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Caught and declared exception in Java?

In Java, if I declare and caught an exception, can I handle the exception in a caller anyway? Or it needs not to be caught to handle it by caller?
class A {
void first() throws Exception {
try {
throw new Exception("my exception")
} catch (Exception e) {
log.message("Error in first()", e.getCouse)
throw e
}
}
}
class B {
Result second(A a) {
try {
a.first()
} catch (Exception e) {
log.message("Caught in B class", e.message)
return new Result(result: null, error: e.message)
}
}
second(A a)
}

You can simply rethrow the exception you've caught (obviously the surrounding method has to permit this via its signature etc.). The exception will maintain the original stack trace.
catch (WhateverException e) {
throw e;
}
You can also wrap the exception in another one AND keep the original stack trace by passing in the Exception as a Throwable as the cause parameter:
try
{
...
}
catch (Exception e)
{
throw new YourOwnException(e);
}

Rethrow Exception Real Application

In Java 7 Rethrow Exception feature added.I know it's concept but I want to see the real application of it and why this feature needed?

I will take examples from here
This is the example:
static class FirstException extends Exception { }
static class SecondException extends Exception { }
public void rethrowException(String exceptionName) throws FirstException, SecondException {
try {
if (exceptionName.equals("First")) {
throw new FirstException();
} else {
throw new SecondException();
}
} catch (FirstException e) {
throw e;
}catch (SecondException e) {
throw e;
}
}
This compiles with both java 6 an 7.
If you want to keep checked exceptions from method signature, you have to keep cumbersome catch clauses in java 6.
In Java 7 you can do it in following way:
public void rethrowException(String exceptionName) throws FirstException, SecondException {
try {
if (exceptionName.equals("First")) {
throw new FirstException();
} else {
throw new SecondException();
}
} catch (Exception e) {
throw e;
}
So your benefit is that you have less cumbersome catch clause.

Use Rethrowing Exceptions with More Inclusive Type Checking feature
in Java SE 7, you can specify the exception types FirstException and SecondException in the throws clause in the rethrowException method declaration
When you want to declare specific exceptions that can be thrown (mainly when you are catching a generic error)
For example see Precise Rethrow example:
public static void precise() throws ParseException, IOException{
try {
new SimpleDateFormat("yyyyMMdd").parse("foo");
new FileReader("file.txt").read();
} catch (Exception e) {
System.out.println("Caught exception: " + e.getMessage());
throw e;
}
}
This also make your code compliant to Sonar's Raw Exception rule.
Note you can similarly catch Throwable

Couple of use-cases:
Using rethrow, you can edit the stacktrace information to make
it more accurate. Also when needed you can hide or rid off the
unnecessary internal details in stack trace.
try { //... }
catch (Exception e) { throw (Exception) e.fillInStackTrace(); }
Real application of fillInStackTrace is very well explained here:
Why is Throwable.fillInStackTrace() method public? Why would someone use it?
Quote from the book "Thinking in Java" written by Bruce Eckel:
If you want to install new stack trace information, you can do so by
calling fillInStackTrace( ), which returns a Throwable object that it
creates by stuffing the current stack information into the old
exception object
Add custom message to the thrown exception. Add custom message to thrown exception while maintaining stack trace in Java
A simple example I can think of:
void fileOperator(String operationType) throws Exception(){ ... }
void fileReader() throws Exception {
try{
fileOperator('r');
}
catch(Exception e){
throw Exception("Failed to read the file", e);
}
}
void fileWriter() throws Exception{
try{
fileOperator('w');
}
catch(Exception e){
throw Exception("Failed to write the file", e);
}
}
Also, we can throw a more specific exception type (say FileReadException, FileWriteException) from the catch block.

How to store an exception

I want to throw an exception and to display it. In my IF block I throw the exception. Do I have to store the exception to pass it as a variable in the method which precedes.
if(count !=0) {
throw new Exception();
eventLogger.logError("The count is not zero",e)
}
else{
// do something else
}
The logger has Throwable error as a parameter.
logError(String description, Throwable error);
How can I pass the exception I thrown to this method

Once the Exception is thrown, your program stops running. Thus you have to change the order of your statements.
if(count !=0) {
Exception e = new Exception();
eventLogger.logError("The count is not zero",e)
throw e;
}
else{
// do something else
}
As you can read in the Java API the class Exception
extends Throwable
EDIT
As Zabuza mentioned, it's possible to handle the exception in a try- catch block. This would be a more elegant way:
try{
if(count !=0) {
throw new Exception();
}else{
// do something else
}
}
catch(Exception e){
eventLogger.logError("The count is not zero",e)
}

