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4.5 Exceptions

Do, or do not... There is no try.

--Yoda (The Empire Strikes Back)

Java's roots are in embedded systems--software that runs inside specialized devices like hand-held computers, cellular phones, and fancy toasters. In those kinds of applications, it's especially important that software errors be handled properly. Most users would agree that it's unacceptable for their phone to simply crash or for their toast (and perhaps their house) to burn because their software failed. Given that we can't eliminate the possibility of software errors, a step in the right direction is to at least try to recognize and deal with the application-level errors that we can anticipate in a methodical and systematic way.

Dealing with errors in a language like C is the responsibility of the programmer. There is no help from the language itself in identifying error types, and there are no tools for dealing with them easily. In C and C++, a routine generally indicates a failure by returning an "unreasonable" value (e.g., the idiomatic -1 or null). As the programmer, you must know what constitutes a bad result, and what it means. It's often awkward to work around the limitations of passing error values in the normal path of data flow.[3] An even worse problem is that certain types of errors can legitimately occur almost anywhere, and it's prohibitive and unreasonable to explicitly test for them at every point in the software.

[3] The somewhat obscure setjmp() and longjmp() statements in C can save a point in the execution of code and later return to it unconditionally from a deeply buried location. In a limited sense, this is the functionality of exceptions in Java.

Java offers an elegant solution to these problems with exception handling. (Java exception handling is similar to, but not quite the same as, exception handling in C++.) An exception indicates an unusual condition or an error condition. Program control becomes unconditionally transferred or thrown to a specially designated section of code where it's caught and handled. In this way, error handling is somewhat orthogonal to the normal flow of the program. We don't have to have special return values for all our methods; errors are handled by a separate mechanism. Control can be passed long distance from a deeply nested routine and handled in a single location when that is desirable, or an error can be handled immediately at its source. There are still a few methods that return -1 as a special value, but these are limited to situations in which there isn't really an error.[4]

[4] For example, the getHeight() method of the Image class returns -1 if the height isn't known yet. No error has occurred; the height will be available in the future. In this situation, throwing an exception would be inappropriate.

A Java method is required to specify the exceptions it can throw (i.e., the ones that it doesn't catch itself); this means that the compiler can make sure we handle them. In this way, the information about what errors a method can produce is promoted to the same level of importance as its argument and return types. You may still decide to punt and ignore obvious errors, but in Java you must do so explicitly.

Exceptions and Error Classes

Exceptions are represented by instances of the class java.lang.Exception and its subclasses. Subclasses of Exception can hold specialized information (and possibly behavior) for different kinds of exceptional conditions. However, more often they are simply "logical" subclasses that exist only to serve as a new exception type (more on that later). Figure 4.1 shows the subclasses of Exception; these classes are defined in various packages in the Java API, as indicated in the diagram.

An Exception object is created by the code at the point where the error condition arises. It can hold whatever information is necessary to describe the exceptional condition, including a full stack trace for debugging. The exception object is passed, along with the flow of control, to the handling block of code. This is where the terms "throw" and "catch" come from: the Exception object is thrown from one point in the code and caught by the other, where execution resumes.

The Java API also defines the java.lang.Error class for eggregious or unrecoverable errors. The subclasses of Error are shown in Figure 4.2. You needn't worry about these errors (i.e., you do not have to catch them); they normally indicate linkage problems or virtual machine errors. An error of this kind usually causes the Java interpreter to display a message and exit.

Exception Handling

The try/catch guarding statements wrap a block of code and catch designated types of exceptions that occur within it:

try { 
    readFromFile("foo"); 
    ... 
}  
catch ( Exception e ) { 
    // Handle error 
    System.out.println( "Exception while reading file: " + e ); 
    ... 
} 

In the above example, exceptions that occur within the body of the try statement are directed to the catch clause for possible handling. The catch clause acts like a method; it specifies an argument of the type of exception it wants to handle, and, if it's invoked, the Exception object is passed into its body as an argument. Here we receive the object in the variable e and print it along with a message.

A try statement can have multiple catch clauses that specify different specific types (subclasses) of Exception:

try { 
    readFromFile("foo"); 
    ... 
}  
catch ( FileNotFoundException e ) { 
    // Handle file not found 
    ... 
}  
catch ( IOException e ) { 
    // Handle read error 
    ... 
}  
catch ( Exception e ) { 
    // Handle all other errors 
    ... 
} 

The catch clauses are evaluated in order, and the first possible (assignable) match is taken. At most one catch clause is executed, which means that the exceptions should be listed from most specific to least. In the above example, we'll assume that the hypothetical readFromFile() can throw two different kinds of exceptions: one that indicates the file is not found; the other indicates a more general read error. Any subclass of Exception is assignable to the parent type Exception, so the third catch clause acts like the default clause in a switch statement and handles any remaining possibilities.

