JLS100714：Note on JLS, Ch11

 

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Hist:

1007014, draft

 

 

 

Chapter 11 Exceptions(异常)

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 exception is said to be thrown from the point where it occurred and said to be caught at the point to which control is transferred.

Programs can also throw exceptions explicitly, using throw statements.

 

The Causes of Exceptions

Three reasons:

1)      an abnormal execution condition was synchronously detected by the JVM.

2)      a throw statement was executed.

3)      an asynchronous exception occurred either because:

a)         the (deprecated) method stop of class Thread was invoked

b)        an internal error has occurred in the vm

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

 

Compile-Time Checking of Exceptions

A compiler for the Java programming language checks, at compile time, that a 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 or constructor must mention the class of that exception or one of the superclasses of the class of that exception.

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.

 

Exception Analysis of Expressions

 

Exception Analysis of Statements

 

Exception Checking

 

Why Errors are Not Checked

The unchecked exception classes which are the error classes 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 program declaring such exceptions would be cluttered, pointlessly.

 

Why Runtime Exceptions are Not Checked

Because having to declare such exceptions would not aid significantly in establishing the correctness of programs. The information available to a compiler, and the level of analysis the compiler performs, are usually not sufficient to establish that such run-time exception cannot occur.

 

Handling of an Exception

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

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.

 

Exceptions are Precise

 

Handling Asynchronous Exceptions

 

The Exception Hierarchy

 

Loading and Linkage Errors

The JVM throws an object that is an instance of a subclass of LinkageError.

 

Virtual Machine Errors

The JVM throws an object that is an instance of a subclass of VirtualMachineError.

 

 

REF

1.       JLS 3nd

2.       JVM 2nd