java - 扩展的 try-with-resources 语句到底捕获了什么？

Question

在下面的代码块中:

try ( /* resources declaration */ ) {
    // some dangerous code
} catch (Exception e) {
    // error handling and reporting
}

如果 try block 中的代码和自动 close() 语句都抛出异常，会发生什么情况？哪个会被 catch block 捕获？两个都？只有其中之一？如果有，是哪一个？

此外，如果 try 成功但 close 不成功怎么办？会进入 catch block 吗？

Answer 1

引自 JLS 部分 14.20.3.1 :

In a basic try-with-resources statement that manages a single resource:

• If the initialization of the resource completes abruptly because of a throw of a value V, then the try-with-resources statement completes abruptly because of a throw of the value V.

• If the initialization of the resource completes normally, and the try block completes abruptly because of a throw of a value V, then:

  • If the automatic closing of the resource completes normally, then the try-with-resources statement completes abruptly because of a throw of the value V.

  • If the automatic closing of the resource completes abruptly because of a throw of a value V2, then the try-with-resources statement completes abruptly because of a throw of value V with V2 added to the suppressed exception list of V.

• If the initialization of the resource completes normally, and the try block completes normally, and the automatic closing of the resource completes abruptly because of a throw of a value V, then the try-with-resources statement completes abruptly because of a throw of the value V.

这意味着如果 try block 中的代码和自动 close() 语句都抛出异常，则 catch 部分将处理 try block 抛出的异常，异常由 suppressed exceptions 中的 close() 抛出.

此外，这意味着如果 try block 成功但自动 close() 失败，catch 仍将被执行并且捕获的异常将是 close() 抛出的异常。

---

这是验证此行为的测试:

public class Main {
    public static void main(String[] args) throws Exception {
        // try block fails and close() fails
        try (T t = new T()) {
            throw new Exception("thrown by try part");
        } catch (Exception e) {
            System.out.println(e.getMessage());
            System.out.println(e.getSuppressed()[0].getMessage());
        }

        // try block is successful but close() fails
        try (T t = new T()) {
            //
        } catch (Exception e) {
            System.out.println(e.getMessage());
        }
    }
}

class T implements AutoCloseable {
    @Override
    public void close() throws Exception {
        throw new Exception("thrown by close");
    }
}

这段代码会打印

thrown by try part
thrown by close
thrown by close

意思是捕获到的异常是第一部分代码的try部分抛出的异常。第二部分，捕获到的异常确实是close()抛出的异常。