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

此外，如果try成功而close不成功怎么办？会进入 catch 块吗？

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 块内的代码和自动 close() 语句都抛出异常，catch 部分将处理 [=16] 抛出的异常=] 块，suppressed exceptions.

中的 close() 抛出异常

此外，这意味着如果 try 块成功但自动 close() 失败， catch 仍将执行并且捕获的异常将是抛出的异常close().

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");
    }
}

此代码将打印

thrown by try part
thrown by close
thrown by close

表示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

Answer 2

try块内部抛出的异常对外抛出。

虽然使用 try-catch-finally，但从 finally 块抛出的异常将向上传播到调用堆栈。

InputStream input = null;

    try {
        input = new FileInputStream("file.txt");

        int data = input.read();
        while(data != -1){
            System.out.print((char) data);
            data = input.read();
        }
    } finally {
        if(input != null){
            input.close();
        }
    }

在这种情况下，如果try块和finally块都发生异常（当InputStream关闭时），最后一个被抛出，即使try块抛出的异常可能更多与传播有关。

try(FileInputStream input = new FileInputStream("file.txt")) {

        int data = input.read();
        while(data != -1){
            System.out.print((char) data);
            data = input.read();
        }
    }

虽然在这种情况下，如果在 try 块和 finally 块中都发生异常，则传播第一个异常

扩展的 try-with-resources 语句到底捕获了什么？

What exactly gets caught in an extended try-with-resources statement?

java

error-handling

try-catch

try-with-resources