I am new to Android mobile application development.
I would like to know, how can I handle exceptions like HttpConnection related exceptions or any other exceptions? Do I need to display an AlertDialog to the user?
Kindly provide a sample code or project source code on how can I handle HttpConnection or similar type of Exceptions.
There are 2 different kinds of exceptions in Java: Checked and Unchecked. There is a big debate over which one is better to use, both arguments are good.
Basically a Checked exception is derived from java.lang.Exception and it requires that if you don't specify your method as "throws MyCheckedException" then you must catch and handle the exception within your method.
// throw the exception
void foo() throws MyCheckedException
{
throw new MyCheckedException();
}
// or handle the exception
void foo()
{
try {
throw new MyCheckedException();
} catch (MyRuntimeException e) {
e.printStackTrace();
}
}
An Unchecked exception, derived from java.lang.RuntimeException, requires neither that you specify "throws" in your method definition nor that you handle it.
void foo()
{
throw new MyUncheckedException();
}
The advantage of Checked is that the compiler will warn you when you haven't handled an exception.
The disadvantage is that you have to declare either a try/catch block or a throws for every Checked exception, and the upper level code can get pretty cumbersome, trying to handle all the different types of Exceptions.
For this reason, if you're careful you might prefer using Unchecked Exceptions.
BTW, you can only choose your exception type when you define your own.
When encountering Exceptions from Java or a 3rd party library, you have to decide how to handle it. e.g. If a 3rd party method throws CheckedException1, then you have to either handle it, or declare the calling method as "throws CheckedException1". If you want to avoid using Checked Exceptions then you can wrap it in an Unchecked Exception and throw that.
void foo() // no throws declaration
{
try {
thirdPartyObj.thirdPartyMethod(); // this throws CheckedException1
}
catch (CheckedException1 e) {
throw new MyUncheckedException(e); // this will wrap the checked in an unchecked.
}
}
Note that you can throw the Unchecked exception without the "throws" declaration. To access the original CheckedException1 from higher up, you can use the .getCause() method of your Unchecked exception.
void caller()
{
try {
foo();
} catch (MyUncheckedException e) {
CheckedException1 ce1 = e.getCause();
ce1.printStackTrace();
}
}
... but because the exception from foo() is Unchecked, you don't have to handle it or declare "throws".
Regarding logging, there are different schools of thought on this.
Log it when the exception occurs (low - level)
Log it when it reaches the top (high - level)
Log it when you have enough information to make an appropriate action and/or a log message. (mid - level)
A good policy I've found is to install an uncaught exception handler which will take care of all uncaught (obviously unchecked) exceptions. This way anything that is missed will be logged and potentially handled before crashing the system.
public class MyExceptionHandler implements UncaughtExceptionHandler
{
#Override
public void uncaughtException(Thread thread, Throwable ex)
{
ex.printStackTrace();
}
}
// In your high-level code
Thread.setDefaultUncaughtExceptionHandler(new MyExceptionHandler());
and all for Exceptions that can be handled gracefully, catch and handle them in a module where you know enough about the situation to possibly correct the problem and try again.
How you handle exception depends on the exception. If the exception is something that you cannot recover from, and the user needs to know about then you could catch the exception and show it in an AlertDialog:
try {
// do something
} catch (SomeImportantException e) {
AlertDialog.Builder builder = new AlertDialog.Builder(this);
builder.setMessage("User friendly text explaining what went wrong.");
AlertDialog alert = builder.create();
alert.show();
}
For more info on the dialog, see creating dialogs.
Alternatively, if the exception is something that you can deal with, you can just log information about the exception and move on.
try {
// do something
} catch (SomeLessImportantException e) {
Log.d(tag, "Failed to do something: " + e.getMessage());
}
You could use the ACRA plugin that offers this functionality or BugSense to gather error reports.
Disclaimer: I am a founder at BugSense.
Related
I got this method:
public void foo(){
int test = 5/0;
}
And I call it like this:
try {
foo();
} catch (Exception e){
e.printStackTrace();
}
This catches the ArithmeticException, but I have seen cases where I call a method inside try-catch, but it doesn't catch the exception. Is there a specific situation in which the exception may not be caught?
