Develop Reference

Java - catch/rethrow behavior [duplicate]

This question already has answers here:
Exception thrown in catch and finally clause
(12 answers)
Closed 1 year ago.
I have this piece of Java code.
void convertFile() {
try{
.....
}catch (Exception e) {
logError("Error in convertJsonFile", e);
throw e;
}finally{
if (writer!=null) {
writer.close();
writer = null;
}
if (fos!=null) {
fos.close();
fos = null;
}
ms2 = System.currentTimeMillis();
logInfo(String.format("Time elapsed: %d seconds.", ((ms2-ms1)/1000)));
logInfo("File conversion complete.");
}
return f + "_changed.xml";
}
The catch block logs and rethrows the Exception.
But it seems the finally block also throws a RuntimeException.
Will the finally block be executed if the catch block rethrows Exception as done here?
If I remember correctly finally block will be executed even in this case.
OK if so... what will happen if we enter the catch block (it rethrows), then we enter the finally block, and the finally block also throws a RuntimeException at this line writer.close(); ? Which exception will be thrown from this whole method - the rethrown one from the catch block, or the RuntimeException from the finally block?!
I think the RuntimeException will be the final outcome of the method and we will never reach the line in the catch block which rethrows. Because I guess the line which rethrows is executed after the finally block. But I am not sure. I got really confused.
Could someone clear my doubts here?
Seems I have forgotten some of these details.
And I have no decent access to the logs, they are in Elastic/Kibana and access to them is a real pain.

Yes, finally block will be executed
The exception from finally block will be thrown in this case. Thus, exception from catch block will be lost.

try-with-resource vs ordinary code with explicit close()-call [duplicate]

This question already has answers here:
What's the purpose of try-with-resources statements?
(7 answers)
Closed 2 years ago.
I have been looking at code and I have seen try with resources. I have used the standard try-catch statement before and it looks like they do the same thing. So my question is Try With Resources vs Try-Catch what are the differences between those, and which is better.
Here is a try with resources :
objects jar = new objects("brand");
objects can= new objects("brand");
try (FileOutputStream outStream = new FileOutputStream("people.bin")){
ObjectOutputStream stream = new ObjectOutputStream(outStream);
stream.writeObject(jar);
stream.writeObject(can);
stream.close();
} catch(FileNotFoundException e) {
System.out.println("sorry it didn't work out");
} catch(IOException f) {
System.out.println("sorry it didn't work out");
}

The main point of try-with-resources is to make sure resources are closed reliably without possibly losing information.
When you don't use try-with-resources there's a potential pitfall called exception-masking. When code in a try block throws an exception, and the close method in the finally also throws an exception, the exception thrown by the try block gets lost and the exception thrown in the finally gets propagated. This is usually unfortunate, since the exception thrown on close is something unhelpful while the informative one is the one thrown from within the try block. (So instead of seeing the SQLException that tells you which referential integrity constraint was violated, you're shown something like BrokenPipeException where closing the resource failed.)
This exception-masking is an annoying problem that try-with-resources prevents from happening.
As part of making sure exception-masking wouldn't lose important exception information, when try-with-resources was developed they had to decide what to do with the exceptions thrown from the close method.
With try-with-resources, if the try block throws an exception and the close method also throws an exception, then the exception from the close block gets tacked on to the original exception:
... there are situations where two independent exceptions can be thrown in sibling code blocks, in particular in the try block of a try-with-resources statement and the compiler-generated finally block which closes the resource. In these situations, only one of the thrown exceptions can be propagated. In the try-with-resources statement, when there are two such exceptions, the exception originating from the try block is propagated and the exception from the finally block is added to the list of exceptions suppressed by the exception from the try block. As an exception unwinds the stack, it can accumulate multiple suppressed exceptions.
On the other hand if your code completes normally but the resource you're using throws an exception on close, that exception (which would get suppressed if the code in the try block threw anything) gets thrown. That means that if you have some JDBC code where a ResultSet or PreparedStatement is closed by try-with-resources, an exception resulting from some infrastructure glitch when a JDBC object gets closed can be thrown and can rollback an operation that otherwise would have completed successfully.
Without try-with-resources whether the close method exception gets thrown is up to the application code. If it gets thrown in a finally block when the try block throws an exception, the exception from the finally block will mask the other exception. But the developer has the option of catching the exception thrown on close and not propagating it.

