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2.10. Exceptions

An exception in the Java Virtual Machine is represented by an instance of the class Throwable or one of its subclasses. Throwing an exception results in an immediate nonlocal transfer of control from the point where the exception was thrown.

Most exceptions occur synchronously as a result of an action by the thread in which they occur. An asynchronous exception, by contrast, can potentially occur at any point in the execution of a program. The Java Virtual Machine throws an exception for one of three reasons:

  • An athrow instruction (§athrow) was executed.
  • An abnormal execution condition was synchronously detected by the Java Virtual Machine. These exceptions are not thrown at an arbitrary point in the program, but only synchronously after execution of an instruction that either:
    • Specifies the exception as a possible result, such as:
      • flower.jpg When the instruction embodies an operation that violates the semantics of the Java programming language, for example indexing outside the bounds of an array.
      • flower.jpg When an error occurs in loading or linking part of the program.
    • Causes some limit on a resource to be exceeded, for example when too much memory is used.
  • An asynchronous exception occurred because:
    • The stop method of class Thread or ThreadGroup was invoked, or
    • An internal error occurred in the Java Virtual Machine implementation.

The stop methods may be invoked by one thread to affect another thread or all the threads in a specified thread group. They are asynchronous because they may occur at any point in the execution of the other thread or threads. An internal error is considered asynchronous (§6.3).

A Java Virtual Machine may permit a small but bounded amount of execution to occur before an asynchronous exception is thrown. This delay is permitted to allow optimized code to detect and throw these exceptions at points where it is practical to handle them while obeying the semantics of the Java programming language.

  • A simple implementation might poll for asynchronous exceptions at the point of each control transfer instruction. Since a program has a finite size, this provides a bound on the total delay in detecting an asynchronous exception. Since no asynchronous exception will occur between control transfers, the code generator has some flexibility to reorder computation between control transfers for greater performance. The paper Polling Efficiently on Stock Hardware by Marc Feeley, Proc. 1993 Conference on Functional Programming and Computer Architecture, Copenhagen, Denmark, pp. 179–187, is recommended as further reading.

Exceptions thrown by the Java Virtual Machine are precise: when the transfer of control takes place, all effects of the instructions executed before the point from which the exception is thrown must appear to have taken place. No instructions that occur after the point from which the exception is thrown may appear to have been evaluated. If optimized code has speculatively executed some of the instructions which follow the point at which the exception occurs, such code must be prepared to hide this speculative execution from the user-visible state of the program.

Each method in the Java Virtual Machine may be associated with zero or more exception handlers. An exception handler specifies the range of offsets into the Java Virtual Machine code implementing the method for which the exception handler is active, describes the type of exception that the exception handler is able to handle, and specifies the location of the code that is to handle that exception. An exception matches an exception handler if the offset of the instruction that caused the exception is in the range of offsets of the exception handler and the exception type is the same class as or a subclass of the class of exception that the exception handler handles. When an exception is thrown, the Java Virtual Machine searches for a matching exception handler in the current method. If a matching exception handler is found, the system branches to the exception handling code specified by the matched handler.

If no such exception handler is found in the current method, the current method invocation completes abruptly (§2.6.5). On abrupt completion, the operand stack and local variables of the current method invocation are discarded, and its frame is popped, reinstating the frame of the invoking method. The exception is then rethrown in the context of the invoker’s frame and so on, continuing up the method invocation chain. If no suitable exception handler is found before the top of the method invocation chain is reached, the execution of the thread in which the exception was thrown is terminated.

The order in which the exception handlers of a method are searched for a match is important. Within a class file, the exception handlers for each method are stored in a table (§4.7.3). At run time, when an exception is thrown, the Java Virtual Machine searches the exception handlers of the current method in the order that they appear in the corresponding exception handler table in the class file, starting from the beginning of that table.

Note that the Java Virtual Machine does not enforce nesting of or any ordering of the exception table entries of a method. The exception handling semantics of the Java programming language are implemented only through cooperation with the compiler (§3.12). When class files are generated by some other means, the defined search procedure ensures that all Java Virtual Machine implementations will behave consistently.

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