It should be obvious, but one beauty of the try/catch statement is that any statement in the try block can assume that all previous statements in the block succeeded. A problem won't arise suddenly because a programmer forgot to check the return value from some method. If an earlier statement fails, execution jumps immediately to the catch clause; later statements are never executed.

Bubbling Up

What if we hadn't caught the exception? Where would it have gone? Well, if there is no enclosing try/catch statement, the exception pops to the top of the method in which it appeared and is, in turn, thrown from that method. In this way, the exception bubbles up until it's caught, or until it pops out of the top of the program, terminating it with a run-time error message. There's a bit more to it than that because, in this case, the compiler would have reminded us to deal with it, but we'll get back to that in a moment.

Let's look at another example. In Figure 4.3, the method getContent() invokes the method openConnection() from within a try/catch statement. openConnection(), in turn, invokes the method sendRequest(), which calls the method write() to send some data.

In this figure, the second call to write() throws an IOException. Since sendRequest() doesn't contain a try/catch statement to handle the exception, it's thrown again, from the point that it was called in the method openConnection(). Since openConnection() doesn't catch the exception either, it's thrown once more. Finally it's caught by the try statement in getContent() and handled by its catch clause.

Since an exception can bubble up quite a distance before it is caught and handled, we may need a way to determine exactly where it was thrown. All exceptions can dump a stack trace that lists their method of origin and all of the nested method calls that it took to arrive there, using the printStackTrace() method.

try {
    // complex task
} catch ( Exception e ) {
    // dump information about exactly where the exception ocurred
    e.printStackTrack( System.err );
    ...
}

The throws Clause and checked Exceptions

I mentioned earlier that Java makes us be explicit about our error handling. But Java is programmer-friendly, and it's not possible to require that every conceivable type of error be handled in every situation. So, Java exceptions are divided into two categories: checked exceptions and unchecked exceptions. Most application level exceptions are checked, which means that any method that throws one, either by generating it itself (as we'll discuss below) or by passively ignoring one that occurs within it, must declare that it can throw that type of exception in a special throws clause in its method declaration. We haven't yet talked in detail about declaring methods; we'll cover that in Chapter 5, Objects in Java. For now all you need know is that methods have to declare the checked exceptions they can throw or allow to be thrown.

Again in Figure 4.3, notice that the methods openConnection() and sendRequest() both specify that they can throw an IOException. If we had to throw multiple types of exceptions we could declare them separated with commas:

void readFile( String s ) throws IOException, InterruptedException { 
    ... 
} 

The throws clause tells the compiler that a method is a possible source of that type of checked exception and that anyone calling that method must be prepared to deal with it. The caller may use a try/catch block to catch it, or it may, itself, declare that it can throw the exception.

Exceptions that are subclasses of the java.lang.RuntimeException class are unchecked. See Figure 4.1 for the subclasses of RuntimeException. It's not a compile-time error to ignore the possibility of these exceptions being thrown; additionally, methods don't have to declare they can throw them. In all other respects, run-time exceptions behave the same as other exceptions. We are perfectly free to catch them if we wish; we simply aren't required to.

Checked exceptions

Exceptions a reasonable application should try to handle gracefully.

Unchecked exception (Runtime exceptions)

Exceptions from which we would not normally expect our software to try to recover.

The category of checked exceptions includes application-level problems like missing files and unavailable hosts. As good programmers (and upstanding citizens), we should design software to recover gracefully from these kinds of conditions. The category of unchecked exceptions includes problems such as "out of memory" and "array index out of bounds." While these may indicate application-level programming errors, they can occur almost anywhere and aren't generally easy to recover from. Fortunately, because there are unchecked exceptions, you don't have to wrap every one of your array-index operations in a try/catch statement.

