Can constructors throw exceptions in Java

Question

Are constructors allowed to throw exceptions

User · Accepted Answer

Yes  constructors can throw exceptions  Usually this means that the new object is immediately eligible for garbage collection  although it may not be collected for some time  of course   It s possible for the  half-constructed  object to stick around though  if it s made itself visible earlier in the constructor  e g  by assigning a static field  or adding itself to a collection    One thing to be careful of about throwing exceptions in the constructor  because the caller  usually  will have no way of using the new object  the constructor ought to be careful to avoid acquiring unmanaged resources  file handles etc  and then throwing an exception without releasing them  For example  if the constructor tries to open a FileInputStream and a FileOutputStream  and the first succeeds but the second fails  you should try to close the first stream  This becomes harder if it s a subclass constructor which throws the exception  of course    it all becomes a bit tricky  It s not a problem very often  but it s worth considering

User · Answer

Yes    Constructors are nothing more than special methods  and can throw exceptions like any other method

User · Answer

Yes  it can throw an exception and you can declare that in the signature of the constructor too as shown in the example below   public class ConstructorTest       public ConstructorTest   throws InterruptedException               System out println  Preparing object                Thread sleep 1000           System out println  Object ready               public static void main String     args                try                       ConstructorTest test   new ConstructorTest                      catch  InterruptedException e                        System out println  Got interrupted

User · Answer

Yes  constructors are allowed to throw exceptions   However  be very wise in choosing what exceptions they should be - checked exceptions or unchecked  Unchecked exceptions are basically subclasses of RuntimeException   In almost all cases  I could not come up with an exception to this case   you ll need to throw a checked exception  The reason being that unchecked exceptions  like NullPointerException  are normally due to programming errors  like not validating inputs sufficiently    The advantage that a checked exception offers is that the programmer is forced to catch the exception in his instantiation code  and thereby realizes that there can be a failure to create the object instance  Of course  only a code review will catch the poor programming practice of swallowing an exception

User · Answer

Absolutely    If the constructor doesn t receive valid input  or can t construct the object in a valid manner  it has no other option but to throw an exception and alert its caller

User · Answer

Yes  they can throw exceptions  If so  they will only be partially initialized and if non-final  subject to attack   The following is from the Secure Coding Guidelines 2 0      Partially initialized instances of a non-final class can be accessed via a finalizer attack  The attacker overrides the protected finalize method in a subclass  and attempts to create a new instance of that subclass  This attempt fails  in the above example  the SecurityManager check in ClassLoader s constructor throws a security exception   but the attacker simply ignores any exception and waits for the virtual machine to perform finalization on the partially initialized object  When that occurs the malicious finalize method implementation is invoked  giving the attacker access to this  a reference to the object being finalized  Although the object is only partially initialized  the attacker can still invoke methods on it  thereby circumventing the SecurityManager check

[java] Can constructors throw exceptions in Java?

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