Is it a bad practice to catch Throwable?

Question

Is it a bad practice to catch Throwable?

For example something like this:

try {
    // Some code
} catch(Throwable e) {
    // handle the exception
}

Is this a bad practice or we should be as specific as possible?

Answer 1

You need to be as specific as possible. Otherwise unforeseen bugs might creep away this way.

Besides, Throwable covers Error as well and that's usually no point of return. You don't want to catch/handle that, you want your program to die immediately so that you can fix it properly.

Answer 2

This is a bad idea. In fact, even catching Exception is usually a bad idea. Let's consider an example:

try {
    inputNumber = NumberFormat.getInstance().formatNumber( getUserInput() );
} catch(Throwable e) {
    inputNumber = 10; //Default, user did not enter valid number
}

Now, let's say that getUserInput() blocks for a while, and another thread stops your thread in the worst possible way ( it calls thread.stop() ). Your catch block will catch a ThreadDeath Error. This is super bad. The behavior of your code after catching that Exception is largely undefined.

A similar problem occurs with catching Exception. Maybe getUserInput() failed because of an InterruptException, or a permission denied exception while trying to log the results, or all sorts of other failures. You have no idea what went wrong, as because of that, you also have no idea how to fix the problem.

You have three better options:

1 -- Catch exactly the Exception(s) you know how to handle:

try {
    inputNumber = NumberFormat.getInstance().formatNumber( getUserInput() );
} catch(ParseException e) {
    inputNumber = 10; //Default, user did not enter valid number
}

2 -- Rethrow any exception you run into and don't know how to handle:

try {
    doSomethingMysterious();
} catch(Exception e) {
    log.error("Oh man, something bad and mysterious happened",e);
    throw e;
}

3 -- Use a finally block so you don't have to remember to rethrow:

 Resources r = null;
 try {
      r = allocateSomeResources();
      doSomething(r);
 } finally {
     if(r!=null) cleanUpResources(r);
 }

Answer 3

Also be aware that when you catch Throwable, you can also catch InterruptedException which requires a special treatment. See Dealing with InterruptedException for more details.

If you only want to catch unchecked exceptions, you might also consider this pattern

try {
   ...
} catch (RuntimeException exception) {
  //do something
} catch (Error error) {
  //do something
}

This way, when you modify your code and add a method call that can throw a checked exception, the compiler will remind you of that and then you can decide what to do for this case.

Answer 4

straight from the javadoc of the Error class (which recommends not to catch these):

 * An <code>Error</code> is a subclass of <code>Throwable</code> 
 * that indicates serious problems that a reasonable application 
 * should not try to catch. Most such errors are abnormal conditions. 
 * The <code>ThreadDeath</code> error, though a "normal" condition,
 * is also a subclass of <code>Error</code> because most applications
 * should not try to catch it. 

 * A method is not required to declare in its <code>throws</code> 
 * clause any subclasses of <code>Error</code> that might be thrown 
 * during the execution of the method but not caught, since these 
 * errors are abnormal conditions that should never occur. 
 *
 * @author  Frank Yellin
 * @version %I%, %G%
 * @see     java.lang.ThreadDeath
 * @since   JDK1.0

Answer 5

It's not a bad practice if you absolutely cannot have an exception bubble out of a method.

It's a bad practice if you really can't handle the exception. Better to add "throws" to the method signature than just catch and re-throw or, worse, wrap it in a RuntimeException and re-throw.

Answer 6

Catching Throwable is sometimes necessary if you are using libraries that throw Errors over-enthusiastically, otherwise your library may kill your application.

However, it would be best under these circumstances to specify only the specific errors thrown by the library, rather than all Throwables.

Answer 7

Throwable is the base class for all classes than can be thrown (not only exceptions). There is little you can do if you catch an OutOfMemoryError or KernelError (see When to catch java.lang.Error?)

catching Exceptions should be enough.

Answer 8

it depends on your logic or to be more specific to your options / possibilities. If there is any specific exception that you can possibly react on in a meaningful way, you could catch it first and do so.

If there isn't and you're sure you will do the same thing for all exceptions and errors (for example exit with an error-message), than it is not problem to catch the throwable.

Usually the first case holds and you wouldn't catch the throwable. But there still are plenty of cases where catching it works fine.

Answer 9

Although it is described as a very bad practice, you may sometimes find rare cases that it not only useful but also mandatory. Here are two examples.

In a web application where you must show a meaning full error page to user. This code make sure this happens as it is a big try/catch around all your request handelers ( servlets, struts actions, or any controller ....)

try{
     //run the code which handles user request.
   }catch(Throwable ex){
   LOG.error("Exception was thrown: {}", ex);
     //redirect request to a error page. 
 }

}

As another example, consider you have a service class which serves fund transfer business. This method returns a TransferReceipt if transfer is done or NULL if it couldn't.