Here is an example to clarify the question:
Does the try-catch block in ActivityA catches the exception in ActivityB? I know the answer is negative. But I want to know the logic behind this.
public class ActivityA extends Activity {
#Override
public void onCreate(Bundle savedInstanceState) {
try {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_a);
startActivity(new Intent(this, ActivityB.class));
} catch (Exception e){
e.printStackTrace();
}
}
}
And ActivityB:
public class ActivityB extends Activity {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_b);
int x = 5/0
}
}
To all cool guys who think this is a duplicate question, please give me a link to a question on StackOverFlow that answers to this very clear question: Does a try-catch catches exceptions that happens in a method which is called inside it?
What may have happened is that you tried to catch in an exception that was not the one that was fired.
In the case of ArithmeticException, if you fire one, for example, NumberFormatException, it won't hold the Exception and will stop your application.
If you catch Exception, Exception itself and all exceptions that are a subtype of Exception will be caught by that block. In Java that will be the so called "checked exceptions".
However, you will not catch those "exceptions" that are only a subtype of Throwable. To be precise, those are not acctually exceptions. They are errors. It is also possible to catch them in a try-catch statement.
Not that neither catching Exception nor Throwable is a good practice. Always catch the specific exceptions and errors you are expecting.
If you specify an exception to be caught, e.g
public void foo(numb, divisor) throws ArithmeticException {
if (divisor == 0) throw new ArithmeticException();
int ans = num / divisor;
}
#Test
public void tesfooWithZeroDivisor() {
try {
foo(5, 0);
} catch (ArithmeticException e) {
System.out.println("Can't divide by zero");
}
}
In this case, you're specifying that you want an ArithmeticException to be caught if thrown. In the event that no ArithmeticException is thrown but a different exception such as IllegalArgumentException, then it will not catch an IllegalArgumentException because you only specify ArithmeticException.
In your case, you used catch (Exception e) which means you didn't specify an exception so it will catch any exception that got thrown regardless.
Is there a specific situation in which the exception may not be caught?
Some situations spring to mind:
The code (e.g. the call to foo()) was not executed. So the exception was not thrown.
The exception was thrown by different code; e.g. in your example, the foo() method is called from somewhere else; e.g. on a different thread stack.
The exception was caught further up the call chain.
You have declared your own exception (e.g. my.pkg.ArithmeticException) and you are catching a different exception to the one thrown as a result
If you do funky things with classloaders, it is possible to load the same class more than once. If you do that, you will have types that have the same name, but are in fact different. If you did that with an exception class, the instanceof test that the handler performs may fail to match.
Does a try-catch catches exceptions that happens in a method which is called inside it?
Yes. If you have implemented it properly; see above.
If you want chapter and verse, read JLS 14.20.
Concerning your updated example, I would not expect the exception handler for ActivityA to see exceptions thrown in ActivityB.onCreate. I would expect onCreate() to be called by a different thread.
You can confirm this by catching the exception in ActivityB.onCreate and looking at the stacktrace.
I am unsure how best to structure custom exceptions. Say i have a situation where there's some library that throws its own custom exception class, with a custom field in it:
public class CustomException extends RuntimeException {
private final String additionalInfo;
public CustomException(String message, String additional) {
super(message);
this.additionalInfo = additional;
}
public String getAdditionalInfo() {
return additionalInfo;
}
}
Dealing with this is pretty straightforward:
try {
something();
} catch (CustomException e) {
LOGGER.error(e.getMessage());
LOGGER.error(e.getAdditionalInfo());
}
So far, so good, but what about when this is wrapped up many layers deep into some sort of service and handled in this way:
// in a subclass of a theoretical ServiceHandler interface
public void handle() throws OperationFailedException {
try {
something();
} catch (RuntimeException e) {
throw new OperationFailedException("The operation failed.", e);
}
}
Further down the callstack the service dispatch code catches this new exception, returns the generic (safe) message to the external caller, and logs the full exception trace somewhere internal, including the cause (our original custom exception type). The problem now is that the extra field in CustomException has gone missing from the logs, because the logging framework is only interested in getMessage() and the callstack.
