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 SE 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 (§14.18).
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 (§11.1). 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 (§14.20).
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 (§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.
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.
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 run-time exception classes.
• 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 unchecked exception classes are the run-time 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 Throwable and all its subclasses 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.
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.
Note that a subclass of Throwable cannot be generic (§8.1.2).
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).
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.
Asynchronous exceptions occur only as a result of:
• An invocation of the (deprecated) stop method of class Thread or ThreadGroup.
The (deprecated) 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 or resource limitation in the Java Virtual Machine that prevents it from implementing the semantics of the Java programming language. In this case, the asynchronous exception that is thrown is an instance of a subclass of VirtualMachineError.
Note that StackOverflowError, a subclass of VirtualMachineError, may be thrown synchronously by method invocation (§15.12.4.5) as well as asynchronously due to native method execution or Java Virtual Machine resource limitations. Similarly, OutOfMemoryError, another subclass of VirtualMachineError, may be thrown synchronously during object creation (§12.5), array creation (§15.10.1, §10.6), class initialization (§12.4.2), and boxing conversion (§5.1.7), as well as asynchronously.
The Java SE platform permits a small but bounded amount of execution to occur before an asynchronous exception is thrown.
Asynchronous exceptions are rare, but proper understanding of their semantics is necessary if high-quality machine code is to be generated.
The delay noted above 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.
The Java programming language requires that a program contains handlers for checked exceptions which can result from execution of a method or constructor (§8.4.6, §8.8.5). This compile-time checking for the presence of exception handlers is designed to reduce the number of exceptions which are not properly handled. For each checked exception which is a possible result, the throws clause for the method or constructor must mention the class of that exception or one of the superclasses of the class of that exception (§11.2.3).
The checked exception classes (§11.1.1) 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 (§8.4.8.3). 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 (§9.4.1).
The unchecked exception classes (§11.1.1) are exempted from compile-time checking.
Error classes are exempted because they can occur at many points in the program and recovery from them is difficult or impossible. A program declaring such exceptions would be cluttered, pointlessly. Sophisticated programs may yet wish to catch and attempt to recover from some of these conditions.
Run-time exception classes are exempted because, in the judgment of the designers of the Java programming language, having to declare such exceptions would not aid significantly in establishing the correctness of programs. Many of the operations and constructs of the Java programming language can result in exceptions at run time. The information available to a Java compiler, and the level of analysis a compiler performs, are usually not sufficient to establish that such run-time exceptions cannot occur, even though this may be obvious to the programmer. Requiring such exception classes to be declared would simply be an irritation to 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 Java 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.
We say that a statement or expression can throw an exception class E if, according to the rules in §11.2.1 and §11.2.2, the execution of the statement or expression can result in an exception of class E being thrown.
We say that a catch clause can catch its catchable exception class(es):
• The catchable exception class of a uni-catch clause is the declared type of its exception parameter (§14.20).
• The catchable exception classes of a multi-catch clause are the alternatives in the union that denotes the type of its exception parameter.
A class instance creation expression (§15.9) can throw an exception class E iff either:
• The expression is a qualified class instance creation expression and the qualifying expression can throw E; or
• Some expression of the argument list can throw E; or
• E is one of the exception types of the invocation type of the chosen constructor (§15.12.2.6); or
• The class instance creation expression includes a ClassBody, and some instance initializer or instance variable initializer in the ClassBody can throw E.
A method invocation expression (§15.12) can throw an exception class E iff either:
• The method invocation expression is of the form Primary . [TypeArguments] Identifier and the Primary expression can throw E; or
• Some expression of the argument list can throw E; or
• E is one of the exception types of the invocation type of the chosen method (§15.12.2.6).
A lambda expression (§15.27) can throw no exception classes.
For every other kind of expression, the expression can throw an exception class E iff one of its immediate subexpressions can throw E.
Note that a method reference expression (§15.13) of the form Primary :: [TypeArguments] Identifier can throw an exception class if the Primary subexpression can throw an exception class. In contrast, a lambda expression can throw nothing, and has no immediate subexpressions on which to perform exception analysis. It is the body of a lambda expression, containing expressions and statements, that can throw exception classes.
A throw statement (§14.18) whose thrown expression has static type E and is not a final or effectively final exception parameter can throw E or any exception class that the thrown expression can throw.
For example, the statement throw new java.io.FileNotFoundException(); can throw java.io.FileNotFoundException only. Formally, it is not the case that it “can throw” a subclass or superclass of java.io.FileNotFoundException.
A throw statement whose thrown expression is a final or effectively final exception parameter of a catch clause C can throw an exception class E iff:
• E is an exception class that the try block of the try statement which declares C can throw; and
• E is assignment compatible with any of C’s catchable exception classes; and
• E is not assignment compatible with any of the catchable exception classes of the catch clauses declared to the left of C in the same try statement.
A try statement (§14.20) can throw an exception class E iff either:
• The try block can throw E, or an expression used to initialize a resource (in a try-with-resources statement) can throw E, or the automatic invocation of the close() method of a resource (in a try-with-resources statement) can throw E, and E is not assignment compatible with any catchable exception class of any catch clause of the try statement, and either no finally block is present or the finally block can complete normally; or
• Some catch block of the try statement can throw E and either no finally block is present or the finally block can complete normally; or
• A finally block is present and can throw E.
An explicit constructor invocation statement (§8.8.7.1) can throw an exception class E iff either:
• Some expression of the constructor invocation’s parameter list can throw E; or
• E is determined to be an exception class of the throws clause of the constructor that is invoked (§15.12.2.6).