You missed something, the finally block. The try-with-resouces will make it something like,
FileOutputStream outStream = null;
try {
outStream = new FileOutputStream("people.bin");
ObjectOutputStream stream = new ObjectOutputStream(outStream);
stream.writeObject(jar);
stream.writeObject(can);
stream.close();
} catch(FileNotFoundException e) {
System.out.println("sorry it didn't work out");
} catch(IOException f) {
System.out.println("sorry it didn't work out");
} finally {
if (outStream != null) {
try {
outStream.close();
} catch (Exception e) {
}
}
}
Which means you really wanted something like (never swallow exceptions),
try (FileOutputStream outStream = new FileOutputStream("people.bin");
ObjectOutputStream stream = new ObjectOutputStream(outStream);) {
stream.writeObject(jar);
stream.writeObject(can);
// stream.close(); // <-- closed by try-with-resources.
} catch(FileNotFoundException e) {
System.out.println("sorry it didn't work out");
e.printStackTrace();
} catch(IOException f) {
System.out.println("sorry it didn't work out");
e.printStackTrace();
}

The only difference is that try-resource is adding automatically resource.close();
as you would do in finally block

Any object (either the class or their superclass) that implements java.lang.AutoCloseable or java.io.Closeable
can only be used in try-with-resource clause.
AutoClosable interface is the parent interface and Closable interface extends the AutoClosable interface.AutoClosable interface has method close which throws Exception while Closable interface has method that throws IOException.
We can also have catch and finally block followed by try-with-resource like ordinary try, catch and finally, but catch and finally block only get executed once the resource declared inside the try-with-resource clause is closed.

Succinctly it is syntactic sugar to support the AutoClosable interface and call the close() method for you for any outcome.

what's try method means in java? [duplicate]

This question already has answers here:
What's the purpose of try-with-resources statements?
(7 answers)
Closed 2 years ago.
I have been looking at code and I have seen try with resources. I have used the standard try-catch statement before and it looks like they do the same thing. So my question is Try With Resources vs Try-Catch what are the differences between those, and which is better.
Here is a try with resources :
objects jar = new objects("brand");
objects can= new objects("brand");
try (FileOutputStream outStream = new FileOutputStream("people.bin")){
ObjectOutputStream stream = new ObjectOutputStream(outStream);
stream.writeObject(jar);
stream.writeObject(can);
stream.close();
} catch(FileNotFoundException e) {
System.out.println("sorry it didn't work out");
} catch(IOException f) {
System.out.println("sorry it didn't work out");
}

The main point of try-with-resources is to make sure resources are closed reliably without possibly losing information.
When you don't use try-with-resources there's a potential pitfall called exception-masking. When code in a try block throws an exception, and the close method in the finally also throws an exception, the exception thrown by the try block gets lost and the exception thrown in the finally gets propagated. This is usually unfortunate, since the exception thrown on close is something unhelpful while the informative one is the one thrown from within the try block. (So instead of seeing the SQLException that tells you which referential integrity constraint was violated, you're shown something like BrokenPipeException where closing the resource failed.)
This exception-masking is an annoying problem that try-with-resources prevents from happening.
As part of making sure exception-masking wouldn't lose important exception information, when try-with-resources was developed they had to decide what to do with the exceptions thrown from the close method.
With try-with-resources, if the try block throws an exception and the close method also throws an exception, then the exception from the close block gets tacked on to the original exception:
... there are situations where two independent exceptions can be thrown in sibling code blocks, in particular in the try block of a try-with-resources statement and the compiler-generated finally block which closes the resource. In these situations, only one of the thrown exceptions can be propagated. In the try-with-resources statement, when there are two such exceptions, the exception originating from the try block is propagated and the exception from the finally block is added to the list of exceptions suppressed by the exception from the try block. As an exception unwinds the stack, it can accumulate multiple suppressed exceptions.
On the other hand if your code completes normally but the resource you're using throws an exception on close, that exception (which would get suppressed if the code in the try block threw anything) gets thrown. That means that if you have some JDBC code where a ResultSet or PreparedStatement is closed by try-with-resources, an exception resulting from some infrastructure glitch when a JDBC object gets closed can be thrown and can rollback an operation that otherwise would have completed successfully.
Without try-with-resources whether the close method exception gets thrown is up to the application code. If it gets thrown in a finally block when the try block throws an exception, the exception from the finally block will mask the other exception. But the developer has the option of catching the exception thrown on close and not propagating it.

You missed something, the finally block. The try-with-resouces will make it something like,
FileOutputStream outStream = null;
try {
outStream = new FileOutputStream("people.bin");
ObjectOutputStream stream = new ObjectOutputStream(outStream);
stream.writeObject(jar);
stream.writeObject(can);
stream.close();
} catch(FileNotFoundException e) {
System.out.println("sorry it didn't work out");
} catch(IOException f) {
System.out.println("sorry it didn't work out");
} finally {
if (outStream != null) {
try {
outStream.close();
} catch (Exception e) {
}
}
}
Which means you really wanted something like (never swallow exceptions),
try (FileOutputStream outStream = new FileOutputStream("people.bin");
ObjectOutputStream stream = new ObjectOutputStream(outStream);) {
stream.writeObject(jar);
stream.writeObject(can);
// stream.close(); // <-- closed by try-with-resources.
} catch(FileNotFoundException e) {
System.out.println("sorry it didn't work out");
e.printStackTrace();
} catch(IOException f) {
System.out.println("sorry it didn't work out");
e.printStackTrace();
}

The only difference is that try-resource is adding automatically resource.close();
as you would do in finally block

Any object (either the class or their superclass) that implements java.lang.AutoCloseable or java.io.Closeable
can only be used in try-with-resource clause.
AutoClosable interface is the parent interface and Closable interface extends the AutoClosable interface.AutoClosable interface has method close which throws Exception while Closable interface has method that throws IOException.
We can also have catch and finally block followed by try-with-resource like ordinary try, catch and finally, but catch and finally block only get executed once the resource declared inside the try-with-resource clause is closed.

