We Keep Coding

Spring Boot Application - Is it considered bad practice to throw EntityExists - or EntityNotFoundException from Service to Controller?

My understanding of these exceptions is if an object in the database that you are looking for doesn't exist or exists these gets thrown? But is it ok for myself to use when I want to handle different cases in MyServiceClass.
Is it bad practice to throw these exceptions or should I create my own Exceptions for let's say if a user dont exist in the database?
How does it work in a real production?
Thanks in advance!

You should only implement a custom exception if it provides a benefit compared to Java's standard exceptions. The class name of your exception should end with Exception.
But it’s sometimes better to catch a standard exception and to wrap it into a custom one. A typical example for such an exception is an application or framework specific business exception. That allows you to add additional information and you can also implement a special handling for your exception class.
When you do that, make sure to set the original exception as the cause. The Exception class provides specific constructor methods that accept a Throwable as a parameter. Otherwise, you lose the stack trace and message of the original exception which will make it difficult to analyze the exceptional event that caused your exception.
public void wrapException(String input) throws MyBusinessException {
try {
// do something
} catch (NumberFormatException e) {
throw new MyBusinessException("A message that describes the error.", e);
}
}
Try not to create new custom exceptions if they do not have useful information for client code.
And if you make a custom exception be sure to:
Document the Exceptions You Specify
Throw Exceptions With Descriptive Messages
Catch the Most Specific Exception First
Don’t Log and Throw

What is the difference between throw and throws Exception? And why does "throws" not require a catch/finally block?

I have a bit of confusion in the following snippet.
try {
public static void showMyName(String str) throws IOException
{
if(str.equals("What is your Name?"))
throw new IOException;
}
}catch(IOException e)
{
System.out.println ("Exception occurs , catch block called")
}
Now, my question is what is difference between throws IOException && throw new IOException
Why don't we require to use catch , finally block for throws IOException ?
How does throws is handle the exception. ?

When an Exception is generated, there are two ways to deal with it.
Handle the Exception - this uses catch block
Declare the Exception - this uses throws block
So, dealing with an already generated exception is done by catch or throws.
On the other hand, throw is used while an Exception is "generated".
Typically, the throw keyword is used when you're instantiating an Exception object or while cascading an already generated exception to the caller.
For example,
public void doSomething(String name) throws SomeException { // Exception declared
if( name == null ){ // cause of the exception
throw new SomeException(); // actual NEW exception object instantiated and thrown
}
// Do something here
.
.
.
}
In the calling method,
public static void main(String[] args) throws SomeException{ // Need to declare exception because you're "throwing" it from inside this method.
try { // try doing some thing that may possibly fail
doSomething(name);
} catch (SomeException e){
throw e;// throw the exception object generated in doSomething()
}
}
Alternately, you can "handle" the exception in the catch block:
public static void main(String[] args) { // No need to declare exception
try { // try doing some thing that may possibly fail
doSomething(name);
} catch (SomeException e){
System.out.println("Exception occurred " + e.getMessage()); // Handle the exception by notifying the user.
}
}
Hope this helps.
Addendum - Exception Handling glossary
try - used for invoking code that can potentially cause exceptions or with resources such as Files/Streams/Network/Database connections.
catch - used immediately after a try block to "handle" or "throw" exceptions. Optional if finally is used.
finally - used immediately after a try or catch block. Optional. Used for executing code that "must" be executed. Code within a finally block always gets executed - even if the try or catch has a return statement.
throw - used to "push" or "cascade" exceptions through the calling methods. Always used inside a method.
throws - used to "declare" that a method will be throwing an exception if something goes wrong. Always used as part of method signature.
Addional Reading
Here's an elaborate article to help you understand the try-catch-finally semantics in java.
Update
To answer your other question,
Why don't we require to use catch , finally block for throws
IOException ?
Think of catch and throws as mutually exclusive constructs. (This is not always true, but for your understanding, it's good to start with this thought.)
You declare a method that it throws IOException. This piece of code is written at a point where you declare the method. And, syntactically, you cannot place a method declaration inside a try block.
How does throws is handle the exception. ?
Like I mentioned before, catch and finally are used during exception handling. throws is just used to cascade the exception back to the caller.

throw new IOException means that you encountered some error condition and decided to break the flow of the program by throwing an exception.
If that exception is a checked exception (i.e. not Error or RuntimeException), you must either catch that exception in a catch block, or declare in the method's signature that the method is expected to throw that exception (or a super-class of that exception). That's what the throws IOException means.

