Why is try-with-resources catch block selectively optional?

Question

I read that the catch block in try-with-resources is optional. I\'ve tried creating a Connection object in a try-with-resources block, with no subsequ

Answer 1

You can create an AutoClosable that does not require an explicit catch-block by declaring your AutoClosable's close() method without any Exception or with a RuntimeException. Without any Exception it is clear that no catch-block is required. Further, the compiler does not statically check for a RuntimeException to be catched (in contrast to checked Exceptions).

Example:

public class AutoClosableDemo
{

    public static void main( final String[] args )
    {
        try (MyAutoCloseable1 mac1 = new MyAutoCloseable1())
        {
            System.out.println( "try-with-resource MyAutoCloseable1" );
        }
        try (MyAutoCloseable2 mac2 = new MyAutoCloseable2())
        {
            System.out.println( "try-with-resource MyAutoCloseable2" );
        }
        // The following is not allowed, because
        // "Unhandled exception type Exception thrown by automatic close() invocation on mac3"
        // try (MyAutoCloseable3 mac3 = new MyAutoCloseable3())
        // {
        // System.out.println( "try-with-resource MyAutoCloseable13" );
        // }
        System.out.println( "done" );
    }

    public static class MyAutoCloseable1 implements AutoCloseable
    {
        @Override
        public void close()
        {
            System.out.println( "MyAutoCloseable1.close()" );
        }
    }

    public static class MyAutoCloseable2 implements AutoCloseable
    {
        @Override
        public void close() throws RuntimeException
        {
            System.out.println( "MyAutoCloseable2.close()" );
        }
    }

    public static class MyAutoCloseable3 implements AutoCloseable
    {
        @Override
        public void close() throws Exception
        {
            System.out.println( "MyAutoCloseable3.close()" );
        }
    }
}