Java try catch finally
Java try, catch and finally blocks help in writing the application code which may throw exceptions in runtime and gives us a chance to either recover from exceptions by executing alternate application logic or handle the exception gracefully to report back to the user. It helps in preventing ugly application crashes.
Note that it is recommended to use the try-with-resources block everytime it is possible to use.
Before going deep into the concept, let us go through the very basic understanding of try-catch blocks and their syntax.
The try block contains the application code which is expected to work in normal conditions. For example, reading a file, writing to databases or performing complex business operations.
A try block is written with the try keyword followed by the curly braces.
The optional catch block(s) follows the try block and MUST handle the checked exceptions thrown by the try block and any possible unchecked exceptions.
An application can go wrong in N different ways. That’s why we can associate multiple catch blocks with a single try block. In each catch block, we can handle one or more specific exceptions in a unique way.
When one catch block handles the exception, the next catch blocks are not executed. Control shifts directly from the executed catch block to execute the remaining part of the program, including the finally block.
try < //code >catch(NullPointerException e) < //handle exception >catch(NumberFormatException e) < //handle exception >catch(Exception e) < //handle exception >An optional finally block gives us a chance to run the code which we want to execute EVERYTIME a try-catch block is completed – either with errors or without any errors.
The finally block statements are guaranteed execution even if we fail to handle the exception successfully in catch block.
try < //open file //read file >catch(Exception e) < //handle exception while reading the file >finally < //close the file >1.4. Only the try block is Mandatory
Please note that only try block is mandatory while catch and finally blocks are optional. With a try block, we can use either a catch block or finally block as needed.
It is possible to have below given both combinations in Java. Both versions are valid.
2. How an exception floats in Java?
Under normal circumstances, when there is an exception occurred during runtime, JVM wraps the error information in an instance of the sub-type of Throwable. This exception object is similar to other Java objects and has fields and methods.
Only difference is that JVM checks their presence and passes control to catch block which can handle this exception type or its parent class types.
When in the application, there is no catch block found for an exception, the uncaught exception is handled by a default exception handler at JVM level. It reports the exception to the user and terminates the application.
3. Different execution flows with try, catch and finally blocks
Let’s see some examples to understand how the executions will flow in different cases.
3.1. try, catch and finally blocks – NO exception occurred
If there is no exception occurred, then JVM will execute only finally block. The catch block will be skipped.
try < System.out.println("try block"); >catch (Exception e) < System.out.println("catch block"); >finally 3.2. try, catch and finally blocks – exception occurred
If there is an exception occurred in try block, then JVM will execute catch block first and then finally block.
try < System.out.println("try block"); throw new NullPointerException("Null occurred"); >catch (Exception e) < System.out.println("catch block"); >finally try block catch block finally block3.3. try and finally blocks – exception NOT handled
If the exception is not handled by any provided catch block, the JVM default exception handler handles it. In this case, finally block will be executed followed by the default exception handling mechanism.
try block finally block Exception in thread "main" java.lang.NullPointerException: Null occurred at com.howtodoinjava.Main.main(Main.java:12)3.4. try, catch and finally blocks – multiple catch blocks
If there are multiple catch blocks associated with the try block, then the exception is handled by the first catch block in the sequence which can handle the exception type or its parent types.
For example, a catch block handling IOException, can handle the exceptions of type FileNotFoundException also because FileNotFoundException extends IOException .
try < System.out.println("try block"); throw new NullPointerException("null occurred"); >catch (NumberFormatException e) < System.out.println("catch block 1"); >catch (NullPointerException e) < System.out.println("catch block 2"); >catch (Exception e) < System.out.println("catch block 3"); >finally try block catch block 2 finally block3.5. try, catch and finally blocks – Exception is thrown from catch block
There may be cases when there is an exception while handling another exception in catch block. How will it handled?
In case of exception in the catch block, the execution is transferred to finally block (if any) associated with the respective catch block. Then the exception is propagated in the method call stack to find a catch block that can handle this exception.
