Exception Handling in Java

Exception Handling in Java

1. Overview

In this tutorial, we’ll go through the basics of exception handling in Java as well as some of its gotchas.

2. First Principles

2.1. What is it?

To better understand exceptions and exception handling, let’s make a real-life comparison.

Imagine that we order a product online, but while en-route, there’s a failure in delivery. A good company can handle this problem and gracefully re-route our package so that it still arrives on time.

Likewise, in Java, the code can experience errors while executing our instructions. Good exception handling can handle errors and gracefully re-route the program to give the user still a positive experience.

2.2. Why use it? 

We usually write code in an idealized environment: the filesystem always contains our files, the network is healthy, and the JVM always has enough memory. Sometimes we call this the “happy path”.

In production, though, filesystems can corrupt, networks break down, and JVMs run out of memory. The wellbeing of our code depends on how it deals with “unhappy paths”.

We must handle these conditions because they affect the flow of the application negatively and form exceptions:

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public static List<Player> getPlayers() throws IOException {     Path path = Paths.get("players.dat");     List<String> players = Files.readAllLines(path);     return players.stream()       .map(Player::new)       .collect(Collectors.toList()); }

This code chooses not to handle the IOException, passing it up the call stack instead. In an idealized environment, the code works fine.

But what might happen in production if players.dat is missing?

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Exception in thread "main" java.nio.file.NoSuchFileException: players.dat <-- players.dat file doesn't exist     at sun.nio.fs.WindowsException.translateToIOException(Unknown Source)     at sun.nio.fs.WindowsException.rethrowAsIOException(Unknown Source)     // ... more stack trace     at java.nio.file.Files.readAllLines(Unknown Source)     at java.nio.file.Files.readAllLines(Unknown Source)     at Exceptions.getPlayers(Exceptions.java:12) <-- Exception arises in getPlayers() method, on line 12     at Exceptions.main(Exceptions.java:19) <-- getPlayers() is called by main(), on line 19

Without handling this exception, an otherwise healthy program may stop running altogether! We need to make sure that our code has a plan for when things go wrong.

Also note one more benefit here to exceptions, and that is the stack trace itself. Because of this stack trace, we can often pinpoint offending code without needing to attach a debugger.

3. Exception Hierarchy

Ultimately, exceptions are just Java objects with all of them extending from Throwable:

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---> Throwable <---               |    (checked)     |               |                  |               |                  |       ---> Exception           Error       |    (checked)        (unchecked)       | RuntimeException   (unchecked)

There are three main categories of exceptional conditions:

• Checked exceptions
• Unchecked exceptions / Runtime exceptions
• Errors

Runtime and unchecked exceptions refer to the same thing. We can often use them interchangeably. 

3.1. Checked Exceptions

Checked exceptions are exceptions that the Java compiler requires us to handle. We have to either declaratively throw the exception up the call stack, or we have to handle it ourselves. More on both of these in a moment.

Oracle’s documentation tells us to use checked exceptions when we can reasonably expect the caller of our method to be able to recover.

A couple of examples of checked exceptions are IOException and ServletException.

3.2. Unchecked Exceptions

Unchecked exceptions are exceptions that the Java compiler does not require us to handle.

Simply put, if we create an exception that extends RuntimeException, it will be unchecked; otherwise, it will be checked.

And while this sounds convenient, Oracle’s documentation tells us that there are good reasons for both concepts, like differentiating between a situational error (checked) and a usage error (unchecked).

Some examples of unchecked exceptions are NullPointerException, IllegalArgumentException, and SecurityException.

3.3. Errors

Errors represent serious and usually irrecoverable conditions like a library incompatibility, infinite recursion, or memory leaks.

And even though they don’t extend RuntimeException, they are also unchecked.

In most cases, it’d be weird for us to handle, instantiate or extend Errors. Usually, we want these to propagate all the way up.

A couple of examples of errors are a StackOverflowError and OutOfMemoryError.

