仅仅在异常状况下才使用Exceptions
- 不要把Exceptions用于ordinary control flow
- 不要写这样的API: 迫使别人把Exceptions用于ordinary control flow
- 应该写这样的API:
have a separate “state-testing” method indicating
whether it is appropriate to invoke the state-dependent method
比如:
the Iterator interface has the state-dependent method next
and the corresponding state-testing method hasNext
have the state-dependent method return an empty optional
or a distinguished value such as null
if it cannot perform the desired computation
java throwable有哪些？
checked exceptions, runtime exceptions, errors
其中 runtime exceptions, errors 都属于unchecked exceptions
在使用java的throwable时，有哪些准则？
- throw checked exceptions for recoverable conditions
- 为你的checked exceptions提供methods，便于caller recoverable
- throw unchecked exceptions for programming errors
- 只定义checked exceptions 或 runtime exceptions，不要定义Error及其子类或其他的throwable及其子类
- 不知道throw哪种exceptions时，就throw RuntimeException
- Avoid unnecessary use of checked exceptions，一种消除 a checked exception的方式是return an optional of the desired result type
- 尽量document all exceptions thrown by each method
- exception的detail message应该包含导致该exception的所有parameters 和 fields 的值，比如IndexOutOfBoundsException的detail message包括 the lower bound, the upper bound, and the index value that failed to lie between the bounds
为什么拥抱使用standard exceptions
- 能否获得高度的code reuse是区分expert programmers和一般programmers的重要属性之一
- Reusing standard exceptions能使你的API更容易learn, use, and read
- fewer exception classes意味着smaller memory footprint 和 less time spent loading classes
- 不要直接 reuse: Exception, RuntimeException, Throwable, or Error
- 常被reused exceptions:
IllegalArgumentException,IllegalStateException,NullPointerException,IndexOutOfBoundsException,ConcurrentModificationException,UnsupportedOperationException
- 不要过度使用exception translation和exception chaining
- 不要忽视异常，在try catch中，一个空的catch块破坏了exception的目的
对exception translation的理解
- higher layers should catch lower-level exceptions and, in their place, throw exceptions that can be explained in terms of the higher-level abstraction
// Exception Translation
try {
... // Use lower-level abstraction to do our bidding
} catch (LowerLevelException e) {
throw new HigherLevelException(...);
}
对exception chaining的理解
- The lower-level exception (the cause) is passed to the higher-level exception
// Exception Chaining
try {
... // Use lower-level abstraction to do our bidding
} catch (LowerLevelException cause) {
throw new HigherLevelException(cause);
}
// Exception with chaining-aware constructor
class HigherLevelException extends Exception {
HigherLevelException(Throwable cause) {
super(cause);
}
}
说说对原子性失败(failure-atomic)的理解
- 一次失败的方法调用应该使该对象处于调用之前的状态，具有这种属性的方法，称为failure-atomic
- 使用failure atomicity时，不应该significantly 增加代码的cost和complexity
如何实现failure-atomic效果？
- design immutable objects
- 在修改对象状态之前，先计算并check parameters的正确性
- 临时copy object，在copy object 计算完成，然后替换掉原来object的内容
- 写recovery code，使该object的状态roll back到原始状态
