翻译自：https://tech.io/playgrounds/929/reactive-programming-with-reactor-3/Error
Error
Description
Reactor ships with several operators that can be used to deal with errors:
propagate errors but also recover from it (eg. by falling back to a different
sequence or by retrying a new Subscription).
Reactor有几个可用于处理错误的运算符：传播错误，但也可以从中恢复（例如，返回到不同的序列或重试新订阅）。
Practice
In the first example, we will return a Mono containing default user Saul
when an error occurs in the original Mono, using the method onErrorReturn.
If you want, you can even limit that fallback to the IllegalStateException
class. Use the User#SAUL constant.
在第一个示例中，当原始Mono中出现错误时，我们将使用onErrorReturn方法返回一个包含默认user Saul的Mono。
如果需要，甚至可以将回退限制为IllegalStateException类。使用用户#SAUL常数。
// Return a Mono containing User.SAUL when an error occurs in the input Mono, else do not change the input Mono.
Mono betterCallSaulForBogusMono(Mono mono) {
return mono.onErrorReturn(User.SAUL);
}
Let's try the same thing with Flux. In this case, we don't just want a single
fallback value, but a totally separate sequence (think getting stale data from a
cache). This can be achieved with onErrorResume, which falls back to a Publisher.
让我们用Flux做同样的尝试。在这种情况下，我们不仅需要单个回退值，还需要一个完全独立的序列
（想想从缓存中获取过时数据）。这可以通过onErrorResume实现，其依赖于Publisher。
Emit both User#SAUL and User#JESSE whenever there is an error in the original FLux:
当原始Flux中出现错误时，发射User#SAUL和User#JESSE：
// Return a Flux containing User.SAUL and User.JESSE when an error occurs in the input Flux, else do not change the input Flux.
Flux betterCallSaulAndJesseForBogusFlux(Flux flux) {
return flux.onErrorResume(throwable -> Flux.just(User.SAUL, User.JESSE));
}
Dealing with checked exceptions is a bit more complicated. Whenever some code that
throws checked exceptions is used in an operator (eg. the transformation function
of a map), you will need to deal with it. The most straightforward way is to make
a more complex lambda with a try-catch block that will transform the checked
exception into a RuntimeException that can be signalled downstream.
处理选中的异常要复杂一些。每当在运算符（例如映射的转换函数）中使用引发选中异常的代码时，
您都需要处理它。最直接的方法是使用try-catch块生成更复杂的lambda，该块将检查的异常转换
为可以向下游发送信号的运行时异常。
There is a Exceptions#propagate utility that will wrap a checked exception into a
special runtime exception that can be automatically unwrapped by Reactor subscribers
and StepVerifier: this avoids seeing an irrelevant RuntimeException in the stacktrace.
有一个Exceptions#propagate实用程序，它将选中的异常包装成一个特殊的运行时异常，该异常可以由
Reactor订阅者和StepVerifier自动展开：这可以避免在堆栈跟踪中看到不相关的运行时异常。
Try to use that on the capitalizeMany method within a map: you'll need to catch
a GetOutOfHereException, which is checked, but the corresponding test still expects
the GetOutOfHereException directly.
尝试在CapitalMany方法上在一个map内部使用：您需要捕获一个GetOutOfHereException，该异常已被选中，
但相应的测试仍然直接预期GetOutOfHereException。
// Implement a method that capitalizes each user of the incoming flux using the
// #capitalizeUser method and emits an error containing a GetOutOfHereException error
Flux capitalizeMany(Flux flux) {
return flux.map(user -> {
try{
return capitalizeUser(user);
}catch (GetOutOfHereException e){
throw Exceptions.propagate(e);
}
});
}
User capitalizeUser(User user) throws GetOutOfHereException {
if (user.equals(User.SAUL)) {
throw new GetOutOfHereException();
}
return new User(user.getUsername(), user.getFirstname(), user.getLastname());
}