Java try with resources on parameters

Question

this is a stupid question, but I can't figure it out. Do I need to close Object passed as parameters inside a try with resources ?

here it is an example:

StringWriter sw = new StringWriter();
try (PrintWriter pw = new PrintWriter(sw)) {
    ...
}

Do I need to call

sw.close();

after the try block ? or does it close automatically all the Autocloseable objects passed as parameters ?

Answer 1

The implicit invocation of close on your PrintWriter will actually close the underlying Writer as well.

See following example:

StringWriter sw = new StringWriter() {
        @Override
        public void close() throws IOException {
            super.close();
            System.out.println("closed");
        }
};
try (PrintWriter pw = new PrintWriter(sw)) {};

Output

closed

Answer 2

PrintWriter 's close method will close StringWriter as well (but... yes, there is nothing to close there). It is not a common case for other wrappers.

You can pass both objects in try block to make sure they will be closed:

try (StringWriter sw = new StringWriter();
     PrintWriter pw = new PrintWriter(sw)) {
    ....
}

Answer 3

Since both of these two classes implement AutoClosable interface, you can try:

try(
StringWriter sw = new StringWriter();
PrintWriter pw = new PrintWriter(sw)
) { ... }

It will automatically close both of these objects.

Answer 4

Add a finally block and close objects passed.This is because finally block is always executed whether there is an exception or not.

StringWriter sw = new StringWriter();
try (PrintWriter pw = new PrintWriter(sw)) {
    ...
}finally{
 sw.close();
}

Answer 5

You can put multiple items inside try-with-resources initialization block:

try (final StringWriter sw = new StringWriter(); PrintWriter pw = new PrintWriter(sw)) {
    // ...
} catch (IOException e) {
    // ...
}

Both of them will be closed automatically.