Exceptions don't work like this. Once you throw an exception, the code gets interrupted and doesn't run any further. You can access the thrown exception in the next try-catch block like this:
try{
//code that may throws an exception like this:
throw new Exception();
}catch(Exception e){
eventLogger.logError("Your message", e);
}
If you wannt to log the exception before you catch it you have to first create it, then log it and finally throw it.
Exception e = new Exception();
eventLogger.logError("your message", e);
throw e;

The logger will never be reached in your example, as the Exception is thrown a line before. To log an error, you'd do so in the catch part of your try - catch block
try {
throw new Exception();
} catch (Exception e) {
eventLogger.logError("The count is not zero",e);
}

When you throw an error, you terminate the execution of the method. That makes your second statement unreachable.
In your method header, declare methodName() throws Exception. Now, wherever this method is called, you know that there is a risk it may throw an exception.
try{
methodName();
}
catch(Exception e){
eventLogger.log(e);
}
Switching the order will not help you because e is not understood.

Exception is throwable so, once throw new Exception(); is executed then
eventLogger.logError("The count is not zero",e)
above statement will be come unreachable. So you need to complete all operations(like method calls.. etc.) before throwing an exception. And you should write the exception printing code in a catch clause.
So finally you need to change the code as follows...
try{
if(count !=0) {
// do method callings
throw new Exception();
}else{
// write alternative flow here
}
}
catch(Exception exception){
logger.error(exception);
eventLogger.logError("The count is not zero",exception);
}

If what you are doing will throw an exception (e.g. calling File.open() or something) then wrap it in a try-catch-finally
bool errorOccurred = false;
try {
// do things that can cause errors here.
} catch (Exception e) {
// things in here happen immediately after an exception in try-block
eventLogger.logError("The count is not zero", e);
errorOccurred = true;
} finally {
// always happens after a try, regaurdless or exception thrown or not
if (errorOccurred) {
/* report error to user or something else */
} else {
/* finish up */
}
}
but if you just want to log that specific condition was met, you can make your own exception object, and pass it to the error logging method without using try catch
if (count != 0) {
Exception ex = new Exception();
// set the properties you need set on ex here
errorLogger.logError("The count is not zero", ex);
} else {
// do something else
}
For the second option, you can use different subclasses the inherit from Exception, or you can even make your own implementation.

rethrow java exception with new message, preserving the exception type if it is in the method declaration list

I am trying to create a helper method that will eliminate the need of having code like this:
void foo() throws ExceptionA, ExceptionB, DefaultException {
try {
doSomething(); // that throws ExceptionA, ExceptionB or others
} catch (Exception e) {
if (e instanceof ExceptionA)
throw new ExceptionA("extra message", e);
if (e instanceof ExceptionB)
throw new ExceptionB("extra message", e);
throw new DefaultException("extra message", e);
}
}
The problem is that I need to maintain the throws list in the function declaration and in the body of the function at the same time. I am looking how to avoid that and to make changing the throws list sufficient and my code to looks like:
void foo() throws ExceptionA, ExceptionB, DefaultException {
try {
doSomething(); // that throws ExceptionA, ExceptionB or others
} catch (Exception e) {
rethrow(DefaultException.class, "extra message", e);
}
}
Where rethrow method will be smart enough to recognize the throws list from the method declaration.
This way when I change the list of type that my method propagates in the throws list I to not need to change the body.
The following is a function that could solve the problem. The problem is because it does not know what type of exception it will throw its throws declaration has to say Exception, but if it does this, the method that is going to use it will need to specify it as well, and the whole idea of using the throws list goes to hell.
Any suggestions how this could be solved?
#SuppressWarnings("unchecked")
public static void rethrow(Class<?> defaultException, String message, Exception e) throws Exception
{
final StackTraceElement[] ste = Thread.currentThread().getStackTrace();
final StackTraceElement element = ste[ste.length - 1 - 1];
Method method = null;
try {
method = getMethod(element);
} catch (ClassNotFoundException ignore) {
// ignore the Class not found exception - just make sure the method is null
method = null;
}
boolean preserveType = true;
if (method != null) {
// if we obtained the method successfully - preserve the type
// only if it is in the list of the thrown exceptions
preserveType = false;
final Class<?> exceptions[] = method.getExceptionTypes();
for (Class<?> cls : exceptions) {
if (cls.isInstance(e)) {
preserveType = true;
break;
}
}
}
if (preserveType)
{
// it is throws exception - preserve the type
Constructor<Exception> constructor;
Exception newEx = null;
try {
constructor = ((Constructor<Exception>) e.getClass().getConstructor());
newEx = constructor.newInstance(message, e);
} catch (Exception ignore) {
// ignore this exception we prefer to throw the original
newEx = null;
}
if (newEx != null)
throw newEx;
}
// if we get here this means we do not want, or we cannot preserve the type
// just rethrow it with the default type
Constructor<Exception> constructor;
Exception newEx = null;
if (defaultException != null) {
try {
constructor = (Constructor<Exception>) defaultException.getConstructor();
newEx = constructor.newInstance(message, e);
} catch (Exception ignore) {
// ignore this exception we prefer to throw the original
newEx = null;
}
if (newEx != null)
throw newEx;
}
// if we get here we were unable to construct the default exception
// there lets log the message that we are going to lose and rethrow
// the original exception
log.warn("this message was not propagated as part of the exception: \"" + message + "\"");
throw e;
}
Update 1:
I can use RuntimeException to avoid the need of throws declaration, but in this case I am losing the type of the exception which is one of the most important points.
Ideas how I can resolve this?

