CHAPTER 11

Exceptions

When a program violates the semantic constraints of the Java programming language, the 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 the Java platform: to provide portability and robustness. Instead, the Java programming language 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.

Programs can also throw exceptions explicitly, using throw statements (§14.17).

Explicit use of throw statements 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 (§14.19). During the process of throwing an exception, the Java virtual machine abruptly completes, one by one, any expressions, statements, method and constructor 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. If no such handler is found, then the method uncaught-Exception 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 exception mechanism of the Java platform is integrated with its synchronization model (§17), so that locks are released as synchronized statements (§14.18) and invocations of synchronized methods (§8.4.3.6, §15.12) 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 (§11.5).

11.1 The Causes of Exceptions

• An abnormal execution condition was synchronously detected by the Java virtual machine. Such conditions arise because:
  • evaluation of an expression violates the normal semantics of the language, such as an integer divide by zero, as summarized in §15.6
  • an error occurs in loading or linking part of the program (§12.2, §12.3)
  • some limitation on a resource is exceeded, such as using too much memory

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.

• A throw statement (§14.17) was executed.
• An asynchronous exception occurred either because:
  • the method stop of class Thread was invoked
  • an internal error has occurred in the virtual machine (§11.5.2)

11.2 Compile-Time Checking of Exceptions

The unchecked exceptions classes are the class RuntimeException and its subclasses, and the class Error and its subclasses. All other exception classes are checked exception classes. The Java API defines a number of exception classes, both checked and unchecked. Additional exception classes, both checked and unchecked, may be declared by programmers. See §11.5 for a description of the exception class hierarchy and some of the exception classes defined by the Java API and Java virtual machine.

The checked exception classes named in the throws clause are part of the contract between the implementor and user of the method or constructor. The throws clause of an overriding method may not specify that this method will result in throwing any checked exception which the overridden method is not permitted, by its throws clause, to throw. When interfaces are involved, more than one method declaration may be overridden by a single overriding declaration. In this case, the overriding declaration must have a throws clause that is compatible with all the overridden declarations (§9.4).

Static initializers (§8.7), class variable initializers, and instance initializers or instance variable initializers within named classes and interfaces (§8.3.2), must not result in a checked exception; if one does, a compile-time error occurs. No such restriction applies to instance initializers or instance variable initializers within anonymous classes (§15.9.5).

11.2.1 Why Errors are Not Checked

11.2.2 Why Runtime Exceptions are Not Checked

For example, certain code might implement a circular data structure that, by construction, can never involve null references; the programmer can then be certain that a NullPointerException cannot occur, but it would be difficult for a compiler to prove it. The theorem-proving technology that is needed to establish such global properties of data structures is beyond the scope of this specification.

11.3 Handling of an Exception

A statement or expression is dynamically enclosed by a catch clause if it appears within the try block of the try statement of which the catch clause is a part, or if the caller of the statement or expression is dynamically enclosed by the catch clause.

The caller of a statement or expression depends on where it occurs:

• If within a method, then the caller is the method invocation expression (§15.12) that was executed to cause the method to be invoked.
• If within a constructor or an instance initializer or the initializer for an instance variable, then the caller is the class instance creation expression (§15.9) or the method invocation of newInstance that was executed to cause an object to be created.
• If within a static initializer or an initializer for a static variable, then the caller is the expression that used the class or interface so as to cause it to be initialized.

The control transfer that occurs when an exception is thrown causes abrupt completion of expressions (§15.6) and statements (§14.1) until a catch clause is encountered that can handle the exception; execution then continues by executing the block of that catch clause. The code that caused the exception is never resumed.

If no catch clause handling an exception can be found, then the current thread (the thread that encountered the exception) is terminated, but only after all finally clauses have been executed and the method uncaughtException has been invoked for the ThreadGroup that is the parent of the current thread.

In situations where it is desirable to ensure that one block of code is always executed after another, even if that other block of code completes abruptly, a try statement with a finally clause (§14.19.2) may be used.

If a try or catch block in a try-finally or try-catch-finally statement completes abruptly, then the finally clause is executed during propagation of the exception, even if no matching catch clause is ultimately found. If a finally clause is executed because of abrupt completion of a try block and the finally clause itself completes abruptly, then the reason for the abrupt completion of the try block is discarded and the new reason for abrupt completion is propagated from there.

The exact rules for abrupt completion and for the catching of exceptions are specified in detail with the specification of each statement in §14 and for expressions in §15 (especially §15.6).

11.3.1 Exceptions are Precise

11.3.2 Handling Asynchronous Exceptions

Proper understanding of the semantics of asynchronous exceptions is necessary if high-quality machine code is to be generated.

Asynchronous exceptions are rare. They occur only as a result of:

• An invocation of the stop methods of class Thread or ThreadGroup
• An internal error (§11.5.2) in the Java virtual machine

The Java platform permits 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.

Like all exceptions, asynchronous exceptions are precise (§11.3.1).

11.4 An Example of Exceptions

class TestException extends Exception {
	TestException() { super(); }
	TestException(String s) { super(s); }
}
class Test {
	public static void main(String[] args) {
		for (int i = 0; i < args.length; i++) {
			try {
				thrower(args[i]);
				System.out.println("Test \"" + args[i] +
					"\" didn't throw an exception");
			} catch (Exception e) {
				System.out.println("Test \"" + args[i] +
					"\" threw a " + e.getClass() +
					"\n        with message: " + e.getMessage());
			}
		}
	}
	static int thrower(String s) throws TestException {
		try {
			if (s.equals("divide")) {
				int i = 0;
				return i/i;
			}
			if (s.equals("null")) {
				s = null;
				return s.length();
			}
			if (s.equals("test"))
				throw new TestException("Test message");
			return 0;
		} finally {
			System.out.println("[thrower(\"" + s +
					"\") done]");
		}
	}
}

divide null not test

[thrower("divide") done]
Test "divide" threw a class java.lang.ArithmeticException
        with message: / by zero
[thrower("null") done]
Test "null" threw a class java.lang.NullPointerException
        with message: null
[thrower("not") done]
Test "not" didn't throw an exception
[thrower("test") done]
Test "test" threw a class TestException
        with message: Test message

The declaration of the method thrower must have a throws clause because it can throw instances of TestException, which is a checked exception class (§11.2). A compile-time error would occur if the throws clause were omitted.

Notice that the finally clause is executed on every invocation of thrower, whether or not an exception occurs, as shown by the "[thrower(...) done]" output that occurs for each invocation.

11.5 The Exception Hierarchy

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

The class Exception is the superclass of all the exceptions that ordinary programs may wish to recover from. The class RuntimeException is a subclass of class Exception. The subclasses of RuntimeException are unchecked exception classes. The subclasses of Exception other than RuntimeException are all checked exception classes.

The class Error and its subclasses are exceptions from which ordinary programs are not ordinarily expected to recover. See the Java API specification for a detailed description of the exception hierarchy.

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) {

11.5.1 Loading and Linkage Errors

• The loading process is described in §12.2.
• The linking process is described in §12.3.
• The class verification process is described in §12.3.1.
• The class preparation process is described in §12.3.2.
• The class initialization process is described in §12.4.

11.5.2 Virtual Machine Errors