Why is catching checked exceptions allowed for cod

问题:

In Java, methods that throw checked exceptions (Exception or its subtypes - IOException, InterruptedException, etc) must declare throws statement:

public abstract int read() throws IOException;

Methods that do not declare throws statement can't throw checked exceptions.

public int read() { // does not compile
    throw new IOException();
}
// Error: unreported exception java.io.IOException; must be caught or declared to be thrown

But catching checked exceptions in safe methods is still legal in java:

public void safeMethod() { System.out.println("I'm safe"); }

public void test() { // method guarantees not to throw checked exceptions
    try {
        safeMethod();
    } catch (Exception e) { // catching checked exception java.lang.Exception
        throw e; // so I can throw... a checked Exception?
    }
}

Actually, no. It's a bit funny: compiler knows that e is not a checked exception and allows to rethrow it. Things are even a bit ridiculous, this code does not compile:

public void test() { // guarantees not to throw checked exceptions
    try {
        safeMethod();
    } catch (Exception e) {        
        throw (Exception) e; // seriously?
    }
}
// Error: unreported exception java.lang.Exception; must be caught or declared to be thrown

The first snippet was a motivation for a question.

Compiler knows that checked exceptions can't be thrown inside a safe method - so maybe it should allow to catch only unchecked exceptions?

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Returning to the main question - are there any reasons to implement catching checked exceptions in this way? Is it just a flaw in the design or am I missing some important factors - maybe backward incompatibilities? What could potentially go wrong if only RuntimeException were allowed to be catched in this scenario? Examples are greatly appreciated.

回答1:

Quoting the Java Language Specification, §11.2.3:

It is a compile-time error if a catch clause can catch checked exception class E1 and it is not the case that the try block corresponding to the catch clause can throw a checked exception class that is a subclass or superclass of E1, unless E1 is Exception or a superclass of Exception.

I'm guessing that this rule originated long before Java 7, where multi-catches did not exist. Therefore, if you had a try block that could throw a multitude of exceptions, the easiest way to catch everything would be to catch a common superclass (in the worst case, Exception, or Throwable if you want to catch Errors as well).

Note that you may not catch an exception type that is completely unrelated to what is actually thrown - in your example, catching any subclass of Throwable that is not a RuntimeException will be an error:

try {
    System.out.println("hello");
} catch (IOException e) {  // compilation error
    e.printStackTrace();
}

---

Edit by OP: The main part of the answer is the fact that question examples work only for Exception class. Generally catching checked exceptions is not allowed in random places of the code. Sorry if I confused somebody using these examples.

回答2:

Java 7 introduced more inclusive exception type checking.

However, in Java SE 7, you can specify the exception types FirstException and SecondException in the throws clause in the rethrowException method declaration. The Java SE 7 compiler can determine that the exception thrown by the statement throw e must have come from the try block, and the only exceptions thrown by the try block can be FirstException and SecondException.

This passage is talking about a try block that specifically throws FirstException and SecondException; even though the catch block throws Exception, the method only needs to declare that it throws FirstException and SecondException, not Exception:

public void rethrowException(String exceptionName)
 throws FirstException, SecondException {
   try {
     // ...
   }
   catch (Exception e) {
     throw e;
   }
 }

This means that the compiler can detect that the only possible exception types thrown in test are Errors or RuntimeExceptions, neither of which need to be caught. When you throw e;, it can tell, even when the static type is Exception, that it doesn't need to be declared or re-caught.

But when you cast it to Exception, this bypasses that logic. Now the compiler treats it as an ordinary Exception which needs to be caught or declared.

The main reason for adding this logic to the compiler was to allow the programmer to specify only specific subtypes in the throws clause when rethrowing a general Exception catching those specific subtypes. However, in this case, it allows you to catch a general Exception and not have to declare any exception in a throws clause, because no specific types that can be thrown are checked exceptions.

回答3:

The issue here is that checked/unchecked exception limitations affect what your code is allowed to throw, not what it's allowed to catch. While you can still catch any type of Exception, the only ones you're allowed to actually throw again are unchecked ones. (This is why casting your unchecked exception into a checked exception breaks your code.)

Catching an unchecked exception with Exception is valid, because unchecked exceptions (a.k.a. RuntimeExceptions) are a subclass of Exception, and it follows standard polymorphism rules; it doesn't turn the caught exception into an Exception, just as storing a String in an Object doesn't turn the String into an Object. Polymorphism means that a variable that can hold an Object can hold anything derived from Object (such as a String). Likewise, as Exception is the superclass of all exception types, a variable of type Exception can hold any class derived from Exception, without turning the object into an Exception. Consider this:

import java.lang.*;
// ...
public String iReturnAString() { return "Consider this!"; }
// ...
Object o = iReturnAString();

Despite the variable's type being Object, o still stores a String, does it not? Likewise, in your code:

try {
    safeMethod();
} catch (Exception e) { // catching checked exception
    throw e; // so I can throw... a checked Exception?
}

What this means is actually "catch anything compatible with class Exception (i.e. Exception and anything derived from it)." Similar logic is used in other languages, as well; for example, in C++, catching a std::exception will also catch std::runtime_error, std::logic_error, std::bad_alloc, any properly-defined user-created exceptions, and so on, because they all derive from std::exception.

tl;dr: You're not catching checked exceptions, you're catching any exceptions. The exception only becomes a checked exception if you cast it into a checked exception type.