I saw many people wrote like:
try {
//something
}
catch (IOException e){
throw new SystemException("IO Error", e);
}
I got "Cannot instantiate the type SystemException" error and it seems SystemException is a abstract class, how can I able to throw it?
Yes, it is an abstract class and that means that it does not make sense to construct an object of type SystemException. It is recommended to use more meaningful exceptions types.
You have mentioned IOException in your code. This means an exception related to an I/O operation and the catcher can act accordingly (maybe a special log level, some I/O cleanup etc.).
In your particular case, I think you should change it to:
try {
//something
}
catch (IOException e) {
// log exception info and other context information here
// e.g. e.printStackTrace();
// just rethrowing the exception (call stack is still there)
throw e;
}
P.S. Quite offtopic, but coming from .NET world I have found about the subtle difference between throw ex; in C# vs. Java.
Related
I am pondering for a while with the following code:
try (Connection conn = dbHandler.getConnection()) {
// do something with the connection
dbConn.commit();
} catch (SQLException e) {
System.err.println("Too bad!");
e.printStackTrace();
}
Consider my application needs to be resilient to some SQL locking/dead-lock situations, so I would need to do some specific handling inside of the catch block. However, I cannot easily identify, if the exception was thrown during the "do something" part of my code or during autoClose(). And of course, each JDBC driver throws slightly different exceptions and if you throw JDBI in the mix, it gets even more complicated. So you can't really rely on different catch clauses do precisely identify, when the exception got thrown.
The only solution I could come up with is this code:
boolean finishedProcessing = false;
try (Connection conn = dbHandler.getConnection()) {
// do something with the connection
dbConn.commit();
finishedProcessing = true;
} catch (SQLException e) {
if (finishedProcessing) {
// this is an exception during autoClose
System.err.println("Too bad!");
e.printStackTrace();
} else {
// handle rollback cases, retries etc
System.err.println("SQL recovery performed.");
throw e;
}
}
Now the same issue comes up with IOExceptions for File* operations and probably in many other cases.
To be honest, I am just hoping I missed something completely obvious and I appreciate any insight from the Java experts out here.
As stated above, the only solution I found so far is introducing state-variables into the code, which doesn't really make sense to me.
I am fully aware of suppressing exceptions from the autoClose process, when an exception got thrown by the try block, however, as stated above the same kind of exception can be thrown, suppressed and both.
Don't use try-with-resources and add a finally block.
Connection conn = null;
try {
conn = dbHandler.getConnection()
// do something with the connection
dbConn.commit();
}
catch (SQLException e) {
// Handle rollback cases, retries, etc.
System.err.println("SQL recovery performed.");
e.printStackTrace();
}
finally {
if (conn != null) {
try {
conn.close();
}
catch (SQLException xSql) {
System.out.println("Failed to close database connection.");
xSql.printStackTrace();
}
}
}
What exactly is eating an exception?
Is it when there's nothing in the catch block or when you don't log the exception?
I understand that this is eating an exception.
try {
//exception code
} catch (Exception e) {
}
However, what about this?
try {
//exception code
} catch (Exception e) {
System.out.println("Exception Thrown");
}
When there is nothing in the catch block.
In general it is necessary to catch the more specific exception if it is possible to recover that exception and if not log the exception and rethrown it.
A program that "eat" exception is a bad practice because could be in an inconsistent state.
I have few similar methods and there calls as follows :
methodThrowingException() throws NullPointerException, InterruptedException, TimeoutException {
<method logic>
}
Calling class :
try{
methodThrowingExceptions();
<some other logic>
}
catch (NullPointerException npx) {
npx.printStackTrace();
log more details...
}
catch (InterruptedException inx) {
inx.printStackTrace();
log more details...
}
catch (TimeoutException tox) {
tox.printStackTrace();
log more details..
}
How (if) can I put all of these three in one Custom Exception?
Other than (1) is there a way to optimise the code so that I need not write the entire same statements for multiple methods?
Since Java 7, you can use a multi-catch block:
catch (NullPointerException | InterruptedException | TimeoutException e) {
e.printStackTrace();
log more details...
}
That said, you should never catch NullPointerException: that is a bug, and if it happens, the exception should bubble up. You can't reasonably expect a NPE to happen.
In addition, doing the same thing for an InterruptedException as for the other exceptions is also very dubious. When catching an InterruptedException, you should reset the interrupted flag on the current thread, and stop what you're doing ASAP.
I'm looking to check what type of exception is retruned in the following code snippet.
I'm not sure exactly how to do it though, or if it's even possible.
try {
//SOME LOGIC
} catch (exception ex) {
System.debug(//EXCEPTION TYPE);
}
Would anyone have any suggestions or advice??
try {
//SOME LOGIC
} catch (Exception ex) {
System.err.println(ex.getClass().getName());
}
Few things:
You have posted exception. I assume you meant Exception.
System.debug does not exist.
I have answered your specific question but obviously this is not standard exception handling code. You will instead output the stacktrace, log the exception or rethrow a different one.
Firstly, within your catch block(s), specify the exception. Semantically, types including Exception are not in camel case.
Secondly, you can obtain the exception type through Exception.getClass().getName():
catch (Exception exception) {
System.out.println(exception.getClass().getName());
// exception.printStackTrace();
// throw exception;
}
Specified by:http://docs.oracle.com/javase/7/docs/api/java/lang/Exception.html
After some searching around I found an answer,
try {
//SOME LOGIC
} catch (Exception ex) {
System.debug(ex.getTypeName());
}
The getTypeName() method does the trick nicely.
I apologise for not specifying I was working in Apex in my question.
When saving an object to database using hibernate, it sometimes fails because of certain fields in the object exceeding the maximum varchar length defined in the database.
Therefore I am using the following approach:
Attempt to save
If getting an DataException, I then truncate the fields in the object to the max length specified in the db definition, then try to save again.
However, in the second save after truncation, I'm getting the following exception:
hibernate: com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Cannot add or update a child row: a foreign key constraint fails
Here's the relevant code, do you notice anything wrong with it?
public static void saveLenientObject(Object obj){
try {
save2(rec);
} catch (org.hibernate.exception.DataException e) {
// TODO Auto-generated catch block
e.printStackTrace();
saveLenientObject(rec, e);
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
private static void saveLenientObject(Object rec, DataException e) {
Util.truncateObject(rec);
System.out.println("after truncation ");
save2(rec);
}
public static void save2(Object obj) throws Exception{
try{
beginTransaction();
getSession().save(obj);
commitTransaction();
}catch(Exception e){
e.printStackTrace();
rollbackTransaction();
//closeSession();
throw e;
}finally{
closeSession();
}
}
All Hibernate exceptions (except for NonUniqueResultException) are irrecoverable. If you get an exception you should close the session and open another one for further operations.
See also:
13.2.3. Exception handling
The hibernate documentaiton is quite clear that once an exception is thrown the session will be left in an inconsistent state so is not safe to use for further operations. I suspect that what you're getting here is that the session is left half saving your first attempt so bad things happen.
Fundamentally you should not rely on database errors to check the length of your fields, instead you should pre-validate this in java code. If you know the lengths enough to truncate, then I suggest you simply call your trucate util every time.
Alternatively use Hibernate Validator to declaratively validate the objects before saving.