Throwing Exceptions

We can throw our own exceptions: either instances of Exception or one of its predefined subclasses, or our own specialized subclasses. All we have to do is create an instance of the Exception and throw it with the throw statement:

throw new Exception(); 

Execution stops and is transferred to the nearest enclosing try/catch statement. (Note that there is little point in keeping a reference to the Exception object we've created here.) An alternative constructor of the Exception class lets us specify a string with an error message:

throw new Exception("Something really bad happened");

By convention, all types of Exception have a String constructor like this. Note that the String message above is somewhat facetious and vague. Normally you won't be throwing a plain old Exception, but a more specific subclass. For example:

public void checkRead( String s ) {  
    if ( new File(s).isAbsolute() || (s.indexOf("..") != -1) ) 
        throw new SecurityException(
           x"Access to file : "+ s +" denied."); 
} 

In the above, we partially implement a method to check for an illegal path. If we find one, we throw a SecurityException, with some information about the transgression.

Of course, we could include whatever other information is useful in our own specialized subclasses of Exception (or SecurityException). Often though, just having a new type of exception is good enough, because it's sufficient to help direct the flow of control. For example, if we are building a parser, we might want to make our own kind of exception to indicate a particular kind of failure.

class ParseException extends Exception {
    ParseException() { 
        super(); }
    ParseException( String desc ) { 
        super( desc ) };
}

See Chapter 5, Objects in Java for a full description of classes and class constructors. The body of our exception class here simply allows a ParseException to be created in the conventional ways that we have created exceptions above. Now that we have our new exception type, we we might guard for it in the following kind of situation:

// Somewhere in our code
...
try {
    parseStream( input );
} catch ( ParseException pe ) {
    // Bad input...
} catch ( IOException ioe ) {
    // Low level communications problem
}

As you can see, although our new exception doesn't currently hold any specialized information about the problem (it certainly could), it does let us distinguish a parse error from an arbitrary communications error in the same chunk of code. You might call this kind of specialization of an exception to be making a "logical" exception.

Re-throwing exceptions

Sometimes you'll want to take some action based on an exception and then turn around and throw a new exception in its place. For example, suppose that we want to handle an IOException by freeing up some resources before allowing the failure to pass on to the rest of the application. You can do this in the obvious way, by simply catching the exception and then throwing it again or throwing a new one.

  *** I was going to say something about fillInStackTrack() here ***

Try Creep

The try statement imposes a condition on the statements they guard. It says that if an exception occurs within it, the remaining statements will be abandoned. This has consequences for local variable initialization. If the compiler can't determine whether a local variable assignment we placed inside a try/catch block will happen, it won't let us use the variable:

void myMethod() { 
    int foo; 
 
    try { 
        foo = getResults(); 
    }  
    catch ( Exception e ) { 
        ... 
    } 
 
    int bar = foo;  // Compile time error--foo may not 
                    // have been initialized 

In the above example, we can't use foo in the indicated place because there's a chance it was never assigned a value. One obvious option is to move the assignment inside the try statement:

try { 
    foo = getResults(); 
    int bar = foo;  // Okay because we only get here 
                    // if previous assignment succeeds 
}  
catch ( Exception e ) { 
    ... 
} 

Sometimes this works just fine. However, now we have the same problem if we want to use bar later in myMethod(). If we're not careful, we might end up pulling everything into the try statement. The situation changes if we transfer control out of the method in the catch clause:

try { 
    foo = getResults(); 
}  
catch ( Exception e ) { 
    ... 
    return; 
} 
 
int bar = foo;  // Okay because we only get here 
                // if previous assignment succeeds 

Your code will dictate its own needs; you should just be aware of the options.

The finally Clause

What if we have some clean up to do before we exit our method from one of the catch clauses? To avoid duplicating the code in each catch branch and to make the cleanup more explicit, Java supplies the finally clause. A finally clause can be added after a try and any associated catch clauses. Any statements in the body of the finally clause are guaranteed to be executed, no matter why control leaves the try body:

try { 
    // Do something here 
}  
catch ( FileNotFoundException e ) { 
    ... 
}  
catch ( IOException e ) { 
    ... 
}  
catch ( Exception e ) { 
    ... 
}  
finally { 
    // Cleanup here 
} 

In the above example the statements at the cleanup point will be executed eventually, no matter how control leaves the try. If control transfers to one of the catch clauses, the statements in finally are executed after the catch completes. If none of the catch clauses handles the exception, the finally statements are executed before the exception propagates to the next level.

If the statements in the try execute cleanly, or even if we perform a return, break, or continue, the statements in the finally clause are executed. To perform cleanup operations, we can even use try and finally without any catch clauses:

try { 
    // Do something here 
    return; 
}  
finally { 
    System.out.println("Whoo-hoo!"); 
} 

Exceptions that occur in a catch or finally clause are handled normally; the search for an enclosing try/catch begins outside the offending try statement.


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