When a program fails due to an uncaught exception, the system automatically prints out the exception’s stack trace. The stack trace contains the exception’s string representation, the result of invoking its toString method. This typically consists of the exception’s class name followed by its detail message. Frequently this is the only information that programmers or site reliability engineers will have when investigating a software failure. If the failure is not easily reproducible, it may be difficult or impossible to get any more information. Therefore, it is critically important that the exception’s toString method return as much information as possible concerning the cause of the failure. In other words, the detail message of an exception should capture the failure for subsequent analysis.
当程序由于未捕获异常而失败时，系统可以自动打印出异常的堆栈跟踪。堆栈跟踪包含异常的字符串表示，这是调用其 toString 方法的结果。这通常包括异常的类名及其详细信息。通常，这是程序员或管理员在调查软件故障时所掌握的唯一信息。如果失败不容易重现，想获得更多的信息会非常困难。因此，异常的 toString 方法返回尽可能多的关于失败原因的信息是非常重要的。换句话说，由失败导致的异常的详细信息应该被捕获，以便后续分析。
To capture a failure, the detail message of an exception should contain the values of all parameters and fields that contributed to the exception. For example, the detail message of an IndexOutOfBoundsException should contain the lower bound, the upper bound, and the index value that failed to lie between the bounds. This information tells a lot about the failure. Any or all of the three values could be wrong. The index could be one less than the lower bound or equal to the upper bound (a “fencepost error”), or it could be a wild value, far too low or high. The lower bound could be greater than the upper bound (a serious internal invariant failure). Each of these situations points to a different problem, and it greatly aids in the diagnosis if you know what sort of error you’re looking for.
要捕获失败，异常的详细消息应该包含导致异常的所有参数和字段的值。 例如，IndexOutOfBoundsException 的详细消息应该包含下界、上界和未能位于下界之间的索引值。这些信息说明了很多关于失败的信息。这三个值中的任何一个或所有值都可能是错误的。索引可以小于或等于上界（「越界错误」），也可以是一个无效值，太小或太大。下界可能大于上界（严重的内部故障）。每一种情况都指向一个不同的问题，如果你知道你在寻找什么样的错误，这对诊断有很大的帮助。
One caveat concerns security-sensitive information. Because stack traces may be seen by many people in the process of diagnosing and fixing software issues, do not include passwords, encryption keys, and the like in detail messages.
提及一个与安全敏感信息有关的警告。因为许多人在诊断和修复软件问题的过程中可能会看到堆栈跟踪，所以 不应包含密码、加密密钥等详细信息。
While it is critical to include all of the pertinent data in the detail message of an exception, it is generally unimportant to include a lot of prose. The stack trace is intended to be analyzed in conjunction with the documentation and, if necessary, source code. It generally contains the exact file and line number from which the exception was thrown, as well as the files and line numbers of all other method invocations on the stack. Lengthy prose descriptions of the failure are superfluous; the information can be gleaned by reading the documentation and source code.
虽然在异常的详细信息中包含所有相关数据非常重要，但通常不需要包含大量的描述。堆栈跟踪将与文档一起分析，如果需要，还将与源代码一起分析。它通常包含抛出异常的确切文件和行号，以及堆栈上所有方法调用的文件和行号。冗长的描述对一个失败问题而言是多余的；可以通过阅读文档和源代码来收集信息。
The detail message of an exception should not be confused with a user-level error message, which must be intelligible to end users. Unlike a user-level error message, the detail message is primarily for the benefit of programmers or site reliability engineers, when analyzing a failure. Therefore, information content is far more important than readability. User-level error messages are often localized, whereas exception detail messages rarely are. One way to ensure that exceptions contain adequate failure-capture information in their detail messages is to require this information in their constructors instead of a string detail message. The detail message can then be generated automatically to include the information. For example, instead of a String constructor, IndexOutOfBoundsException could have had a constructor that looks like this:
异常的详细信息不应该与用户层的错误消息混淆，因为用户层错误消息最终必须被用户理解。与用户层错误消息不同，详细消息主要是为程序员或管理员在分析故障时提供的。因此，信息内容远比可读性重要。用户层错误消息通常是本地化的，而异常详细信息消息很少本地化。确保异常在其详细信息中包含足够的故障捕获信息的一种方法是，在其构造函数中配置，而不是以传入字符串方式引入这些信息。之后可以自动生成详细信息来包含细节。例如，IndexOutOfBoundsException 构造函数不包含 String 参数，而是像这样：
/**
* Constructs an IndexOutOfBoundsException.
**
@param lowerBound the lowest legal index value
* @param upperBound the highest legal index value plus one
* @param index the actual index value
*/
public IndexOutOfBoundsException(int lowerBound, int upperBound, int index) {
// Generate a detail message that captures the failure
super(String.format("Lower bound: %d, Upper bound: %d, Index: %d",lowerBound, upperBound, index));
// Save failure information for programmatic access
this.lowerBound = lowerBound;
this.upperBound = upperBound;
this.index = index;
}
As of Java 9, IndexOutOfBoundsException finally acquired a constructor that takes an int valued index parameter, but sadly it omits the lowerBound and upperBound parameters. More generally, the Java libraries don’t make heavy use of this idiom, but it is highly recommended. It makes it easy for the programmer throwing an exception to capture the failure. In fact, it makes it hard for the programmer not to capture the failure! In effect, the idiom centralizes the code to generate a high-quality detail message in the exception class, rather than requiring each user of the class to generate the detail message redundantly.
从 Java 9 开始，IndexOutOfBoundsException 最终获得了一个接受 int 值索引参数的构造函数，但遗憾的是它忽略了下界和上界参数。更一般地说，Java 库不会大量使用这个习惯用法，但是强烈推荐使用它。它使程序员很容易通过抛出异常来捕获失败。事实上，它使程序员不想捕获失败都难！实际上，这个习惯用法将集中在异常类中生成高质量的详细信息，而不是要求该类的每个用户都生成冗余的详细信息。
译注：IndexOutOfBoundsException 有关 int 参数的构造函数源码
/**
* Constructs a new {@code IndexOutOfBoundsException} class with an
* argument indicating the illegal index.
*
* <p>The index is included in this exception's detail message. The
* exact presentation format of the detail message is unspecified.
*
* @param index the illegal index.
* @since 9
*/
public IndexOutOfBoundsException(int index) {
super("Index out of range: " + index);
}
As suggested in Item 70, it may be appropriate for an exception to provide accessor methods for its failure-capture information (lowerBound, upperBound, and index in the above example). It is more important to provide such accessor methods on checked exceptions than unchecked, because the failure-capture information could be useful in recovering from the failure. It is rare (although not inconceivable) that a programmer might want programmatic access to the details of an unchecked exception. Even for unchecked exceptions, however, it seems advisable to provide these accessors on general principle (Item 12, page 57).
正如 Item-70 中建议的，异常为其故障捕获信息提供访问器方法是适合的（上面示例中的下界、上界和索引）。在 checked 异常上提供此类访问器方法比 unchecked 异常上提供此类访问器方法更为重要，因为故障捕获信息可能有助于程序从故障中恢复。程序员可能希望通过编程访问 unchecked 异常的详细信息，但这是很少见的（尽管是可以想象的）。然而，即使对于 unchecked 异常，根据一般原则，提供这些访问器也是可以的（Item-12，第 57 页）。
Back to contents of the chapter（返回章节目录）
- Previous Item（上一条目）：Item 74: Document all exceptions thrown by each method（为每个方法记录会抛出的所有异常）
- Next Item（下一条目）：Item 76: Strive for failure atomicity（尽力保证故障原子性）