4. Handling Exceptions

In the Java API, there are plenty of places where things can go wrong, and some of these places are marked with exceptions, either in the signature or the Javadoc:

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/**  * @exception FileNotFoundException ...  */ public Scanner(String fileName) throws FileNotFoundException {    // ... }

As stated a little bit earlier, when we call these “risky” methods, we must handle the checked exceptions, and we may handle the unchecked ones. Java gives us several ways to do this:

4.1. throws

The simplest way to “handle” an exception is to rethrow it:

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public int getPlayerScore(String playerFile)   throws FileNotFoundException {        Scanner contents = new Scanner(new File(playerFile));     return Integer.parseInt(contents.nextLine()); }

Because FileNotFoundException is a checked exception, this is the simplest way to satisfy the compiler, but it does mean that anyone that calls our method now needs to handle it too!

parseInt can throw a NumberFormatException, but because it is unchecked, we aren’t required to handle it.

4.2. try–catch

If we want to try and handle the exception ourselves, we can use a try-catch block. We can handle it by rethrowing our exception:

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public int getPlayerScore(String playerFile) {     try {         Scanner contents = new Scanner(new File(playerFile));         return Integer.parseInt(contents.nextLine());     } catch (FileNotFoundException noFile) {         throw new IllegalArgumentException("File not found");     } }

Or by performing recovery steps:

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public int getPlayerScore(String playerFile) {     try {         Scanner contents = new Scanner(new File(playerFile));         return Integer.parseInt(contents.nextLine());     } catch ( FileNotFoundException noFile ) {         logger.warn("File not found, resetting score.");         return 0;     } }

4.3. finally

Now, there are times when we have code that needs to execute regardless of whether an exception occurs, and this is where the finally keyword comes in.

In our examples so far, there ‘s been a nasty bug lurking in the shadows, which is that Java by default won’t return file handles to the operating system.

Certainly, whether we can read the file or not, we want to make sure that we do the appropriate cleanup!

Let’s try this the “lazy” way first:

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public int getPlayerScore(String playerFile)   throws FileNotFoundException {     Scanner contents = null;     try {         contents = new Scanner(new File(playerFile));         return Integer.parseInt(contents.nextLine());     } finally {         if (contents != null) {             contents.close();         }     } }

Here, the finally block indicates what code we want Java to run regardless of what happens with trying to read the file.

Even if a FileNotFoundException is thrown up the call stack, Java will call the contents of finally before doing that.

We can also both handle the exception and make sure that our resources get closed:

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public int getPlayerScore(String playerFile) {     Scanner contents;     try {         contents = new Scanner(new File(playerFile));         return Integer.parseInt(contents.nextLine());     } catch (FileNotFoundException noFile ) {         logger.warn("File not found, resetting score.");         return 0;     } finally {         try {             if (contents != null) {                 contents.close();             }         } catch (IOException io) {             logger.error("Couldn't close the reader!", io);         }     } }

Because close is also a “risky” method, we also need to catch its exception!

This may look pretty complicated, but we need each piece to handle each potential problem that can arise correctly.

4.4. try-with-resources

Fortunately, as of Java 7, we can simplify the above syntax when working with things that extend AutoCloseable:

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public int getPlayerScore(String playerFile) {     try (Scanner contents = new Scanner(new File(playerFile))) {       return Integer.parseInt(contents.nextLine());     } catch (FileNotFoundException e ) {       logger.warn("File not found, resetting score.");       return 0;     } }

When we place references that are AutoClosable in the try declaration, then we don’t need to close the resource ourselves.

We can still use a finally block, though, to do any other kind of cleanup we want.

Check out our article dedicated to try-with-resources to learn more.

4.5. Multiple catch Blocks

Sometimes, the code can throw more than one exception, and we can have more than one catch block handle each individually:

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public int getPlayerScore(String playerFile) {     try (Scanner contents = new Scanner(new File(playerFile))) {         return Integer.parseInt(contents.nextLine());     } catch (IOException e) {         logger.warn("Player file wouldn't load!", e);         return 0;     } catch (NumberFormatException e) {         logger.warn("Player file was corrupted!", e);         return 0;     } }

Multiple catches give us the chance to handle each exception differently, should the need arise.

Also note here that we didn’t catch FileNotFoundException, and that is because it extends IOException. Because we’re catching IOException, Java will consider any of its subclasses also handled.

