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CHAPTER 11

Exceptions

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 (§8.4.3.5, §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).

11.1 The Causes of Exceptions

An exception is thrown for one of three reasons:

Exceptions are represented by instances of the class Throwable and instances of its subclasses. These classes are, collectively, the exception classes.

11.2 Compile-Time Checking of Exceptions

The Java language checks, at compile time, that a Java program contains handlers for checked exceptions, by analyzing which checked exceptions can result from execution of a method or constructor. For each checked exception which is a possible result, the throws clause for the method (§8.4.4) or constructor (§8.6.4) must mention the class of that exception or one of the superclasses of the class of that exception. This compile-time checking for the presence of exception handlers is designed to reduce the number of exceptions which are not properly handled.

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 standard Java API defines a number of exception classes, both checked and unchecked. Additional exception classes, both checked and unchecked, may be declared by Java programmers. See §11.5 for a description of the Java exception class hierarchy and the exception classes defined by the standard 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).

Variable initializers for fields (§8.3.2) and static initializers (§8.5) must not result in a checked exception; if one does, a compile-time error occurs.

11.2.1 Why Errors are Not Checked

Those unchecked exception classes which are the error classes (Error and its subclasses) are exempted from compile-time checking because they can occur at many points in the program and recovery from them is difficult or impossible. A Java program declaring such exceptions would be cluttered, pointlessly.

11.2.2 Why Runtime Exceptions are Not Checked

The runtime exception classes (RuntimeException and its subclasses) are exempted from compile-time checking because, in the judgment of the designers of Java, having to declare such exceptions would not aid significantly in establishing the correctness of Java programs. Many of the operations and constructs of the Java language can result in runtime exceptions. The information available to a Java compiler, and the level of analysis the compiler performs, are usually not sufficient to establish that such runtime exceptions cannot occur, even though this may be obvious to the Java programmer. Requiring such exception classes to be declared would simply be an irritation to Java programmers.

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 Java Language Specification.

11.3 Handling of an Exception

When an exception is thrown, control is transferred from the code that caused the exception to the nearest dynamically-enclosing catch clause of a try statement (§14.18) that handles the 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:

Whether a particular catch clause handles an exception is determined by comparing the class of the object that was thrown to the declared type of the parameter of the catch clause. The catch clause handles the exception if the type of its parameter is the class of the exception or a superclass of the class of the exception. Equivalently, a catch clause will catch any exception object that is an instanceof (§15.19.2) the declared parameter type.

The control transfer that occurs when an exception is thrown causes abrupt completion of expressions (§15.5) 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 (§20.21.31) 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 (§14.18.2) 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 §15.5).

11.3.1 Exceptions are Precise

Exceptions in Java are precise: when the transfer of control takes place, all effects of the statements executed and expressions evaluated before the point from which the exception is thrown must appear to have taken place. No expressions, statements, or parts thereof 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 expressions or statements 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 Java program.

11.3.2 Handling Asynchronous Exceptions

Most exceptions in Java occur synchronously as a result of an action by the thread in which they occur, and at a point in the Java 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 Java program.

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

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 InternalError is considered asynchronous so that it may be handled using the same mechanism that handles the stop method, as will now be described.

Java 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 language.

A simple implementation might poll for asynchronous exceptions at the point of each control transfer instruction. Since a Java 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 Mark 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

Consider the following example: 


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]");
		}
	}
}
If we execute the test program, passing it the arguments: 

divide null not test
it produces the output: 


[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
This example declares an exception class TestException. The main method of class Test invokes the thrower method four times, causing exceptions to be thrown three of the four times. The try statement in method main catches each exception that the thrower throws. Whether the invocation of thrower completes normally or abruptly, a message is printed describing what happened.

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

The possible exceptions in a Java program are organized in a hierarchy of classes, rooted at class Throwable (§11.5, §20.22), a direct subclass of Object. The classes Exception and Error are direct subclasses of Throwable. The class RuntimeException is a direct subclass of Exception.

The exception classes declared by the standard packages java.lang, java.util, java.io and java.net are called the standard exception classes.

Java 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 Java'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.

11.5.1 The Classes Exception and RuntimeException

The class Exception is the superclass of all the exceptions that ordinary programs may wish to recover from.

11.5.1.1 Standard Runtime Exceptions

The class RuntimeException is a subclass of class Exception. The subclasses of RuntimeException are unchecked exception classes.

Package java.lang defines the following standard unchecked runtime exceptions, which, like all other classes in package java.lang, are implicitly imported and therefore may be referred to by their simple names:

Package java.util defines the following additional standard unchecked runtime exceptions:

11.5.1.2 Standard Checked Exceptions

The standard subclasses of Exception other than RuntimeException are all checked exception classes.

Package java.lang defines the following standard exceptions, which, like all other classes in package java.lang, are implicitly imported and therefore may be referred to by their simple names:

Package java.io defines the following additional standard exceptions:

The standard package java.net defines the following additional subclasses of java.io.IOException:

u java.net.MalformedURLException: A string that was provided as a URL, or as part of a URL, had an inappropriate format or specified an unknown protocol.

11.5.2 The Class Error

The class Error and its standard subclasses are exceptions from which ordinary programs are not ordinarily expected to recover. 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) {
to catch all exceptions from which recovery may be possible without catching errors from which recovery is typically not possible.

Package java.lang defines all the error classes described here. These classes, like all other classes in package java.lang, are implicitly imported and therefore may be referred to by their simple names.

11.5.2.1 Loading and Linkage Errors

A Java Virtual Machine throws an object that is an instance of a subclass of LinkageError when a loading, linkage, preparation, verification or initialization error occurs:

11.5.2.2 Virtual Machine Errors

A Java Virtual Machine throws an object that is an instance of a subclass of the class VirtualMachineError when an internal error or resource limitation prevents it from implementing the semantics of the Java Language. This language specification and the Java Virtual Machine Specification define the following virtual machine errors:

A sophisticated Java program may be designed to handle OutOfMemoryError and attempt to recover from it, perhaps by carefully dropping references to objects.

We are exploring enhancements to Java to simplify handling of out-of-memory conditions. One possibility would be to support automatic suspension of a thread which encounters an OutOfMemoryError and allow another thread to handle the error situation. Such a technique might also permit a Java program to recover from a StackOverflowError if this overflow does not result from a nonterminating recursion. Suggestions for other approaches are welcomed.

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Java Language Specification (HTML generated by Suzette Pelouch on February 24, 1998)
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