Java未报告的异常
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Java unreported exception
提问by Sejanus
While learning Java I stumble upon this error quite often. It goes like this:
在学习 Java 时，我经常偶然发现这个错误。它是这样的：
Unreported exception java.io.FileNotFound exception; must be caught or declared to be thrown.
未报告的异常 java.io.FileNotFound 异常；必须被捕获或声明被抛出。
java.io.FileNotFound is just an example, I've seen many different ones. In this particular case, code causing the error is:
java.io.FileNotFound 只是一个例子，我见过很多不同的例子。在这种特殊情况下，导致错误的代码是：
OutputStream out = new BufferedOutputStream(new FileOutputStream(new File("myfile.pdf")));
Error always disappears and code compiles & runs successfully once I put the statement inside try/catch block. Sometimes it's good enough for me, but sometimes not.
一旦我将语句放入 try/catch 块中，错误总是消失并且代码编译并成功运行。有时这对我来说已经足够了，但有时却不够。
First, examples I'm learning from do not always use try/catch and should work nevertheless, apparently.
首先，我正在学习的示例并不总是使用 try/catch 并且显然应该可以工作。
Whats more important, sometimes when I put whole code inside try/catch it cannot work at all. E.g. in this particular case I need to out.close();in finally{ }block; but if the statement above itself is inside the try{ }, finally{}doesnt "see" outand thus cannot close it.
更重要的是，有时当我将整个代码放入 try/catch 时，它根本无法工作。例如，在这种特殊情况下，我需要out.close(); 在finally{}块中；但是如果上面的语句本身在try{ }内部，finally{}不会“看到”出来，因此无法关闭它。
My first idea was to import java.io.FileNotFound;or another relevant exception, but it didnt help.
我的第一个想法是导入 java.io.FileNotFound；或其他相关的例外，但它没有帮助。
采纳答案by cletus
What you're referring to are checked exceptions, meaning they must be declared or handled. The standard construct for dealing with files in Java looks something like this:
您所指的是检查异常，这意味着它们必须被声明或处理。Java 中处理文件的标准结构如下所示：
InputStream in = null;
try {
in = new InputStream(...);
// do stuff
} catch (IOException e) {
// do whatever
} finally {
if (in != null) {
try {
in.close();
} catch (Exception e) {
}
}
}
Is it ugly? Sure. Is it verbose? Sure. Java 7 will make it a little better with ARM blocks but until then you're stuck with the above.
丑吗？当然。它很冗长吗？当然。Java 7 会用 ARM 块让它更好一点，但在那之前你会被上面的内容困住。
You can also let the caller handle exceptions:
您还可以让调用者处理异常：
public void doStuff() throws IOException {
InputStream in = new InputStream(...);
// do stuff
in.close();
}
although even then the close()should probably be wrapped in a finallyblock.
尽管即便如此，也close()应该将其包裹在一个finally块中。
But the above function declaration says that this method can throw an IOException. Since that's a checked exception the caller of this function will need to catchit (or declare it so its caller can deal with it and so on).
但是上面的函数声明说这个方法可以抛出一个IOException. 由于这是一个检查异常，这个函数的调用者将需要catch它（或声明它以便它的调用者可以处理它等等）。
回答by Tom Hawtin - tackline
Java's checked exceptions make programmers address issues like this. (That's a good thing in my opinion, even if sweeping bugs under the carpet is easier.)
Java 的检查异常使程序员可以解决这样的问题。（在我看来，这是一件好事，即使扫地毯下的虫子更容易。）
You should take some appropriate action if a failure occurs. Typically the handling should be at a different layer from where the exception was thrown.
如果发生故障，您应该采取一些适当的措施。通常，处理应该在与抛出异常不同的层。
Resource should be handled correctly, which takes the form:
应该正确处理资源，它采用以下形式：
acquire();
try {
use();
} finally {
release();
}
Never put the acquire()within the try block. Never put anything between the acquire()and try(other than a simple assign). Do not attempt to release multiple resources in a single finallyblock.
切勿将 放在acquire()try 块内。永远不要在acquire()and之间放置任何东西try（除了简单的赋值）。不要尝试在单个finally块中释放多个资源。
So, we have two different issues. Unfortunately the Java syntax mixes up the two. The correct way to write such code is:
所以，我们有两个不同的问题。不幸的是，Java 语法将两者混为一谈。编写此类代码的正确方法是：
try {
final FileOutputStream rawOut = new FileOutputStream(file);
try {
OutputStream out = new BufferedOutputStream(rawOut);
...
out.flush();
} finally {
rawOut.close();
}
} catch (FileNotFoundException exc) {
...do something not being able to create file...
} catch (IOException exc) {
...handle create file but borked - oops...
}