If such catch block is found then exception is handled, else JVM default exception handler handles the exception and terminates the application.
try < System.out.println("try block"); throw new NullPointerException("NullPointerException occured"); >catch (NullPointerException e) < System.out.println("catch block 1"); throw new NumberFormatException("NumberFormatException occurred"); >catch (Exception e) < System.out.println("catch block 2"); >finally try block catch block 1 finally block Exception in thread "main" java.lang.NumberFormatException: NumberFormatException occurred at com.howtodoinjava.Main.main(Main.java:18)4. Java 7 try-with-resources
For AutoCloseable resources, such as streams, Java SE 7 introduced try-with-resources statements which is recommended ways to handle exceptions in mentioned scenarios. In this approach, we are not required to close the streams and JVM does it for us. It eliminates the need for finally blocks.
In try-with-resources, a resource is opened in try block inside small brackets and finally block is completely gone.
try (BufferedReader br = new BufferedReader(new FileReader("C:/temp/test.txt"))) < String sCurrentLine; while ((sCurrentLine = br.readLine()) != null) < System.out.println(sCurrentLine); >> catch (IOException e) The catch Blocks
You associate exception handlers with a try block by providing one or more catch blocks directly after the try block. No code can be between the end of the try block and the beginning of the first catch block.
try < >catch (ExceptionType name) < >catch (ExceptionType name)
Each catch block is an exception handler that handles the type of exception indicated by its argument. The argument type, ExceptionType , declares the type of exception that the handler can handle and must be the name of a class that inherits from the Throwable class. The handler can refer to the exception with name .
The catch block contains code that is executed if and when the exception handler is invoked. The runtime system invokes the exception handler when the handler is the first one in the call stack whose ExceptionType matches the type of the exception thrown. The system considers it a match if the thrown object can legally be assigned to the exception handler’s argument.
The following are two exception handlers for the writeList method:
try < >catch (IndexOutOfBoundsException e) < System.err.println("IndexOutOfBoundsException: " + e.getMessage()); >catch (IOException e)
Exception handlers can do more than just print error messages or halt the program. They can do error recovery, prompt the user to make a decision, or propagate the error up to a higher-level handler using chained exceptions, as described in the Chained Exceptions section.
Catching More Than One Type of Exception with One Exception Handler
In Java SE 7 and later, a single catch block can handle more than one type of exception. This feature can reduce code duplication and lessen the temptation to catch an overly broad exception.
In the catch clause, specify the types of exceptions that block can handle, and separate each exception type with a vertical bar ( | ):
catch (IOException|SQLException ex)
Note: If a catch block handles more than one exception type, then the catch parameter is implicitly final . In this example, the catch parameter ex is final and therefore you cannot assign any values to it within the catch block.
Java catch exception line
правильно ли понимаю, что когда я работаю с проектом, в котором есть несколько потоков исполнения, может быть вот такая ситуация. Один из этих потоков запускается и завершается успешно, а затем выбрасывает исключение внутри блока try-catch. Оставшиеся потоки исполнения продолжают свою работу, но никакой код в блоке finally не выполняется. Тогда блок finally при обработке исключений не будет выполнен?
я читаю про исключения на 1м и в принципе понимаю, но не очень. ps: зачем только я начал с java core. pss: если вы это читаете, и я до сих пор на первом, то либо я прохожу другой курс, либо читаю книгу по джаве, параллельно проходя этот курс, либо решил взять перерыв на неопределенный срок времени. никогда не сдамся)
Есть подозрение, что так будет правильнее.
обращу внимание на некоторую неточность. цитата «Создание исключения При исполнении программы исключение генерируется JVM или вручную, с помощью оператора throw» в java исключения это тоже объекты поэтому создается исключение так же как объект new Exception. а бросается в программе с помощью оператора throw. обычно эти операции объединяют в одну throw new Exception(«aaa»);
если что я пишу это с 3 уровня. Под конец лекций я читал статью про бафридер, после нашел там ссылку на потоки вводов, а потом чтобы понять что там говориться ввел гугл про исключение и нашел эту статью, спасибо автору, это статья очень помогла. PS если ты читаешь этот комментарий и видишь что у меня нет прогресса(то есть если я все еще на 3 уровне или чуточку больше), то скажи мне, что я нуб и не дошел до 40 лвла