A simple 'try with resources' in Java example

How to use Java's try-with-resources statement

To use Java's try-with-resources language feature, the following rules apply:

• All objects managed by a try with resources statement must implement the AutoCloseable interface.
• Multiple AutoCloseable objects can be created within Java's try with resources block.
• Objects declared in a try with resources statement have scope within the try block, but not the catch and finally blocks.
• The close() method of objects declared in a try with resources block is invoked regardless of whether an exception is thrown during execution.
• If an exception is thrown in the close() method, it may get categorized as a suppressed exception.

The AutoCloseable interface

AutoCloseable is a functional interface that defines only one method: close(). Here's a simple example of an AutoCloseable class named Door:

class Door implements AutoCloseable {

  public void swing() {
     System.out.print("The door is swinging. ");
   }
 
   public void close() {
     System.out.print("Now the door is closed. ");
  }
 }
 

Try-with-resources example

The following code uses the AutoCloseable Door class in a try with resources statement:

public class TryWithResourcesExample {
   public static void main(String[] args) throws Exception {
 
     try (Door door = new Door()) {
       door.swing();
     } catch (Exception e) { /* do something */ }
     } finally { /* do something */ }
 
   }
 }

When this try with resources example runs, the output of this code is:

Now the door is swinging. Now the door is closed.

In the code above, here are the steps that make this happen:

• The developer creates and initializes the Door instance in the try with resources block.
• The developer explicitly invokes the swing() method.
• The JVM implicitly calls the close() method after the try block completes.

The close() method is always invoked, regardless of whether a checked or unchecked exception is thrown in one of the try, catch or finally blocks. This simplifies resource management code and makes the application more effective.

The resource referenced by the AutoCloseable object will always be closed if a try with resources statement is used, and potential memory leaks commonly caused by a misallocation of resources are eliminated.