You are pretty much right on. Except for one thing I'll mention in a bit.
throws is as much a part of the method API as the name and the parameters. Clients know if they call that method, they need to handle that exception--by simply throwing it also or by catching it and handling it (which may in fact entail the throwing of another exception wrapping the original). throws is addressed at compile time.
throw is the actual act of letting the runtime know something bad happened--that the exceptional condition we were worried about has in fact taken place. So it needs to be dealt with at runtime.
But you weren't quite right when you said, "Throws in method signature should always appear if there exist a throw statement in the method." That is often true but not always. I could also call another method that throws an exception within my method, and if I don't catch it, my method needs to throw it. In that case, there is no explicit throw of the same exception by me.
The final point is that you only need to declare an exception in throws when the exception is a checked exception--meaning it is from the other side of the Exception class hierarchy from RuntimeException. Common checked exceptions are IOException and SQLException. Checked exceptions must be listed in the throws part of the method signature if you don't handle them yourself. Anything subclassing RuntimeException--like NoSuchElementException in your example and also the hated NullPointerException--is an unchecked exception and doesn't have to be caught or thrown or anything.
Typically, you use checked exceptions for recoverable problems (where the client knows what can happen and can gracefully handle the problem and move on) and unchecked exceptions for catastrophic problems (like can't connect to the database).
If you can get past all the AOP stuff, this is a great discussion of how you use checked and unchecked exceptions effectively.

The first biggest difference is throw can throw exception like you throw stone in river,
and throws state that this method have chances to throw exception but may not also... and thats why throws dont need try catch because it state already that it gona throw some kind of exception.

throws keyword means that this method will throw any Exception that will have to be handled in a higher level.
public static void showMyName(String str) throws IOException
If another method call this method, it has to catch this exception or throw it one more time (by typing throws in its signature).
Catch the exception or keep throwing it, depends on your business logic.

Is there any documentation for what runtime exceptions or underlying causes can be thrown when using Hessian?

I have a Java 1.4 web app that uses Hessian to make web service calls. I'm trying to write it as robust and transparent as possible. I don't want any hessian exceptions to make it out of my delegate class when calling the proxy. I want to be able to catch any Hessian runtime exceptions, unwrap them and either rethrow some of the major exceptions (e.g., ConnectException) or rewrap them in another exception. But finding these exceptions are a bit of a challenge.
Every method I have in my delegating class has this sort of structure.
public MyResult myMethod(MyArgsType myArgs)
throws ConnectException
{
try
{
return proxy.myMethod(myArgs);
}
catch (HessianRuntimeException ex)
{
Throwable cause = ex.getCause();
if (cause instanceof ConnectException)
throw (ConnectException)cause;
throw new MyRuntimeException(cause);
}
}
This has been working fine in my own tests but some other runtime exceptions are making it out and I'm missing them. For instance, we recently found that it may throw a HessianConnectionException caused by a SocketException (the remote server was down). I was fully expecting that what I currently have would have caught that. The HessianConnectionException doesn't derive from the HessianRuntimeException so now I have to add that exception to be caught.
public MyResult myMethod(MyArgsType myArgs)
throws SocketException, ConnectException
{
try
{
return proxy.myMethod(myArgs);
}
catch (HessianException ex) // HessianConnectionException derives from HessianException
{
Throwable cause = ex.getCause();
if (cause instanceof SocketException)
throw (SocketException)cause;
throw new MyRuntimeException(cause);
}
catch (HessianRuntimeException ex)
{
Throwable cause = ex.getCause();
if (cause instanceof ConnectException)
throw (ConnectException)cause;
throw new MyRuntimeException(cause);
}
}
Ok that's fine with me, it had to be done... but where does it end? I can't find any documentation on what other runtime exceptions that I need to be aware of or any of their underlying causes.
I'm hoping just catching HessianException and HessianRuntimeException would be enough since those are the only runtime exceptions that I can find listed here. But with these newly found issues, I'm not sure what exceptions I would want to unwrap since it seems like it's a mixed bag (as far as I knew, SocketException and ConnectException were different representations of the same problem).
Is there any documentation in what runtime exceptions are thrown when calling methods through the proxy and all their underlying causes?