Let’s say, though, that we need to treat FileNotFoundException differently from the more general IOException:

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public int getPlayerScore(String playerFile) {     try (Scanner contents = new Scanner(new File(playerFile)) ) {         return Integer.parseInt(contents.nextLine());     } catch (FileNotFoundException e) {         logger.warn("Player file not found!", e);         return 0;     } catch (IOException e) {         logger.warn("Player file wouldn't load!", e);         return 0;     } catch (NumberFormatException e) {         logger.warn("Player file was corrupted!", e);         return 0;     } }

Java lets us handle subclass exceptions separately, remember to place them higher in the list of catches.

4.6. Union catch Blocks

When we know that the way we handle errors is going to be the same, though, Java 7 introduced the ability to catch multiple exceptions in the same block:

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public int getPlayerScore(String playerFile) {     try (Scanner contents = new Scanner(new File(playerFile))) {         return Integer.parseInt(contents.nextLine());     } catch (IOException | NumberFormatException e) {         logger.warn("Failed to load score!", e);         return 0;     } }

5. Throwing Exceptions

If we don’t want to handle the exception ourselves or we want to generate our exceptions for others to handle, then we need to get familiar with the throw keyword.

Let’s say that we have the following checked exception we’ve created ourselves:

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public class TimeoutException extends Exception {     public TimeoutException(String message) {         super(message);     } }

and we have a method that could potentially take a long time to complete:

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public List<Player> loadAllPlayers(String playersFile) {     // ... potentially long operation }

5.1. Throwing a Checked Exception

Like returning from a method, we can throw at any point.

Of course, we should throw when we are trying to indicate that something has gone wrong:

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public List<Player> loadAllPlayers(String playersFile) throws TimeoutException {     while ( !tooLong ) {         // ... potentially long operation     }     throw new TimeoutException("This operation took too long"); }

Because TimeoutException is checked, we also must use the throws keyword in the signature so that callers of our method will know to handle it.

5.2. Throwing an Unchecked Exception

If we want to do something like, say, validate input, we can use an unchecked exception instead:

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public List<Player> loadAllPlayers(String playersFile) throws TimeoutException {     if(!isFilenameValid(playersFile)) {         throw new IllegalArgumentException("Filename isn't valid!");     }          // ... }

Because IllegalArgumentException is unchecked, we don’t have to mark the method, though we are welcome to.

Some mark the method anyway as a form of documentation.

5.3. Wrapping and Rethrowing

We can also choose to rethrow an exception we’ve caught:

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public List<Player> loadAllPlayers(String playersFile)   throws IOException {     try {         // ...     } catch (IOException io) {              throw io;     } }

Or do a wrap and rethrow:

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public List<Player> loadAllPlayers(String playersFile)   throws PlayerLoadException {     try {         // ...     } catch (IOException io) {              throw new PlayerLoadException(io);     } }

This can be nice for consolidating many different exceptions into one.

5.4. Rethrowing Throwable or Exception

Now for a special case.

If the only possible exceptions that a given block of code could raise are unchecked exceptions, then we can catch and rethrow Throwable or Exception without adding them to our method signature:

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public List<Player> loadAllPlayers(String playersFile) {     try {         throw new NullPointerException();     } catch (Throwable t) {         throw t;     } }

While simple, the above code can’t throw a checked exception and because of that, even though we are rethrowing a checked exception, we don’t have to mark the signature with a throws clause.

This is handy with proxy classes and methods. More about this can be found here.

5.5. Inheritance

When we mark methods with a throws keyword, it impacts how subclasses can override our method.

In the circumstance where our method throws a checked exception:

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public class Exceptions {     public List<Player> loadAllPlayers(String playersFile)       throws TimeoutException {         // ...     } }

A subclass can have a “less risky” signature:

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public class FewerExceptions extends Exceptions {       @Override     public List<Player> loadAllPlayers(String playersFile) {         // overridden     } }

But not a “more riskier” signature:

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public class MoreExceptions extends Exceptions {            @Override     public List<Player> loadAllPlayers(String playersFile) throws MyCheckedException {         // overridden     } }

This is because contracts are determined at compile time by the reference type. If I create an instance of MoreExceptions and save it to Exceptions:

1 2	Exceptions exceptions = new MoreExceptions(); exceptions.loadAllPlayers("file");

Then the JVM will only tell me to catch the TimeoutException, which is wrong since I’ve said that MoreExceptions#loadAllPlayers throws a different exception.

Simply put, subclasses can throw fewer checked exceptions than their superclass, but not more.