I'm guessing that code where you're doing real work (ie. the part where you're not tinkering with exceptions) looks like this.
public void doSomeWork( ... ) throws ExceptionA, ExceptionB, DefaultException
{
try
{
// some code that could throw ExceptionA
...
// some code that could throw OtherExceptionA
...
// some code that could throw ExceptionB
...
// some code that could throw OtherExceptionB
}
catch (Exception e)
{
if( e instanceof ExceptionA )
{
throw new ExceptionA("extra message", e);
}
if( e instanceof ExceptionB )
{
throw new ExceptionB("extra message", e);
}
throw new DefaultException("extra message", e);
}
}
There are two better approaches
First Approach
public void doSomeWork( ... ) throws ExceptionA, ExceptionB, DefaultException
{
// some code that could throw ExceptionA
...
try
{
// some code that could throw OtherExceptionA
...
}
catch (Exception e)
{
throw new DefaultException("extra message", e);
}
// some code that could throw ExceptionB
...
try
{
// some code that could throw OtherExceptionB
}
catch (Exception e)
{
throw new DefaultException("extra message", e);
}
}
Second Approach
public void doSomeWork( ... ) throws ExceptionA, ExceptionB, DefaultException
{
try
{
// some code that could throw ExceptionA
...
// some code that could throw OtherExceptionA
...
// some code that could throw ExceptionB
...
// some code that could throw OtherExceptionB
}
catch (OtherExceptionA | OtherExceptionB e)
{
throw new DefaultException("extra message", e);
}
}
The first approach is good if you want to continue execution at all costs and catch and wrap RuntimeExceptions if you run into them. Generally you don't want to do this, and it's better to let them propagate up, as you probably can't handle them.
The second approach is generally the best. Here you're explicitly pointing out which exceptions you can handle, and dealing with them by wrapping them. Unexpected RuntimeExceptions propagate up, as they should unless you have some way of dealing with them.
Just a general comment: playing with StackTraceElements isn't considered to be a great idea. You may end up getting an empty array from Thread.currentThread().getStackTrace() (although you most likely will not if using a modern Oracle JVM), and the depth of the calling method isn't always length-2, it may be length-1 particularly in older versions of the Oracle JVM.
You can read more about this problem in this question.

To elaborate on what )some) people are telling you, this is MyFunctionFailedException, ofcourse it should be named something more sensible:
public class MyFunctionFailedException extends Exception {
public MyFunctionFailedException(String message, Throwable cause) {
super(message, cause);
}
}
Then your catch block becomes something like this.
try {
...
} catch (Exception e) {
throw new MyFunctionFailedException("extra message", e);
}
If you really want to rethrow a lower level exception, you should use multiple catch blocks. Be aware tho' that not all types of Exceptions necessarily has a constructor that let's you add a cause. And you really should think about why it makes sense for your method to let for instance an uncaught SQLException bubble up the call stack.

In Java, if a general exception is caught and rethrown, will outer methods still be able to catch specific exceptions?

In Java, if a general exception is caught and rethrown, will outer methods still be able to catch specific exceptions?
In other words, can I do this:
try {
try {
//...
} catch (Exception e) {
//...
throw e;
}
} catch (SpecificException e) {
//...
}

re-throwing an exception does not change anything about it (it's still the same object originally thrown).

While jtahlborn answer is correct, there is one more appreciation: the compiler will see that you are throwing an exception of the generic type (even if at runtime it can be only of the specific class) and will force you to declare the generic exception in the method header.
private void test() throws FileNotFoundException {
try {
throw new FileNotFoundException("Es una exception");
} catch (IOException e) {
throw e; <-- Error because the method only throws
FileNotFoundException, not IOException
}
}
e is indeed FileNotFoundException, but as it is declared as IOException the compiler works with the broader class. What you can do is "cast" the exception.
throw (FileNotFoundException) e;

Eclipse marks the "throw e" in the inner catch as an unhandled exception, BUT it does catch the exception because when I run this it prints "It worked!". Thanks #jtahlborn. Unfortunately this means that there will still need to be an unnecessary try/catch block somewhere.
public class Tester {
public static void main(String[] args) {
try {
try {
throw new SpecificException("Test!");
} catch (Exception e) {
throw e;
}
} catch (SpecificException e) {
System.out.println("It worked!");
}
}
}