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Trail: Essential Classes
Lesson: Exceptions
Section: How to Throw Exceptions
Exceptions
What Is an Exception?
The Catch or Specify Requirement
Catching and Handling Exceptions
The try Block
The catch Blocks
The finally Block
Putting It All Together
Specifying the Exceptions Thrown by a Method
How to Throw Exceptions
Chained Exceptions
Creating Exception Classes
Unchecked Exceptions — The Controversy
Advantages of Exceptions
Summary
Questions and Exercises
Home Page > Essential Classes > Exceptions
Chained Exceptions

> An application often responds to an exception by throwing another exception. In effect, the first exception causes the second exception. It can be very helpful to know when one exception causes another. Chained Exceptions help the programmer do this.

> The following are the methods and constructors in `Throwable` that support chained exceptions.

> >     Throwable getCause()
> >     Throwable initCause(Throwable)
> >     Throwable(String, Throwable)
> >     Throwable(Throwable)

> The `Throwable` argument to `initCause` and the `Throwable` constructors is the exception that caused the current exception. `getCause` returns the exception that caused the current exception, and `initCause` returns the current exception.

> The following example shows how to use a chained exception.

> >     try {

> >     } catch (IOException e) {
> >         throw new SampleException("Other IOException", e);
> >     }

> In this example, when an `IOException` is caught, a new `SampleException` exception is created with the original cause attached and the chain of exceptions is thrown up to the next higher level exception handler.

> ### Accessing Stack Trace Information

> Now let's suppose that the higher-level exception handler wants to dump the stack trace in its own format.

> > Definition: A stack trace provides information on the execution history of the current thread and lists the names of the classes and methods that were called at the point when the exception occurred. A stack trace is a useful debugging tool that you'll normally take advantage of when an exception has been thrown.

> The following code shows how to call the `getStackTrace` method on the exception object.

> >     catch (Exception cause) {
> >         StackTraceElement elements[] = cause.getStackTrace();
> >         for (int i = 0, n = elements.length; i < n; i++) {
> >             System.err.println(elements[i].getFileName() + ":"
> >                           + elements[i].getLineNumber()
> >                           + ">> "
> >                           + elements[i].getMethodName() + "()");
> >         }
> >     }

> ### Logging API

> The next code snippet logs where an exception occurred from within the `catch` block. However, rather than manually parsing the stack trace and sending the output to `System.err()`, it sends the output to a file using the logging facility in the `java.util.logging` package.

> >     try {
> >         Handler handler = new FileHandler("OutFile.log");
> >         Logger.getLogger("").addHandler(handler);

> >     } catch (IOException e) {
> >         Logger logger = Logger.getLogger("package.name");
> >         StackTraceElement elements[] = e.getStackTrace();
> >         for (int i = 0, n = elements.length; i < n; i++) {
> >             logger.log(Level.WARNING,
> >                        elements[i].getMethodName());
> >         }
> >     }

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