Why doesn't catching Exception catch RuntimeException?

问题:

This is very odd to me. RuntimeException inherits from Exception, which inherits from Throwable.

catch(Exception exc) { /* won't catch RuntimeException */ 

but

catch(Throwable exc) { /* will catch RuntimeException */ 

I know RuntimeException is special in that it's unchecked. But to my understanding that applies just to whether exceptions have to be declared, not whether they are caught. And even then, I don't know why this logic would break on catching Throwable.

This is pretty relevant to me since I have a situation where RuntimeExceptions can be thrown in a terminal operation. I'm not sure the name for this pattern, but something like, my class EmailRoller takes an array of Callbacks. The code looks like this:

for(Callback cb : callbacks) {     try {         cb.call(item);     }     catch(Exception exc) {         logger.error("Error in callback: ", exc);    } } 

So this is a case where something like an OOME needs to fly through, because if one of these callbacks consumes all machine memory, that sure as heck is going to affect the running of the other ones. But a NullPointerException? Or an IndexOutOfBoundsException? Those will affect the callback but won't prevent the others from running.

Also, this is a bit of an enterprise design. Different programmers or teams can add callbacks to process the item, but they should be isolated from each other. This means, as the programmer responsible for insulating these callbacks from each other, I shouldn't rely on them to make sure errors don't slip through. Catching Exception should be about the right line, but it isn't because RuntimeException slips through. So my more general question is: what's a good pattern here? Just catch(Exception | RuntimeException exc), which I believe is a syntax error because of the inheritance?

回答1:

The premise of the question is flawed, because catching Exception does catch RuntimeException. Demo code:

public class Test {     public static void main(String[] args) {         try {             throw new RuntimeException("Bang");         } catch (Exception e) {             System.out.println("I caught: " + e);         }     } } 

Output:

I caught: java.lang.RuntimeException: Bang 

Your loop will have problems if:

• callbacks is null
• anything modifies callbacks while the loop is executing (if it were a collection rather than an array)

Perhaps that's what you're seeing?

回答2:

catch (Exception ex) { ... } 

WILL catch RuntimeException.

Whatever you put in catch block will be caught as well as the subclasses of it.

回答3:

Catching Exception will catch a RuntimeException

回答4:

I faced similar scenario. It was happening because classA's initilization was dependent on classB's initialization. When classB's static block faced runtime exception, classB was not initialized. Because of this, classB did not throw any exception and classA's initialization failed too.

class A{//this class will never be initialized because class B won't intialize   static{     try{       classB.someStaticMethod();     }catch(Exception e){       sysout("This comment will never be printed");     }  } }  class B{//this class will never be initialized  static{     int i = 1/0;//throw run time exception   }   public static void someStaticMethod(){} } 

And yes...catching Exception will catch run time exceptions as well.

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