1. 常見錯誤分類

一般的，errors可以分為以下幾類：

user input errors

device errors or physical limitations

code errors

2. 常用錯誤處理方式

2.1 error code

一種常用的錯誤處理方法是返回error code，由calling method根據error code做不同處理。

但是有些情形下error code并不方便使用，比如如何取分錯誤碼和與錯誤碼相同的有效值。

2.2 exception handling

The mission of exception handling is to transfer control from where the error occured to an error handler that can deal with the situation.

當出現錯誤時，程序應當恢復到某個安全狀態并重新來過，或者保存當前工作安全退出，但是這并不容易，關鍵就是程序控制權如何從觸發錯誤的地方跳轉到處理錯誤的地方。

Java allows every method an alternative exit path if it is unable to complete its task in the normal way：

首先，throws an object that encapsulates the error information

然后，the exception-handling mechanism begins its search for an exception handler that can deal with this particular error condition

注意，這條alternative exit path與正常的程序邏輯無關，the method exits immediately, and it does not return its normal value, and the execution does not resume at the code that called the method

3. Java exception hierarchy

3.1 Throwable

Throwable是整個Java exception hierarchy的基類，其下分為：

Error

Exception

3.2 Error

the error hierarchy describes internal errors and resource exhaustion situations inside the Java runtime system.

You should not throw an object of this type. There is little you can do if sucn an internal error occurs, beyond notifying the user and trying to terminate the program gracefully.

3.3 Exception

Exception可以分為兩類：

RuntimeException

其他

RuntimeException意味著編碼錯誤，比如

a bad cast, an out-of-bounds array access, a null pointer access等。

其他Exception一般是出現了某種意外，some bad things happened, 比如，打開一個不存在的文件。

為什么打開不存在的文件不是RuntimeException？因為這不是你的代碼能控制的，你先校驗文件是否存在也沒用，可能你校驗時是存在的，但你打開時就不存在了。

4. Checked exception vs unchecked exception

Error和RuntimeException這兩支，我們稱為unchecked exception.

除Error和RuntimeException這兩支外的其他Exception，我們稱為checked exception.

The compiler checks that you provide exception handlers for all checked exceptions:

對于Error，你無能為力，所以是unchecked exception

對于RuntimeException，編碼錯誤是你的責任，你應當避免它們出現，所以也是unchecked exception

對于其他exceptions，你必須做好處理它們的準備，所以是checked exception

5. Checked exception declaration

對于你的方法可能拋出的checked exceptions，你必須在method declaration中通過throws聲明出來。

如果可能拋出多個checked exceptions，那么需要都列出來，使用逗號分隔。

public Image loadImage(String name) throws FileNotFoundException, EOFException {...}

注意，unchecked exception不應當出現在你的throws聲明中：

對于Error，你無能為力

對于RuntimeException，你應當避免它們出現，而不是聲明可能拋出它們。

6. Throw an exception

如何拋出exception呢？很簡單：

find an appropriate exception class

make an object of that class

throw it

沒有合適的standard exception class可用？沒有關系，你可以自定義一個

class FileFormatException extends IOException {

public FileFormatException() {}

public FileFormatException(String gripe) {

super(gripe);

}

}

一般的，我們為自定義的exception class提供兩個constructors：a default constructor and a constructor with a detailed message.

7. Catch exceptions

try {

xxx

} catch (ExceptionType1 | ExceptionType 2 e) {

xxx

} catch (ExceptionType3 e) {

xxx

}

對于checked exceptions，如果你知道如何處理它們，那么你可以catch它們，這樣就不用拋出它們了。

As a general rule, you should catch those exceptions that you know how to handle and propagate those that you do not know how to handle.

8. Rethrow exceptions

try {

xxx

} catch (SQLException e) {

throw new ServletException("xxx error").initCause(e);

}

這是rethrow exception的常用方式。通過這種方式，我們可以包裝出一個更抽象的exception，或者把一個checked exception轉換成一個RuntimeException.

9. finally

try {

try {

xxx

} finally {

xxx

}

} catch (Exception e) {

xxx

}

the inner try block has a single responsibility: to make sure that the resources are released

the outer try block has a single responsibility: to ensure that errors are reported.

注意，the body of the finally clause is intended for cleaning up resources. Don't put statements that change the control flow (return, throw, break, continue) inside a finally clause.

10. try-with-Resources

try (Resource res = xxx) {

xxx

}

從Java 7開始，try-finally結構可以簡化為try-with-Resources.

要求Resource必須是AutoCloseable的實現類，when the try block exits, then res.close() is called automatically.

A difficulty arises when the try block throws an exception and the close method also throws an exception. The original exception is rethrown, and any exceptions thrown by close methods are considered "suppressed". They are automatically caught and added to the original exception with the addSuppressed method.

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