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How to detect initialization order of fields? [message #1794235]

Mon, 27 August 2018 19:03

Sergey Toshin

Messages: 56
Registered: May 2015

Member

Hi, I have a testing class like it

class Test {
private Container c1 = new Container(this.c2);
private Container c2 = new Container();
}

In the provided example c2 should be initialized before c1 (doesn't matter of their declaration order), but are there any default Eclipse JDT methods/APIs that can provide me that order? Or such order doesn't exists, but it's created in runtime (e.g. c1 is being initialized, but stopped until c2 is initialized)?

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Re: How to detect initialization order of fields? [message #1794238 is a reply to message #1794235]

Mon, 27 August 2018 22:00

Stephan Herrmann

Messages: 1853
Registered: July 2009

Senior Member

Field initialization is performed in the order of declarations in the source code (see JLS §12.5 step 4).

Note, that removing the "this." prefix from the forward reference to c2, yields this error:

        private Container c1 = new Container(c2);
                                             ^^
Cannot reference a field before it is defined

IOW, using "this.c2" is a trick to bypass the rules from JLS §8.3.3.

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Re: How to detect initialization order of fields? [message #1794277 is a reply to message #1794238]

Tue, 28 August 2018 12:11

Sergey Toshin

Messages: 56
Registered: May 2015

Member

Thanks, but it's description how it should work anywhere, but not how it really works in Eclipse. I'm using Eclipse JDT in my own project, and I have FieldDeclaration[] for a class, so I want to resolve order of initializations (because previously I initialized fields by ascending of their array indexes, but it appeared to be wrong, because some fields are referenced before they were initialized in my tool). That's why I'm trying to find a solution how using JDT I can resolve that order

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Re: How to detect initialization order of fields? [message #1794279 is a reply to message #1794277]

Tue, 28 August 2018 12:16

Stephan Herrmann

Messages: 1853
Registered: July 2009

Senior Member

Sergey Toshin wrote on Tue, 28 August 2018 14:11

Thanks, but it's description how it should work anywhere, but not how it really works in Eclipse.

What exactly is not working that way in Eclipse?
Do we generate wrong byte code?

Quote:

I'm using Eclipse JDT in my own project, and I have FieldDeclaration[] for a class, so I want to resolve order of initializations

Are you interested in the correct order according to JLS (textual source code order), or the semantically meaningful order (with no dereference before initialization)?

As mentioned, when saying "this.c2" this is not guaranteed to be the same order, but when saying "c2", differences between both orders will raise a compile error.

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Re: How to detect initialization order of fields? [message #1794280 is a reply to message #1794279]

Tue, 28 August 2018 12:19

Stephan Herrmann

Messages: 1853
Registered: July 2009

Senior Member

IOW: Eclipse doesn't compute the meaningful order of fields, it only checks if the actual textual order violates that order wrt simple name references (where "this.c2" is *not* a simple name reference).

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Re: How to detect initialization order of fields? [message #1794282 is a reply to message #1794280]

Tue, 28 August 2018 12:32

Sergey Toshin

Messages: 56
Registered: May 2015

Member

Quote:

the semantically meaningful order (with no dereference before initialization)

I believe I want that. So in the provided example

class Test {
    private Container c1 = new Container(this.c2);
    private Container c2 = new Container();
}

Field declarations should be sorted to
1. private Container c2 = new Container();
2. private Container c1 = new Container(this.c2);

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Re: How to detect initialization order of fields? [message #1794284 is a reply to message #1794282]

Tue, 28 August 2018 12:40

Sergey Toshin

Messages: 56
Registered: May 2015

Member

I had idea to retrieve all SimpleNames (which are variable names) in initializers using a visitor, and then sort them to ensure that dependencies are already initialized. But it seems to be monkey codes, so I wanted to know is there a better approach

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Re: How to detect initialization order of fields? [message #1794285 is a reply to message #1794282]

Tue, 28 August 2018 12:41

Stephan Herrmann

Messages: 1853
Registered: July 2009

Senior Member

Sergey Toshin wrote on Tue, 28 August 2018 14:32

Field declarations should be sorted to
1. private Container c2 = new Container();
2. private Container c1 = new Container(this.c2);

There is no unique order fulfilling your criteria, and in case of cycles there may not even exist any solution.

To implement your own strategy you'd need to get a resolved AST (i.e., with bindings) and search for field references inside initializers by yourself (using bindings is more precise than relying on names). This is the preferred way if you want to solve this automatically.

A human should simply remove the "this." prefix and change order until it's accepted by the compiler. At that point you will have an order that fulfills your criteria.

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Re: How to detect initialization order of fields? [message #1794296 is a reply to message #1794285]

Tue, 28 August 2018 14:03

Ed Willink

Messages: 7655
Registered: July 2009

Senior Member

Hi

I think you are assuming that Java is going to be very clever when it only requires you the programmer to be competent.

In general "private Container c1 = new Container(this.c2);" does not work since "this" has yet to be fully initialized. For non-trivial initializations you need to sequence them as constructor statements rather than field initializers. It is then very obvious what has been subtly sequenced. Usually attempts to use "this" during construction are discouraged/inhibited since use of a partially constructed "this" is dangerous. However in some circumstances, particularly with the aid of a static method you can use a partially constructed "this" that violates OO integrity but may be useful when you know what you are doing. If Java prohibited such well-designed hazardous usage that would be painful. e.g. A has a final @NonNull reference to B and vice-versa.

Regards

Ed Willink

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