulated data will always be more flexible than static method calls which can only ever deal with specific pre-defined types.

Further reading


The only exception to this rule is when a static method is used for object creation in place of the new keyword[3]. This is because the new keyword is already a static call. However, even here a non-static factory is often preferable for testing purposes[4][5].


  1. Popov, N. (2014) Don't be STUPID: GRASP SOLID! [online]. Available from:
  2. Rogers, P. (2001) Encapsulation is not information hiding [online]. Available from:
  3. Sonmez, J. (2010) Static Methods Will Shock You [online]. Available from:
  4. Hevery, M. (2008) Static Methods are Death to Testability [online]. Available from:
  5. Butler, T. (2013) Are Static Methods/Variables bad practice? [online]. Available from:
{ return (int) \pow(2, $retries - 1) * 1000; } public function __invoke(RequestInterface $request, array $options): PromiseInterface { if (!isset($options['retries'])) { $options['retries'] = 0; } $fn = $this->nextHandler; return $fn($request, $options) ->then( $this->onFulfilled($request, $options), $this->onRejected($request, $options) ); } /** * Execute fulfilled closure */ private function onFulfilled(RequestInterface $req, array $options): callable { return function ($value) use ($req, $options) { if (!\call_user_func( $this->decider, $options['retries'], $req, $value, null )) { return $value; } return $this->doRetry($req, $options, $value); }; } /** * Execute rejected closure */ private function onRejected(RequestInterface $req, array $options): callable { return function ($reason) use ($req, $options) { if (!\call_user_func( $this->decider, $options['retries'], $req, null, $reason )) { return \GuzzleHttp\Promise\rejection_for($reason); } return $this->doRetry($req, $options); }; } private function doRetry(RequestInterface $request, array $options, ResponseInterface $response = null) { $options['delay'] = \call_user_func($this->delay, ++$options['retries'], $response); return $this($request, $options); }}