Try-with-resources scope of resource

问题:

In the try-with-resources construct of Java 7, I can declare a resource in the try statement, and it will be closed automatically when it goes out of scope.

However, I don't find any indication of the scope of the resource made available. Specifically, is it available in the catch/finally blocks of the try block where it is declared?

I tried the following in Eclipse Kepler, but it's giving a mixed impression:

Resource variable is made available by Content Assist (Code Completion):

Quick Fix suggests changing to resource variable, but this recursively produces the same problem it's trying to fix:

I would like to know what the correct scope limitation is, before raising a bug in the Eclipse Bug Tracker.

回答1:

This syntax is called Extended try-with-resources

As per JLS:

try ResourceSpecification
    Block
Catchesopt
Finallyopt

Will be translated to:

try {
    try ResourceSpecification
        Block
}
Catchesopt
Finallyopt

So, in your example, your resource will be limited to inner try block, so not available for outer try/catch/finally.

EDIT:

my question does not have nested try blocks

By explicitly adding catch/finally block in your code, you are introducing nested try blocks.

回答2:

Update from 2017 after Java 9 release

Now with Java 9 we have more syntactic sugar and we can have a resource declared outside the try-catch block but still handled properly. That's why with Java 9 the Try-With-Resources has been improved introducing a new syntax:

InputStream stream = new MyInputStream(...)
try (stream) {
   // do something with stream being sure that is going to be closed at the end
} catch(IOException e) {
   // you can surely use your resource here
}

Note that this syntax will result in a compile time error for Java version 8 or minor

This is more "natural" way of writing even though in most use cases we don't need the resource outside the scope of the try block. The only restriction is that the reader variable should be effectively final or just final.

Anyway with this syntax you can surely have your resource used also in the catch and finally block

回答3:

The correct scope limitation is within the declaration part (...) and the actual try block.

The JLS states

The scope of a variable declared in the ResourceSpecification of a try-with-resources statement (§14.20.3) is from the declaration rightward over the remainder of the ResourceSpecification and the entire try block associated with the try-with-resources statement.

So from the point it is declared in the ResourceSpecification (...) of the try onwards until the final closing } bracket of the try Block.

TryWithResourcesStatement:
    try ResourceSpecification Block Catchesopt Finallyopt

ResourceSpecification:
    ( Resources ;opt )

Resources:
    Resource
    Resource ; Resources

Resource:
    VariableModifiersopt Type VariableDeclaratorId = Expression

回答4:

In addition to @Nambari's answer:

A try-with-resources statement can have catch and finally blocks just like an ordinary try statement. In a try-with-resources statement, any catch or finally block is run after the resources declared have been closed.

That pretty much explains the behaviour, your resource goes out of scope in your explicit catch/finally block.

Reference