try { // call to the ServiceHandler
sendResponseToClient(findServiceHandler(rpcName).handle(args));
} catch (Throwable th) {
LOGGER.error(th);
sendErrorToClient("An internal error occurred with incident ID: <id>");
}
I can think of a few of solutions:
1) Make the thing that logs the service exceptions aware of this special exception type so it can log it
2) Override getMessage() so it returns super.getMessage() + getAdditionalInfo()
3) Everywhere something is called that could throw one of these i rethrow it as something else with the additional field baked into the message:
try {
something();
} catch (CustomException ce) {
throw new OperationFailedException("The operation failed because " + ce.getAdditionalInfo(), ce);
} catch (RuntimeException e) {
throw new OperationFailedException("The operation failed", e);
}
The actual use case is a native code library with a few hundred functions, and they're called literally everywhere throughout this app. This exception type is a general "the api has failed in an unexepected way" error, so it's unchecked, and in day to day operation i wouldn't expect to ever see one.
(1) seems like it's easy to miss out new types in future, if in a year's time someone adds another custom exception type they need to be aware of all places that might log it and write the corresponding handler code there. (2) feels a little fishy, since you'd be creating an exception with one message, and getting a different message back. Applying the pattern in (3) to it seems like it will produce a lot of clutter, particularly if i have a couple of these custom exception types to worry about.
Which approach should i use? Are there other options, or would you recommend restructuring something to get around this?
There are essentially two cases for an exception:
1) You can recover
2) You can't, you just want your application to exit
If you can recover, you may want to think about using a checked exception.
Then you have to deal with the exception and the compiler will tell you where to catch it.
Otherwise, if the application just crashes you may want to add this additional info to the message you're giving to the superclass constructor.
I've coded a method with a catch-all handler, but I need to rethrow the exception as if it were unhandled, so that a caller (much) further up the call stack can handle it. The trivial way to do this is simply:
try {
...
} catch (Exception ex) {
// do something here...
// and rethrow
throw ex;
}
But the problem is that, because of the throw statement, Java requires this method to declare itself as throws Exception, which in turn, requires all the callers to handle the exception or declare themselves as throws Exception. And so on up the call chain...
Is there any simple way to rethrow the exception as if the current method did not handle it?
You have exactly two options with (checked) exceptions:
Handle them in the method via a try/catch (which may include rethrowing as a different exception type)
Declare that the method throws the exception.
If you want to rethrow the exception as if this method did not catch it, your only option is 2.
Note: you only want to catch (Exception e) if a method in the try block actually throws Exception. Otherwise, catch the specific exception types.
You could do what #radoh has said and just wrap into a RuntimeException, but one downside of this is your stacktrace is now polluted and will show the offending line to be where you declare throw new RuntimeException(ex).
An alternative is to use Lomboks SneakyThrows mechanism, like this:
public static void main(String[] args) {
methodWithException();
}
private static void methodWithException() {
try {
throw new Exception("Hello");
} catch (Exception e) {
Lombok.sneakyThrow(e);
}
}
Your stacktrace will remain intact, but you no longer need to declare throws Exception.
It's worth reading the documentation on why you should/shouldn't do this
I'm beginner in Java and Android. My problem is when I use setDefaultUncaughtExceptionHandler in my code, some functions still need a try/catch block surrounding it, but I want throw all my exceptions to UncaughtException thread.
public class MyAlarmReciever extends BroadcastReceiver {
#Override
public void onReceive(Context context, Intent intent)
{
Thread.setDefaultUncaughtExceptionHandler(new UnCaughtException(context));
try
{
String imageURL = MyWebService.readFeed();
DownloadAndSet.downloadFile(imageURL);
}
catch(Throwable e)
{
throw new RuntimeException(e);
}
Toast.makeText(context, "Alarm Triggered", Toast.LENGTH_LONG).show();
}
}
Java distinguishes checked and unchecked exceptions. Checked Exceptions have to be caught, no matter what.
Correction: Or you have to add the throws clause to the method. This postpones the urge to catch that exception to the caller of your method.
If you want them to be handled in the UncaughtExceptionHandler, you can "forward" them:
try{
// blah "throws some checked exception type"
} catch ( Throwable e ) {
// throw e; <- This will not work :( unless you add the "throws" clause.
throw new RuntimeException(e);
}
Unfortunately, just throwing the same Exception won't work, because you'd have to add the throws clause, which you do not want. You'll have to wrap it in a RuntimeException.
