Closing Streams

Upon opening a stream, your program may acquire resources such as file handles or sockets.

You must remember to close your stream to release these resources.

FileReader fr= new FileReader("input.txt"); 
BufferedReader bReader = new BufferedReader(fr); 
// ...
bReader.close();

For output streams, close ensures any written output is flushed to its destination before your program finishes.

One use of a finally block is to ensure that a stream is closed whether or not an exception has occurred.

Since the close() method itself may throw an exception, your code may end up unwieldy with a try/catch block embedded within a finally block.

BufferedReader bufIn = null;
try {
  bufIn = new BufferedReader(new FileReader("input.txt"));
  String line;
  while ((line = bufIn.readLine()) != null) {
    System.out.println(line);
  }
}
catch (IOException e) {
  System.err.println(e);
}
finally {
  if (bufIn != null) {
    try {
      bufIn.close();
    }
    catch (IOException e) {
      System.err.println(e);
    }
  }
}

The `AutoCloseable` Interface and `try`-with-resources¶

Java 7 introduced the AutoCloseable interface:

public interface AutoCloseable {
  void close() throws Exception;
}

Implementors of AutoCloseable can take advantage of try-with-resources syntax.

If an implementing object is declared within parentheses after the try keyword, Java will automatically invoke its close method regardless of how the try/catch terminates.

try ( BufferedReader bufIn = new BufferedReader(new FileReader("input.txt")) ) {
  String line;
  while ((line = bufIn.readLine()) != null) {
    System.out.println(line);
  }
}
catch (IOException e) {
  System.err.println(e);
}

Any catch or finally blocks associated with a try-with-resources statement will run after the resources are closed.
Since the closeable object must be declared in the try, it will be out of scope anywhere else.

Practice Exercise¶

The examples here of opening a file with a BufferedReader, reading a line at a time until the end of the file, and closing the file, is code you can copy and paste with minimal modification into any program that reads text data.

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Closing Streams

The AutoCloseable Interface and try-with-resources¶

Practice Exercise¶

The `AutoCloseable` Interface and `try`-with-resources¶