异常(Exceptions)

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