Programs must not catch java.lang.NullPointerException. A NullPointerException exception thrown at runtime indicates the existence of an underlying null pointer dereference that must be fixed in the application code (see rule EXP01-J. Do not use a null in a case where an object is required for more information). Handling the underlying null pointer dereference by catching the NullPointerException rather than fixing the underlying problem is inappropriate for several reasons. First, catching NullPointerException adds significantly more performance overhead than simply adding the necessary null checks [Bloch 2008]. Second, when multiple expressions in a try block are capable of throwing a NullPointerException, it is difficult or impossible to determine which expression is responsible for the exception because the NullPointerException catch block handles any NullPointerException thrown from any location in the try block. Third, programs rarely remain in an expected and usable state after a NullPointerException has been thrown. Attempts to continue execution after first catching and logging (or worse, suppressing) the exception rarely succeed.
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This noncompliant code example is derived from the logging service null object design pattern described by Henney [Henney 2003]. The logging service is composed of two classes: one that prints the triggering activity's details to a disk file using the FileLog class and another that prints to the console using the ConsoleLog class. An interface, Log, defines a write() method that is implemented by the respective log classes. Method selection occurs polymorphically at runtime. The logging infrastructure is subsequently used by a Service class.
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Some system designs require returning a value from a method rather than implementing do-nothing behavior. One acceptable approach is use of an exceptional value object that throws an exception before the method returns [Cunningham 1995]. This can be a useful alternative to returning null.
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ERR08-EX1: Task processing threads such as worker threads in a thread pool or the Swing event dispatch thread are permitted to catch RuntimeException when they call untrusted code through an abstraction such as the Runnable interface [Goetz 2006, p. 161].
ERR08-EX2: Systems that require substantial fault tolerance or graceful degradation are permitted to catch and log general exceptions such as Throwable at appropriate levels of abstraction. For example:
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Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
ERR08-J | medium | likely | medium | P12 | L1 |
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