11. Exceptions



If anything can go wrong, it will.
—Finagle's Law (often incorrectly attributed to Murphy, whose law is
rather different—which only goes to show that Finagle was right)

When a Java program violates the semantic constraints of the Java language, a Java Virtual Machine signals this error to the program as an exception. An example of such a violation is an attempt to index outside the bounds of an array. Some programming languages and their implementations react to such errors by peremptorily terminating the program; other programming languages allow an implementation to react in an arbitrary or unpredictable way. Neither of these approaches is compatible with the design goals of Java: to provide portability and robustness. Instead, Java specifies that an exception will be thrown when semantic constraints are violated and will cause a non-local transfer of control from the point where the exception occurred to a point that can be specified by the programmer. An exception is said to be thrown from the point where it occurred and is said to be caught at the point to which control is transferred.

Java programs can also throw exceptions explicitly, using throw statements (§14.16). This provides an alternative to the old-fashioned style of handling error conditions by returning funny values, such as the integer value -1 where a negative value would not normally be expected. Experience shows that too often such funny values are ignored or not checked for by callers, leading to programs that are not robust, exhibit undesirable behavior, or both.

Every exception is represented by an instance of the class Throwable or one of its subclasses; such an object can be used to carry information from the point at which an exception occurs to the handler that catches it. Handlers are established by catch clauses of try statements (§14.18). During the process of throwing an exception, a Java Virtual Machine abruptly completes, one by one, any expressions, statements, method and constructor invocations, static initializers, and field initialization expressions that have begun but not completed execution in the current thread. This process continues until a handler is found that indicates that it handles that particular exception by naming the class of the exception or a superclass of the class of the exception. If no such handler is found, then the method uncaughtException(§20.21.31) is invoked for the ThreadGroup that is the parent of the current thread-thus every effort is made to avoid letting an exception go unhandled.

The Java exception mechanism is integrated with the Java synchronization model (§17), so that locks are released as synchronized statements (§14.17) and invocations of synchronized methods (§, §15.11) complete abruptly.

This chapter describes the different causes of exceptions (§11.1). It details how exceptions are checked at compile time (§11.2) and processed at run time (§11.3). A detailed example (§11.4) is then followed by an explanation of the exception hierarchy and the standard exception classes (§11.5).