String FoundtransferService.doTransfer( fundtransferVO);

Now imaging you get a List of fund transfers from user and you must use above service to do them all.

for(FundTransferVO fundTransferVO : fundTransferVOList){
   FoundtransferService.doTransfer( foundtransferVO);
}

But what will happen if any exception happens? You should not stop, as one transfer may have been success and one may not, you should keep go on through all user List, and show the result to each transfer. So you end up with this code.

for(FundTransferVO fundTransferVO : fundTransferVOList){
    FoundtransferService.doTransfer( foundtransferVO);
 }catch(Throwable ex){
    LOG.error("The transfer for {} failed due the error {}", foundtransferVO, ex);
  }
}

You can browse lots of open source projects to see that the throwable is really cached and handled. For example here is a search of tomcat,struts2 and primefaces:

https://github.com/apache/tomcat/search?utf8=%E2%9C%93&q=catch%28Throwable https://github.com/apache/struts/search?utf8=%E2%9C%93&q=catch%28Throwable https://github.com/primefaces/primefaces/search?utf8=%E2%9C%93&q=catch%28Throwable

Answer 10

The question is a bit vague; are you asking "is it OK to catch Throwable", or "is it OK to catch a Throwable and not do anything"? Many people here answered the latter, but that's a side issue; 99% of the time you should not "consume" or discard the exception, whether you are catching Throwable or IOException or whatever.

If you propagate the exception, the answer (like the answer to so many questions) is "it depends". It depends on what you're doing with the exception—why you're catching it.

A good example of why you would want to catch Throwable is to provide some sort of cleanup if there is any error. For example in JDBC, if an error occurs during a transaction, you would want to roll back the transaction:

try {
  …
} catch(final Throwable throwable) {
  connection.rollback();
  throw throwable;
}

Note that the exception is not discarded, but propagated.

But as a general policy, catching Throwable because you don't have a reason and are too lazy to see which specific exceptions are being thrown is poor form and a bad idea.

Answer 11

Generally speaking you want to avoid catching Errors but I can think of (at least) two specific cases where it's appropriate to do so:

• You want to shut down the application in response to errors, especially AssertionError which is otherwise harmless.
• Are you implementing a thread-pooling mechanism similar to ExecutorService.submit() that requires you to forward exceptions back to the user so they can handle it.

Answer 12

If we use throwable, then it covers Error as well and that's it.

Example.

    public class ExceptionTest {
/**
 * @param args
 */
public static void m1() {
    int i = 10;
    int j = 0;
    try {
        int k = i / j;
        System.out.println(k);
    } catch (Throwable th) {
        th.printStackTrace();
    }
}

public static void main(String[] args) {
    m1();
}

}

Output:

java.lang.ArithmeticException: / by zero
at com.infy.test.ExceptionTest.m1(ExceptionTest.java:12)
at com.infy.test.ExceptionTest.main(ExceptionTest.java:25)

Answer 13

Throwable is the superclass of all the errors and excetions. If you use Throwable in a catch clause, it will not only catch all exceptions, it will also catch all errors. Errors are thrown by the JVM to indicate serious problems that are not intended to be handled by an application. Typical examples for that are the OutOfMemoryError or the StackOverflowError. Both are caused by situations that are outside of the control of the application and can’t be handled. So you shouldn't catch Throwables unless your are pretty confident that it will only be an exception reside inside Throwable.

Answer 14

While it is generally bad practice to catch Throwable (as elucidated by the numerous answers on this question), the scenarios where catching Throwable is useful are quite common. Let me explain one such case that I use at my job, with a simplified example.

Consider a method that performs the addition of two numbers, and after successful addition, it sends an email alert to certain people. Assume that the number returned is important and used by the calling method.

public Integer addNumbers(Integer a, Integer b) {
    Integer c = a + b;          //This will throw a NullPointerException if either 
                                //a or b are set to a null value by the
                                //calling method
    successfulAdditionAlert(c);
    return c;
}

private void successfulAdditionAlert(Integer c) {
    try {
        //Code here to read configurations and send email alerts.
    } catch (Throwable e) {
        //Code to log any exception that occurs during email dispatch
    }
}

The code to send email alerts reads a lot of system configurations and hence, there could be a variety of exceptions thrown from that block of code. But we do not want any exception encountered during alert dispatch to propagate to the caller method, as that method is simply concerned with the sum of the two Integer values it provides. Hence, the code to dispatch email alerts is placed in a try-catch block, where Throwable is caught and any exceptions are merely logged, allowing the rest of the flow to continue.