I assume you have done web searches etc., so here is a fallback position in case you don't find the documentation you are looking for.
After your existing catches, catch RuntimeException. Test whether the Exception's class name begins with "Hessian" and/or check its package name. If it is not Hessian-originated, rethrow.
After you know it is a Hessian exception you can do something similar to your current strategy, looking through the cause chain for something that makes sense in your context or at least is from a non-Hessian package.

Handling checked exceptions

I'm reading exception handling in the book "A programmer guide to Java SCJP certificate". The author wrote that :
If a checked exception is thrown in a method, it must be handled in one of three ways:
1.By using a try block and catching the exception in a handler and dealing with it
2.By using a try block and catching the exception in a handler, but throwing
another exception that is either unchecked or declared in its throws clause
3.By explicitly allowing propagation of the exception to its caller by declaring it
in the throws clause of its method header
I understood clearly the first and third, but the second made me a lot of confused. My concerns are that :
-It's still alright even if I don't throw any other unchecked exceptions, so why do we have to throw another exception at here?
-Why do we have to re-declare the exception that we have caught, in throws clause? I think it's over by the handler.
Thanks to everyone.

The book just list three allowed options.
-It's still alright even if I don't throw any other unchecked exceptions,
so why do we have to throw another
exception at here?
You might want to throw another more descriptive exception, for example adding more info.
-Why do we have to re-declare the exception that we have caught, in
throws clause? I think it's over by
the handler.
You don't have to re-declare. But if the new exception you are throwing is checked, then you must declare it in the throws clause. In fact the exception you just caught doesn't need to be declared even if checked.

You may want to do this to catch a checked exception and throw another checked exception of a different kind. Perhaps you want to throw your own exception rather than a different one.
public void doSomething() throws MyCheckedException {
try {
doSomethingThatThrowsSpecificCheckedException();
} catch(SpecificCheckedException e) {
throw new MyCheckedException();
}
}
Or you can throw an unchecked exception (something that is or extends RuntimeException).
public void doSomething() {
try {
doSomethingThatThrowsSpecificCheckedException();
} catch(SpecificCheckedException e) {
throw new RuntimeException();
}
}

First of all, you should declare in the throw clause the exception that you throw, not the one you caught, assuming you are throwing a checked exception.
Second, you don't have to do that. It is just one of the three options.
Why would you do that? Usually this is done between the application layers. For example, Hibernate catches SQLExceptions and rethrows them as unchecked HibernateException, so that code that calls Hibernate methods doesn't have to be polluted with the try/catches for SQLExceptions. Another option is to translate a low-level exception into some business logic exception that can be handled up the stack. This allows for the better isolation of the business logic from the low level implementation details.

