48. Anatomy of an Exception

Throwable and all of its descendants provide a number of useful methods. Let’s start off with a very basic try-catch block so that we can then dissect the NullPointerException:

try {
    def person = null
    person.getName()
} catch (any) {
    println "Exception received: ${any.class.name}"
    println()
    println "getMessage() returns: ${any.getMessage()}"
    println()
    println "getCause() returns: ${any.getCause()}"
    println()
    println 'getStackTrace() returned:'
    for (item in any.getStackTrace()) {
        println "${item.getFileName()}\t${item.getLineNumber()}\t${item.getClass\
Name()}\t${item.getMethodName()}\t${item.isNativeMethod()}"
    }
}

any.class.name

Tells us the type of exception that was raised

We get: java.lang.NullPointerException

getMessage()

Returns the message that was set by the thrower of the exception

We get: Cannot invoke method getName() on null object

getCause()

This isn’t always set but can be handy to know. Essentially this is set if the exception is “relaying” another exception.

We get: null

for (item in any.getStackTrace()) {..}

The getStackTrace() method returns an array of java.lang.StackTraceElement. Each StackTraceElement can then be further dissected to see what was happening when the exception occurred.

In my example I output a tab separated row for each StackTraceElement - the methods that I call should be reasonably self-explanatory but you can always check the Java API for more information.

To just dump out the stack trace I could have just used printStackTrace()