All checked exceptions in your code must be caught.
Further reading on checked vs unchecked exceptions:
http://www.javapractices.com/topic/TopicAction.do?Id=129
http://docs.oracle.com/javase/tutorial/essential/exceptions/runtime.html
When you call a method that throws an exception, requires you to either handle the exception using try/catch or throw the same exception.
As you don't want to handle the exception in your code and want all exceptions to be handled by your Default Exception handler so you need to add throws to all your methods where you don't want to catch the exception.
The documentation says:
Set the default handler invoked when a thread abruptly terminates due to an uncaught exception, and no other handler has been defined for that thread.
It doesn't relate to exceptions you have to catch. An example of an exception you have to catch/throw is IOException. An example of an exception you don't is NullPointerException.
So if your code causes a NullPointerException, the default handler will deal with it. If your code (potentially) causes an IOException, you have to deal with it then and there (either by catching or throwing).
What I have always done in my programs is create a exception handler method and call it every time I make a try/catch block. Many times I have had Thread.sleep() methods and I just send the exception to a common place to do "global" handling. In your global exception handler, you can also refer to this method.
Keep in mind that you might not always want to use this method because things like file streams might throw errors if a file already exists and you would want to take a different approach such as naming it something else than just stopping the program.
public static void main(String[] args) {
Thread.setDefaultUncaughtExceptionHandler(new UncaughtExceptionHandler() {
#Override
public void uncaughtException(Thread t, Throwable e) {
catchException(e);
}
});
try {
Thread.sleep(1000);// Just an example of a possible exception thrower
}
catch (InterruptedException e) {
catchException(e);
}
}
public static void catchException(Throwable e) {
// Deal with the exception here
System.out.println("Oh no! You broke the program!!!");
System.out.println("Here is the error btw: " + e.getMessage());
}
If you are using Eclipse, you can set the default automatic code generation for try/catch blocks to automatically include this method:
You can also set project specific settings if you don't want this behaviour for all of your projects.
Reading about Exceptions in this book, I found this statement:
Checked exceptions are checked by the compiler at compile time.
and
The compiler does not check unchecked exceptions at compile time.
So, if also we can say that IOException or SQLException are below the Checked Exceptions class tree. How would java compiler know there will be an exception and no for IllegalArgumentException which might remain inside the code as my understanding.
Also, what does exactly the fact of being obligatory to get the Checked exceptions caught and not for Unchecked mean?
A checked exception requires that you add a throws declaration to the method signature, or you add a try catch block around it.
public void checked() throws IOException {
throw new IOException(); // compiles just fine
}
public void checked() {
try {
throw new IOException();
} catch (IOException e) {
// ...
}
}
This will not work:
public void checkedWithoutThrows() {
throw new IOException(); // will not compile
}
An unchecked exception does not need this.
public void unchecked() {
throw new RuntimeException();
}
And, just for completeness, the way to determine unchecked from checked exceptions is that unchecked exceptions all descend from RuntimeException at some point or another.
exceptions in java all work the same way.
the difference between checked and unchecked exceptions (which are all subclasses of RuntimeException) if that for a method that throws a checked exception whoever calls that method has to either try/catch the exception, or declare his own method as throwing that exception.
so if i have a method:
void throwsACheckedException() throws SomeCheckedException;
whoever calls it must do one of 2 things. either:
try {
throwsACheckedException();
} catch (SomeCheckedException e) {
//do something
}
or
void someCallingMethod() throws SomeCheckedException { //pass it on
throwsACheckedException();
}
unchecked exceptions you dont have to declare, and whoever calls the method does not have to explicitly catch. for example:
void someInnocentLookingMethod() {
throw new NullPointerException("surprise!"); //...extends RuntimeException
}
and then you can simply invoke it, without the try/catch hassle:
void unsuspectingVictim() {
someInnocentLookingMethod();
}
unchecked exceptions are usually used for things that can creep up on you at any point and so forcing developers to try/catch them would make the code very tedious (NullPointerException, for example), although there are those who thing checked exceptions are evil entirely :-)
For Checked exceptions, you see error at compile time and says that you have to handle them.
Run time exceptions do not give any error or warning to handle in your code.
Compiler expects exceptions (Checked exceptions) and asks you to handle it.