Any other statement S can throw an exception class E iff an expression or statement immediately contained in S can throw E.
It is a compile-time error if a method or constructor body can throw some exception class E when E is a checked exception class and E is not a subclass of some class declared in the throws clause of the method or constructor.
It is a compile-time error if a lambda body can throw some exception class E when E is a checked exception class and E is not a subclass of some class declared in the throws clause of the function type targeted by the lambda expression.
It is a compile-time error if a class variable initializer (§8.3.2) or static initializer (§8.7) of a named class or interface can throw a checked exception class.
It is a compile-time error if an instance variable initializer (§8.3.2) or instance initializer (§8.6) of a named class can throw a checked exception class, unless the named class has at least one explicitly declared constructor and the exception class or one of its superclasses is explicitly declared in the throws clause of each constructor.
Note that no compile-time error is due if an instance variable initializer or instance initializer of an anonymous class (§15.9.5) can throw an exception class. In a named class, it is the responsibility of the programmer to propagate information about which exception classes can be thrown by initializers, by declaring a suitable throws clause on any explicit constructor declaration. This relationship between the checked exception classes thrown by a class’s initializers and the checked exception classes declared by a class’s constructors is assured for an anonymous class declaration, because no explicit constructor declarations are possible and a Java compiler always generates a constructor with a suitable throws clause for the anonymous class declaration based on the checked exception classes that its initializers can throw.
It is a compile-time error if a catch clause can catch checked exception class E1 and it is not the case that the try block corresponding to the catch clause can throw a checked exception class that is a subclass or superclass of E1, unless E1 is Exception or a superclass of Exception.
It is a compile-time error if a catch clause can catch an exception class E1 and a preceding catch clause of the immediately enclosing try statement can catch E1 or a superclass of E1.
A Java compiler is encouraged to issue a warning if a catch clause can catch checked exception class E1 and the try block corresponding to the catch clause can throw checked exception class E2, where E2 <: E1, and a preceding catch clause of the immediately enclosing try statement can catch checked exception class E3, where E2 <: E3 <: E1.
Example 11.2.3-1. Catching Checked Exceptions
import java.io.*;
class StaticallyThrownExceptionsIncludeSubtypes {
public static void main(String[] args) {
try {
throw new FileNotFoundException();
} catch (IOException ioe) {
// "catch IOException" catches IOException
// and any subtype.
}
try {
throw new FileNotFoundException();
// Statement "can throw" FileNotFoundException.
// It is not the case that statement "can throw"
// a subtype or supertype of FileNotFoundException.
} catch (FileNotFoundException fnfe) {
// ... Handle exception ...
} catch (IOException ioe) {
// Legal, but compilers are encouraged to give
// warnings as of Java SE 7, because all subtypes of
// IOException that the try block "can throw" have
// already been caught by the prior catch clause.
}
try {
m();
// m's declaration says "throws IOException", so
// m "can throw" IOException. It is not the case
// that m "can throw" a subtype or supertype of
// IOException (e.g. Exception).
} catch (FileNotFoundException fnfe) {
// Legal, because the dynamic type of the exception
// might be FileNotFoundException.
} catch (IOException ioe) {
// Legal, because the dynamic type of the exception
// might be a different subtype of IOException.
} catch (Throwable t) {
// Can always catch Throwable.
}
}
static void m() throws IOException {
throw new FileNotFoundException();
}
}
By the rules above, each alternative in a multi-catch clause (§14.20) must be able to catch some exception class thrown by the try block and uncaught by previous catch clauses. For example, the second catch clause below would cause a compile-time error because exception analysis determines that SubclassOfFoo is already caught by the first catch clause:
try { ... }
catch (Foo f) { ... }
catch (Bar | SubclassOfFoo e) { ... }
When an exception is thrown (§14.18), control is transferred from the code that caused the exception to the nearest dynamically enclosing catch clause, if any, of a try statement (§14.20) that can handle 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:
• 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 (§12.4).
Whether a particular catch clause can handle an exception is determined by comparing the class of the object that was thrown to the catchable exception classes of the catch clause. The catch clause can handle the exception if one of its catchable exception classes 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 (§15.20.2) one of its catchable exception classes.
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.
All exceptions (synchronous and asynchronous) 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 program.
If no catch clause that can handle an exception can be found, then the current thread (the thread that encountered the exception) is terminated. Before termination, all finally clauses are executed and the uncaught exception is handled according to the following rules:
• If the current thread has an uncaught exception handler set, then that handler is executed.
• Otherwise, the method uncaughtException is invoked for the ThreadGroup that is the parent of the current thread. If the ThreadGroup and its parent ThreadGroups do not override uncaughtException, then the default handler’s uncaughtException method is invoked.
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 (§14.20.2) 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 (Blocks and Statements) and for expressions in §15 (Expressions) (especially §15.6).
Example 11.3-1. Throwing and Catching Exceptions
The following program 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.
class TestException extends Exception {
TestException() { super(); }
TestException(String s) { super(s); }
}
class Test {
public static void main(String[] args) {
for (String arg : args) {
try {
thrower(arg);
System.out.println("Test \"" + arg +
"\" didn't throw an exception");
} catch (Exception e) {
System.out.println("Test \"" + arg +
"\" 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 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
The declaration of the method thrower must have a throws clause because it can throw instances of TestException, which is a checked exception class (§11.1.1). A compiletime 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.