java - 为什么不能在 try-with-resources try 子句中重用引用变量？

Question

我注意到这不会编译:

PrintWriter printWriter = new PrintWriter("test.txt");
printWriter.append('a');
printWriter.close();
printWriter = null;

try(printWriter = new PrintWriter("test.txt")) {

}

与:Error:(17, 24) java: <identifier> expected

此时只有一个新变量起作用:

try(PrintWriter printWriter2 = new PrintWriter("test.txt")) {

}

我的直觉是只需要一个新对象，但显然需要一个新引用。显然这是编译时类型检查的结果，但为什么旧引用不起作用？

Answer 1

答案很简单，因为它会在资源尝试完成后关闭。更具体地说，

JLS 14.20.3. try-with-resources部分说(粗体添加)，

A ResourceSpecification declares one or more local variables with initializer expressions to act as resources for the try statement.

A resource declared in a ResourceSpecification is implicitly declared final (§4.12.4) if it is not explicitly declared final.

The type of a variable declared in a ResourceSpecification must be a subtype of AutoCloseable, or a compile-time error occurs.