Great question, and one which good java programmers should get their head around.
It's all about adhering to the method signature that defines the method's contract with its caller, and which includes what exceptions you are going to throw.
Option 1 is dealing with the exception
Option 2 is not dealing with the exception, but keeping the same contract
Option 3 is not dealing with the exception and changing your contract
A implementation of the pattern in option 2 would be:
public interface Server {
public void useServer() throws ServerException;
}
public class ExplodingClient {
private Server server = new ServerImpl();
public void doIt() throws ClientException {
try {
server.useServer();
} catch (ServerException e) {
// Our contract doesn't allow throwing ServerException,
// so wrap it in an exception we are allowed by contract to throw
throw new ClientException(e);
}
}
}
public class SilentClient {
private Server server = new ServerImpl();
public void doIt() {
try {
server.useServer();
} catch (ServerException e) {
// Our contract doesn't allow throwing any Exceptions,
// so wrap it in a RuntimeException
throw new RuntimeException(e);
}
}
}

By using a try block and catching the
exception in a handler, but throwing
another exception that is either
unchecked or declared in its throws
clause.
Handling the exception in Java can be done in two ways :
Wrapping it in try-catch-finally block.
Declaring the method to throw ( using throws) the exception to the caller of method to handle.
-It's still alright even if I don't throw any other unchecked exceptions,
so why do we have to throw another
exception at here?
Throwing another exception means describing more about it. Also, to let the caller of the method know that this particular exception was generated.
-Why do we have to re-declare the exception that we have caught, in
throws clause? I think it's over by
the handler.
Redeclaring exception that you just caught in the catch block is to make the caller of this method alert that this method could throw a particluar exception. So be prepare to handle it.
You must go over this Jon Skeet's post : Sheer Evil: Rethrowing exceptions in Java
Remember, you are never forced to handle the unchecked exceptions, compiler forces you to just catch the checked one.

Spring Frame work Wraps Checked Exceptions inside RuntimeExceptions

Have this method call:
->
simpleJdbcTemplate.queryForInt(SQL,null);
->
queryForInt() method in the springs SimpleJdbcTemplate throws a DataAccessException which is a runtime exception. I want to propegate exceptions to the view tier of the application since Spring frame work Wraps Checked Exceptions inside RuntimeExceptions I stuck here.
How do I do this?
Explanation 1:
The value-add provided by the Spring Framework's JDBC abstraction framework- they say The Spring Framework takes care of all except 3 and 6. 3 and 6 need to be coded by an application developer
Define connection parameters
Open the connection
Specify the statement
Prepare and execute the statement
Set up the loop to iterate through the results (if any)
Do the work for each iteration
Process any exception
Handle transactions
Close the connection
But if I encounter a situation where the connection to the database losses after certain time the program started. Then a runtime exception will be thrown when a call to the above method made.since I don't handle the exception I cannot inform the user interface (view).

Just because Spring throws a runtime exception doesn't mean you cannot catch it. If you want to do something special for DataAccessExceptions, you can certainly do that:
try {
// query logic
} catch (DataAccessException ex) {
// handle the exception
}
If you're using Spring's MVC framework, it may be worth looking into the ExceptionResolver interface. It's a mechanism for deciding how to handle all those uncaught exceptions thrown by the lower layers of the application. It gives you one last chance to display a different view based on exceptions that are thrown.

It depends if your view tier catches checked exceptions (any subclass of throwable that does not subclass RuntimeException or Error, or are not instances of RuntimeException or Error directly) or unchecked exceptions (RuntimeException or Errors or subclasses of these Throwable subclasses).
Generally, you'll either have something like this:
try {
//... processing
} catch(Exception/RuntimeException e) {
// propagate the exception to the view in a meaningful manner
}
If this is the case, for a runtime exception, you don't have to do anything - the block will catch the runtime exception.
If you want to convert it to checked, assuming you're using a version of Java that supports wrapped exceptions, all you have to do is:
try {
//...spring code
} catch(DataAccessException e) {
throw new Exception(e);
}
Then, your layer above this processing will catch it as a checked exception.

Do you just want to be able to access the original exception information in your View? If so, you should be able to invoke getCause() on the RuntimeException to get the underlying checked Exception that caused it. Otherwise you would need to add a "throws" declaration to your methods that are utilizing SimpleJdbcTemplate, catch DataAccessException and rethrow the checked Exceptions that are wrapped.