Java - 最终变量可以在静态初始化块中初始化吗？

Question

Based on my understanding of the Java language, static variables can be initialized in static initialization block .

However, when I try to implement this in practice ( static variables that are final too), I get the error shown in the screenshot below:

Answer 1

Yes of course: static final variables can be initialized in a static block but .... you have implicit GOTOs in that example ( try/catch is essentially a 'GOTO catch if something bad happens' ).

If an exception is thrown your final variables will not be initialized.

Note that the use of static constructs goes against Object-Oriented dogma. It may complicate your testing and make debugging more difficult.

Answer 2

You can do this but you need to exit the static block by throwing an exception - you can rethrow the exception that was caught or a new one. Generally this exception must be a RuntimeException . You really should not catch a generic Exception but more specific exception(s) that might be thrown from within your try block. Finally, if a static initializer throws an exception then it will render the class unusable during that specific run because the JVM will only attempt to initialize your class once. Subsequent attempts to use this class will result in another exception, such as NoClassDefFoundError .

So, to work, your initializer should read something like this:

static {
    try {
        ...
    } catch (Exception e) {
        e.PrintStackTrace();
        throw new InitializationFailedException("Could not init class.", e);
    }
}

Assuming that InitializationFailedException is a custom RuntimeException , but you could use an existing one.

Answer 3

public class MyClass
{
    private static final SomeClass myVar;

    static
    {
        Object obj = null;  // You could use SomeClass, but I like Object so you can reuse it
        try
        {
            obj = new SomeClass(...);    
        }
        catch(WhateverException err)
        {
            // Possibly nested try-catches here if the first exception is recoverable...
            // Print an error, log the error, do something with the error
            throw new ExceptionInInitializerError(err); 
        }
        finally
        {
            myVar = (SomeClass) obj;
        }
    }
}

Assuming no where upstream is in a position to catch either an ExceptionInInitializationError or a general Exception then the program should not ever try to use myVar . If however those are caught and the program doesn't end, then you need to code to watch for and handle myVar being null (or be happy with NullPointerExceptions coming out all over).

I'm not sure there is a good way to handle this.

Answer 4

Can you put the declaration in the finally block?

try {
    //load file
} catch(IOException e) {
    // horay
} finally {
    HOST=config.get......
}