11.1

The Kinds and Causes of Exceptions

EXCEPTIONS

invocations, 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 (§11.2). If no such handler is found, then the exception may be handled by one of a hierarchy of uncaught exception handlers (§11.3) - thus every effort is made to avoid letting an exception go unhandled.

The exception mechanism of the Java SE platform is integrated with its synchronization model (§17.1), so that monitors are unlocked as synchronized statements (§14.19) and invocations of synchronized methods (§8.4.3.6, §15.12) complete abruptly.

11.1 The Kinds and Causes of Exceptions

11.1.1 The Kinds of Exceptions

An exception is represented by an instance of the class Throwable (a direct subclass of Object) or one of its subclasses.

Throwable and all its subclasses are, collectively, the exception classes.

The classes Exception and Error are direct subclasses of Throwable.

Exception is the superclass of all the exceptions from which ordinary programs may wish to recover.

Error is the superclass of all the exceptions from which ordinary programs are not ordinarily expected to recover.

Error and all its subclasses are, collectively, the error classes.

The class Error is a separate subclass of Throwable, distinct from Exception in the class hierarchy, to allow programs to use the idiom "} catch (Exception e) {" (§11.2.3) to catch all exceptions from which recovery may be possible without catching errors from which recovery is typically not possible.

The class RuntimeException is a direct subclass of Exception. RuntimeException is the superclass of all the exceptions which may be thrown for many reasons during expression evaluation, but from which recovery may still be possible.

RuntimeException and all its subclasses are, collectively, the runtime exception classes.

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EXCEPTIONS

The Causes of Exceptions 11.1.2

The unchecked exception classes are the runtime exception classes and the error classes.

The checked exception classes are all exception classes other than the unchecked exception classes. That is, the checked exception classes are all subclasses of Throwable other than RuntimeException and its subclasses and Error and its subclasses.

Programs can use the pre-existing exception classes of the Java SE platform API in throw statements, or define additional exception classes as subclasses of Throwable or of any of its subclasses, as appropriate. To take advantage of compile-time checking for exception handlers (§11.2), it is typical to define most new exception classes as checked exception classes, that is, as subclasses of Exception that are not subclasses of RuntimeException.

11.1.2 The Causes of Exceptions

An exception is thrown for one of three reasons:

•A throw statement (§14.18) was executed.

•An abnormal execution condition was synchronously detected by the Java virtual machine, namely:

evaluation of an expression violates the normal semantics of the Java programming language (§15.6), such as an integer divide by zero.

an error occurs while loading, linking, or initializing part of the program (§12.2, §12.3, §12.4); in this case, an instance of a subclass of LinkageError is thrown.

an internal error or resource limitation prevents the Java virtual machine from implementing the semantics of the Java programming language; in this case, an instance of a subclass of VirtualMachineError is thrown.

These exceptions are not thrown at an arbitrary point in the program, but rather at a point where they are specified as a possible result of an expression evaluation or statement execution.

• An asynchronous exception occurred (§11.1.3).

11.1.3 Asynchronous Exceptions

Most exceptions occur synchronously as a result of an action by the thread in which they occur, and at a point in the program that is specified to possibly result in such an exception. An asynchronous exception is, by contrast, an exception that can potentially occur at any point in the execution of a program.

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