Succinctly it is syntactic sugar to support the AutoClosable interface and call the close() method for you for any outcome.

Does finally completely execute if an exception is thrown within finally block

so I have a bit of code here and I'm not sure entirely how it would react in the event that the reader.close() method throws an exception.
public void someMethod(String s) throws IOException{
BufferedReader reader = Files.newBufferedReader(filePath,cs);
listRWLock.readLock().lock();
try{
//miscellaneous code involving reading
}finally{
reader.close()
listRWLock.readLock().unlock()
}
}
ListRWLock is a ReentrantReadWriteLock. In the event that the reader.close() method throws an exception, would the statement after it fail to execute? I've tried searching for the topic, and while I've gotten something about finally executing in the event of return statements, I haven't managed to find details on what happens if an exception is thrown within the finally block.
Thanks in advance.

Basically, finally clauses are there to ensure proper release of a resource. However, if an exception is thrown inside the finally block, that guarantee goes away.
An issue for which there's no really neat solution is that code in the finally block could itself throw an exception. In this case, the exception in the finally block would be thrown from the exception instead of any exception occurring inside the try block. Since code in the finally block is intended to be "cleanup" code, we could decide to treat exceptions occurring there as secondary, and to put an excplicit catch:
public int readNumber(File f) throws IOException, NumberFormatException {
BufferedReader br = new BufferedReader(new
InputStreamReader(new FileInputStream(f), "ASCII"));
try {
return Integer.parseInt(br.readLine());
} finally {
try { br.close(); } catch (IOException e) {
// possibly log e
}
}
}
Some other things to note about finally blocks:
The same 'overriding' problem that we mentioned with exceptions
occurs when returning a value from a finally block: this would
override any return value that the code in the try block wanted to
return. In practice, returning a value from a finally clause is rare
and not recommended.
Actually exiting the program (either by calling System.exit() or by
causing a fatal error that causes the process to abort: sometimes
referred to informally as a "hotspot" or "Dr Watson" in Windows)
will prevent your finally block from being executed!
There's nothing to stop us nesting try/catch/finally blocks (for
example, putting a try/finally block inside a try/catch block, or
vice versa), and it's not such an uncommon thing to do.

You can do something like this:
try{
//miscellaneous code involving reading
}finally{
handlePossibleException(reader);
listRWLock.readLock().unlock()
}
handlePossibleException(BufferedReader reader) {
try {
if (reader != null) {
reader.close();
}
} catch( Exception e ) {
log.e( "reader.close() Exception: ", e );
}
}

You should test this,
but if try throws IO Exception, then your reader wont close.
Maybe have
catch(IOException ex){
reader.close()

An exception can happen anywhere in your code, including finally block, so you have to catch it as anywhere else in your code. Check the following post to get some ideas about the ways you can handle such situation:
throws Exception in finally blocks

how to avoid program termination when exception occur in java

I am doing an app for finding BPM in Android. Based on the input of the file null pointer exception and IO exception is getting thrown. When the exception occurs the program is terminated, even if I surround the code with a try/catch block. My requirement is that the program not terminate, and continue to the other parts.
My Code:
try {
mp3file.seekMP3Frame();
sourcefile = new File("/sdcard/a.mp3");
mp3file = new MP3File(sourcefile);
AbstractID3v2 tag = mp3file.getID3v2Tag();
text=tag.getFrame("TBPM").toString();
}
catch (NullPointerException e) {
System.out.println("null pointer exception");
}
catch (IOException e1) {
System.out.println("IO exception");
}
........ // other coding

If the error occurs between the try - catch block :
You are only catching 2 exceptions here (NullPointerException and IOException).
Any other runtime exception between the try - catch block can still cause your program to crash. catch java.lang.Throwable to ensure all exceptions are catched.
If the error doesn't occur between the try - catch block :
Look in the stacktrace for the line in your class where the error occurs, and implement "proper" error handling there.
If your application crashes, you'll always see a stacktrace in logcat. That stacktrace will be able to tell you what line of code is causing the crash.

Use finally block in your code which is like
`try{
} catch{
} finally{
}`
finally will execute even if your try block throws exception or even if it doesn't.