在调用超级构造函数时尝试使用资源

Question

Is there any good way of using try-with-resources when opening an InputStream in a constructor and then passing that to a super constructor?

Basically what I want to do is this:

public class A {
    public A(InputStream stream) {
        // Do something with the stream but don't close it since we didn't open it
    }
}

public class B {
    public B(File file) {
        // We open the stream so we need to ensure it's properly closed
        try (FileInputStream stream = new FileInputStream(file)) {
            super(new FileInputStream(file));
        }
    }
}

But, of course, since super must be the first statement in the constructor this isn't allowed. Is there any good way of achieving this?

Answer 1

Consider using a static factory method instead of using the constructor directly. Make at least B 's constructor private, and create a method such as

private B(InputStream is) {
    super(is);
    // Whatever else is needed
}

public static B newInstance(File file) {
    B result;
    try (FileInputStream stream = new FileInputStream(file)) {
        result = new B(stream);
    }
    // Further processing
    return result;
}

Answer 2

Another way to go :

public class A {
    protected A(){
        // so he can't be called from the outside, subclass ensure that init is done properly.
    }

    public A(InputStream stream) {
        init(stream);
    }
    // not be able to call it from outside
    protected final init(InputStream is){
        //here goes the code
    }
}

public class B {
    public B(File file) {
        // We open the stream so we need to ensure it's properly closed
        try (FileInputStream stream = new FileInputStream(file)) {
            init(stream);
        }
    }
}

I'm posting this here as a possible answer, however here i'm consdering :

1. You can update A's code
2. You're moving constructor's code to an init method, thanks to protected empty arg constructor, only subclasses have to handle the call to init properly. Some may see that as not so well designed. My point is as soon your subclassing something, you have to know more about it that just when you just using it.

Answer 3

Sadly I do not have a compiler on hand to test on but could you not do as follows.

public class B {
    private static InputStream file2stream(File f){
        // We open the stream so we need to ensure it's properly closed
        try (FileInputStream stream = new FileInputStream(file)) {
            return stream;
        }catch(/*what you need to catch*/){
             //cleanup
             // possibly throw runtime exception
        }
    }
    public B(File file) {
        super(file2stream(file))
    }
}