JDK 的 try-with-resources 示例中的错误做法？ [复制]

Question

我在 Java 的 try-with-resources 文档中找到了这个示例：

static String readFirstLineFromFile(String path) throws IOException {
    try (BufferedReader br = new BufferedReader(new FileReader(path))) {
        return br.readLine();
    }
}

如果BufferedReader的构造函数抛出异常，则FileReader持有的资源不会被释放。那么这样写不是一个不好的做法，而不是：

static String readFirstLineFromFile(String path) throws IOException {
    try (FileReader fr = new FileReader(path);
         BufferedReader br = new BufferedReader(fr)) {
        return br.readLine();
    }
}

Answer 1

确实，刚刚试了一下：

public class MyFileReader extends FileReader {

    public MyFileReader(String fileName) throws FileNotFoundException {
        super(fileName);
    }

    @Override
    public void close() throws IOException {
        System.out.println("Closing MyFileReader");
        super.close();
    }

}

public class MyBufferedReader extends BufferedReader {

    public MyBufferedReader(Reader in) {
        super(in);
        throw new RuntimeException();
    }

    @Override
    public void close() throws IOException {
        System.out.println("Closing MyBufferedReader");
        super.close();
    }

}

public String test(String path) throws IOException {
    try (BufferedReader br = new MyBufferedReader(new MyFileReader(path))) {
        return br.readLine();
    }
}

MyFileReader 和 MyBufferedReader 都没有关闭...很好！

同时：

public String test(String path) throws IOException {
    try (FileReader fr = new MyFileReader(path); BufferedReader br = new MyBufferedReader(fr)) {
        return br.readLine();
    }
}

MyFileReader 已关闭。

BufferedReader 构造函数确实可以抛出异常，请参阅BufferedReader(Reader in, int sz) constructor（虽然不是来自BufferedReader(Reader in) constructor，但您链接的文档仍然应该提醒您这个可能的问题恕我直言）。

看起来你赢得了raise an issue 的权利:)

Answer 2

很遗憾，你是对的。

以下示例显示了这种行为 - Internal 的实例永远不会关闭。

public class Test {
    public static void main(String[] args) {
        try (External external = new External(new Internal())) {
        }
    }
}

class External implements Closeable {
    private Internal internal;
    public External(Internal internal) {
        this.internal = internal;
        throw new RuntimeException("boom");
    }

    @Override
    public void close() {
        System.out.println("External.close()");
        internal.close();
    }
}

class Internal implements Closeable {
    @Override
    public void close() {
        System.out.println("Internal.close()");
    }
}