静态初始化是良好的编程实践吗？

Question

In Java, we use the static initialization block:

private static final ApiKey API_KEY;

static {
    API_KEY = new ApiKey();
}

I was wondering that

• Is it a good programming practice?
• Where should we use this pattern?

Thanks in advance.

Answer 1

To some extent it's a matter of taste. To me it's fine so long as:

• You keep the field final, as you have done
• You make sure the object referenced is immutable and thread-safe

Statics tend to make writing good tests harder. If you ever find you want to start modifying static state then you probably need to look at the design again.

Consider looking at Google Guice and its very nice Singleton implementation .

Of course if your application is a 10 line single-class experiment then this matters a whole lot less.

Note that in your example, you could simplify to:

private static final ApiKey API_KEY = new ApiKey();

That's not always possible though. Perhaps you have omitted some more complex initialization code? In which case Guice would again be worth a look.

Answer 2

You could avoid using a static initializer block completely by using the following code:

private static final ApiKey API_KEY = new ApiKey();

or

private static final ApiKey API_KEY = createNewApiKey();

if the API key creation requires more than just a constructor call. That makes the code more readable, IMHO. But it doesn't matter much.

The static initializer is useful when two static fields depend on the same initialization code:

static {
    // compute some values
    A = somePartOfTheComputedValues();
    B = someOtherPartOfTheComputedValues();
}

But even then, A and B could perhaps be refactored into a single object, which would be created in a single method.

Answer 3

I like using enums whenever possible.

Instead of

class ApiKey {        
    private static final ApiKey API_KEY;

    static {
        API_KEY = new ApiKey();
    }

I would write

enum ApiKey {
    INSTANCE;